C L I N I C A L
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E X P E R I M E N T A L
OPTOMETRY RESEARCH PAPER
Monocular and binocular reading performance in subjects with normal binocular vision Clin Exp Optom 2014; 97: 341–348 Jan Johansson MSc Tony Pansell PhD Jan Ygge PhD MD Gustaf Öqvist Seimyr PhD, MA Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden E-mail: [email protected]
Submitted: 28 June 2013 Revised: 18 November 2013 Accepted for publication: 21 November 2013
DOI:10.1111/cxo.12137 Background: It is well known that problems with binocular vision can cause issues for reading; less known is to what extent binocular vision improves reading performance. The purpose of this study was to explore the role of binocularity by directly comparing monocular and binocular reading in subjects with typical reading skills and normal binocular vision. A secondary purpose was to assess any asymmetry in monocular performance and its association with the sighting dominant eye. Methods: In a balanced repeated measures experiment, 18 subjects read paragraphs of text under monocular and binocular conditions. All subjects went through an optometric examination before inclusion. Reading speed and eye movements were recorded with an eye tracker. Results: The mean difference in reading speed (2.1 per cent) between monocular (dominant and non-dominant eye averaged) and binocular reading speed was not significant. A significant difference in reading speed was found between binocular and the non-dominant eye, as determined by the far sighting test (p = 0.03). Monocular reading showed significantly increased (8.9 per cent) fixation duration (p < 0.01) and longer regressive saccades by 0.43 character spaces (p < 0.01). Reading with the non-dominant eye, as determined by the near sighting test, showed increased progressive saccade length by 0.2 characters compared to the dominant eye (p = 0.03). No other significant differences between dominant and non-dominant eyes were found. The agreement between the faster reading eye and ocular dominance was 44 to 56 per cent depending on whether dominance was determined at near or far. Conclusion: The outcomes suggest that in subjects with normal binocular vision, there is no marked enhancement in reading performance by binocular vision when reading paragraphs of text. Furthermore, the monocular reading performance appears to be close to equal and any small differences in performance appear not to be strongly associated with ocular dominance.
Key words: binocular, eye movements, monocular, ocular dominance, sighting dominance, reading, reading speed Binocular vision offers several advantages compared to monocular vision. Our visual field is extended by having two eyes and the offset in the overlapping retinal images allows the brain to discriminate depth in the visual scene. Having two visual inputs of the same stimulus also allows for binocular summation which results in higher visual acuity,1–4 greater contrast sensitivity,1,5–7 and faster processing speed of visual stimuli.8 Having two visual inputs can also be disadvantageous. When the retinal images do not match, we get blurry and strained vision, in severe cases even double vision. It is well known that problems with binocular vision can cause issues for reading; less known is if binocular vision improves reading perfor-
mance. Is binocular vision really advantageous for reading? This knowledge may be of importance when reading is used as a measure of visual function. In the current experiment, the intention is to explore what can be expected in terms of a binocular advantage, when reading is symptom free and binocular vision parameters are within physiological normal values. If binocular vision improves reading performance, it would be rewarding to understand in which way binocularity enhances the reading process. Most models of reading do not take binocular aspects into account. If binocular vision does not improve reading performance, it would be interesting to know why. Could it also be that monocular
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
reading may be faster than binocular reading in subjects without problems with binocular vision? Previous research offers little guidance regarding what the advantages of binocular vision could be for a typical reader with normal binocular status and reading skill. We have only found four studies which have investigated monocular and binocular reading of continuous text. Spache9 had 23 children of marked eye preference orally read texts (Gray’s Oral Check Test) binocularly and monocularly. The results showed that 43 per cent read significantly better with the preferred eye than with both. Robinson10 had 75 students read the same texts orally under monocular and binocular conditions. Clinical and Experimental Optometry 97.4 July 2014
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Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
No significant differences were found for reading speed or reading errors between right eye, left eye or binocular reading. Heller and Radach11 had eight subjects silently read texts (200 lines) under binocular and monocular left and right conditions. In that experiment, eye movements were studied and monocular reading (left and right eye averaged) showed an increase of fixation duration (six per cent), number of fixations per line (10 per cent) and the proportion of regressive saccades (16 per cent). This was considered to be evidence of monocular reading being more difficult; however, it is difficult to draw clear conclusions from these three studies, since there was no description of the visual or binocular status of the participants. Second, it is not clear if texts of equal characteristics and difficulty were used for the repeated reading sessions. Kanonidou, Proudlock and Gottlob12 included 20 subjects with normal binocular vision as controls in an experiment regarding reading strategies in strabismic amblyopia. The subjects read paragraphs of texts (Brothers Grimm fairy tales) on a screen 1.2 metres from the subject, while eye movements were recorded. A decrease in reading speed (five per cent) was found during monocular reading (dominant and non-dominant eye averaged) for the controls along with an increased fixation duration (five per cent), decreased progressive saccade amplitude (two per cent) and increased rate of regressive saccades per line (3.5 per cent). When comparing dominant and non-dominant eyes, the reading speed, fixation duration and proportion of regressive saccades were equal but the nondominant eye displayed a shorter progressive saccade length by 0.1 degrees (2.6 per cent). The method of determining dominance was not described. The paragraphs of text were of comparable length but any variability in readability index between the texts was not described, which makes it more difficult to evaluate the differences in reading performance. Three additional studies have investigated effects on letter, word and sentence reading. Jones and Lee13 had 10 subjects orally read letters on cardboard cards under binocular and monocular conditions in bright and dim light. Any effects of ocular dominance were controlled in the experiment by having half of the subjects use their dominant eye, as determined by a sighting task, and half use their non-dominant eye. The results showed that letter identification errors Clinical and Experimental Optometry 97.4 July 2014
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under monocular conditions increased slightly in bright light but the difference was more pronounced in dim light. Sheedy et al.14 had 13 subjects silently read word charts (Bailey–Lovie) under binocular and monocular conditions. The results showed a three per cent decrease in reading speed for the best performing eye in the monocular compared to the binocular condition. More recently, Jainta and Jaschinski15 had 13 subjects silently read single sentences (Potsdam Sentence Corpus) under binocular and monocular (dominant and non-dominant eye) conditions in a haploscope. Ocular dominance was determined with a sighting test. The results of monocular reading showed increased first fixation duration, as well as increased progressive saccade length and disconjugacy after saccades. No significant differences were found between the performances in dominant and nondominant eyes. Another finding was the influence of heterophoria on binocular co-ordination and fixation duration. While the first fixation duration increased for orthophoric readers at monocular reading, it decreased in readers with greater heterophorias. It was speculated that higher demands on the binocular system due to heterophoria were lessened under monocular conditions, leading to reduced fixation duration. One aspect that is close at hand when comparing monocular and binocular performance is the potential effect of ocular dominance. There are different ways of classifying and testing ocular dominance and the fact that a subject’s dominance may change with different tests and test conditions16 adds to the complexity of determining ocular dominance. In experimental settings, where one eye needs to be selected, it is frequently recommended to choose the dominant eye, as defined by sighting dominance tests. The wide application of sighting dominance tests may be related to their ease of use for the clinician and the patient, as well as their ability to provide clear-cut responses in most cases. An issue with the sighting test is that it assesses the preference of an eye in a monocular task, that is, it forces the subject to choose an eye, when in fact most everyday visual tasks, for example, reading, involve both eyes. What speaks for a superior performance of the sighting dominant eye is research that compared the function of dominant and non-dominant eyes.17–19 A study of the effect of ocular dominance on latency and amplitude of visual
evoked potentials found shorter latency as well as greater amplitude for the dominant eye.20 Other studies have found poor agreement between the sighting dominant eye and performance superiority14 and it has been suggested that the dominant eye has no unique role in vision other than in monocular tasks.21 In this paper, we refer to the dominant eye as classified by sighting tests. Acknowledging the limitations of defining ocular dominance based on this test, we wished to explore its possible connection to reading performance, given its wide application in clinical and experimental settings. Reading is a very complex task and reading rate and properties of reading eye movements are strongly influenced by lexical and cognitive factors in the process. Still, the visual input is a starting point that needs to provide sensory input at an adequate speed and effort to allow for efficient higher-order processing. In the present project, we focus on the contribution of binocular vision. The main objective of this study is to study the differences in reading performance between monocular and binocular reading in the typical reader. The secondary objective is to assess any asymmetry in monocular performance, that is, if either eye is superior in performance and how this might be associated with the dominant eye. METHODS The experiment was designed as a balanced repeated-measurements study, where each subject silently read texts under all conditions (monocular right, monocular left and binocular) while eye movements were recorded. The conditions were repeated three times, so that each repetition began with a different condition and no condition was immediately repeated. Each text was read once by each subject. The inclusion criteria for participation were that the subjects are subjectively asymptomatic when reading, have normal binocular status, have not been diagnosed with ocular disease or reading difficulties and have an ability to read Swedish text fluently. Subjects requiring spectacles were excluded to avoid quality issues with the eye movement recordings due to the frame obstructing the eye tracker’s view of the eye. Subjects were allowed to wear contact lenses. The study adhered to the tenets of the Declaration of Helsinki and was approved by © 2014 The Authors
Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
nya butik ovanpå en djup källare. Varje natt kom det massor av möss upp från källaren och in i butiken. Där mumsade de sedan friskt i sig av äpplen, nötter och vattenmeloner. Inte ens grönsaker och potatisar lät de bli att knapra pä. Mellan midnatt och soluppgången gick inga varor i butiken säkra för de små gnagarna. Så länge det hördes ljud på gatorna och bilar körde förbi höll sig mössen helt tysta i källaren. Men så snart som den gamla klockan i stadshuset slagit tolv, och det blev tyst på gatan, då strömmade de fram och njot av de söta godsakerna under rejäla fester. Resterna som väntade på grönsakshandlaren om morgnarna när han steg in i butiken fyllde honom med förtvivlan. I sina försök att skydda sig mot mössen placerade han ut fällor
1024 px = 25.2 deg @ 60 cm
I den lilla staden hade en grönsakshandlare öppnat sin
runtomkring i butiken.
1280 px = 31.3 deg @ 60 cm Figure 1. Stimulus used in the experiment with screen dimensions and visual angles
the regional ethics review board in Stockholm, EPN (2012/1447-31/1).
SUBJECTS A total of 18 subjects were included in the study. Another five subjects participated but were excluded from the analysis; three due to not meeting the criteria for vision and two due to the quality of the eye movement recordings. The mean age of the subjects was 25.2 ± 4.5 years and 12 subjects (67 per cent) were female. All subjects had normal binocular status with stereovision of 60 seconds of arc or better and were fluent in Swedish.
missible occluder allowing binocular recording, which is required by the Tobii eye tracker. The recorded eye movement data were processed using Visiolyzer, a software developed at our laboratory. Fixations were defined according to a fixation dispersion model, that is, when the centre of gravity of recorded fixation points stayed within a radius of 0.75 degrees (2.7 character spaces) for a minimum of 50 milliseconds. Progressive saccades were defined as a rightward movement between two adjacent fixations and regressive saccades were defined as a leftward movement between two adjacent fixations.
Apparatus
Stimuli
A Tobii 1750 eye tracker (Tobii Technology, Stockholm, Sweden) was used for eye movement recording. It is a display mounted tracker that records eye movements at 50 Hz using infra-red video technology. The accuracy is 0.5 degrees and the resolution is 0.25 degrees according to the manufacturer’s specifications. Each recording was preceded by a binocular calibration using a nine-point sequence. During monocular reading, one of the eyes was covered by an infra-red trans-
Nine texts from the International Reading Speed Texts (IReST)22 were used in the experiment. The IReST texts are validated in 17 languages including Swedish. The Swedish texts have the exact same number of words (146), characters (684) and lines (16) and all texts have the same readability index (LIX 35). The mean word length is 4.61 ± 0.01 characters. The texts were presented as a single paragraph subtending 14.3 by 17.1 degrees on the integrated screen of the eye
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
tracker, which was placed 60 cm from the eyes of the subject (Figure 1). The characters were dark on a white background (Weber contrast -0.98). The font was Helvetica and the mean character width was 0.28 degrees.
Procedure Before starting the experiment, the subjects were informed verbally and in writing of the details of the experiment. All subjects gave informed consent prior to the experiment. Next, the subject’s visual status was assessed, including a brief history, visual acuity at far and near (Logarithmic Visual Acuity Chart 2000 ‘ETDRS’ 4 m/40 cm), stereoscopic visual acuity (TNO random dot test), cover test, fusional reserves, eye motility, near point of convergence and near point of accommodation (RAF ruler) and a suppression test (Bagolini striated lenses). Symptoms experienced during day-to-day near work were assessed using a translated version of the Revised Convergence Insufficiency Symptom Survey (CISS).23 If a subject wore contact lenses, these were kept on during the vision screening. Ocular dominance was assessed at far (400 cm) and near (40 to 50 cm). At far, the hole-in-card sighting test21,24,25 was used. In this test, the subject held a card (20.0 by 12.8 cm) with both hands at arm’s length looking at a black cross at four metres through a 3.0 cm diameter hole in the centre of the card. The subject was instructed to look with both eyes at all times. When the subject reported to have aligned the hole in the card and the black cross, each eye was covered in turn and the subject was asked to report which eye kept the alignment. This eye was recorded as the dominant eye. The test was repeated four times for all subjects. For near dominance assessment the near hole-in-card sighting test24 was used. The procedure was the same as in the far assessment. The subject was then seated comfortably and unrestrained in a steady chair in front of the eye tracker. The position of the eye tracker was adjusted horizontally and vertically to optimise the signal quality. A test run was made for the subject to get acquainted with the procedure. Initially there was the calibration procedure, which was triggered by the test leader. Next, the subject read nine texts under the different reading conditions. The subject started each reading session by pressing the space bar on a keyboard, whereby a fixation cross was presented on Clinical and Experimental Optometry 97.4 July 2014
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Table 1. Summary of visual status
0
23 6 5 20 -14 12 -8 0 0 60 -0.10 -0.08 0.00 0.02 0.08 0.02 23 18
16 9
10 5
5 35
16 -10
-12 25
20 -6
-6 -6
0 0
0 30
60 -0.10
-0.20 0.00
0.00 -0.08
-0.08 -0.10
-0.08 0.02
-0.08 -0.10
0.00 23
38 16
0.10
0.10 22
17
4
11 6 5 20 -6 16 -6 0 0 60 0.00 0.10 0.10
19
15
0.02
10 5 20 -12 10 -8 -4 0 30 -0.10 -0.08 -0.10 19
-0.18
12
14
-0.08 -0.08
9
6 5
10 16
35 -8
-6 10
12 -8
-3 -3
1 0
0 60
30 -0.20
0.00 0.06 0.00
-0.08 -0.10 -0.12
-0.02 0.00
0.06 31 13
0.00 22 12
-0.06
6
16 9 5 16 -8 14 -8 0 0 60 -0.08 0.02 0.02 22
-0.20
8
11
-0.08 -0.14
6
6 5
5 20
30 -8
-6 12
20 -8
-6 0
0 0
0 30
30 -0.14
0.00 -0.06 0.00
-0.10 -0.08
20
-0.10
-0.08 -0.08 26 9
10
-0.18
-0.24 -0.10 27
-0.06 -0.12
7
10 10 5 35 -4 25 -6 0 0 60 -0.10 -0.08 -0.08
8
8
-0.18
9 5 20 -6 16 -6 0 0 60 -0.14 -0.10 -0.10 30
-0.16
7
7
-0.10 -0.10
6 5 25 -8 18 -8 -1 0 30 -0.16 -0.12 -0.12 -0.26 23 6
-0.18
-0.26 -0.26
18 10
6 5
5 20
35 -14
-10 16
35 -10
-8 0
0 0
0 30
15 -0.10
-0.10 0.00 0.00
-0.08 -0.08 -0.20
-0.26
29 5
0.00 23 4
-0.14 -0.12
2
11 7 5 30 -16 30 -10 -2 0 60 -0.20 -0.20 -0.18 -0.30 24 3
-0.28 -0.18
10 5
8 5
5 25
25 -8
-10 14
14 -8
-6 0
0 0
0 30
30 -0.20
-0.10 0.02
-0.10 0.00
0.00 -0.30
Left Eye Binoc. Right Eye
-0.30 -0.18 -0.28 28
-0.18 -0.20 23
2
IBM SPSS version 19 was used for statistical analysis. The significance level was set to five per cent (α = 0.05) and tests were two-sided. Parametric data were analysed using paired t-tests. Reading speed was calculated as words read per minute (WPM), comprehension
1
Statistical analysis
Left Eye
A relative score27 was applied on the test responses, that is, the response at each trial was set as -1 in the case of left eye dominance and +1 in the case of right eye dominance. The relative score was calculated as the algebraic sum of responses divided by the number of trials. This resulted in a score ranging from -1 in the case of left eye dominance in all trials, zero in the case of no dominance (50–50 mixed responses) and +1 in the case of right eye dominance in all trials.
Right Eye
Ocular dominance
Visual Acuity Near (LogMar)
The mean interocular difference in visual acuity was logMAR 0.03 (1.5 optotypes) and none of the subjects differed more than one line (five optotypes) at near. The mean binocular summation effect in visual acuity was 13 per cent, that is, the minimum angle of resolution (MAR) was smaller (better) by this percentage during the binocular condition. All subjects reported that their eyes were healthy and not under any treatment. One subject scored two points above the cut-off point for normal level of symptoms on the CISS26 but was still included as the optometric measures were within normal limits (Table 1).
Visual Acuity Far (LogMar)
Visual status
Subject Age
the centre of the screen. A moment later the full text was presented on the screen. The subject was instructed to fixate the cross carefully and then to start reading at his or her own pace as soon as the text appeared and to finish by pressing the space bar again as soon as the last word was read. The subjects were instructed to read the texts for comprehension and were informed that they would need to answer questions about the content after finishing reading. There were four multiple-choice questions with three alternatives for each text. After each text was finished, the subject answered the multiple-choice questions in writing. The test leader was present at all times giving instructions before each reading session and monitoring the eye movements online.
Stereo Covertest Covertest Breakpoint Breakpoint Breakpoint Breakpoint Nearpoint Nearpoint Symptom 40 cm Divergence Convergence Convergence Divergence Convergence Accommodation Score Vision Test 400 cm (Sec. Arc.) (Prism (Prism 400 cm 400 cm (Prism 40 cm (Prism 40 cm (Prism (cm) (cm) Binoc. Diopt.) Diopt.) (Prism Diopt.) Diopt.) Diopt.) Diopt.)
Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
18 17 16
300
15 14 13
11 10
Near
9
Far
8 7
Words per minute
280
12
260
240
6 5 4
220
3 2 1 -1
-0.5
200 0
0.5
1
Binocular
Monocular
Figure 2. Relative score on ocular dominance for subjects 1 to 18. Negative scores indicate left dominance and positive scores indicate right dominance. A score of zero indicates no dominance, that is, equal number of responses left and right.
Figure 3. Mean binocular and monocular reading speed. Error bars represent 95% confidence interval.
assessment. In the far-sighting test, all subjects consistently chose the same eye in all trials, while the near-sighting test indicated less consistency with two subjects (subject 10 and 11) showing weaker dominance and one subject showing no dominance (subject 5) (Figure 2).
determined at near, there was a significant difference (p = 0.03) in progressive saccade length. Reading with the nondominant eye slightly increased the saccade length by 0.06 degrees (0.2 characters) (Table 3). The agreement between the dominant eye and faster reading speed was 56 per cent with dominance determined at distance and 44 per cent with dominance determined at near.
scores were enumerated as the percentage of correct answers and eye movement parameters were averaged for the texts. Each subject read three texts under binocular conditions and six texts under monocular conditions (three with either eye). Reading speed, comprehension scores and eye movement parameters were averaged over the texts within the conditions before statistical comparison. The analysis of monocular versus binocular performance included monocular performance as the mean of dominant and non-dominant eyes and secondly monocular performance for dominant and non-dominant eyes separately. RESULTS
Ocular dominance According to the hole-in-card sighting test performed at distance, 12 subjects (67 per cent) were right eye dominant and six subjects (33 per cent) were left eye dominant. In the near sighting test, the distribution was 11 subjects (61 per cent) right eye dominant, six subjects (33 per cent) left eye dominant and one subject (six per cent) with no eye dominance. Five subjects (28 per cent) switched dominance between far and near
Monocular versus binocular Monocular reading speed (dominant and non-dominant averaged) decreased by 2.1 per cent compared to binocular but the difference was not significant (Figure 3). There was no difference in comprehension between the conditions. There were small but significant differences in the fixation durations (p < 0.01) and the lengths of regressive saccades (p = 0.01) between the conditions. Reading monocularly increased the fixation duration by 16.6 ms and the regressive saccades became 0.12 degrees (0.4 characters) longer. None of the other eye movement measures differed significantly (Table 2).
Dominant versus non-dominant With dominance determined at distance, there were no significant differences between the monocular conditions for any of the measures but with dominance
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Dominant and non-dominant versus binocular With dominance determined at distance, there was a significant difference in reading speed between reading with the nondominant eye and reading binocularly (p = 0.03) (Figure 4). There were also significant differences in fixation durations for the dominant (p < 0.01) and the non-dominant (p < 0.01) eyes compared with reading binocularly. With dominance determined at near, there were significant differences between reading binocularly and with the dominant (p < 0.01) and non-dominant (p < 0.01) eyes for fixation durations; for the non-dominant eye there was also a significant difference in regressive saccade length (p < 0.01) (Tables 2 and 3). Clinical and Experimental Optometry 97.4 July 2014
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Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
Binocular reading
Monocular reading
269.6 ± 33.5
264.2 ± 36.3
94.9 ± 7.4
92.8 ± 6.2
Reading speed (WPM) Comprehension (%)
0.96 ± 0.11
0.94 ± 0.13
187.4 ± 23.5
204.0 ± 29.2
Progressive saccade length (char.)
7.4 ± 0.9
7.4 ± 1.0
Regressive saccade length (char.)
4.8 ± 1.4
5.2 ± 1.6
Proportion of regressive saccades
0.2 ± 0.1
0.2 ± 0.1
Fixations per word Fixation duration (ms)
Table 2. Binocular and monocular reading performance
Dominance determined at:
Distance
Reading speed (WPM) Comprehension (%) Fixations per word Fixation duration (ms)
Near
Dominant eye reading
Non-dominant eye reading
Dominant eye reading
Non-dominant eye reading
267.7 ± 41.8
260.6 ± 34.1
260.6 ± 38.7
267.7 ± 37.6
92.1 ± 10.2
93.5 ± 5.2
92.1 ± 9.8
93.5 ± 5.9
0.93 ± 0.14
0.95 ± 0.14
0.95 ± 0.15
0.92 ± 0.13
205.0 ± 29.3
203.0 ± 30.1
204.4 ± 29.6
203.7 ± 29.9
Progressive saccade length (char.)
7.4 ± 1.0
7.4 ± 1.1
7.3 ± 1.0
7.5 ± 1.1
Regressive saccade length (char.)
5.4 ± 1.8
5.1 ± 1.4
5.0 ± 1.6
5.4 ± 1.6
Proportion of regressive saccades
0.2 ± 0.1
0.2 ± 0.1
0.2 ± 0.1
0.2 ± 0.1
Table 3. Dominant and non-dominant reading performance
DISCUSSION 300
Words per minute
280
260
240
220
200 Binocular
Dominant eye Non-dominant eye Dominant eye Non-dominant eye (distance) (distance) (near) (near)
Figure 4. Reading speed for binocular, dominant and nondominant eyes (as determined by distance and near testing). Error bars represent 95% confidence interval.
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The main objective of this study was to explore the differences in reading performance between monocular and binocular reading with the intention to identify if and in what way binocularity enhances reading performance. Second, we wanted to explore any tendency for monocular performance superiority and its association with ocular sighting dominance. The position of the subject was unrestrained to achieve a nearto-normal reading situation. The stimuli were texts of readability equivalent to fiction books and presented at high contrast and at an acuity level well above the subject’s threshold. To deal with the fact that reading performance may be influenced by reading flexibility and strategy, the subjects were instructed to read at their own pace and to read for comprehension. Additionally, they were informed that they would need to answer questions about the content after the reading. Through these factors, the intention was to limit what differed between the © 2014 The Authors
Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
reading sessions down to the actual visual condition, that is, monocular or binocular. We used reading speed, as it is a measure of practical and clinical relevance that indicates at what speed the reader can process written information and it is used in the evaluation of visual function, for example, before and after treatment of ophthalmological issues. Additionally, the reading eye movements were analysed with the intention to understand possible mechanisms behind differences in reading performance. Mean monocular reading speed was marginally slower than binocular by 2.1 per cent. This non-significant difference was even smaller than found in the control group by Kanonidou, Proudlock and Gottlob.12 The mean absolute difference was six words per minute, which is far lower than the standard deviation and the clinical significance is questionable when reading short paragraphs of text. An additional consideration is that the monocular condition in this experiment was in an unadapted state and still the monocular-binocular difference was small. Sheedy et al.14 found that five days of occlusion tended to slightly improve monocular performance in tasks benefitting from stereopsis but the binocular performance was still superior. Even though reading may differ from tasks requiring stereopsis, we believe it cannot be excluded that time for adaptation may have served to further reduce the performance gap. None of the reading conditions differed significantly in the percentage of correctly answered questions. This finding combined with the finding of equal reading speed in monocular and binocular conditions indicates that reading performance is not immediately affected by covering one of the eyes. The fixation duration increased significantly during monocular reading. This is in line with previous research comparing monocular and binocular reading.11,12,15 It has been established previously that the duration of reading fixations mainly increases with increased effort to comprehend. In addition, there may be an effect of binocular co-ordination on fixation duration, that is, that monocular and binocular fixation duration may differ with the size of heterophoria.15 In this experiment, we did not find an association between the size of exophoria and a decrease in fixation duration when switching to the monocular condition. This may be due to quite small amounts of heterophoria in this sample.
Despite the increased fixation duration during monocular reading, it did not lead to a significant difference in reading speed from the binocular condition. The mechanism behind this remains to be understood. Why was there not a greater advantage for binocular reading? Binocular performance advantages can be achieved through binocular concordance, summation and binocular disparity stereopsis. For reading, binocular summation seems to be the most plausible mechanism for a binocular advantage. Previous research on summation effects found a decrease in summation effect as the task at hand gets more complex; for example, as in pattern recognition1,13 and as the stimulus takes on higher contrast.6,28,29 Reading being a complex task and texts being presented at high contrasts may touch on these previous findings as a partial explanation of the marginal binocular advantage found in this experiment. Another contributing explanation may be the variability found in reading speed. This may stem from the texts, which apart from being relatively short and easy to comprehend, are presented in visually favourable contrast conditions. All together, this may mean that there was an overcapacity in the processing of the text, allowing flexibility for the subjects to vary the reading speed. A future experiment where the text is visually harder to process may reduce this flexibility and more clearly display any advantage of binocular vision.
Role of ocular dominance The only significant difference in mean reading speed (3.4 per cent) was between the binocular condition and the nondominant eye as defined by the hole-in-card sighting test at far. The absolute difference was small in comparison to the standard deviation in reading speed, which makes the practical effect questionable. In this experiment, we found a weak agreement between reading performance superiority and ocular dominance as was found by Sheedy et al.14 An interesting observation was that among the subjects with greatest interocular differences in reading speed (more than 25 words per minute), the majority switched dominance between far and near testing. In this case, the faster reading eye and the dominant eye, as determined by the far test, agreed more strongly. Additionally, we noted a weak tendency that the greater the interocular difference in reading speed, the more likely the subject read faster monocularly than binocularly,
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
which was also observed by Spache,9 when studying reading in subjects with marked ocular dominance. These observations indicate that there may be subgroups of subjects with different levels of asymmetry in performance, despite normal binocular vision, possibly related to dominance. An explanation of the quite low agreement between performance and dominance may be that the sighting dominance test is not really applicable to reading, where both eyes are used simultaneously. Another explanation may be that ocular dominance has less impact on performance in subjects with normal binocular vision, that is, where both eyes have equal preconditions to contribute. On the other hand, in subjects who do demonstrate an obvious asymmetry in monocular performance, the sighting test may better predict performance superiority. Further research with more subjects will be required to draw further conclusions on ocular dominance versus performance.
Limitations This study was carried out as a preliminary experiment to explore the role of binocularity and dominance in reading, when binocular vision is close to optimal. We found minimal and mostly non-significant differences between the reading conditions. Despite using a repeated-measurements experimental design, which allows for a relatively small number of subjects to achieve statistical validity (1 - β error probability), the possibility remains that there are differences among the conditions that we did not find. Given that the significant differences we found were small, we believe the sample size was sufficient for this experiment. Second, the participants were all healthy, of young mean age and with a good level of binocular vision. This limits the extent to which the findings can be generalised to other populations. CONCLUSION The outcomes of this experiment suggest that in subjects with normal binocular vision and typical reading skills, monocular and binocular reading performances are close to equal when reading short paragraphs of text. Thus, it seems that binocular vision does not markedly enhance reading performance when reading conditions are favourable. Furthermore, the monocular reading performances appear to be close to equal and Clinical and Experimental Optometry 97.4 July 2014
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Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
any small differences in performance seem not to be strongly associated with ocular dominance. REFERENCES 1. Frisén L, Lindblom B. Binocular summation in humans: evidence for a hierarchic model. J Physiol 1988; 402: 773–782. 2. Horowitz MW. An analysis of the superiority of binocular over monocular visual acuity. J Exp Psychol 1949; 39: 581–596. 3. Barany E. A theory of binocular visual acuity and an analysis of the variability of visual acuity. Acta Ophthalmol (Copenh) 1946; 24: 63–92. 4. Heravian JS, Jenkins TC, Douthwaite WA. Binocular summation in visually evoked responses and visual acuity. Ophthalmic Physiol Opt 1990; 10: 257– 261. 5. Blake R, Sloane M, Fox R. Further developments in binocular summation. Percept Psychophys 1981; 30: 266–276. 6. Banton T, Levi DM. Binocular summation in vernier acuity. J Opt Soc Am A 1991; 8: 673–680. 7. Home R. Binocular summation: a study of contrast sensitivity, visual acuity and recognition. Vision Res 1978; 18: 579–585. 8. Woodman W, Young M, Kelly K, Simoens J, Yolton RL. Effects of monocular occlusion on neural and motor response times for two-dimensional stimuli. Optom Vis Sci 1990; 67: 169–178. 9. Spache G. One-eyed and two-eyed reading. J Educ Res 1944; 37: 616–618. 10. Robinson HM. Factors related to monocular and binocular reading efficiency. Am J Optom Arch Am Acad Optom 1951; 28: 337–346. 11. Heller D, Radach R. Eye movements in reading. Are two eyes better than one? In: Becker W, Deubel H, Mergner T, eds. Current Oculomotor Research. New York: Plenum Press, 1999. p 341–348. 12. Kanonidou E, Proudlock FA, Gottlob I. Reading strategies in mild to moderate strabismic amblyopia: an eye movement investigation. Invest Ophthalmol Vis Sci 2010; 51: 3502–3508. 13. Jones RK, Lee DN. Why two eyes are better than one: the two views of binocular vision. J Exp Psychol Hum Percept Perform 1981; 7: 30–40. 14. Sheedy JE, Bailey IL, Buri M, Bass E. Binocular vs. monocular task performance. Am J Optom Physiol Opt 1986; 63: 839–846. 15. Jainta S, Jaschinski W. Individual differences in binocular coordination are uncovered by directly comparing monocular and binocular reading conditions. Invest Ophthalmol Vis Sci 2012; 53: 5762– 5769. 16. Evans BJ. Monovision: a review. Ophthalmic Physiol Opt 2007; 27: 417–439. 17. Oishi A, Tobimatsu S, Arakawa K, Taniwaki T, Kira J-I. Ocular dominancy in conjugate eye movements at reading distance. Neurosci Res 2005; 52: 263–268. 18. Porac C, Coren S. Monocular asymmetries in recognition after an eye movement: Sighting dominance and dextrality. Percept Psychophys 1979; 25: 55–59. 19. Money J. Studies on the function of sighting dominance. Q J Exp Psychol 1972; 24: 454–464. 20. Jagadamba A, Karthiyanee K. Effect of ocular dominance on the latency and amplitude of visual evoked potentials. IOSR J Dent Med Sci 2012; 2: 19–24.
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21. Mapp AP, Ono H, Barbeito R. What does the dominant eye dominate? A brief and somewhat contentious review. Percept Psychophys 2003; 65: 310– 317. 22. Trauzettel-Klosinski S, Dietz K. Standardized assessment of reading performance: the New International Reading Speed Texts IReST. Invest Ophthalmol Vis Sci 2012; 53: 5452–5461. 23. Borsting EJ, Rouse MW, Mitchell GL, Scheiman M, Cotter S, Cooper J, Kulp MT et al. Validity and reliability of the revised convergence insufficiency symptom survey in children aged 9 to 18 years. Optom Vis Sci 2003; 80: 832–838. 24. Rice ML, Leske DA, Smestad CE, Holmes JM. Results of ocular dominance testing depend on assessment method. J AAPOS 2008; 12: 365–369. 25. Ehrenstein WH, Arnold-Schulz-Gahmen BE, Jaschinski W. Eye preference within the context of binocular functions. Graefes Arch Clin Exp Ophthalmol 2005; 243: 926–932. 26. Rouse MW, Borsting EJ, Mitchell GL, Scheiman M, Cotter S, Cooper J, Kulp MT et al. Validity and reliability of the revised convergence insufficiency symptom survey in adults. Ophthalmic Physiol Opt 2004; 24: 384–390. 27. Zeri F, De Luca M, Spinelli D, Zoccolotti P. Ocular dominance stability and reading skill: a controversial relationship. Optom Vis Sci 2011; 88: 1353–1362. 28. Blake R, Fox R. The psychophysical inquiry into binocular summation. Atten Percept Psychophys 1973; 14: 161–185. 29. Cagenello R, Arditi A, Halpern DL. Binocular enhancement of visual acuity. J Opt Soc Am A Opt Image Sci Vis 1993; 10: 1841–1848.
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
A N D
E X P E R I M E N T A L
OPTOMETRY RESEARCH PAPER
Monocular and binocular reading performance in subjects with normal binocular vision Clin Exp Optom 2014; 97: 341–348 Jan Johansson MSc Tony Pansell PhD Jan Ygge PhD MD Gustaf Öqvist Seimyr PhD, MA Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden E-mail: [email protected]
Submitted: 28 June 2013 Revised: 18 November 2013 Accepted for publication: 21 November 2013
DOI:10.1111/cxo.12137 Background: It is well known that problems with binocular vision can cause issues for reading; less known is to what extent binocular vision improves reading performance. The purpose of this study was to explore the role of binocularity by directly comparing monocular and binocular reading in subjects with typical reading skills and normal binocular vision. A secondary purpose was to assess any asymmetry in monocular performance and its association with the sighting dominant eye. Methods: In a balanced repeated measures experiment, 18 subjects read paragraphs of text under monocular and binocular conditions. All subjects went through an optometric examination before inclusion. Reading speed and eye movements were recorded with an eye tracker. Results: The mean difference in reading speed (2.1 per cent) between monocular (dominant and non-dominant eye averaged) and binocular reading speed was not significant. A significant difference in reading speed was found between binocular and the non-dominant eye, as determined by the far sighting test (p = 0.03). Monocular reading showed significantly increased (8.9 per cent) fixation duration (p < 0.01) and longer regressive saccades by 0.43 character spaces (p < 0.01). Reading with the non-dominant eye, as determined by the near sighting test, showed increased progressive saccade length by 0.2 characters compared to the dominant eye (p = 0.03). No other significant differences between dominant and non-dominant eyes were found. The agreement between the faster reading eye and ocular dominance was 44 to 56 per cent depending on whether dominance was determined at near or far. Conclusion: The outcomes suggest that in subjects with normal binocular vision, there is no marked enhancement in reading performance by binocular vision when reading paragraphs of text. Furthermore, the monocular reading performance appears to be close to equal and any small differences in performance appear not to be strongly associated with ocular dominance.
Key words: binocular, eye movements, monocular, ocular dominance, sighting dominance, reading, reading speed Binocular vision offers several advantages compared to monocular vision. Our visual field is extended by having two eyes and the offset in the overlapping retinal images allows the brain to discriminate depth in the visual scene. Having two visual inputs of the same stimulus also allows for binocular summation which results in higher visual acuity,1–4 greater contrast sensitivity,1,5–7 and faster processing speed of visual stimuli.8 Having two visual inputs can also be disadvantageous. When the retinal images do not match, we get blurry and strained vision, in severe cases even double vision. It is well known that problems with binocular vision can cause issues for reading; less known is if binocular vision improves reading perfor-
mance. Is binocular vision really advantageous for reading? This knowledge may be of importance when reading is used as a measure of visual function. In the current experiment, the intention is to explore what can be expected in terms of a binocular advantage, when reading is symptom free and binocular vision parameters are within physiological normal values. If binocular vision improves reading performance, it would be rewarding to understand in which way binocularity enhances the reading process. Most models of reading do not take binocular aspects into account. If binocular vision does not improve reading performance, it would be interesting to know why. Could it also be that monocular
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
reading may be faster than binocular reading in subjects without problems with binocular vision? Previous research offers little guidance regarding what the advantages of binocular vision could be for a typical reader with normal binocular status and reading skill. We have only found four studies which have investigated monocular and binocular reading of continuous text. Spache9 had 23 children of marked eye preference orally read texts (Gray’s Oral Check Test) binocularly and monocularly. The results showed that 43 per cent read significantly better with the preferred eye than with both. Robinson10 had 75 students read the same texts orally under monocular and binocular conditions. Clinical and Experimental Optometry 97.4 July 2014
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No significant differences were found for reading speed or reading errors between right eye, left eye or binocular reading. Heller and Radach11 had eight subjects silently read texts (200 lines) under binocular and monocular left and right conditions. In that experiment, eye movements were studied and monocular reading (left and right eye averaged) showed an increase of fixation duration (six per cent), number of fixations per line (10 per cent) and the proportion of regressive saccades (16 per cent). This was considered to be evidence of monocular reading being more difficult; however, it is difficult to draw clear conclusions from these three studies, since there was no description of the visual or binocular status of the participants. Second, it is not clear if texts of equal characteristics and difficulty were used for the repeated reading sessions. Kanonidou, Proudlock and Gottlob12 included 20 subjects with normal binocular vision as controls in an experiment regarding reading strategies in strabismic amblyopia. The subjects read paragraphs of texts (Brothers Grimm fairy tales) on a screen 1.2 metres from the subject, while eye movements were recorded. A decrease in reading speed (five per cent) was found during monocular reading (dominant and non-dominant eye averaged) for the controls along with an increased fixation duration (five per cent), decreased progressive saccade amplitude (two per cent) and increased rate of regressive saccades per line (3.5 per cent). When comparing dominant and non-dominant eyes, the reading speed, fixation duration and proportion of regressive saccades were equal but the nondominant eye displayed a shorter progressive saccade length by 0.1 degrees (2.6 per cent). The method of determining dominance was not described. The paragraphs of text were of comparable length but any variability in readability index between the texts was not described, which makes it more difficult to evaluate the differences in reading performance. Three additional studies have investigated effects on letter, word and sentence reading. Jones and Lee13 had 10 subjects orally read letters on cardboard cards under binocular and monocular conditions in bright and dim light. Any effects of ocular dominance were controlled in the experiment by having half of the subjects use their dominant eye, as determined by a sighting task, and half use their non-dominant eye. The results showed that letter identification errors Clinical and Experimental Optometry 97.4 July 2014
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under monocular conditions increased slightly in bright light but the difference was more pronounced in dim light. Sheedy et al.14 had 13 subjects silently read word charts (Bailey–Lovie) under binocular and monocular conditions. The results showed a three per cent decrease in reading speed for the best performing eye in the monocular compared to the binocular condition. More recently, Jainta and Jaschinski15 had 13 subjects silently read single sentences (Potsdam Sentence Corpus) under binocular and monocular (dominant and non-dominant eye) conditions in a haploscope. Ocular dominance was determined with a sighting test. The results of monocular reading showed increased first fixation duration, as well as increased progressive saccade length and disconjugacy after saccades. No significant differences were found between the performances in dominant and nondominant eyes. Another finding was the influence of heterophoria on binocular co-ordination and fixation duration. While the first fixation duration increased for orthophoric readers at monocular reading, it decreased in readers with greater heterophorias. It was speculated that higher demands on the binocular system due to heterophoria were lessened under monocular conditions, leading to reduced fixation duration. One aspect that is close at hand when comparing monocular and binocular performance is the potential effect of ocular dominance. There are different ways of classifying and testing ocular dominance and the fact that a subject’s dominance may change with different tests and test conditions16 adds to the complexity of determining ocular dominance. In experimental settings, where one eye needs to be selected, it is frequently recommended to choose the dominant eye, as defined by sighting dominance tests. The wide application of sighting dominance tests may be related to their ease of use for the clinician and the patient, as well as their ability to provide clear-cut responses in most cases. An issue with the sighting test is that it assesses the preference of an eye in a monocular task, that is, it forces the subject to choose an eye, when in fact most everyday visual tasks, for example, reading, involve both eyes. What speaks for a superior performance of the sighting dominant eye is research that compared the function of dominant and non-dominant eyes.17–19 A study of the effect of ocular dominance on latency and amplitude of visual
evoked potentials found shorter latency as well as greater amplitude for the dominant eye.20 Other studies have found poor agreement between the sighting dominant eye and performance superiority14 and it has been suggested that the dominant eye has no unique role in vision other than in monocular tasks.21 In this paper, we refer to the dominant eye as classified by sighting tests. Acknowledging the limitations of defining ocular dominance based on this test, we wished to explore its possible connection to reading performance, given its wide application in clinical and experimental settings. Reading is a very complex task and reading rate and properties of reading eye movements are strongly influenced by lexical and cognitive factors in the process. Still, the visual input is a starting point that needs to provide sensory input at an adequate speed and effort to allow for efficient higher-order processing. In the present project, we focus on the contribution of binocular vision. The main objective of this study is to study the differences in reading performance between monocular and binocular reading in the typical reader. The secondary objective is to assess any asymmetry in monocular performance, that is, if either eye is superior in performance and how this might be associated with the dominant eye. METHODS The experiment was designed as a balanced repeated-measurements study, where each subject silently read texts under all conditions (monocular right, monocular left and binocular) while eye movements were recorded. The conditions were repeated three times, so that each repetition began with a different condition and no condition was immediately repeated. Each text was read once by each subject. The inclusion criteria for participation were that the subjects are subjectively asymptomatic when reading, have normal binocular status, have not been diagnosed with ocular disease or reading difficulties and have an ability to read Swedish text fluently. Subjects requiring spectacles were excluded to avoid quality issues with the eye movement recordings due to the frame obstructing the eye tracker’s view of the eye. Subjects were allowed to wear contact lenses. The study adhered to the tenets of the Declaration of Helsinki and was approved by © 2014 The Authors
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Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
nya butik ovanpå en djup källare. Varje natt kom det massor av möss upp från källaren och in i butiken. Där mumsade de sedan friskt i sig av äpplen, nötter och vattenmeloner. Inte ens grönsaker och potatisar lät de bli att knapra pä. Mellan midnatt och soluppgången gick inga varor i butiken säkra för de små gnagarna. Så länge det hördes ljud på gatorna och bilar körde förbi höll sig mössen helt tysta i källaren. Men så snart som den gamla klockan i stadshuset slagit tolv, och det blev tyst på gatan, då strömmade de fram och njot av de söta godsakerna under rejäla fester. Resterna som väntade på grönsakshandlaren om morgnarna när han steg in i butiken fyllde honom med förtvivlan. I sina försök att skydda sig mot mössen placerade han ut fällor
1024 px = 25.2 deg @ 60 cm
I den lilla staden hade en grönsakshandlare öppnat sin
runtomkring i butiken.
1280 px = 31.3 deg @ 60 cm Figure 1. Stimulus used in the experiment with screen dimensions and visual angles
the regional ethics review board in Stockholm, EPN (2012/1447-31/1).
SUBJECTS A total of 18 subjects were included in the study. Another five subjects participated but were excluded from the analysis; three due to not meeting the criteria for vision and two due to the quality of the eye movement recordings. The mean age of the subjects was 25.2 ± 4.5 years and 12 subjects (67 per cent) were female. All subjects had normal binocular status with stereovision of 60 seconds of arc or better and were fluent in Swedish.
missible occluder allowing binocular recording, which is required by the Tobii eye tracker. The recorded eye movement data were processed using Visiolyzer, a software developed at our laboratory. Fixations were defined according to a fixation dispersion model, that is, when the centre of gravity of recorded fixation points stayed within a radius of 0.75 degrees (2.7 character spaces) for a minimum of 50 milliseconds. Progressive saccades were defined as a rightward movement between two adjacent fixations and regressive saccades were defined as a leftward movement between two adjacent fixations.
Apparatus
Stimuli
A Tobii 1750 eye tracker (Tobii Technology, Stockholm, Sweden) was used for eye movement recording. It is a display mounted tracker that records eye movements at 50 Hz using infra-red video technology. The accuracy is 0.5 degrees and the resolution is 0.25 degrees according to the manufacturer’s specifications. Each recording was preceded by a binocular calibration using a nine-point sequence. During monocular reading, one of the eyes was covered by an infra-red trans-
Nine texts from the International Reading Speed Texts (IReST)22 were used in the experiment. The IReST texts are validated in 17 languages including Swedish. The Swedish texts have the exact same number of words (146), characters (684) and lines (16) and all texts have the same readability index (LIX 35). The mean word length is 4.61 ± 0.01 characters. The texts were presented as a single paragraph subtending 14.3 by 17.1 degrees on the integrated screen of the eye
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
tracker, which was placed 60 cm from the eyes of the subject (Figure 1). The characters were dark on a white background (Weber contrast -0.98). The font was Helvetica and the mean character width was 0.28 degrees.
Procedure Before starting the experiment, the subjects were informed verbally and in writing of the details of the experiment. All subjects gave informed consent prior to the experiment. Next, the subject’s visual status was assessed, including a brief history, visual acuity at far and near (Logarithmic Visual Acuity Chart 2000 ‘ETDRS’ 4 m/40 cm), stereoscopic visual acuity (TNO random dot test), cover test, fusional reserves, eye motility, near point of convergence and near point of accommodation (RAF ruler) and a suppression test (Bagolini striated lenses). Symptoms experienced during day-to-day near work were assessed using a translated version of the Revised Convergence Insufficiency Symptom Survey (CISS).23 If a subject wore contact lenses, these were kept on during the vision screening. Ocular dominance was assessed at far (400 cm) and near (40 to 50 cm). At far, the hole-in-card sighting test21,24,25 was used. In this test, the subject held a card (20.0 by 12.8 cm) with both hands at arm’s length looking at a black cross at four metres through a 3.0 cm diameter hole in the centre of the card. The subject was instructed to look with both eyes at all times. When the subject reported to have aligned the hole in the card and the black cross, each eye was covered in turn and the subject was asked to report which eye kept the alignment. This eye was recorded as the dominant eye. The test was repeated four times for all subjects. For near dominance assessment the near hole-in-card sighting test24 was used. The procedure was the same as in the far assessment. The subject was then seated comfortably and unrestrained in a steady chair in front of the eye tracker. The position of the eye tracker was adjusted horizontally and vertically to optimise the signal quality. A test run was made for the subject to get acquainted with the procedure. Initially there was the calibration procedure, which was triggered by the test leader. Next, the subject read nine texts under the different reading conditions. The subject started each reading session by pressing the space bar on a keyboard, whereby a fixation cross was presented on Clinical and Experimental Optometry 97.4 July 2014
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Table 1. Summary of visual status
0
23 6 5 20 -14 12 -8 0 0 60 -0.10 -0.08 0.00 0.02 0.08 0.02 23 18
16 9
10 5
5 35
16 -10
-12 25
20 -6
-6 -6
0 0
0 30
60 -0.10
-0.20 0.00
0.00 -0.08
-0.08 -0.10
-0.08 0.02
-0.08 -0.10
0.00 23
38 16
0.10
0.10 22
17
4
11 6 5 20 -6 16 -6 0 0 60 0.00 0.10 0.10
19
15
0.02
10 5 20 -12 10 -8 -4 0 30 -0.10 -0.08 -0.10 19
-0.18
12
14
-0.08 -0.08
9
6 5
10 16
35 -8
-6 10
12 -8
-3 -3
1 0
0 60
30 -0.20
0.00 0.06 0.00
-0.08 -0.10 -0.12
-0.02 0.00
0.06 31 13
0.00 22 12
-0.06
6
16 9 5 16 -8 14 -8 0 0 60 -0.08 0.02 0.02 22
-0.20
8
11
-0.08 -0.14
6
6 5
5 20
30 -8
-6 12
20 -8
-6 0
0 0
0 30
30 -0.14
0.00 -0.06 0.00
-0.10 -0.08
20
-0.10
-0.08 -0.08 26 9
10
-0.18
-0.24 -0.10 27
-0.06 -0.12
7
10 10 5 35 -4 25 -6 0 0 60 -0.10 -0.08 -0.08
8
8
-0.18
9 5 20 -6 16 -6 0 0 60 -0.14 -0.10 -0.10 30
-0.16
7
7
-0.10 -0.10
6 5 25 -8 18 -8 -1 0 30 -0.16 -0.12 -0.12 -0.26 23 6
-0.18
-0.26 -0.26
18 10
6 5
5 20
35 -14
-10 16
35 -10
-8 0
0 0
0 30
15 -0.10
-0.10 0.00 0.00
-0.08 -0.08 -0.20
-0.26
29 5
0.00 23 4
-0.14 -0.12
2
11 7 5 30 -16 30 -10 -2 0 60 -0.20 -0.20 -0.18 -0.30 24 3
-0.28 -0.18
10 5
8 5
5 25
25 -8
-10 14
14 -8
-6 0
0 0
0 30
30 -0.20
-0.10 0.02
-0.10 0.00
0.00 -0.30
Left Eye Binoc. Right Eye
-0.30 -0.18 -0.28 28
-0.18 -0.20 23
2
IBM SPSS version 19 was used for statistical analysis. The significance level was set to five per cent (α = 0.05) and tests were two-sided. Parametric data were analysed using paired t-tests. Reading speed was calculated as words read per minute (WPM), comprehension
1
Statistical analysis
Left Eye
A relative score27 was applied on the test responses, that is, the response at each trial was set as -1 in the case of left eye dominance and +1 in the case of right eye dominance. The relative score was calculated as the algebraic sum of responses divided by the number of trials. This resulted in a score ranging from -1 in the case of left eye dominance in all trials, zero in the case of no dominance (50–50 mixed responses) and +1 in the case of right eye dominance in all trials.
Right Eye
Ocular dominance
Visual Acuity Near (LogMar)
The mean interocular difference in visual acuity was logMAR 0.03 (1.5 optotypes) and none of the subjects differed more than one line (five optotypes) at near. The mean binocular summation effect in visual acuity was 13 per cent, that is, the minimum angle of resolution (MAR) was smaller (better) by this percentage during the binocular condition. All subjects reported that their eyes were healthy and not under any treatment. One subject scored two points above the cut-off point for normal level of symptoms on the CISS26 but was still included as the optometric measures were within normal limits (Table 1).
Visual Acuity Far (LogMar)
Visual status
Subject Age
the centre of the screen. A moment later the full text was presented on the screen. The subject was instructed to fixate the cross carefully and then to start reading at his or her own pace as soon as the text appeared and to finish by pressing the space bar again as soon as the last word was read. The subjects were instructed to read the texts for comprehension and were informed that they would need to answer questions about the content after finishing reading. There were four multiple-choice questions with three alternatives for each text. After each text was finished, the subject answered the multiple-choice questions in writing. The test leader was present at all times giving instructions before each reading session and monitoring the eye movements online.
Stereo Covertest Covertest Breakpoint Breakpoint Breakpoint Breakpoint Nearpoint Nearpoint Symptom 40 cm Divergence Convergence Convergence Divergence Convergence Accommodation Score Vision Test 400 cm (Sec. Arc.) (Prism (Prism 400 cm 400 cm (Prism 40 cm (Prism 40 cm (Prism (cm) (cm) Binoc. Diopt.) Diopt.) (Prism Diopt.) Diopt.) Diopt.) Diopt.)
Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
18 17 16
300
15 14 13
11 10
Near
9
Far
8 7
Words per minute
280
12
260
240
6 5 4
220
3 2 1 -1
-0.5
200 0
0.5
1
Binocular
Monocular
Figure 2. Relative score on ocular dominance for subjects 1 to 18. Negative scores indicate left dominance and positive scores indicate right dominance. A score of zero indicates no dominance, that is, equal number of responses left and right.
Figure 3. Mean binocular and monocular reading speed. Error bars represent 95% confidence interval.
assessment. In the far-sighting test, all subjects consistently chose the same eye in all trials, while the near-sighting test indicated less consistency with two subjects (subject 10 and 11) showing weaker dominance and one subject showing no dominance (subject 5) (Figure 2).
determined at near, there was a significant difference (p = 0.03) in progressive saccade length. Reading with the nondominant eye slightly increased the saccade length by 0.06 degrees (0.2 characters) (Table 3). The agreement between the dominant eye and faster reading speed was 56 per cent with dominance determined at distance and 44 per cent with dominance determined at near.
scores were enumerated as the percentage of correct answers and eye movement parameters were averaged for the texts. Each subject read three texts under binocular conditions and six texts under monocular conditions (three with either eye). Reading speed, comprehension scores and eye movement parameters were averaged over the texts within the conditions before statistical comparison. The analysis of monocular versus binocular performance included monocular performance as the mean of dominant and non-dominant eyes and secondly monocular performance for dominant and non-dominant eyes separately. RESULTS
Ocular dominance According to the hole-in-card sighting test performed at distance, 12 subjects (67 per cent) were right eye dominant and six subjects (33 per cent) were left eye dominant. In the near sighting test, the distribution was 11 subjects (61 per cent) right eye dominant, six subjects (33 per cent) left eye dominant and one subject (six per cent) with no eye dominance. Five subjects (28 per cent) switched dominance between far and near
Monocular versus binocular Monocular reading speed (dominant and non-dominant averaged) decreased by 2.1 per cent compared to binocular but the difference was not significant (Figure 3). There was no difference in comprehension between the conditions. There were small but significant differences in the fixation durations (p < 0.01) and the lengths of regressive saccades (p = 0.01) between the conditions. Reading monocularly increased the fixation duration by 16.6 ms and the regressive saccades became 0.12 degrees (0.4 characters) longer. None of the other eye movement measures differed significantly (Table 2).
Dominant versus non-dominant With dominance determined at distance, there were no significant differences between the monocular conditions for any of the measures but with dominance
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Dominant and non-dominant versus binocular With dominance determined at distance, there was a significant difference in reading speed between reading with the nondominant eye and reading binocularly (p = 0.03) (Figure 4). There were also significant differences in fixation durations for the dominant (p < 0.01) and the non-dominant (p < 0.01) eyes compared with reading binocularly. With dominance determined at near, there were significant differences between reading binocularly and with the dominant (p < 0.01) and non-dominant (p < 0.01) eyes for fixation durations; for the non-dominant eye there was also a significant difference in regressive saccade length (p < 0.01) (Tables 2 and 3). Clinical and Experimental Optometry 97.4 July 2014
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Binocular reading
Monocular reading
269.6 ± 33.5
264.2 ± 36.3
94.9 ± 7.4
92.8 ± 6.2
Reading speed (WPM) Comprehension (%)
0.96 ± 0.11
0.94 ± 0.13
187.4 ± 23.5
204.0 ± 29.2
Progressive saccade length (char.)
7.4 ± 0.9
7.4 ± 1.0
Regressive saccade length (char.)
4.8 ± 1.4
5.2 ± 1.6
Proportion of regressive saccades
0.2 ± 0.1
0.2 ± 0.1
Fixations per word Fixation duration (ms)
Table 2. Binocular and monocular reading performance
Dominance determined at:
Distance
Reading speed (WPM) Comprehension (%) Fixations per word Fixation duration (ms)
Near
Dominant eye reading
Non-dominant eye reading
Dominant eye reading
Non-dominant eye reading
267.7 ± 41.8
260.6 ± 34.1
260.6 ± 38.7
267.7 ± 37.6
92.1 ± 10.2
93.5 ± 5.2
92.1 ± 9.8
93.5 ± 5.9
0.93 ± 0.14
0.95 ± 0.14
0.95 ± 0.15
0.92 ± 0.13
205.0 ± 29.3
203.0 ± 30.1
204.4 ± 29.6
203.7 ± 29.9
Progressive saccade length (char.)
7.4 ± 1.0
7.4 ± 1.1
7.3 ± 1.0
7.5 ± 1.1
Regressive saccade length (char.)
5.4 ± 1.8
5.1 ± 1.4
5.0 ± 1.6
5.4 ± 1.6
Proportion of regressive saccades
0.2 ± 0.1
0.2 ± 0.1
0.2 ± 0.1
0.2 ± 0.1
Table 3. Dominant and non-dominant reading performance
DISCUSSION 300
Words per minute
280
260
240
220
200 Binocular
Dominant eye Non-dominant eye Dominant eye Non-dominant eye (distance) (distance) (near) (near)
Figure 4. Reading speed for binocular, dominant and nondominant eyes (as determined by distance and near testing). Error bars represent 95% confidence interval.
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The main objective of this study was to explore the differences in reading performance between monocular and binocular reading with the intention to identify if and in what way binocularity enhances reading performance. Second, we wanted to explore any tendency for monocular performance superiority and its association with ocular sighting dominance. The position of the subject was unrestrained to achieve a nearto-normal reading situation. The stimuli were texts of readability equivalent to fiction books and presented at high contrast and at an acuity level well above the subject’s threshold. To deal with the fact that reading performance may be influenced by reading flexibility and strategy, the subjects were instructed to read at their own pace and to read for comprehension. Additionally, they were informed that they would need to answer questions about the content after the reading. Through these factors, the intention was to limit what differed between the © 2014 The Authors
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Monocular and binocular reading performance Johansson, Pansell, Ygge and Seimyr
reading sessions down to the actual visual condition, that is, monocular or binocular. We used reading speed, as it is a measure of practical and clinical relevance that indicates at what speed the reader can process written information and it is used in the evaluation of visual function, for example, before and after treatment of ophthalmological issues. Additionally, the reading eye movements were analysed with the intention to understand possible mechanisms behind differences in reading performance. Mean monocular reading speed was marginally slower than binocular by 2.1 per cent. This non-significant difference was even smaller than found in the control group by Kanonidou, Proudlock and Gottlob.12 The mean absolute difference was six words per minute, which is far lower than the standard deviation and the clinical significance is questionable when reading short paragraphs of text. An additional consideration is that the monocular condition in this experiment was in an unadapted state and still the monocular-binocular difference was small. Sheedy et al.14 found that five days of occlusion tended to slightly improve monocular performance in tasks benefitting from stereopsis but the binocular performance was still superior. Even though reading may differ from tasks requiring stereopsis, we believe it cannot be excluded that time for adaptation may have served to further reduce the performance gap. None of the reading conditions differed significantly in the percentage of correctly answered questions. This finding combined with the finding of equal reading speed in monocular and binocular conditions indicates that reading performance is not immediately affected by covering one of the eyes. The fixation duration increased significantly during monocular reading. This is in line with previous research comparing monocular and binocular reading.11,12,15 It has been established previously that the duration of reading fixations mainly increases with increased effort to comprehend. In addition, there may be an effect of binocular co-ordination on fixation duration, that is, that monocular and binocular fixation duration may differ with the size of heterophoria.15 In this experiment, we did not find an association between the size of exophoria and a decrease in fixation duration when switching to the monocular condition. This may be due to quite small amounts of heterophoria in this sample.
Despite the increased fixation duration during monocular reading, it did not lead to a significant difference in reading speed from the binocular condition. The mechanism behind this remains to be understood. Why was there not a greater advantage for binocular reading? Binocular performance advantages can be achieved through binocular concordance, summation and binocular disparity stereopsis. For reading, binocular summation seems to be the most plausible mechanism for a binocular advantage. Previous research on summation effects found a decrease in summation effect as the task at hand gets more complex; for example, as in pattern recognition1,13 and as the stimulus takes on higher contrast.6,28,29 Reading being a complex task and texts being presented at high contrasts may touch on these previous findings as a partial explanation of the marginal binocular advantage found in this experiment. Another contributing explanation may be the variability found in reading speed. This may stem from the texts, which apart from being relatively short and easy to comprehend, are presented in visually favourable contrast conditions. All together, this may mean that there was an overcapacity in the processing of the text, allowing flexibility for the subjects to vary the reading speed. A future experiment where the text is visually harder to process may reduce this flexibility and more clearly display any advantage of binocular vision.
Role of ocular dominance The only significant difference in mean reading speed (3.4 per cent) was between the binocular condition and the nondominant eye as defined by the hole-in-card sighting test at far. The absolute difference was small in comparison to the standard deviation in reading speed, which makes the practical effect questionable. In this experiment, we found a weak agreement between reading performance superiority and ocular dominance as was found by Sheedy et al.14 An interesting observation was that among the subjects with greatest interocular differences in reading speed (more than 25 words per minute), the majority switched dominance between far and near testing. In this case, the faster reading eye and the dominant eye, as determined by the far test, agreed more strongly. Additionally, we noted a weak tendency that the greater the interocular difference in reading speed, the more likely the subject read faster monocularly than binocularly,
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which was also observed by Spache,9 when studying reading in subjects with marked ocular dominance. These observations indicate that there may be subgroups of subjects with different levels of asymmetry in performance, despite normal binocular vision, possibly related to dominance. An explanation of the quite low agreement between performance and dominance may be that the sighting dominance test is not really applicable to reading, where both eyes are used simultaneously. Another explanation may be that ocular dominance has less impact on performance in subjects with normal binocular vision, that is, where both eyes have equal preconditions to contribute. On the other hand, in subjects who do demonstrate an obvious asymmetry in monocular performance, the sighting test may better predict performance superiority. Further research with more subjects will be required to draw further conclusions on ocular dominance versus performance.
Limitations This study was carried out as a preliminary experiment to explore the role of binocularity and dominance in reading, when binocular vision is close to optimal. We found minimal and mostly non-significant differences between the reading conditions. Despite using a repeated-measurements experimental design, which allows for a relatively small number of subjects to achieve statistical validity (1 - β error probability), the possibility remains that there are differences among the conditions that we did not find. Given that the significant differences we found were small, we believe the sample size was sufficient for this experiment. Second, the participants were all healthy, of young mean age and with a good level of binocular vision. This limits the extent to which the findings can be generalised to other populations. CONCLUSION The outcomes of this experiment suggest that in subjects with normal binocular vision and typical reading skills, monocular and binocular reading performances are close to equal when reading short paragraphs of text. Thus, it seems that binocular vision does not markedly enhance reading performance when reading conditions are favourable. Furthermore, the monocular reading performances appear to be close to equal and Clinical and Experimental Optometry 97.4 July 2014
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