REFERENCES. Amblyopic abnormality involves neural mechanisms concerned with movement processing. INGO RENTSCHLER, RUDOLF

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1 Volume 20 Number 5 Reports 695 terrupt the light quite as abruptly as a mechanical shutter. There is reason to believe, however, that the differences in waveform, particularly in the first section of the responses, cannot be attributed only to stimulus factors. The differences seem too large. Furthermore it is now recognized that the role of blinks is not merely to periodically interrupt the light. Psychophysical experiments have demonstrated a transient increase in visual threshold that is time-locked to blinks." The importance of blinks and their relation to vision deserves further investigation. I thank Susan McCarthy for her assistance and suggestions. From the Department of Psychology, Northeastern University, Boston. This research was supported by grant EY from the National Eye Institute. Submitted for publication Aug. 20, Reprint requests: John C. Armington, Department of Psychology, Northeastern University, Boston, Mass Key words: visual evoked potential, blinks, artifact rejection REFERENCES 1. Alpern M: Eye movements. In Visual Psychophysics, Jameson D and Hurvich LM, editors. Berlin, 1972, Springer-Verlag, pp Ditchburn RW: Eye Movements and Visual Perception. Oxford, 1973, Clarendon Press, pp Yarbus DL: Eye Movements and Vision. New York, 1967, Plenum Publishing Corp., p Armington JC and Bloom MB: Relations between the amplitudes of spontaneous saccades and visual responses. J Opt Soc Am 64:1263, Armington JC: Using the Lab-8 for experimentation with the human visual system. Behav Res Methods lustrum 4:61, Offner FF: The EEG as potential mapping: the value of the average monopolar reference. EEG Clin Neurophysiol 2:215, Perl ER and Casby JU: Localization of cerebral electrical activity: the acoustic cortex of cat. J Neurophysiol 17:429, Jasper HH: Report of the committee on methods of clinical examination in electroencephalography. EEG Clin Neurophysiol 10:370, Armington JC: Potentials accompany eye movement. In Frontiers in Visual Science, Cool SJ and Smith EL III, editors. New York, 1978, Springer- Verlag, pp Spekreijse H, Estevez O, and Reits D: Visual evoked potentials and the physiological analysis of visual processes in man. In Visual Evoked Potentials in Man: New Developments, Desmedt JE, editor. Oxford, 1977, Clarendon Press, pp Volkmann FC, Riggs LA, and Moore RK: A comparison of saccades and blinks in suppression of vision. INVEST OPHTHALMOL VIS SCI 18 (April Suppl.): 140, Amblyopic abnormality involves neural mechanisms concerned with movement processing. INGO RENTSCHLER, RUDOLF HILZ, AND HANS BRETTEL. In strabismic amblyopia the detection of apparent movement for counterphase gratings is considerably more impaired than the detection of pattern. No such anomaly is found for the detection of changes from a blank field when gradual or abrupt onsets or offsets of the temporal grating presentation are used. Similarly, normal peripheral vision is relatively poor in detecting alternation of spatial phase. It is concluded that the observed movement abnormality does not reflect a loss in sensitivity of transient mechanisms but rather a visualinsensitivity tospatialphase. This would explain why perceptual distortions and low optotype acuity occur in amblyopes with normal contrast sensitivity. The dramatic loss of visual function in amblyopia is conventionally characterized by reduced acuity and spatial contrast sensitivity. More specifically, it is assumed that high spatial frequencies in the contrast sensitivity function would be most affected by the visual anomaly. This acuity interpretation 1 of amblyopia meets a difficulty: perceptual distortions of suprathreshold grating stimuli 2 and strongly reduced acuity for single optotypes 3 have been reported from amblyopes with normal contrast sensitivity. In this study we provide evidence that spatiotemporal aspects of amblyopic vision may account for the discrepancy. Two distinct thresholds may be found for a temporally modulated grating pattern. At one contrast the spatial structure of the pattern is recognized and at another contrast flicker or movement just becomes visible. This psychophysical dichotomy possibly reflects the activity of two neural subsystems. Indeed, as Enroth-Cugell and Robson 4 have shown, there is neurophysiologic evidence for the existence of parallel X- (sustained) and Y- (transient) subsystems in the visual system. Compared with Y-cells, X-cells generally have smaller receptive fields and are more sensitive to stationary stimuli, whereas they respond less strongly to quickly moving targets. 5 The authors of recent studies 1 ' e ~ 8 agree that both flicker (or movement) and pattern detection are similarly affected in amblyopic vision. In the presence of sharp temporal transients, however, /81/ $00.60/ Assoc. for Res. in Vis. and Ophthal., Inc.

2 696 Reports Invest. Ophthalmol. Vis. Sci. May 1981 spatial frequency [cpd] Fig. 1. Contrast threshold functions for pattern and movement detection. Subjects R. B. and A. R. suffer from moderate amblyopia (squares, amblyopic eye; circles, normal eye). The stimulus was a sine-wave grating counterphased at 1.5 Hz. Two kinds of threshold are plotted: thresholds for recognizing the spatial structure of the pattern (filled symbols) and thresholds for seeing movement (open symbols). Amblyopic threshold elevation for pattern detection (solid lines) and movement detection (dashed lines) is replotted at the bottom. flicker and pattern thresholds of the amblyopic and the normal eye seem to be the same at low spatial frequencies. It is this observation (which is not confirmed by the study of Hilz et al., 6 using a low temporal rate of modulation) from which it is concluded that transient channels are essentially normal 1 ' 7 in amblyopia. Wood and Kulikowski 8 have drawn attention to the difficulties that even experienced normal observers (let alone amblyopes) have in setting movement and flicker or pattern thresholds at high rates of temporal modulation. Therefore we have re-examined the problem of transient sensitivity in amblyopia by contrasting the results of two methods of investigating the sustained-transient dichotomy. One is the above-mentioned, requiring two different psychophysical criteria. The other uses a single detection criterion but determines the grating contrast sensitivity when abrupt or gradual onsets and offsets of the temporal presentation are used. 9 ' 10 We show that in all strabismic amblyopes tested, the detection of apparent movement is considerably more impaired than the detection of pattern or sharp temporal transients. Method. Vertical sine-wave gratings with an average luminance of 16 cd/m 2 were displayed on an oscilloscope screen subtending 6.4 by 5.1 degrees. A large, moderately illuminated light background surrounded the display. The observation distance was 0.9 m. In one experimental situation the gratings were sinusoidally counterphased. Two kinds of contrast threshold 11 were determined: thresholds for seeing movement and thresholds for recognizing the pattern striation. The subjects were instructed not to respond to uniform flicker of the display and to detect movement at low spatial frequencies through horizontally moving shadows. A rate of 1.5 Hz was found most suitable to allow for a distinct separation of pattern and movement thresholds over the entire range of spatial frequencies tested. As motion detection is assumed to be mediated by a "transient mechanism," 12 ' 13 amblyopic sensitivity to temporal transients was investigated in another experiment. Two stimuli 10 were used: the first (S-type) had the Gaussian temporal waveform exp [-(t - 0.5) 2 / ]; the second (T-type), consisting of 1 cycle of an 8.0 Hz square-wave, had an abrupt onset and offset. The duration of the S-type stimulus was restricted to 1 sec. Both stimuli had identical spatial components (i.e., gratings). They were used in a simple contrastdetection experiment. The observer was not required to use different criteria for each stimulus type but simply to establish the contrast level at which he could just detect the occurrence of a change in the uniformly illuminated display screen. Four strabismic amblyopes served as observers. All of them were examined ophthalmologically and wore their corrective lenses throughout the experiments (Table I). Each experiment consisted of two threshold settings at various test spatial frequencies that were used individually in ascending and descending order. The good and the amblyopic eyes were tested alternately, first for pattern (sustained) and then for movement (transient) sensitivity.

3 Volume 20 Number 5 Reports 697 pat- mov C.K. S-T C.K. D TD O 10 "5 0) I spatial frequency [cpd] spatial frequency [cpd) ^ I B) ^ spatial frequency [cpd] spatial frequency [cpd] Fig. 2. Contrast threshold functions for pattern and movement detection as compared with the detection of temporal transients. Subjects C. K. (A) and V. T. (B) suffer from high amblyopia (squares, amblyopic eye; circles, normal eye). The graphs show pattern and movement thresholds for 1.5 Hz counteiphase gratings at the left. Simple detection thresholds for gratings turned on and off gradually (S-type, closed symbols) or abruptly (T-type, open symbols) are plotted at the right. Mean S.E.: C. K., 7% (N = 4, all functions) and V. T., 13% (N = 6, all functions). Data of C. K. were obtained in June 1980; data of V. T. in November Results. Fig. 1 shows the results of measuring pattern and movement thresholds with a counteiphase stimulus in two subjects (R. B. and A. R.) suffering from moderate amblyopia. The amblyopic differed from the normal threshold functions in that they showed elevated thresholds for both pattern and movement detection at all but the veiy lowest spatial frequencies. Most significant, however, is the observation that movement sensitivity was more impaired than pattern detection in both observers. This is revealed most clearly by the inset curves replotting the log threshold elevation (i.e., the logarithm of the ratio of the modulation threshold of the amblyopic to the nonamblyopic eye) as a function of spatial frequency. Fig. 2 plots the results from two experiments for comparison. The graphs show pattern and movement thresholds with counterphase stimuli at the left. Simple detection thresholds for gratings turned on and off gradually (S-type) or abruptly (T-type) are presented at the right. Two high

4 698 Reports Invest. Ophthabnol. Vis. Sci. April spatial frequency [cpd] spatial frequency [cpd] Fig. 3. Contrast threshold functions in central and peripheral vision of a normal observer. R. H. monocularly observed the 3-degree circular testfieldcentrally (circles) orfixateda target separated by 8 degrees from the temporal edge of the test field (squares). Pattern (closed symbols) and movement thresholds (open symbols) for 1.5 Hz counterphased gratings are at the left; simple detection thresholds for gratings turned on and off gradually (S-type, closed symbols) or abruptly (T-type, open symbols) are at the right. Inset curves indicate peripheral threshold elevation for pattern or S-type thresholds (solid lines) and movement or T-type thresholds (dashed lines). amblyopes (C. K. and V. T.) served as observers. For both observers the reduction of T-sensitivity in the amblyopic eye (squares) was not greater than that of S-sensitivity. At low spatial frequencies one subject (V. T.) showed virtually no reduction at all, whereas T-sensitivity was significantly depressed in the other (C. K.). By contrast, the loss in movement detection was, for both observers and at all spatial frequencies tested, considerably stronger than that for pattern detection. Moreover, it should be noted that almost normal pattern thresholds were obtained from the amblyopic eye of V. T., whereas pattern and S-type thresholds were elevated in the amblyopic eye of C. K. Given the fact that all amblyopes tested were eccentric fixators, the observed differences in amblyopic movement and T-type sensitivities might have been predicted if the mechanisms underlying movement sensitivity are relatively concentrated in the center of the retina as compared with the distribution of T-type mechanisms. Fig. 3 shows that this is actually the case. Off-axis observation by a normal observer similarly reduced pattern, S-type, and T-type sensitivities, whereas the loss in movement sensitivity was significantly stronger. Discussion. Detection of apparent movement in counterphase gratings is impaired considerably more than pattern detection in all the amblyopic eyes tested. This is the most significant result of our study. The observed movement anomaly seems to be at odds with the findings of Hess et al., 13 who claimed that movement perception is normal in strabismic amblyopes. These authors used drifting gratings, but the difference in experimental conditions would not account for the apparent discrepancy, since counterphased gratings of unit contrast can be synthesized from the combination of two half-contrast gratings drifting in opposite directions, which are detected independently. l4 Hess et al., 13 however, restricted their study to the extremely low spatial frequency of 0.2 cy/deg. Whether amblyopes show any anomaly under this condition may just be a matter of subject variations. Indeed, our observer A. R. showed identical movement thresholds of the bad and the good eye at 0.2 cy/deg, although his amblyopic movement sensitivity was significantly reduced already at 0.5 cy/deg (Fig. 1). At present we see no explanation for such individual differences. Our results obtained with S- and T-type stimulation (Fig. 2) are consistent with the observations

5 Volume 20 Number 5 Reports 699 Table I. Conditions of amblyopes participating in the experiments Subject Correction R +1.5 sph L +2.0 sph R +1.0 sph L +1.0 sph R +1.0 sph L +2.5 sph Piano Snellen acuity R 1.0 L0.5 R 1.2 L0.5 R 1.2 L0.1 R0.2 L 1.2 Fixation pattern of amblyopic eye Angle of squint, corneal reflection (degrees) Surgical treatment R. B. A. R. C. K. V. T. 1 cleg nasal Steady Parafoveolar Wandering Paramacular Steady Peripheral eccentric Wandering No Yes Yes Yes sph = spherical lens. of Levi and Harwerth 7 and of Thomas 1 in that transient and sustained as well as flicker and pattern sensitivities are similarly affected in the amblyopic eye. This correspondence is not surprising, since Levi and Harwerth used on-off modulation at a rate of 10 Hz and Thomas used square-wave modulation at 5 Hz. Thus sharp temporal transients did occur in both experimental situations, and the observers detected them according to a flicker criterion that did not discriminate apparent movement against the percept of a homogeneously flickering field. From the data displayed in Fig. 3 it would seem that unlike the sensitivity to temporal transients, the detection of apparent movement at slowly modulated counterphase gratings is a typical function of central vision. If so, we might predict a significant loss of this function in eccentrically fixating amblyopes. It should be noted, however, that the observed loss in movement sensitivity (Fig. 2) does not strongly correlate with the amount of off-axis fixation (Table I). We therefore conclude that as far as movement sensitivity is concerned, amblyopic vision is functionally comparable to peripheral vision but that the degree of eccentric fixation does not fully account for the loss in visual function. The functional significance of the observed amblyopic movement anomaly remains to be explained. The fact that it is more prominent at higher spatial frequencies and at a slow rate of temporal modulation suggests that it will affect the visual analysis of form rather than that of motion perception. On the other hand, the anomaly was found in a subject (V. T.) with almost normal pattern sensitivity for gratings. The apparent contradiction disappears if the fact is considered that a complex pattern (e.g., a Snellen target) is incompletely represented by its power spectrum, with the phase spectrum accounting for the difference. This suggests that the detection of movement at phase-alternating gratings reflects visual sensitivity to spatial phase. A deficit of the amblyopic eye in processing spatial phase would account for the perceptual distortions and low optotype acuity in amblyopes with normal contrast sensitivity. 10 Although the sustained-transient dichotomy is questionable, 16 neurophysiologic research has clearly established the existence of at least two sets of mutually exclusive cells in the retinogeniculocortical pathway. One (X) maintains spatial phase (or position) and a second (Y) does not. 4 Apparent motion perception requires position detection, and position sensitivity is substandard in at least some amblyopes. 17 The main finding of the present study (that apparent movement sensitivity in amblyopia is reduced) is thus consistent with the hypothesis that it is the X- and not the Y-system that is impaired in amblyopia. We are grateful to Terry Caelli, Fergus Campbell, John Daugman, and two anonymous reviewers for their helpful comments on an earlier draft. From the University of Munich, Institut fur Medizinische Psychologie and Institut fur Medizinische Optik, Munich, West Germany. Supported by SFB 50, A 2.5, Deutsche Forschungsgemeinschaft. I. R. holds a DFG Heisenberg Research Fellowship. Submitted for publication Jan. 11, Present address (H. B.): Gesellschaft fuer Strahlen und Umweltforschung, Munich. Reprint requests: Dr. Ingo Rentschler, Institut fur Medizinische Psychologie, Schillerstrasse 42/111, FRG 8000 Miinchen. Key words: strabismic amblyopia, movement detection, transient mechanisms, spatial phase, visual acuity REFERENCES 1. Thomas J: Normal and amblyopic contrast sensitivity functions in central and peripheral retinas. INVEST OPHTHALMOL VIS SCI 17:746, Hess RF, Campbell FW, and Greenhalgh T: On the nature of the neural abnormality in human amblyopia; neural aberrations and neural sensitivity loss. Pfluegers Arch 377:201, 1978.

6 700 Reports Invest. Ophthalmol. Vis. Sci. May Rentschler I, Hilz R, and Brettel H: Spatial tuning properties in human amblyopia cannot explain the loss of optotype acuity. Behav Brain Res 1:433, Enroth-Cugell C and Robson JG: The contrast sensitivity of retinal ganglion cells of the cat. J Physiol 187:517, Lehmkuhle S, Kratz KE, Mangel SC, and Sherman SM: Spatial and temporal sensitivity of X- and Y-cells in dorsal lateral geniculate of the cat. J Neurophysiol 43:520, Hilz R, Rentschler I, and Brettel H: Myopic and strabismic amblyopia: substantial differences in human visual development. Exp Brain Res 30:445, Levi DM and Harwerth RS: Spatio-temporal interactions in anisometropic and strabismic amblyopia. INVEST OPHTHALMOL VIS SCI 16:90, Wood ITJ and Kulikowski JJ: Pattern and movement detection in patients with reduced visual acuity. Vision Res 18:331, Breitmeyer B and Julesz B: The role of on and off transients in determining the psychophysical spatial frequency response. Vision Res 15:411, Wilson HR: Quantitative characterization of two types of linespread function near the fovea. Vision Res 18:971, Kulikowski JJ: Apparent fineness of briefly presented gratings: balance between movement and pattern channels. Vision Res 15:673, Tolhurst DJ: Separate channels for the analysis of the shape and the movement of a moving visual stimulus. J Physiol 231:385, Hess RF, Howell ER, and Kitchin JE: On the relationship between pattern and movement perception in strabismic amblyopia. Vision Res 18:375, Levinson E and Sekuler R: The independence of channels in human vision selective for direction of movement. J Physiol 250:347, Brettel H, Caelli T, Hilz R, and Rentschler I: Amplitude and phase transmission in the human visual system: the two-dimensional case. (In preparation.) 16. Lennie P: Parallel visual pathways: a review. Vision Res 20:561, Freeman RD and Bradley A: Monocularly deprived humans: nondeprived eye has supernormal vernier acuity. J Neurophysiol 43:1645, 1980.