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Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
Visual capability to receive character information Part I: How many characters can we recognize at a glance? TADAHIKO FUKUDA
a
a
Faculty of Environmental Information, Keio University , Endoh 5322, Fujisawa-shi, Kanagawa-ken, 252, Japan Published online: 31 May 2007.
To cite this article: TADAHIKO FUKUDA (1992) Visual capability to receive character information Part I: How many characters can we recognize at a glance?, Ergonomics, 35:5-6, 617-627, DOI: 10.1080/00140139208967841 To link to this article: http://dx.doi.org/10.1080/00140139208967841
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
ERGONOMICS,
1992, VOL. 35, NOS 5/6, 617-627
Visual capability to receive character information Part I: How many characters can we recognize at a glance? T ADAHIKO
FUKUDA
Faculty of Environmental Information, Keio University, Endoh 5322, Fujisawa-shi, Kanagawa-ken 252, Japan
Downloaded by [Universitaetsbibiothek Bonn] at 01:15 11 March 2015
Keywords: Information capacity; Memory span; Character recognition; Lateral interference; VDT. A study was made on the capability to receive character information and the factors restricting it. The study showed that the capability indicated by the memory span was limited by the average number of characters for words that are made up of individual characters, and calculated in terms of information quantity, there was no difference among the individual characters. The differences in memory span, depending on the size of the pattern presented, was negligible. The difference in the way characters were arranged produced the difference in memory span saturation. This phenomenon is explained by the nature of the lateral interference effect working among the adjacent characters.
1. Introduction Characters are now displayed in a variety of modes other than characters printed on paper such as CRT displays. In books and other printed forms, we can 'read' the information expressed by the characters as we wish. However, we are 'compelled to read' rather than 'choose to read' characters on a television screen, VDT screen, videotext, etc. Whereas printed matter represents active reception of character information, a television display represents, a more passive reception form. When receiving character information actively, readers are allowed to spend as much time as they wish, or to read repeatedly, until they completely understand. When readers are required to receive character information passively, their intentions are almost" neglected. A person probably feels extremely unsettled if he fails to understand correctly the contents in the sentence. These circumstances must be considered when presenting character information through some technical process. Easily received transmission of information by humans can only be possible by fully considering the limits and characteristics of human capability to receive such information, apart from technical solutions. Based on an understanding of these requirements. this paper studies the basic problems concerning capability to receive character information. There have been many studies on the attention span, though the stimulus pattern is composed mainly by dots something like these. For example, Ohyama et al. (1981) obtained the attention span for each SOA condition and for the no-mask condition from the pooled data of the three subjects, using linear interpolation. Klahr (1973) studied attention span by reaction time. On the other hand, immediate memory span has not yet been studied, as far as this author knows. So, the present study was undertaken.
2. Method of describing the capability to receive character information The ultimate goal in describing the capability to receive character information must 0014-0139/92 $3·00
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1992 Taylor & Francis Ltd.
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cover an understanding of the contents of the sentences. As a basic consideration leading to that point, this paper discusses cases using character groups randomly selected from character groups of the same type as information. The number of objects that can be understood at one glance when a large number are shown together is referred to alternately as the span of attention, span of perception or span of apprehension. Compared with this, the number of things that can be memorized (and collated or reproduced) at one time without repetition is called the span of immediate memory, or merely memory span (Woodworth and Schlosberg 1954). The question of the mere number of characters falls under only the span of attention (or attention span). However, when the question is amplified to involve character names as well, the range falls under the memory span. It is clear that the memory span is treated as a method for describing the capability to receive character information. The receiving ability relative to character information will be described by the number of characters that can be memorized and reproduced immediately after glancing at a pattern consisting of random character groups.
3. Experimental method The three kinds of patterns with different layouts shown in figure I were produced on slides using a fixed treatment process. As shown in figure 2, these slides were projected on a screen using a tachistoscope to improve projection.
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The subject observed them with both eyes under natural visual field conditions at a viewing distance of 285 em. In the centre of the pattern, a laser beam spot was shown dimming to the lowest possible brightness within a range easily seen. This point was made a fixation point. The spot could be driven randomly in terms of two dimensions by an electrical signal, to place it in the centre of the pattern presented.
How many characters can we recognize at a glance?
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Screen
Exposure duration Measuring system Exposure duration
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Figure 2. Schematic diagram of the experimental apparatus.
Three kinds of characters were used in forming the patterns-alphabet characters (capitals), Japanese hirakana characters, and Chinese kanji characters. These were placed randomly in a straight string, random string, and random two-dimensions as shown in figure I. The character groups forming patterns were randomly selected, and hirakana and alphabet characters were used in the same frequencies. Kanji characters were selected from those in daily usage. Regarding the spatial relationship, the selection of directions was made random so that the random strings would come into contact with each other in eight directions vertically, horizontally and diagonally. In the random patterns, two-dimensional character co-ordinates were chosen by applying a two-digit random number table using a fixed algorithm. The aspect ratio of the patterns was made 3:4, nearly matching the expansion of the visual field. These were presented on a screen by means of expanded projections to 10° (maximum visual angle diameter) and 50°. Each subject verified that characters constituting patterns could easily be read in all positions when they were presented independently within the regulated visual field in this section. The character size corresponded to an approximate height of 2° when projected by expanding to 50° of maximum visual angle diameter. The duration of the pattern presentation was 200 ms considering the non-responsive period of saccadic eye movement. In the first experiment, the subjects were asked to report orally the number of characters they perceived from the pattern shown on the screen. In the second experiment the subjects were asked to report in handwriting as many characters as possible in their proper spatial relationships from the patterns shown on the screen. The subjects were told the following in accordance with Hunter and Sigler's instructions (Hunter and Sigler 1940): 'Characters will be shown on the screen for a short period of time. Look at the' characters, then report what characters you saw and in what position of your visual field. First, pay attention to what characters you saw.' There were ten test cycles for each measurable point. The sequence of the number of character patterns presented was random. There were three subjects in the experiments. One of them was a male in his late 305, and the other two were females
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in their early 20s. They were confirmed to have normally functioning vision in an advance test.
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4. Results and discussions 4.1. Experiment 1 The attention span was obtained for a number of patterns for comparison with memory span. An example of the results is shown in figure 3. Figure 3 shows the relationship between the percentage of correct answers and the number of characters presented on the screen. As shown in this figure, the attention span is about twice as large as that of immediate memory. as for 100 per cent or 50 per cent of the correct answers. This tendency was common for all patterns employed in this experiment. 100
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4.2. Experiment 2 The memory span was obtained for parameters using character kinds (alphabet, Japanese hirakana, and Chinese kanji), character layouts (straight strings, random strings, and two-dimensional random layout patterns), and pattern size (maximum visual angles 10° and 50 in diameter). In terms of-memory span, character groups reported at individual measurement points with correct character names and respective spatial relationships were considered correct answers. These were expressed with values obtained by averaging ten measurements (referred to as the average number of correct answer characters). In the calculation results of the relationship between the number of characters presented and the memory span indicated by the average number of correct-answer characters, minute characteristic differences were found for each set of test conditions (details are described below). However, it was verified that no major differences existed among test conditions and among subjects as far as qualitative basic characteristics were concerned. Examples of the test results are shown in figures 4 a and b. Figures 4 a and b show the results when a pattern of characters arranged in a straight string and one with characters arranged randomly in two dimensions were 0
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respectively presented in visual angle diameters of 100 and 50 0 , and with exposure durations of 200 ms. Each plot shows average values of the three subjects. Based on this data, the fol1owing can be understood regarding memory span.
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4.2.1. Difference in memory span by character size: As shown in figures 4 a and b. the relationship between the memory span shown by the average number of correctanswer characters and the number of characters presented shows a saturation characteristic. And in spite of the number of characters presented, only about five alphabet characters, four Japanese hirakana characters, and two Chinese kanji characters could be recognized. A comparison of these values with the average number of characters in words comprising various characters is interesting. As one example, table 1 presents the results of a survey made selecting 2000 words each consisting of alphabet, Japanese hirakana and Chinese kanji characters taken from respective dictionaries. A study was made on the average number of characters these words contained. The saturated values of the average number of correct-answer characters shown in figures 4a and b coincided extremely well with the average number of characters in the words shown in table 1. It can basically be concluded that the memory span corresponds to one word regardless of character types. Conversely, it is safe to assume that a word consists of the number of characters that corresponds with the memory span. Certainly, a careful study is needed regarding these causal relationships. The author merely wishes to mention as criteria that the two have a quantitatively corresponding relationship.
Table I.
Average number of characters per one word. A verage number of
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Word
Alphabet
English French German
Hirakana Kanji
5·12 4·87
6·39 4·26 2·98
Table 2 presents calculations of approximate information quantities for one each of alphabet, Japanese hirakana and Chinese kanji characters. The alphabet values were obtained on the assumption that the alphabet contains 26 characters and that their appearance frequencies were equal. Calculations were made using 76 Japanese hirakana characters, 1820 (educational) or 4500 (commonly used) Chinese kanji characters. Table 2. Information quantity for one character. Kind of characters
Number of characters
Alphabet Hirakana Kanji
26 75 1850'
4000
Information quantity (bits) 4·70
6·23 10·85 11·97
How many characters can we recognize at a glance?
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Expressing the relationship between the presented number of characters shown in figures 4 a and b, and the average number of correct-answer characters by the information quantity shown in table 2. Figures 5 a and b were obtained. As shown in figures 5 a and b, the difference by character kind becomes extremely small when the average number of correct-answer characters is expressed by information quantity. The information quantity of characters that can be read at a glance is approximately twenty to thirty bits regardless of the character kind.
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Based on these results, the memory span by character kind shows different values in terms of number of characters. However, when the number of characters comprising words and when the information quantities of individual characters are both made units, the values become equal.
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4.2.2. Visual field size and memory span: When a pattern is presented in a visual field whose maximum visual angle diameter is within 10D (shown by solid lines in figure 4 a and b), all the characters are presented in what is called the central visual field. By contrast, when the pattern is expanded and presented in a visual field whose maximum visual angle diameter is within SOD (shown by broken lines in figure 4 a and b), each character becomes five times larger than the former. However, the bulk of the character groups can be caught by the retinal location referred to as the peripheral visual field. Comparing the performance of both, asshown in figures 4 a and b, the results are slightly better when all the character groups are viewed in the central visual field, even when the characters are small. This trend is the same even when the character types and character arrangements forming patterns are changed. Although not shown in figures 4 a and b, other tests have confirmed that the difference was not caused by pattern presentation time nor by the subjects themselves. This is logical when one considers that the central visual field has excellent character recognition capability and that the character recognition function lowers the larger the eccentric distance from the central visual field is for the peripheral visual field. However, the difference is not so large. In facts it should look rather small. All the character groups should have a receiving capability equivalent to that when all character groups are projected in the central visual field, even when the bulk of the characters are projected in the peripheral visual field. 4.2.3. Relationship between character arrangement and memory span: The memory span shown by the average number of correct answer characters is expressed by a saturation characteristic as indicated by the examples in figures 4 a and b. However, by strictly analysing the results obtained for individual patterns, these results show a trend which differs slightly depending on how the characters are arranged. Generally speaking, the relationship between the average number of correctanswer characters and the number of presented characters is approximated by two straight lines as in the examples in figures 4 a and b. Comparing the slope of the straight line in the saturation region, for example, a clear difference exists between patterns in figures 4 a and b. Figure 6 can be obtained when the slopes only are compared.
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How many characters can we recognize at a glance?
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As shown in figures 4 a and b, the slope in a random pattern has a slightly positive value. As the number of presented characters increases, the memory span expands, even though it is a small increase. Against this, the straight string pattern slope has a negative value. Conversely, the memory span shrinks as the number of presented characters increases. In a random string pattern, the results show a characteristic just halfway between the two as shown in figure 6. Intuitively, a straight string pattern is forecasted to cause a lesser burden on observers than a random pattern, at least in terms of determining the relationship of object positions to which attention must be paid. One may be tempted to think that the memory span will also expand when the number of characters to be presented increases in this case. However, in reality, the trend is diametrically opposite. This is not contradictory considering the nature of a lateral interference effect, which can be considered a type of spatial masking between two adjacent characters that provides mutual interference and lowers the recognition rate (Bouma 1970, Fukuda 1979). This lateral interference effect has a fixed order and forms a uniform network over a large part of the retina. When character spacing is the same, the maximum interference effect works between characters that are arranged in a radial pattern when viewed from the fixation point (the fovea on the retina), making mutual recognition difficult. Lateral interference is minimized between characters arranged in a pattern directly crossing this, and mutua) recognition becomes relatively easy compared with the former method. In addition. these lateral interference effects are known to always work towards the fixation point from outside the visual field. The characters outside the visual field can be more easily recognized than those inside, within a fixed visual field in which single characters can be recognized (Bouma 1973, Fukuda 1979). These characteristics are shown schematically in figure 7.
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626
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Figure 8 shows the relationship between memory span and the characteristics of lateral interference effects. The number of characters that can be received increases slightly along with the increase in the number of presented characters in the case of two-dimensional random patterns, which requires that careful attention be paid to the widest and most disorderly objects in terms of attention distribution relative to the visual objects. The number of characters that can be accepted decreases from a straight string pattern, which places the lightest burden on the subjects on this point as the number of characters increases. The principal reason is that the lateral interference effect is clearly reflected is this phenomenon.
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5. Conclusions A study was made on the capability to receive character information and the factors restricting it. This mainly involved the conditions surrounding the character pattern spatial presentation and the relationship between the types of characters and the memory span. The study showed that the capability to receive character information indicated the memory span was limited to a certain numer of characters-approximately five alphabet characters, about four Japanese hirakana characters, and about two Chinese kanji characters. These values correspond to the average number of characters for words that are made up of individual characters. Calculated in terms of information quantity, there were no differences among the individual characters. The difference in memory span, depending upon the size of the patterns presented) seems to be negligible even though the patterns projected in the central visual field showed slightly better performance than those patterns in which the bulk of the characters were projected in the peripheral visual field. The difference in the way characters were arranged produced the difference in memory span saturation. It was determined that the nature of the lateral interference effect working between the adjacent characters was the reason. This knowledge is an extremely basic
How many characters can we recognize at a glance?
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characteristic in receiving character information. It not only serves as a basis for analysing the receiving capability of the visual and sensory systems, but is also effective when considering a method of character information display. References H. 1970, Interaction effect is parafoveal letter recognition, Nature, 226 (II), 177-178. H. 1973, Visual interference in the parafoveal recogni tion of initial and final letters of words. Vision Research, 13, 767-782. FUKUDA, T. 1979, Visual interference in the peripheral recognition of embedded letters, Journal of ITE ofJapan, 33(9), 726-731. HUNTER, W. S. and SIGLER, M. 1940, The span of visual discrimination as a function of time and intensity of stimulation, Journal of Experimental Psychology, 26, 160-179. KuHR, D. J973, Quantification processes, in: W. G. Chase (ed.), Visual Information Processing (Academic Press, New York). OHYAMA, T. KIKUCHI, T. and ICHIHARA, S. 1981, Span of attention, backward masking and reaction time, Perception and Psychophysics, 29,(2), J06- J 12. WOODWORTH, R. S. and SCHLOSBERG, H. 1954, Experimental Psychology (rev. edn) (Holt, Rinehart and Winston, New York).
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BOUMA, BOUMA,
Manuscript received I September 1990. Manuscript accepted 27 November 1990.
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Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
Visual capability to receive character information Part I: How many characters can we recognize at a glance? TADAHIKO FUKUDA
a
a
Faculty of Environmental Information, Keio University , Endoh 5322, Fujisawa-shi, Kanagawa-ken, 252, Japan Published online: 31 May 2007.
To cite this article: TADAHIKO FUKUDA (1992) Visual capability to receive character information Part I: How many characters can we recognize at a glance?, Ergonomics, 35:5-6, 617-627, DOI: 10.1080/00140139208967841 To link to this article: http://dx.doi.org/10.1080/00140139208967841
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
ERGONOMICS,
1992, VOL. 35, NOS 5/6, 617-627
Visual capability to receive character information Part I: How many characters can we recognize at a glance? T ADAHIKO
FUKUDA
Faculty of Environmental Information, Keio University, Endoh 5322, Fujisawa-shi, Kanagawa-ken 252, Japan
Downloaded by [Universitaetsbibiothek Bonn] at 01:15 11 March 2015
Keywords: Information capacity; Memory span; Character recognition; Lateral interference; VDT. A study was made on the capability to receive character information and the factors restricting it. The study showed that the capability indicated by the memory span was limited by the average number of characters for words that are made up of individual characters, and calculated in terms of information quantity, there was no difference among the individual characters. The differences in memory span, depending on the size of the pattern presented, was negligible. The difference in the way characters were arranged produced the difference in memory span saturation. This phenomenon is explained by the nature of the lateral interference effect working among the adjacent characters.
1. Introduction Characters are now displayed in a variety of modes other than characters printed on paper such as CRT displays. In books and other printed forms, we can 'read' the information expressed by the characters as we wish. However, we are 'compelled to read' rather than 'choose to read' characters on a television screen, VDT screen, videotext, etc. Whereas printed matter represents active reception of character information, a television display represents, a more passive reception form. When receiving character information actively, readers are allowed to spend as much time as they wish, or to read repeatedly, until they completely understand. When readers are required to receive character information passively, their intentions are almost" neglected. A person probably feels extremely unsettled if he fails to understand correctly the contents in the sentence. These circumstances must be considered when presenting character information through some technical process. Easily received transmission of information by humans can only be possible by fully considering the limits and characteristics of human capability to receive such information, apart from technical solutions. Based on an understanding of these requirements. this paper studies the basic problems concerning capability to receive character information. There have been many studies on the attention span, though the stimulus pattern is composed mainly by dots something like these. For example, Ohyama et al. (1981) obtained the attention span for each SOA condition and for the no-mask condition from the pooled data of the three subjects, using linear interpolation. Klahr (1973) studied attention span by reaction time. On the other hand, immediate memory span has not yet been studied, as far as this author knows. So, the present study was undertaken.
2. Method of describing the capability to receive character information The ultimate goal in describing the capability to receive character information must 0014-0139/92 $3·00
e
1992 Taylor & Francis Ltd.
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cover an understanding of the contents of the sentences. As a basic consideration leading to that point, this paper discusses cases using character groups randomly selected from character groups of the same type as information. The number of objects that can be understood at one glance when a large number are shown together is referred to alternately as the span of attention, span of perception or span of apprehension. Compared with this, the number of things that can be memorized (and collated or reproduced) at one time without repetition is called the span of immediate memory, or merely memory span (Woodworth and Schlosberg 1954). The question of the mere number of characters falls under only the span of attention (or attention span). However, when the question is amplified to involve character names as well, the range falls under the memory span. It is clear that the memory span is treated as a method for describing the capability to receive character information. The receiving ability relative to character information will be described by the number of characters that can be memorized and reproduced immediately after glancing at a pattern consisting of random character groups.
3. Experimental method The three kinds of patterns with different layouts shown in figure I were produced on slides using a fixed treatment process. As shown in figure 2, these slides were projected on a screen using a tachistoscope to improve projection.
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The subject observed them with both eyes under natural visual field conditions at a viewing distance of 285 em. In the centre of the pattern, a laser beam spot was shown dimming to the lowest possible brightness within a range easily seen. This point was made a fixation point. The spot could be driven randomly in terms of two dimensions by an electrical signal, to place it in the centre of the pattern presented.
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Screen
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Three kinds of characters were used in forming the patterns-alphabet characters (capitals), Japanese hirakana characters, and Chinese kanji characters. These were placed randomly in a straight string, random string, and random two-dimensions as shown in figure I. The character groups forming patterns were randomly selected, and hirakana and alphabet characters were used in the same frequencies. Kanji characters were selected from those in daily usage. Regarding the spatial relationship, the selection of directions was made random so that the random strings would come into contact with each other in eight directions vertically, horizontally and diagonally. In the random patterns, two-dimensional character co-ordinates were chosen by applying a two-digit random number table using a fixed algorithm. The aspect ratio of the patterns was made 3:4, nearly matching the expansion of the visual field. These were presented on a screen by means of expanded projections to 10° (maximum visual angle diameter) and 50°. Each subject verified that characters constituting patterns could easily be read in all positions when they were presented independently within the regulated visual field in this section. The character size corresponded to an approximate height of 2° when projected by expanding to 50° of maximum visual angle diameter. The duration of the pattern presentation was 200 ms considering the non-responsive period of saccadic eye movement. In the first experiment, the subjects were asked to report orally the number of characters they perceived from the pattern shown on the screen. In the second experiment the subjects were asked to report in handwriting as many characters as possible in their proper spatial relationships from the patterns shown on the screen. The subjects were told the following in accordance with Hunter and Sigler's instructions (Hunter and Sigler 1940): 'Characters will be shown on the screen for a short period of time. Look at the' characters, then report what characters you saw and in what position of your visual field. First, pay attention to what characters you saw.' There were ten test cycles for each measurable point. The sequence of the number of character patterns presented was random. There were three subjects in the experiments. One of them was a male in his late 305, and the other two were females
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in their early 20s. They were confirmed to have normally functioning vision in an advance test.
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4. Results and discussions 4.1. Experiment 1 The attention span was obtained for a number of patterns for comparison with memory span. An example of the results is shown in figure 3. Figure 3 shows the relationship between the percentage of correct answers and the number of characters presented on the screen. As shown in this figure, the attention span is about twice as large as that of immediate memory. as for 100 per cent or 50 per cent of the correct answers. This tendency was common for all patterns employed in this experiment. 100
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4.2. Experiment 2 The memory span was obtained for parameters using character kinds (alphabet, Japanese hirakana, and Chinese kanji), character layouts (straight strings, random strings, and two-dimensional random layout patterns), and pattern size (maximum visual angles 10° and 50 in diameter). In terms of-memory span, character groups reported at individual measurement points with correct character names and respective spatial relationships were considered correct answers. These were expressed with values obtained by averaging ten measurements (referred to as the average number of correct answer characters). In the calculation results of the relationship between the number of characters presented and the memory span indicated by the average number of correct-answer characters, minute characteristic differences were found for each set of test conditions (details are described below). However, it was verified that no major differences existed among test conditions and among subjects as far as qualitative basic characteristics were concerned. Examples of the test results are shown in figures 4 a and b. Figures 4 a and b show the results when a pattern of characters arranged in a straight string and one with characters arranged randomly in two dimensions were 0
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respectively presented in visual angle diameters of 100 and 50 0 , and with exposure durations of 200 ms. Each plot shows average values of the three subjects. Based on this data, the fol1owing can be understood regarding memory span.
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4.2.1. Difference in memory span by character size: As shown in figures 4 a and b. the relationship between the memory span shown by the average number of correctanswer characters and the number of characters presented shows a saturation characteristic. And in spite of the number of characters presented, only about five alphabet characters, four Japanese hirakana characters, and two Chinese kanji characters could be recognized. A comparison of these values with the average number of characters in words comprising various characters is interesting. As one example, table 1 presents the results of a survey made selecting 2000 words each consisting of alphabet, Japanese hirakana and Chinese kanji characters taken from respective dictionaries. A study was made on the average number of characters these words contained. The saturated values of the average number of correct-answer characters shown in figures 4a and b coincided extremely well with the average number of characters in the words shown in table 1. It can basically be concluded that the memory span corresponds to one word regardless of character types. Conversely, it is safe to assume that a word consists of the number of characters that corresponds with the memory span. Certainly, a careful study is needed regarding these causal relationships. The author merely wishes to mention as criteria that the two have a quantitatively corresponding relationship.
Table I.
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Table 2 presents calculations of approximate information quantities for one each of alphabet, Japanese hirakana and Chinese kanji characters. The alphabet values were obtained on the assumption that the alphabet contains 26 characters and that their appearance frequencies were equal. Calculations were made using 76 Japanese hirakana characters, 1820 (educational) or 4500 (commonly used) Chinese kanji characters. Table 2. Information quantity for one character. Kind of characters
Number of characters
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Expressing the relationship between the presented number of characters shown in figures 4 a and b, and the average number of correct-answer characters by the information quantity shown in table 2. Figures 5 a and b were obtained. As shown in figures 5 a and b, the difference by character kind becomes extremely small when the average number of correct-answer characters is expressed by information quantity. The information quantity of characters that can be read at a glance is approximately twenty to thirty bits regardless of the character kind.
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Based on these results, the memory span by character kind shows different values in terms of number of characters. However, when the number of characters comprising words and when the information quantities of individual characters are both made units, the values become equal.
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4.2.2. Visual field size and memory span: When a pattern is presented in a visual field whose maximum visual angle diameter is within 10D (shown by solid lines in figure 4 a and b), all the characters are presented in what is called the central visual field. By contrast, when the pattern is expanded and presented in a visual field whose maximum visual angle diameter is within SOD (shown by broken lines in figure 4 a and b), each character becomes five times larger than the former. However, the bulk of the character groups can be caught by the retinal location referred to as the peripheral visual field. Comparing the performance of both, asshown in figures 4 a and b, the results are slightly better when all the character groups are viewed in the central visual field, even when the characters are small. This trend is the same even when the character types and character arrangements forming patterns are changed. Although not shown in figures 4 a and b, other tests have confirmed that the difference was not caused by pattern presentation time nor by the subjects themselves. This is logical when one considers that the central visual field has excellent character recognition capability and that the character recognition function lowers the larger the eccentric distance from the central visual field is for the peripheral visual field. However, the difference is not so large. In facts it should look rather small. All the character groups should have a receiving capability equivalent to that when all character groups are projected in the central visual field, even when the bulk of the characters are projected in the peripheral visual field. 4.2.3. Relationship between character arrangement and memory span: The memory span shown by the average number of correct answer characters is expressed by a saturation characteristic as indicated by the examples in figures 4 a and b. However, by strictly analysing the results obtained for individual patterns, these results show a trend which differs slightly depending on how the characters are arranged. Generally speaking, the relationship between the average number of correctanswer characters and the number of presented characters is approximated by two straight lines as in the examples in figures 4 a and b. Comparing the slope of the straight line in the saturation region, for example, a clear difference exists between patterns in figures 4 a and b. Figure 6 can be obtained when the slopes only are compared.
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As shown in figures 4 a and b, the slope in a random pattern has a slightly positive value. As the number of presented characters increases, the memory span expands, even though it is a small increase. Against this, the straight string pattern slope has a negative value. Conversely, the memory span shrinks as the number of presented characters increases. In a random string pattern, the results show a characteristic just halfway between the two as shown in figure 6. Intuitively, a straight string pattern is forecasted to cause a lesser burden on observers than a random pattern, at least in terms of determining the relationship of object positions to which attention must be paid. One may be tempted to think that the memory span will also expand when the number of characters to be presented increases in this case. However, in reality, the trend is diametrically opposite. This is not contradictory considering the nature of a lateral interference effect, which can be considered a type of spatial masking between two adjacent characters that provides mutual interference and lowers the recognition rate (Bouma 1970, Fukuda 1979). This lateral interference effect has a fixed order and forms a uniform network over a large part of the retina. When character spacing is the same, the maximum interference effect works between characters that are arranged in a radial pattern when viewed from the fixation point (the fovea on the retina), making mutual recognition difficult. Lateral interference is minimized between characters arranged in a pattern directly crossing this, and mutua) recognition becomes relatively easy compared with the former method. In addition. these lateral interference effects are known to always work towards the fixation point from outside the visual field. The characters outside the visual field can be more easily recognized than those inside, within a fixed visual field in which single characters can be recognized (Bouma 1973, Fukuda 1979). These characteristics are shown schematically in figure 7.
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Figure 8 shows the relationship between memory span and the characteristics of lateral interference effects. The number of characters that can be received increases slightly along with the increase in the number of presented characters in the case of two-dimensional random patterns, which requires that careful attention be paid to the widest and most disorderly objects in terms of attention distribution relative to the visual objects. The number of characters that can be accepted decreases from a straight string pattern, which places the lightest burden on the subjects on this point as the number of characters increases. The principal reason is that the lateral interference effect is clearly reflected is this phenomenon.
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5. Conclusions A study was made on the capability to receive character information and the factors restricting it. This mainly involved the conditions surrounding the character pattern spatial presentation and the relationship between the types of characters and the memory span. The study showed that the capability to receive character information indicated the memory span was limited to a certain numer of characters-approximately five alphabet characters, about four Japanese hirakana characters, and about two Chinese kanji characters. These values correspond to the average number of characters for words that are made up of individual characters. Calculated in terms of information quantity, there were no differences among the individual characters. The difference in memory span, depending upon the size of the patterns presented) seems to be negligible even though the patterns projected in the central visual field showed slightly better performance than those patterns in which the bulk of the characters were projected in the peripheral visual field. The difference in the way characters were arranged produced the difference in memory span saturation. It was determined that the nature of the lateral interference effect working between the adjacent characters was the reason. This knowledge is an extremely basic
How many characters can we recognize at a glance?
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characteristic in receiving character information. It not only serves as a basis for analysing the receiving capability of the visual and sensory systems, but is also effective when considering a method of character information display. References H. 1970, Interaction effect is parafoveal letter recognition, Nature, 226 (II), 177-178. H. 1973, Visual interference in the parafoveal recogni tion of initial and final letters of words. Vision Research, 13, 767-782. FUKUDA, T. 1979, Visual interference in the peripheral recognition of embedded letters, Journal of ITE ofJapan, 33(9), 726-731. HUNTER, W. S. and SIGLER, M. 1940, The span of visual discrimination as a function of time and intensity of stimulation, Journal of Experimental Psychology, 26, 160-179. KuHR, D. J973, Quantification processes, in: W. G. Chase (ed.), Visual Information Processing (Academic Press, New York). OHYAMA, T. KIKUCHI, T. and ICHIHARA, S. 1981, Span of attention, backward masking and reaction time, Perception and Psychophysics, 29,(2), J06- J 12. WOODWORTH, R. S. and SCHLOSBERG, H. 1954, Experimental Psychology (rev. edn) (Holt, Rinehart and Winston, New York).
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BOUMA, BOUMA,
Manuscript received I September 1990. Manuscript accepted 27 November 1990.
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