IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-26, NO. 11, NOVEMBER 1979
601
Visual Field Plotting Using Eye Movement Response M. EDWARD JERNIGAN,
Abstract-Traditional techniques for visual field plotting are described as acting in opposition to the underlying perceptual task. An approach is described which encourages the natural eye movement response and objectively classifies it as either an acquisition or a search response. Empirical results demonstrate the reliability of eye movement response as an index of perception of peripheral stimuli A visual field plotting example is given and implications for clinical practice are discussed.
INTRODUCTION IN attempting to assess performance of the visual system for the purpose of diagnosing visual disease, it is important to be aware of the perceptual context in which the subject is operating. Measurements of vision made in an unnatural perceptual environment are especially prone to error when subjective response is relied on to obtain the desired information. In visual field plotting, the subject is asked to report whether a peripheral stimulus is visible, yet he is told he must not look at it. While it is certainly possible for subjects to report visibility while maintaining fixation on a central target, the perceptual task is unnatural. This conflict between task and natural response is the source of serious difficulty in traditional techniques of visual field plotting, or perimetry. A brief discussion of visual fields and the historical need for continuous fixation leads into a discussion of a 'new approach to the problem which employs a more natural visual task as well as a more objective scheme for deciding whether a peripheral stimulus was perceived. THE VISUAL FIELD The visual field is defined as the region of space that is visible when the eye is fixated. Quantitatively, it is plotted by detennining the sensitivity to point light stimuli at locations in a region around the point of fixation. A visual field plot is a map of retinal sensitivity, and is usually represented by a plot of isopters, contours of constant sensitivity, on polar coordinates. Fig. 1 shows a typical plot of the central visual field for a normal subject's right eye. An alternate representation of retinal sensitivity plots sensitivity versus eccentricity along a selected meridian, as in Fig. 2. Since defects in the visual field can usually be traced to pathology in the visual pathway, from the retina to the visual cortex, the interpretation of visual fields is important in ophthalmic and neurologic examinations. Knowledge of the topology of the visual pathway enables the Manuscript received March 13, 1978; revised April 13, 1979. This paper is part of a dissertation submitted to the Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge, MA, in partial fulfillment of the requirements for the Ph.D. degree. The author is with the Department of Systems Design, University of Waterloo, Waterloo, Ont., Canada.
MEMBER, IEEE
examiner to associate specific field defects with lesions at specific sites in the pathway [9]. In order to obtain a visual field plot, the visibility of stimuli of specific contrast, size, and color, presented at various locations in the field, must be determined. By varying contrasts it is then possible to determine threshold sensitivity. Since sensitivity, in general, varies with retinal position, it is important to be sure that the stimulus is presented at a specific position and that the perceptual decision is made while the position is maintained. When this requirement is coupled with reliance on the subject's verbal response, it is easy to see why the continuous fixation requirement was adopted. Even if the subject is sufficiently reliable and sophisticated to report perception prior to any eye movement, it is often difficult for him to know whether or not he made an eye movement before seeing the stimulus. Recent instruments for visual field plotting allow for accurate control of stimulus contrast and position as well as some means of monitoring subject fixation (some rely on visual observation by the operator while at least one employs a photoelectric fixation monitor) [31, [5]. These efforts have been supplemented by short (less than 100 ms) stimulus presentation times to eliminate the possibility of saccadic response enabling perception of an otherwise subthreshold stimulus (minimum response latencies are about 150 ms). In all cases the subject is instructed to maintain central fixation and respond either manually or verbally when the stimulus is perceived. Greve's thorough work on static perimetry includes a detailed description of three clinical instruments [4]. More recently, attempts have been made to further automate perimetry and improve its reliability. Fankhauser et al. describe a technique in which the patient's responses are processed in the context of any available a priori information about the visual field [2]. An interactive procedure employing context and redundancy of target presentation is then followed to obtain a reliable field plot in spite of variability in patient response. Their technique seems to hold the most promise for problems associated with subjective response and the necessity to maintain fixation. The approach proposed in this paper is directed at eliminating, rather than compensating, for these two problem areas. EYE MOVEMENT APPROACH TO FIELD PLOTTING
The fundamental piece of information necessary in visual field plotting is whether the subject perceives a stimulus presented at a given location in the field. The fixation requirement of traditional techniques hopes to achieve not only accurate stimulus position, but also a reliable subjective response
0018-9294/79/1100-0601$00.75 © 1979 IEEE
602
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-26, NO. 11, NOVEMBER 1979
7w
UP
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2700 Fig. 1. Typical visual field plot, right eye central 300 field. Dark lines represent isopters, contours of constant sensitivity. Shaded ellipse corresponds to physiological blind spot. SENSITIVITY
2P0 300 200 100 0 10° 30° Fig. 2. Typical contrast sensitivity plot. Note sharp drop at blind spot around 150 right of center.
to the appropriate peripheral perception. But it asks the subject to behave unnaturally, to suppress the strong natural urge to look at, make an eye movement to, any new stimulus appearing in the periphery. The measurement process is disrupting the very perceptual phenomenon it is designed to measure. A more satisfying approach to the problem may be to present the desired stimulus, encourage the natural eye movement response and devise a scheme for distinguishing between responses which indicate that the stimulus was above threshold and perceived, and responses, such as searching behavior, which indicate that the stimulus was subthreshold. Rather than relying on the subject's introspective decision, the scheme monitors oculomotor response. The response is processed and the necessary information is extracted to make an automatic objective decision. The subject is presented with a central fixation target and his eye position is monitored. While he is fixating the central target, a near threshold, point light stimulus is presented at a desired position in the visual field. To encourage the natural eye movement response, the central fixation target is extin-
LEFT
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RESP TARG Fig. 3. Simple acquisition of perceived stimulus. Upper trace: vertical component, up and down indicate 200 movement. Lower trace: horizontal component, right and left indicate 200 movement. RESP: subject's manual response indicates target seen. TARG: target presentation. Abscissa marked in seconds. Inset above is a vector diagram of the plotted saccade; horizontal and vertical axes of the visual field are as drawn.
guished when the peripheral stimulus is presented. If the stimulus is above threshold, the subject will make one or more eye movements in a characteristic acquisition sequence to fixate the stimulus (see Fig. 3). If the stimulus is below threshold, the subject may continue to fixate the central region (even with the fixation target extinguished), or, more likely, he will initiate a sequence of searching saccadic eye movements in an effort to locate the target (see Fig. 4). To determine whether the stimulus was above or below threshold, then, it is necessary to classify the saccadic response sequence as either acquisition or search. Before discussing the nature of the eye movement analysis, consider the relative merits and potential problems of this approach. As mentioned earlier, the approach is matched to the perceptual task and exploits the natural eye movement response rather than acting in opposition to it. It is also relatively objective in that a decision is made automatically, based on the oculomotor response, rather than relying on the subject's manual or verbal response. While it is true that subjects have some conscious control over their eye movements and can suppress the natural response, yielding a false negative decision, there is little likelihood that a false positive will result. As long as the stimulus position is unknown to the subject prior to presentation, there is little chance that an initial search saccade would be confused with an initial acquisition saccade. In any case, one would expect problems with malingerers to
JERNIGAN: VISUAL FIELD PLOTTING
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Fig. 5. Schematic diagram of saccade showing significant parameters. HOR
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Fig. 4. Search sequence. Note blink coincident with third saccade (see Fig. 3).
be diminished if a natural response is encouraged rather than prohibited. ANALYSIS TECHNIQUES In analyzing the eye movement response to visual field stimuli, it is possible to take two approaches, one which concentrates on the characteristics of the initial saccade, and the other which is more global and considers the structure of the entire saccadic sequence. A conceptually simple position comparison scheme, described as impractical below, is one local scheme. The parameters of the initial saccade and subsequent fixation are shown in Fig. 5. Since the proposed visual field plotting scheme is designed to encourage eye movement response, it might be expected that discriminating between acquisition and search behavior on the basis of the initial saccade would be difficult without detailed position information. On the other hand, comparison of eye movement records of entire response sequences suggests that there are significant structural differences that might be exploited (compare Figs. 3 and 4). Before developing the decision algorithm, however, it is necessary to be more explicit about the meaning of structure of the saccadic sequence. The structure refers to relative measures among the saccades in the sequence. Temporal information, latencies between saccades, and relative magnitude and direction measures are used in the structural analysis. By using information about all of the saccades in the sequence, a reliable discrimination might be made using less precise measures of any single saccade than those required for
any local initial saccade based scheme. Practical problems associated with implementing the scheme stem from the need for monitoring and analyzing eye move-
ments. If complete eye movement information is available, a simple comparison of stimulus position to eye position after the initial saccade is the obvious approach to determining whether the stimulus was perceived without searching. Unfortunately, the technology of eye movement monitoring devices imposes severe practical constraints. Highly accurate devices which are linear over the central 200 field, such as those of Cornsweet and Crane, and Merchant et al., are not only expensive, but rather complex to use [1], [71. Such instruments may be unacceptable as components of a clinical instrument. Other optical devices are available, but are also relatively expensive and complex for clinical application [11]. Electrooculography is another accepted technique for eye movement monitoring, but it requires electrodes that may not be desirable in a clinical environment, and may not be sufficiently accurate for a direct position comparison scheme. The eye movement analysis procedure should, therefore, require minimal eye movement information on which to base decisions. An experiment was undertaken to collect data to test various analysis techniques. Actual eye movement responses to nearthreshold peripheral stimuli were recorded using a headmounted optical eye movement monitoring technique' [11]. Point-light stimuli were presented at positions throughout the central (150) visual field. A total of 1456 responses were recorded. Several local schemes, using information about the initial saccade and subsequent fixation, were implemented. In each case, precise position information was ignored in order to be consistent with the expressed goal of obtaining a clinically practical procedure. In fact, the actual position of the stimulus was not considered except to establish the true classification of the response for evaluation purposes. The eye movement signal was sampled every 2 ms and digitized. A digital preprocessing algorithm was applied to detect saccades and fixations while screening out blink artifacts such as that shown in Fig. 4. The preprocessing is a sequential difference thresholding scheme which easily discriminates between the funda-
1The
eye movement monitor is a Biometrics Model 200, modified to record horizontal and vertical components from the same eye.
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-26, NO. 11, NOVEMBER 1979
mentally step-like nature of a fixation-saccade-fixation sequence and the pulse-like nature of a blink. Details of the implementation, including processing of the eye movement signal, are described elsewhere [6]. As expected, these local schemes were not particularly successful when deprived of accurate position information. The simplest approach assumed that the latency between stimulus presentation and initial oculomotor response would be greater for search responses. For an empirically optimum threshold, acquisition responses were correctly identified on 55 percent of the trials, and search responses were correctly identified on 75 percent of the trials. A more successful approach relied on duration of the fixation following the initial saccade, with allowance made for normal corrective saccades. In a search sequence, the first new fixation usually lasts just long enough to establish that the stimulus is not present at the new location, after which a second saccade occurs. In an acquisition sequence, the new fxation is on target and is likely to be held until the stimulus is extinguished. True seen rate (correct identification of acquisition responses) was 91 percent while true miss was 84 percent. This performance was encouraging since the decision was based on temporal eye movement information which can be easily obtained with simple photoelectric monitoring techniques. Note, however, that this scheme is actually somewhat global in that it must allow for corrective saccades, otherwise, the brief fixation (80-150 ms) between initial saccade and the correction saccade would result in a search classification. By extending the analysis to deal with the entire sequence, then, we might expect even better classification performance. From this more global perspective, an acquisition response is characterized by a single relatively long fixation (of the stimulus, although remember that actual position is ignored) which begins at least 160 ms after stimulus presentation. It is sometimes followed by an early return to center with possible refixation of the peripheral stimulus. When the stimulus is extinguished, the central fixation target is refixated. Fig. 6 shows the algorithm which sequentially analyzes the eye movement response. The first three steps detect an initial saccade and screen for normal correction saccades using relative latency, magnitude, and direction information. If a new noncorrective saccade occurs too soon (within 300 ms) a SEARCH decision is returned, otherwise, the new saccade is analyzed according to whether it occurs during or after stimulus presentation. A premature return to center is provided for in the left branch (BEFORE TOFF). In the right branch, the occurrence of a third major saccade results in a SEARCH decision. Finally, once it has been determined that the saccadic sequence has the expected acquisition structure, a fixation duration criterion discriminates between single fixation search sequences which are usually less than 500 ms and true acquisitions. The major complication is the necessity of detecting and allowing for the corrective saccades which are part of the normal acquisition response. When applied to the data, this approach resulted in a true seen rate of 95 percent and a true miss rate of 97 percent. This compares favorably, not only to the local schemes described above, but also to the results of the subject's own response, where on 32 percent of the cases where search be-
.START DETECT SACCADE NONE
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Fig. 6. Decision algorithm functional flow chart.
havior indicated that the stimulus was not perceived, the subject responded "seen" (true miss rate = 68 percent). Since the primary goal of visual field plotting is the detection of relative scotomas, where the stimulus is below threshold, it is especially important to achieve the highest possible true miss rate. A VISUAL FIELD PLOTTING EXAMPLE The results of the last section verify the potential of eye movement response as an index of perception of point-light stimuli in the visual field. In this section, the technique will be applied to obtain a simple plot of a subject's central field (left eye), including the normal physiological blind spot. Fig. 7 shows the target positions for the example. Clearly, there are too few targets for an exhaustive field examination, but the positions chosen are adequate to demonstrate the technique, particularly in the area of the normal blind spot. Background illumination was 2 cd/M2. Four trials were performed, each consisting of the 32 targets. Target intensity was decremented in 0.2 log unit steps between trials, with the first trial, or maximum intensity, set at 18 cd/m2. The decision algorithm described in the last section was used to classify the responses. The plots of Fig. 8(a)-(d) show the target positions labeled by the response classifications, or decisions, for the four stimulus levels. Unlabeled points were decided as seen points. Two other types of response classifications are shown. M indicates a simple miss, based on either no eye movement response, or a searching response. The second category, invalid trial, consists of eye movement responses involving antici-
patory saccades occurring either prior
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JERNIGAN: VISUAL FIELD PLOTTING
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Fig. 7. Shown are 32 target positions for visual field plotting example. Dashed ellipse indicates position of normal blind spot for the left eye.
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or with a latency less than 160 ms. These trials are not necessarily misses, but they must be invalidated and reexamined because of the shift in eye position before the target could be perceived. Combining the results of Fig. 8 with results of retests of the indictated points, the plot of Fig. 9 is obtained. The four intensity isopters are labeled by the neutral density filters used to attenuate the original 18 cd/M2 stimulus. The subject's blind spot is clearly shown, centered at a point 130 left of, and 20 below center. Horizontal extent is about 60 and vertical extent is about 80, well within normal limits.
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IMPLICATIONS FOR CLINICAL APPLICATION 0 0 0 0 There are several procedural and instrumental implications 0 M of the eye movement processing results for clinical visual field 00 00 00 MM M plotting. Target presentation should be modified to enhance 0 0 0 0 x eye movement response rather than to avoid it. Increased tar0 0 0 x get duration and elimination of the central fixation point during target presentation are, therefore, recommended. The order of presentation of targets should be unknown to the subND =0.4 ject in order to prevent successful prediction of target posi(c) tion. In other respects, the target presentation procedure should adhere to the principles described by Greve [4]. The significant instrumental requirement is the necessity of an eye movement monitor. The degree of sophistication re0 M < quired depends on the desired reliability of the field plots. 0 0 Mass screening requirements are likely to be different from 0 x M 0 those of detailed diagnosis and research applications. As the Mx 00 M M MM performance of the algorithm of the last section indicates, M 0 0 0 a useful clinical screening instrument can be built around a M 0 M M simple and inexpensive eye movement monitor. Temporal 0 X information and coarse position measurements are adequate to achieve a false seen rate of 3 percent. Specifically, eye movement monitor response time should be on the order of ND =0.6 5 ms, and accuracy need be no better than a few degrees with (d) crosstalk nonlinearities of 20 percent. Note that, although Fig. 8. Four stimulus intensity levels. 0 is the stimulus perceived (acthe monitor may be nonlinear, it is presumed stable in the quisition response), M is the stimulus not perceived (search response), sense that a constant eye position results in a constant output and X is the invalid trial (anticipatory saccade). m
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-26, NO. 11, NOVEMBER 1979
606
the oculomotor response to stimuli presented in a homonymous field defect would be necessary. The successful implementation of a reliable and convenient clinical visual fields procedure would have a significant impact on ophthalmologic and optometric practice. Visual fields could become part of normal visual examinations, greatly facilitating early diagnosis of glaucoma, as well as other visual diseases. REFERENCES [1] T. N. Cornsweet and H. D. Crane, "Accurate two-dimensional eye tracker using first and fourth Purkinje images," J. Opt. Soc. Amer., vol. 63, pp. 921-928, Aug. 1973. [2] F. Fankhauser, P. Koch, and A. Roulier, "On automation of perimetry," Albrecht V. Graefes Arch. Klin. Exp. Opthal., vol. 04
184, pp. 126-150, 1972.
[31 J. Gloster, "Flash Perimetry," Brit. J. Ophthal., vol. 54, pp. 649-658, 1970. [4] E. L. Greve, Single and Multiple Stimulus Static Perimetry in Glaucoma; The Two Phases of Visual Field Examination. The Hague, The Netherlands: Dr. W. Junk B. V., 1973.
[51 D. 0. Harrington, The Visual Fields. St. Louis, MO: Mosby,
Fig. 9. Visual field plot obtained from Fig. 8. Isopters for 0.0, 0.2, 0.4, and 0.6 neutral density filters (log units of intensity attenuation). See text.
that the structural pattern of saccades (changes) and fixations (no change) is retained. Simple remote photoelectric monitoring schemes are more than adequate. The final decision algorithm can be implemented with available integrated circuit components. A combined stimulus projector, eye movement monitor, and signal processing electronics for reso
analysis could be produced and marketed as a selfcontained instrument. The decision algorithm may be implemented in real time, allowing a stimulus presentation rate of about 12 per min. Since typically four stimulus intensities must be presented at each retinal location to determine the sensitivity threshold, three positions may be determined per minute. The time required for a complete field plot depends on the desired resolution. For example, to plot 45 positions requires 15 min. A potential limitation of the eye movement analysis procedure is raised by the "blind sight" oculomotor response, a residual oculomotor response which seems to correlate with the position of a stimulus presented within a homonymous field defect caused by postchiasmal lesions of the visual pathway [8], [10]. There are two alternatives. The eye movement analysis approach may be limited to application in field plotting for diagnosis of retinal defects such as those of glaucoma. Alternatively, a more elaborate scheme could be developed to discriminate between normal acquisition responses and the "blind sight" response. More specific data on sponse
1971. [6] M. E. Jernigan, "Eye movement analysis in plotting the visual field," Ph.D. dissertation, Dep. Elec. Eng., Massachusetts Inst. Tech., Cambridge, MA, 1975. [7] J. Merchant, R. Morrissette, and J. L. Porterfield, "Remote measurement of eye direction allowing subject motion over one cubic foot of space," IEEE Trans. Biomed. Eng., vol. BME-21, pp. 309-317, July 1974. [8] E. Poppel, R. Held, and D. Frost, "Residual visual function after brain wounds involving the central visual pathways in man," Nature, vol. 243, pp. 295-296, June 1973. [9] C. W. Rucker, The Interpretation of Visual Fields, AAOO Continuing Education Programs, 1957. [10] M. D. Sanders, E. K. Warrington, and J. Marshall, "'Blindsight': Vision in a field defect," Lancet, pp. 707-708, Apr. 20, 1974. [11] L. R. Young and D. Sheena, "Eye-movement measurement techniques,"Amer. Psychol., vol. 30, pp. 315-330, Mar. 1975.
M. Edward Jernigan (M'77) was born in Quantico, VA, in 1947. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from the Massachusetts Institute of Technology, Cambridge, in 1969, 1971, and 1975, respectively. * In 1976 he was a Postdoctoral Research Associate at the Research Laboratory of Electronics, Massachusetts Institute of Technology, working in the area of pattern recognition and eye movement analysis. Since then he has been an Assistant Professor in the Department of Systems Design, University of Waterloo, Waterloo, Ont., Canada. His research interests are in the areas of pattern recognition, image processing, and communications, with particular interests in biomedical applications, biophysical signal analysis, and visual perception. Dr. Jernigan is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi. He is a Registered Professional Engineer.
601
Visual Field Plotting Using Eye Movement Response M. EDWARD JERNIGAN,
Abstract-Traditional techniques for visual field plotting are described as acting in opposition to the underlying perceptual task. An approach is described which encourages the natural eye movement response and objectively classifies it as either an acquisition or a search response. Empirical results demonstrate the reliability of eye movement response as an index of perception of peripheral stimuli A visual field plotting example is given and implications for clinical practice are discussed.
INTRODUCTION IN attempting to assess performance of the visual system for the purpose of diagnosing visual disease, it is important to be aware of the perceptual context in which the subject is operating. Measurements of vision made in an unnatural perceptual environment are especially prone to error when subjective response is relied on to obtain the desired information. In visual field plotting, the subject is asked to report whether a peripheral stimulus is visible, yet he is told he must not look at it. While it is certainly possible for subjects to report visibility while maintaining fixation on a central target, the perceptual task is unnatural. This conflict between task and natural response is the source of serious difficulty in traditional techniques of visual field plotting, or perimetry. A brief discussion of visual fields and the historical need for continuous fixation leads into a discussion of a 'new approach to the problem which employs a more natural visual task as well as a more objective scheme for deciding whether a peripheral stimulus was perceived. THE VISUAL FIELD The visual field is defined as the region of space that is visible when the eye is fixated. Quantitatively, it is plotted by detennining the sensitivity to point light stimuli at locations in a region around the point of fixation. A visual field plot is a map of retinal sensitivity, and is usually represented by a plot of isopters, contours of constant sensitivity, on polar coordinates. Fig. 1 shows a typical plot of the central visual field for a normal subject's right eye. An alternate representation of retinal sensitivity plots sensitivity versus eccentricity along a selected meridian, as in Fig. 2. Since defects in the visual field can usually be traced to pathology in the visual pathway, from the retina to the visual cortex, the interpretation of visual fields is important in ophthalmic and neurologic examinations. Knowledge of the topology of the visual pathway enables the Manuscript received March 13, 1978; revised April 13, 1979. This paper is part of a dissertation submitted to the Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge, MA, in partial fulfillment of the requirements for the Ph.D. degree. The author is with the Department of Systems Design, University of Waterloo, Waterloo, Ont., Canada.
MEMBER, IEEE
examiner to associate specific field defects with lesions at specific sites in the pathway [9]. In order to obtain a visual field plot, the visibility of stimuli of specific contrast, size, and color, presented at various locations in the field, must be determined. By varying contrasts it is then possible to determine threshold sensitivity. Since sensitivity, in general, varies with retinal position, it is important to be sure that the stimulus is presented at a specific position and that the perceptual decision is made while the position is maintained. When this requirement is coupled with reliance on the subject's verbal response, it is easy to see why the continuous fixation requirement was adopted. Even if the subject is sufficiently reliable and sophisticated to report perception prior to any eye movement, it is often difficult for him to know whether or not he made an eye movement before seeing the stimulus. Recent instruments for visual field plotting allow for accurate control of stimulus contrast and position as well as some means of monitoring subject fixation (some rely on visual observation by the operator while at least one employs a photoelectric fixation monitor) [31, [5]. These efforts have been supplemented by short (less than 100 ms) stimulus presentation times to eliminate the possibility of saccadic response enabling perception of an otherwise subthreshold stimulus (minimum response latencies are about 150 ms). In all cases the subject is instructed to maintain central fixation and respond either manually or verbally when the stimulus is perceived. Greve's thorough work on static perimetry includes a detailed description of three clinical instruments [4]. More recently, attempts have been made to further automate perimetry and improve its reliability. Fankhauser et al. describe a technique in which the patient's responses are processed in the context of any available a priori information about the visual field [2]. An interactive procedure employing context and redundancy of target presentation is then followed to obtain a reliable field plot in spite of variability in patient response. Their technique seems to hold the most promise for problems associated with subjective response and the necessity to maintain fixation. The approach proposed in this paper is directed at eliminating, rather than compensating, for these two problem areas. EYE MOVEMENT APPROACH TO FIELD PLOTTING
The fundamental piece of information necessary in visual field plotting is whether the subject perceives a stimulus presented at a given location in the field. The fixation requirement of traditional techniques hopes to achieve not only accurate stimulus position, but also a reliable subjective response
0018-9294/79/1100-0601$00.75 © 1979 IEEE
602
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-26, NO. 11, NOVEMBER 1979
7w
UP
VER
DOWN RIGHT
HOR
2700 Fig. 1. Typical visual field plot, right eye central 300 field. Dark lines represent isopters, contours of constant sensitivity. Shaded ellipse corresponds to physiological blind spot. SENSITIVITY
2P0 300 200 100 0 10° 30° Fig. 2. Typical contrast sensitivity plot. Note sharp drop at blind spot around 150 right of center.
to the appropriate peripheral perception. But it asks the subject to behave unnaturally, to suppress the strong natural urge to look at, make an eye movement to, any new stimulus appearing in the periphery. The measurement process is disrupting the very perceptual phenomenon it is designed to measure. A more satisfying approach to the problem may be to present the desired stimulus, encourage the natural eye movement response and devise a scheme for distinguishing between responses which indicate that the stimulus was above threshold and perceived, and responses, such as searching behavior, which indicate that the stimulus was subthreshold. Rather than relying on the subject's introspective decision, the scheme monitors oculomotor response. The response is processed and the necessary information is extracted to make an automatic objective decision. The subject is presented with a central fixation target and his eye position is monitored. While he is fixating the central target, a near threshold, point light stimulus is presented at a desired position in the visual field. To encourage the natural eye movement response, the central fixation target is extin-
LEFT
0LO
1.0
2-0
3.0
RESP TARG Fig. 3. Simple acquisition of perceived stimulus. Upper trace: vertical component, up and down indicate 200 movement. Lower trace: horizontal component, right and left indicate 200 movement. RESP: subject's manual response indicates target seen. TARG: target presentation. Abscissa marked in seconds. Inset above is a vector diagram of the plotted saccade; horizontal and vertical axes of the visual field are as drawn.
guished when the peripheral stimulus is presented. If the stimulus is above threshold, the subject will make one or more eye movements in a characteristic acquisition sequence to fixate the stimulus (see Fig. 3). If the stimulus is below threshold, the subject may continue to fixate the central region (even with the fixation target extinguished), or, more likely, he will initiate a sequence of searching saccadic eye movements in an effort to locate the target (see Fig. 4). To determine whether the stimulus was above or below threshold, then, it is necessary to classify the saccadic response sequence as either acquisition or search. Before discussing the nature of the eye movement analysis, consider the relative merits and potential problems of this approach. As mentioned earlier, the approach is matched to the perceptual task and exploits the natural eye movement response rather than acting in opposition to it. It is also relatively objective in that a decision is made automatically, based on the oculomotor response, rather than relying on the subject's manual or verbal response. While it is true that subjects have some conscious control over their eye movements and can suppress the natural response, yielding a false negative decision, there is little likelihood that a false positive will result. As long as the stimulus position is unknown to the subject prior to presentation, there is little chance that an initial search saccade would be confused with an initial acquisition saccade. In any case, one would expect problems with malingerers to
JERNIGAN: VISUAL FIELD PLOTTING
603 POSITION
UP
MAGNI1
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TIME DOWN RIGHT DURATION
Fig. 5. Schematic diagram of saccade showing significant parameters. HOR
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0.0 RESP TARG
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Fig. 4. Search sequence. Note blink coincident with third saccade (see Fig. 3).
be diminished if a natural response is encouraged rather than prohibited. ANALYSIS TECHNIQUES In analyzing the eye movement response to visual field stimuli, it is possible to take two approaches, one which concentrates on the characteristics of the initial saccade, and the other which is more global and considers the structure of the entire saccadic sequence. A conceptually simple position comparison scheme, described as impractical below, is one local scheme. The parameters of the initial saccade and subsequent fixation are shown in Fig. 5. Since the proposed visual field plotting scheme is designed to encourage eye movement response, it might be expected that discriminating between acquisition and search behavior on the basis of the initial saccade would be difficult without detailed position information. On the other hand, comparison of eye movement records of entire response sequences suggests that there are significant structural differences that might be exploited (compare Figs. 3 and 4). Before developing the decision algorithm, however, it is necessary to be more explicit about the meaning of structure of the saccadic sequence. The structure refers to relative measures among the saccades in the sequence. Temporal information, latencies between saccades, and relative magnitude and direction measures are used in the structural analysis. By using information about all of the saccades in the sequence, a reliable discrimination might be made using less precise measures of any single saccade than those required for
any local initial saccade based scheme. Practical problems associated with implementing the scheme stem from the need for monitoring and analyzing eye move-
ments. If complete eye movement information is available, a simple comparison of stimulus position to eye position after the initial saccade is the obvious approach to determining whether the stimulus was perceived without searching. Unfortunately, the technology of eye movement monitoring devices imposes severe practical constraints. Highly accurate devices which are linear over the central 200 field, such as those of Cornsweet and Crane, and Merchant et al., are not only expensive, but rather complex to use [1], [71. Such instruments may be unacceptable as components of a clinical instrument. Other optical devices are available, but are also relatively expensive and complex for clinical application [11]. Electrooculography is another accepted technique for eye movement monitoring, but it requires electrodes that may not be desirable in a clinical environment, and may not be sufficiently accurate for a direct position comparison scheme. The eye movement analysis procedure should, therefore, require minimal eye movement information on which to base decisions. An experiment was undertaken to collect data to test various analysis techniques. Actual eye movement responses to nearthreshold peripheral stimuli were recorded using a headmounted optical eye movement monitoring technique' [11]. Point-light stimuli were presented at positions throughout the central (150) visual field. A total of 1456 responses were recorded. Several local schemes, using information about the initial saccade and subsequent fixation, were implemented. In each case, precise position information was ignored in order to be consistent with the expressed goal of obtaining a clinically practical procedure. In fact, the actual position of the stimulus was not considered except to establish the true classification of the response for evaluation purposes. The eye movement signal was sampled every 2 ms and digitized. A digital preprocessing algorithm was applied to detect saccades and fixations while screening out blink artifacts such as that shown in Fig. 4. The preprocessing is a sequential difference thresholding scheme which easily discriminates between the funda-
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eye movement monitor is a Biometrics Model 200, modified to record horizontal and vertical components from the same eye.
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mentally step-like nature of a fixation-saccade-fixation sequence and the pulse-like nature of a blink. Details of the implementation, including processing of the eye movement signal, are described elsewhere [6]. As expected, these local schemes were not particularly successful when deprived of accurate position information. The simplest approach assumed that the latency between stimulus presentation and initial oculomotor response would be greater for search responses. For an empirically optimum threshold, acquisition responses were correctly identified on 55 percent of the trials, and search responses were correctly identified on 75 percent of the trials. A more successful approach relied on duration of the fixation following the initial saccade, with allowance made for normal corrective saccades. In a search sequence, the first new fixation usually lasts just long enough to establish that the stimulus is not present at the new location, after which a second saccade occurs. In an acquisition sequence, the new fxation is on target and is likely to be held until the stimulus is extinguished. True seen rate (correct identification of acquisition responses) was 91 percent while true miss was 84 percent. This performance was encouraging since the decision was based on temporal eye movement information which can be easily obtained with simple photoelectric monitoring techniques. Note, however, that this scheme is actually somewhat global in that it must allow for corrective saccades, otherwise, the brief fixation (80-150 ms) between initial saccade and the correction saccade would result in a search classification. By extending the analysis to deal with the entire sequence, then, we might expect even better classification performance. From this more global perspective, an acquisition response is characterized by a single relatively long fixation (of the stimulus, although remember that actual position is ignored) which begins at least 160 ms after stimulus presentation. It is sometimes followed by an early return to center with possible refixation of the peripheral stimulus. When the stimulus is extinguished, the central fixation target is refixated. Fig. 6 shows the algorithm which sequentially analyzes the eye movement response. The first three steps detect an initial saccade and screen for normal correction saccades using relative latency, magnitude, and direction information. If a new noncorrective saccade occurs too soon (within 300 ms) a SEARCH decision is returned, otherwise, the new saccade is analyzed according to whether it occurs during or after stimulus presentation. A premature return to center is provided for in the left branch (BEFORE TOFF). In the right branch, the occurrence of a third major saccade results in a SEARCH decision. Finally, once it has been determined that the saccadic sequence has the expected acquisition structure, a fixation duration criterion discriminates between single fixation search sequences which are usually less than 500 ms and true acquisitions. The major complication is the necessity of detecting and allowing for the corrective saccades which are part of the normal acquisition response. When applied to the data, this approach resulted in a true seen rate of 95 percent and a true miss rate of 97 percent. This compares favorably, not only to the local schemes described above, but also to the results of the subject's own response, where on 32 percent of the cases where search be-
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havior indicated that the stimulus was not perceived, the subject responded "seen" (true miss rate = 68 percent). Since the primary goal of visual field plotting is the detection of relative scotomas, where the stimulus is below threshold, it is especially important to achieve the highest possible true miss rate. A VISUAL FIELD PLOTTING EXAMPLE The results of the last section verify the potential of eye movement response as an index of perception of point-light stimuli in the visual field. In this section, the technique will be applied to obtain a simple plot of a subject's central field (left eye), including the normal physiological blind spot. Fig. 7 shows the target positions for the example. Clearly, there are too few targets for an exhaustive field examination, but the positions chosen are adequate to demonstrate the technique, particularly in the area of the normal blind spot. Background illumination was 2 cd/M2. Four trials were performed, each consisting of the 32 targets. Target intensity was decremented in 0.2 log unit steps between trials, with the first trial, or maximum intensity, set at 18 cd/m2. The decision algorithm described in the last section was used to classify the responses. The plots of Fig. 8(a)-(d) show the target positions labeled by the response classifications, or decisions, for the four stimulus levels. Unlabeled points were decided as seen points. Two other types of response classifications are shown. M indicates a simple miss, based on either no eye movement response, or a searching response. The second category, invalid trial, consists of eye movement responses involving antici-
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or with a latency less than 160 ms. These trials are not necessarily misses, but they must be invalidated and reexamined because of the shift in eye position before the target could be perceived. Combining the results of Fig. 8 with results of retests of the indictated points, the plot of Fig. 9 is obtained. The four intensity isopters are labeled by the neutral density filters used to attenuate the original 18 cd/M2 stimulus. The subject's blind spot is clearly shown, centered at a point 130 left of, and 20 below center. Horizontal extent is about 60 and vertical extent is about 80, well within normal limits.
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IMPLICATIONS FOR CLINICAL APPLICATION 0 0 0 0 There are several procedural and instrumental implications 0 M of the eye movement processing results for clinical visual field 00 00 00 MM M plotting. Target presentation should be modified to enhance 0 0 0 0 x eye movement response rather than to avoid it. Increased tar0 0 0 x get duration and elimination of the central fixation point during target presentation are, therefore, recommended. The order of presentation of targets should be unknown to the subND =0.4 ject in order to prevent successful prediction of target posi(c) tion. In other respects, the target presentation procedure should adhere to the principles described by Greve [4]. The significant instrumental requirement is the necessity of an eye movement monitor. The degree of sophistication re0 M < quired depends on the desired reliability of the field plots. 0 0 Mass screening requirements are likely to be different from 0 x M 0 those of detailed diagnosis and research applications. As the Mx 00 M M MM performance of the algorithm of the last section indicates, M 0 0 0 a useful clinical screening instrument can be built around a M 0 M M simple and inexpensive eye movement monitor. Temporal 0 X information and coarse position measurements are adequate to achieve a false seen rate of 3 percent. Specifically, eye movement monitor response time should be on the order of ND =0.6 5 ms, and accuracy need be no better than a few degrees with (d) crosstalk nonlinearities of 20 percent. Note that, although Fig. 8. Four stimulus intensity levels. 0 is the stimulus perceived (acthe monitor may be nonlinear, it is presumed stable in the quisition response), M is the stimulus not perceived (search response), sense that a constant eye position results in a constant output and X is the invalid trial (anticipatory saccade). m
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the oculomotor response to stimuli presented in a homonymous field defect would be necessary. The successful implementation of a reliable and convenient clinical visual fields procedure would have a significant impact on ophthalmologic and optometric practice. Visual fields could become part of normal visual examinations, greatly facilitating early diagnosis of glaucoma, as well as other visual diseases. REFERENCES [1] T. N. Cornsweet and H. D. Crane, "Accurate two-dimensional eye tracker using first and fourth Purkinje images," J. Opt. Soc. Amer., vol. 63, pp. 921-928, Aug. 1973. [2] F. Fankhauser, P. Koch, and A. Roulier, "On automation of perimetry," Albrecht V. Graefes Arch. Klin. Exp. Opthal., vol. 04
184, pp. 126-150, 1972.
[31 J. Gloster, "Flash Perimetry," Brit. J. Ophthal., vol. 54, pp. 649-658, 1970. [4] E. L. Greve, Single and Multiple Stimulus Static Perimetry in Glaucoma; The Two Phases of Visual Field Examination. The Hague, The Netherlands: Dr. W. Junk B. V., 1973.
[51 D. 0. Harrington, The Visual Fields. St. Louis, MO: Mosby,
Fig. 9. Visual field plot obtained from Fig. 8. Isopters for 0.0, 0.2, 0.4, and 0.6 neutral density filters (log units of intensity attenuation). See text.
that the structural pattern of saccades (changes) and fixations (no change) is retained. Simple remote photoelectric monitoring schemes are more than adequate. The final decision algorithm can be implemented with available integrated circuit components. A combined stimulus projector, eye movement monitor, and signal processing electronics for reso
analysis could be produced and marketed as a selfcontained instrument. The decision algorithm may be implemented in real time, allowing a stimulus presentation rate of about 12 per min. Since typically four stimulus intensities must be presented at each retinal location to determine the sensitivity threshold, three positions may be determined per minute. The time required for a complete field plot depends on the desired resolution. For example, to plot 45 positions requires 15 min. A potential limitation of the eye movement analysis procedure is raised by the "blind sight" oculomotor response, a residual oculomotor response which seems to correlate with the position of a stimulus presented within a homonymous field defect caused by postchiasmal lesions of the visual pathway [8], [10]. There are two alternatives. The eye movement analysis approach may be limited to application in field plotting for diagnosis of retinal defects such as those of glaucoma. Alternatively, a more elaborate scheme could be developed to discriminate between normal acquisition responses and the "blind sight" response. More specific data on sponse
1971. [6] M. E. Jernigan, "Eye movement analysis in plotting the visual field," Ph.D. dissertation, Dep. Elec. Eng., Massachusetts Inst. Tech., Cambridge, MA, 1975. [7] J. Merchant, R. Morrissette, and J. L. Porterfield, "Remote measurement of eye direction allowing subject motion over one cubic foot of space," IEEE Trans. Biomed. Eng., vol. BME-21, pp. 309-317, July 1974. [8] E. Poppel, R. Held, and D. Frost, "Residual visual function after brain wounds involving the central visual pathways in man," Nature, vol. 243, pp. 295-296, June 1973. [9] C. W. Rucker, The Interpretation of Visual Fields, AAOO Continuing Education Programs, 1957. [10] M. D. Sanders, E. K. Warrington, and J. Marshall, "'Blindsight': Vision in a field defect," Lancet, pp. 707-708, Apr. 20, 1974. [11] L. R. Young and D. Sheena, "Eye-movement measurement techniques,"Amer. Psychol., vol. 30, pp. 315-330, Mar. 1975.
M. Edward Jernigan (M'77) was born in Quantico, VA, in 1947. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from the Massachusetts Institute of Technology, Cambridge, in 1969, 1971, and 1975, respectively. * In 1976 he was a Postdoctoral Research Associate at the Research Laboratory of Electronics, Massachusetts Institute of Technology, working in the area of pattern recognition and eye movement analysis. Since then he has been an Assistant Professor in the Department of Systems Design, University of Waterloo, Waterloo, Ont., Canada. His research interests are in the areas of pattern recognition, image processing, and communications, with particular interests in biomedical applications, biophysical signal analysis, and visual perception. Dr. Jernigan is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi. He is a Registered Professional Engineer.
