The effect of blur and contrast of VEP latency: comparison between check and sinusoidal grating patterns

Electroencephalography and clinical Neurophysiology, 1987, 68:247-255 Elsevier Scientific Publishers Ireland, Ltd.

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EEG 03167

The effect of blur and contrast on V E P latency: comparison between check and sinusoidal grating patterns P. Bobak a,1, I. Bodis-Wollner a,b and S. Guillory a " Department of Ophthalmology and b Department of Neurology, Mount Sinai School of Medicine, City University of New York, New York, N Y (U.S.A.) (Accepted for publication: 7 November, 1986)

Summary Pattern defocusing was used to evaluate the contribution of different spatial frequency components in checks to VEP latency. Latency shifts with increasing blur ( - 2 . 5 to + 2.5 diopters) were determined for sinusoidal grating and check patterns equated in fundamental spatial frequency. With both check and grating patterns, the effect of blur was greater the higher the spatial frequency. Given an equal fundamental spatial frequency, however, the latency of checks was more effected. This latency difference between check and sine patterns was pronounced at low fundamental spatial frequencies (large pattern) and decreased with higher spatial frequencies (small pattern). Latencies were then compared for patterns which were defocused vs. simply reduced in contrast. Results show that the increase in latency with defocused large checks is due to both fundamental and higher harmonic spatial components but with small checks, to the fundamental spatial frequency alone. Key words: Visual evoked potential; Latency; Refractive error; Contrast; Check and sinusoidal grating patterns

Visual evoked potential (VEP) measurements are routinely used in charting the development of normal infant visual acuity (Sokol and Dobson 1976; Harter et al. 1977; Sokol and Jones 1979) and in the evaluation of neuroophthalmological disorders. The widely accepted methodology is based on counterphase modulation of a repetitive pattern and measurement of the VEP 'latency' (American EEG Soc. Recommended Standards 1979-1982). In clinical testing, the most common visual stimulus is an alternating checkerboard pattern while sinusoidal gratings, favored by visual physiologists, are less frequently used. A check pattern is complicated. The constituent gratings include: (a) the fundamental spatial frequency which is angled obliquely, and (b) higher harmonics which approximate vertical and horizontal angles (i.e., check edges) more with each increasing i Present address: Department of Ophthalmology, University of Illinois at Chicago, Chicago, IL 60612, U.S.A.

Correspondence to: Ivan Bodis-Wollner, M.D., 1200 Fifth Avenue, The Neurology Suite, New York, NY 10029 (U.S.A.).

odd harmonic (Kelly 1976; De Valois et al. 1979; May et al. 1979; Camisa et al. 1981). With such a complex stimulus, it is not immediately obvious which of the different spatial frequency components determines the VEP response. To investigate the contribution of different check components to the VEP, we used defocusing with spherical lenses as the primary methodology. Several previous studies have already shown that blur alters the VEP response. VEP amplitude, for example, is especially affected when small checks (Harter and White 1968; Adachi-Usami 1979), fine square-wave gratings (Rentschler and Spinelli 1978), and fine sinusoidal gratings (Freeman and Thibos 1975) are blurred. Defocusing also increases VEP latency (Collins et al. 1979), especially with smaller check sizes (Sokol and Moskowitz 1981). Given the spatial frequency dependence of blur on the VEP, we studied 3 check sizes. Any difference in the relative contribution of fundamental and harmonic components to the VEP with pattern size could thus be evaluated. VEP latencies to

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check and simple sinusoidal grating patterns of equal fundamental spatial frequencies were harmonically related. In addition, the effect of reducing pattern contrast on VEP latency to the same spatial frequencies was also determined. This comparison allowed us to evaluate how VEP latency with blur was determined by the magnitude of contrast reduction for different spatial frequencies. Methods

Stimuli (A) Sinusoidal grating patterns.

A special purpose oscilloscope with white P4 phosphor was used to present the gratings which were counterphase modulated at 1 Hz (1 Hz = 2 reversals/sec). The spatial frequency of the gratings included 2.3, 4.6 and 6.9 c/deg. The circular stimulus field subtended 3.7 o and was surrounded with an evenly illuminated translucent screen. Mean screen luminance was 171 c d / m 2 and the surround, 0.2 log units dimmer. (B) Check patterns. A television unit with a pattern generator was used to present the check patterns which were counterphase modulated at 1 Hz. The fundamental spatial frequency of the check pattern was defined as the number of check diagonals subtended in 1° of visual angle. The spatial frequencies were 2.3, 4.6 and 6.9 c/deg, the same as for the gratings. In each session, the responses to check and sinusoidal grating patterns of the same spatial frequency were recorded. The circular stimulus field subtended 4 ° and was surrounded with an evenly illuminated translucent screen as described above. Mean screen luminance was 171 c d / m 2.

Measurements Monocular VEPs were recorded with the active electrode positioned at Z5, the reference electrode at Z63, and the forehead was grounded (' Z' refers to the midline and the number, to the percentage inion-to-nasion distance). Signal amplification was with a preamplifier with bandpass half amplitude settings at 0.3 Hz (low) and 100 Hz (high). The VEP wave form was labeled for successive positive and negative deflections according to the nomen-

P. BOBAK ET AL.

clature of Cigfinek (1961). As in the majority of transient VEP studies, latency was defined by the timing of P1, the major positive wave, relative to the stimulus trigger. The recording epoch was 500 msec with averaging ranging from 128 to 864 modulations cycles depending on the expected amplitude of the signal.

Subjects Twelve subjects participated in these studies with an age range of 18-45 years. Five of the observers were tested in all 9 conditions of the defocused experiments for both check and sinusoidal grating patterns as detailed below. Five observers were tested in all conditions in which the contrast of the pattern was varied. All participants had an acuity of 20/20 with no visual complaints. In addition, several observers had a complete ophthalmologic exam with normal results.

Procedures The observer was seated 144 cm from the display when the grating patterns were viewed. The viewing distance for the 2.3 and 4.6 c/deg checks was 167 cm and for the 6.9 c/deg checks, 240 cm. The longer viewing distance for the 6.9 c/deg checks was to prevent distortion of the pattern due to the resolution limit of the T.V. system. Since the shortest distance was 144 cm and accommodation is at that point minimal (0.7 diopters), the longer viewing distances for the check patterns did not have an appreciable confounding effect (i.e., image distortion due to an inability to accommodate). Regardless of viewing distance, the stimulus field size was relatively constant (3.70-4 °). The right eye of each observer was tested while a translucent patch was placed over the left eye. A translucent patch eliminates undesirable effects which can occur with opaque patching some of which include changes in accommodation and eye movements in the open eye (Lehmkuhle and Fox 1976). (A) Grating contrast. The effect of changing the contrast on VEP latency was determined for 2.3, 4.6 and 6.9 c/deg sinusoidal gratings. The contrast of the pattern was varied in 6 dB (0.3 log unit) steps and ranged from 58% to 1.8%. Contrast

E F F E C T OF' B L U R A N D C O N T R A S T ON VEP L A T E N C Y

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levels were randomly presented. (B) Defocused patterns. For sinusoidal gratings, the following contrasts were used: 2.3 c/de& 58%; 4.6 c/deg, 62%; and 6.9 c/deg, 58%. The contrast of the check patterns was 60%. These particular contrasts were used as the VEP is not yet saturated for the pattern sizes used in this study. Contrast, defined as the difference between maximum and minimum luminance over their sum, was determined by the contrast vs. voltage calibrations described by Bodis-Wollner and Hendley (1979). First baseline latencies (emmetropic) were established for each condition. The observer then wore image-distorting spherical lenses in a trial frame. Preliminary experiments showed that with a + 5 diopter lens, the VEP was indistinguishable from noise at 4.6 c/deg and 6.9 c/deg. The main experiments were therefore done with + 1, + 1.5, +2, +2.5, - 1 , -1.5, - 2 , and - 2 . 5 diopter lenses. (C) Cycloplegia. The effect of defocusing the sinusoidal grating patterns on VEP latency was compared with and without cycloplegia in two observers. A 3 mm artificial pupil was used in both conditions. In addition to the observers' own refractive error correction, a + 0.7D lens was used with cycloplegia for the viewing distance of 1.44 m. Two sessions were completed under cycloplegia with the 2.3 c/deg and 4.6 c/deg sinusoidal grating patterns, and one session with the 6.9 c/deg pattern for each of the previously detailed defocused conditions.

correlation coefficient was based on the specified degrees of freedom defined as the number of latency observations per diopter strength x the number of diopter strengths - 2. For the 2.3 c/deg and 4.6 c/deg patterns, 23 df were used for both positive and negative lens conditions. For the 6.9 c/deg patterns, 18 df were used for both positive and negative lens conditions. Given previous work indicating that blur increases latency (Sokol and Moskowitz 1981), a positive correlation between blur and latency would be expected. The statistical tests were therefore one-tailed. (B) To evaluate if the regression line slopes with positive lenses were significantly different for any two spatial frequency combinations for both the check and sinusoidal grating patterns, the difference in latency for each diopter strength was computed pairwise for each spatial frequency. A regression analysis was then computed between diopter strength (the predictor variable) and the latency differences (the criterion variable). A significant correlation coefficient between diopter strength and the pairwise latency differences is an indication that there is a departure from parallelism between the two slopes: the difference in latency between the two spatial frequencies is greater for lenses of higher than lower diopter strength. The critical correlation coefficient for comparisons with the 6.9 c/deg pattern was based on 18 df (5 latency difference observations x 4 diopter strengths- 2), while the comparison between the 2.3 c/deg and 4.6 c/deg patterns was based on 23 df. Previous work indicates that the effect of blur is greater for high (small checks) than for low (large checks) spatial frequencies (Harter and White 1968; Sokol and Moskowitz 1981). A one-tailed correlation test was therefore used since the slope of the high minus low spatial frequency differences should be positive. (C) To evaluate if the regression line slopes were significantly different for check and sine patterns of the same fundamental frequency, a linear regression analysis between diopter strength of positive lenses (the predictor variable) and the latency difference between sine and check patterns of the same fundamental spatial frequency (the criterion variable) were computed as in (B). The critical correlation coefficient with the 6.9 c/deg

Statistical measurements (A) The relationship between VEP latency (the criterion variable) and diopter strength (the predictor variable) was determined with a leastsquares regression line for each spatial frequency and for both check and sinusoidal grating patterns. Regression lines were computed separately for positive and negative lens conditions. The correlation coefficient between diopter strength and latency was used as an indication of the strength of the regression function in predicting latencies accurately: a significant correlation coefficient indictes that the prediction error is minimal or the latencies predicted from the regression line will closely approximate actual latencies. The critical

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P. B O B A K E T A L .

patterns was based on 18 df and with the 2.3 c/deg and 4.6 c/deg patterns, on 23 df as described above. Since there was no clear prediction for the check vs. grating difference in latency with blur, the statistical tests were two-tailed.

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Results

The latency of the P1 component of the transient VEP is affected both by defocusing the pattern and by reducing its contrast. VEP latency increases with blur using either sinusoidal gratings or checks as stimuli. This increase in latency is illustrated in Fig. 1 where the VEP wave forms of one observer are compared with defocused and emmetropic viewing of the 2.3 c/deg check pattern. In Fig. 2, the mean latency of all observers is represented as a function of diopter strength for both check and sinusoidal grating patterns. These results are summarized in Table I by the regression line values. It is obvious that a steeper regression line slope indicates that VEP latency becomes longer with a smaller increase in diopter strength.

VEP latency with defocused checks It can be seen in Fig. 2 that VEP latency is more affected by positive than by negative lenses. The slopes of the regression lines with negative lenses (Table IA) are relatively 'flat' regardless of spatial frequency. Steeper regression line slopes

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Fig. 1. V E P w a v e f o r m s o f o n e o b s e r v e r a r e c o m p a r e d w i t h e m m e t r o p i c a n d w i t h b l u r r e d viewing. W i t h b l u r ( + 1 . 5 d i o p t e r s ) , the l a t e n c y o f the P1 c o m p o n e n t is i n c r e a s e d 14 m s e c relative to the n o b l u r c o n d i t i o n (0 diopters). Positive deflections are 'down'.

are found with positive lenses for all spatial frequencies (Table IC). Moreover, the correlation coefficient is statistically significant with positive lenses for each spatial frequency; the regression line is therefore a good representation of how latency increases with diopter strength. These resuits indicate that accommodation to maintain a

TABLE I R e g r e s s i o n line e q u a t i o n s a n d c o r r e l a t i o n c o e f f i c i e n t s b e t w e e n d i o p t e r s t r e n g t h a n d l a t e n c y . (A) C h e c k p a t t e r n

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As with checks (a) the regression line slopes are steeper with positive (Table ID) than with negative (Table IB) lenses for 4.6 c/deg and 6.9 c/deg grating patterns and (b) there is a progressive increase in the steepness of the regression line slope with an increase in spatial frequency (Table ID). It can be seen in Table IIB that there is a significant departure from parallelism between the slopes of the 2.3 c/deg and 6.9 c/deg gratings and between the 4.6 c/deg and 2.3 c/deg gratings.

VEP latency during cycloplegia In two observers, the EP latency-blur functions were compared with and without cycloplegia. This

focused image is stronger with negative than with positive lenses. A second noteworthy finding is that there is a disproportionately greater change in VEP latency with blur for high spatial frequencies; the slopes of the regression lines in Table IC become steeper with increasing spatial frequency. There is a statistically significant divergence of slopes between 2.3

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helped us to determine how much of the spatial frequency differences with blur were due to accommodation. The data of one observer are shown in Fig. 3. Without cycloplegia (a) latency is less affected with negative than with positive lenses for all spatial frequencies and (b) there is a greater increase in latency as a function of blur for higher than for lower spatial frequencies. These results are consistent with those of the other 5 observers shown in Fig. 2. With cycloplegia, a more dramatic increase in latency with blur is seen for 2.3 c / d e g and 4.6 c / d e g gratings. In this particular observer, the ability to accommodate resulted in a greater spatial frequency difference in latency with blur. In the other observer, however, the cyclopleged an uncyclopleged curves were similar; the ability to accommodate did not add to the spatial frequency difference in curves. This difference between subjects in the ability to accommodate to blur is consistent with the findings of Millodot and Newton (1981) although their data were obtained with negative lenses.

Comparison between grating and check patterns Using either check or grating patterns, the effect of blur is greater the higher the spatial frequency as discussed above. Given an equal fundamental spatial frequency, however, checks are more affected by blur than the corresponding gratings (Table IC and D). This difference is the most pronounced at the lowest spatial frequency (2.3 c/deg). With the highest fundamental spatial frequency, however, the latency vs. blur functions for check and grating patterns are equal. These results are summarized in Table IIC; there is a decrease in the latency difference slope between check and grating patterns with an increase in fundamental spatial frequency. A significant departure from parallelism is found only between the slopes of the 2.3 c / d e g patterns. This differential effect of blur on checks vs. gratings as a function of fundamental spatial frequency is one of the main findings of this study.

P. BOBAK ET AL.

function of grating contrast (note log scale) is plotted for all spatial frequencies. The slopes of the regression fines relating contrast and VEP latency are nearly identical for the 3 spatial frequencies ranging from - 0 . 1 4 to -0.16. It is apparent, however, that the similarity between the slopes of the 3 contrast functions occurs only above a contrast level of 7.25%. Below this point, decreasing pattern contrast results in a steep increase in latency with the 6.9 c / d e g gratings, no change in latency with the 4.6 c / d e g gratings, and a steep decrease followed by a sharp increase in latency with the 2.3 c / d e g grating.

Compar&on between latency changes with contrast reduction and defocusing In comparing VEP latencies obtained with contrast reduction (Fig. 4) and defocusing (Fig. 2), it 140

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is evident that for a given latency equating blur and contrast attenuation is not constant across spatial frequencies. Equal blur corresponds to a greater contrast reduction at high than at low spatial frequencies. For example, using 1 diopter of blur, VEP latency to the 2.3 c/deg and 4.6 c/deg gratings is equivalent to that of a pattern of 29% contrast but only to a contrast of 7% at 6.9 c/deg. Similarly, using 1.5 diopters of blur, the same latency is obtained by a contrast of 58% for 2.3 c/deg gratings, a contrast of 14.5% for 4.6 c/deg gratings, and a dramatically reduced 3.6% contrast for 6.9 c/deg gratings.

is due only to a difference in the amplitude of the fundamental spatial frequency. In Fig. 4, for example, a 50% decrease in grating contrast increases VEP latency by only 3 msec. The difference in latency between checks and gratings especially with lenses of higher diopter strength (Fig. 2) is too large to be attributed only to a 20% difference in fundamental frequency amplitude between the two pattern types. This differential effect of blur depending on pattern type therefore suggests that at low spatial frequencies, the higher harmonics of the check pattern contribute to the VEP response. VEP latency with blur, however, is not determined only by a higher harmonic: the blur functions are different for 2.3 c/deg checks and for the 6.9 c/deg grating, the third harmonic of 2.3 c/deg checks. The amplitude of the 6.9 c/deg component as the third harmonic is one-third that of the fundamental spatial frequency. In Fig. 4, the latency difference between a 6.9 c/deg grating pattern of 59% and 20% contrast (corresponding to the amplitude one-third the contrast) is about 5 msec. In Fig. 2, however, a 5 msec change in latency occurs with only about 0.5 diopters of blur for the 6.9 c/deg grating pattern but for the 2.3 c/deg check pattern, over 1.5 diopters of blur is needed to achieve the same latency change. VEP latency to the 2.3 c/deg blurred check pattern therefore resuits from the simultaneous presence or interaction between the fundamental and higher harmonic components. This finding that the higher harmonics contribute to the VEP response differs from the results of Sokol and Moskowitz (1981) which suggest that the increase in latency with check defocusing is due mainly to the fundamental spatial frequency: VEP latency increased only when the angles of the fundamental spatial frequency were blurred with a cylinder lens. This was true for check sizes larger (48') and comparable (12') to those investigated in our study. Our results, however, favoring a higher harmonic contribution are consistent with the findings of Spekreijse et al. (1973) May et al. (1979), and May et al. (1984). May et al. (1979), for example, found in one observer that relative to the emmetropic baseline, VEP latency increased when the fundamental check components were

Discussion

Several threshold and suprathreshold studies have tried to evaluate which spatial component in the complex check pattern dominates the visual response. We used VEPs to study this issue. The VEP is a neurophysiological measure with response characteristics which are frequently in agreement with single-cell findings. An example which is directly relevant to this study is the increase in responsiveness which is found in cat cortical cells with increasing levels of contrast (Dean 1981). Consistent with this finding that contrast is a stimulus trigger for higher order neurons, we found an increase in VEP latency with simple contrast reduction and with pattern blur. Furthermore, the magnitude of this latencyblur effect is highly dependent on the size of the pattern elements, as we shall discuss.

Latency and refractive error Low ]undamental spatial frequencies. Without blur, VEP latency was nearly equal for check and corresponding grating patterns. With blur, however, there was a marked increase in latency for 2.3 c/deg and 4.6 c/deg checks compared with sinusoidal gratings of the same fundamental spatial frequency (Fig. 2). This difference between pattern types became insignificant for the highest explored spatial frequency, 6.9 c/deg. The amplitude of the fundamental check component is 81% that of a simple grating of equal contrast (De Valois et al. 1979). It is unlikely, however, that the latency difference between check and grating VEPs

254 blurred with a cylinder lens and decreased when the higher harmonics were blurred. They suggested that the various check components may be processed by pathways with different latencies which are asymmetrically changed with cylinder blurring. Also supporting a higher harmonic contribution with complex stimuli was the finding by May et al. (1984) that VEPs could be elicited by the separate harmonic components of a 2.5 c/deg square-wave grating (comparable to our 2.3 c/deg patterns). The component stimuli included a 2.5 c/deg sinusoidal grating and a corresponding harmonic (' missing fundamental') wave form. The VEP response to the square-wave grating in fact had a double P1 latency peak reflecting the difference in response latency to the fundamental and higher harmonic components. In the no-blur conditions of this study, however, the VEP latency to the 2.3 c/deg checks was identical to that of the 2.3 c/deg sinusoidal grating. Moreover, the VEP response to the checks was simple in morphology (i.e., no P1 double peak). This seems to suggest that there is no effect of higher harmonics in the check pattern. We found that the higher harmonics of the check pattern began to influence VEP latency only when the pattern was blurred. Such a result could be explained for instance, if the amplitude and phase of the higher harmonics were asymmetrically changed with respect to the fundamental as suggested by May et al. (1979). High fundamental spatial frequencies. With the smallest check size, 6.9 c/deg, the higher harmonics do not contribute to the VEP response. Sharpedged checks, however, can be seen with emmetropic viewing and with blurring lenses of low diopter strength. The higher harmonics are therefore visually resolvable. This lack of harmonic contribution to the VEP response is consistent with the results of May et al. (1984): for the highest explored fundamental spatial frequencies in square-wave gratings (3.5 c/deg and 4.5 c/deg), the corresponding harmonic ('missing fundamental') wave forms did not even elicit a VEP response. This suggests that there may in fact be no contribution of higher harmonics to the VEP response elicited with 6.9 c/deg checks even in the no-blur condition when they are visually resolvable.

P. BOBAKET AL. Latency as a function of grating contrast For a given level of grating contrast, the latency of the VEP increases with spatial frequency (Fig. 4). Similar results were reported by Parker and Salzen (1977). We did not, therefore, find a 'tuning' of spatial frequency as was demonstrated by Jones and Keck (1978): VEP latency was the shortest around 4 c/deg, the peak of the contrast sensitivity curve. Our range of spatial frequencies was not large enough, however, so that an unequivable decrease in latency at mid-spatial frequencies could have been demonstrated. Within the contrast range of 7.25-58%, the slope of the regression line between latency and the log of grating contrast is nearly the same for all 3 spatial frequencies. This similarity of slopes among spatial frequencies is seen in the data of Kulikowski (1977) for 'on-off' presentation although our slopes for comparable spatial frequencies are less steep. Because of this large difference in latency between spatial frequencies for a given level of contrast combined with the relatively 'flat' slopes relating contrast to latency, a criterion latency across spatial frequency cannot be established by adjusting grating contrast. Similar findings were reported by Jones and Keck (1978) where latency was the shortest with a 4 c/deg grating pattern regardless of the grating contrast of other spatial frequencies. Below 7.25% contrast, the functions relating latency to contrast are markedly different for each spatial frequency. More detailed sampling of this contrast range is needed to clarify these frequency-dependent differences. Contrast attenuation as a function of blur We compared the latency of the VEP to sinusoidal grating patterns which were defocused versus reduced in contrast. We found that for a given diopter strength, latency was matched with gratings of a lower contrast as the spatial frequency of the grating pattern increased. Latency comparisons were restricted to contrast levels between 7 and 58%; The slope of the regression line relating latency to log contrast was constant between spatial frequencies within, this contrast range. These results agree with Campbell and Green (1965) who reported that contrast sensitivity drops in an accelerated manner with blur as a function of

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spatial frequency. We did, in fact, compare the contrast sensitivity of one observer under emmetropic and defocused conditions ( + 2 diopters) with results confirming Campbell and Green. These results suggest that when a check pattern is blurred with a spherical lens, contrast attenuation is greater for the higher harmonic components. The greater contrast attenuation of higher harmonics with blur, however, is not a sufficient explanation for the increase in latency with check defocusing as discussed earlier. In summary, we compared the latency of the VEP to defocused checks and sinusoidal gratings of the same fundamental spatial frequency. We found that the increase in latency with defocusing large checks must be due to an interaction between the fundamental and higher harmonic components and with smaller checks, to the fundamental frequency alone.

refractive error on the visual evoked response. Brit. med. J., 1979, 1: 231-232. Dean, A.F. The relationship between response amplitude and contrast for cat striate cortical neurons. J. Physiol. (Lond.), 1981, 318: 413-427. De Valois, K.K., De Valois, R.L. and Yund, E.W. Responses of striate cortex cells to grating and checkerboard patterns. J. Physiol. (Lond.), 1979, 291: 483-505. Freeman, R.D. and Thibos, L.N. Visual evoked responses in humans with abnormal visual experience. J. Physiol. (Lond.), 1975, 247: 711-724. Harter, M.R. and White, C.T. Effects of contour sharpness and check-size on visually evoked cortical potentials. Vision Res., 1968, 8: 701-711. Harter, M., Deaton, F. and Odom, J. Maturation of evoked potentials and visual preference in 6-45-day-old infants: effects of check size, visual acuity and refractive error. Electroenceph. clin. Neurophysiol., 1977, 42: 595-607. Jones, R. and Keck, M.J. Visual evoked response as a function of grating spatial frequency. Invest. Ophthal. Vis. Sci., 1978, 17: 652-659. Kelly, D.H. Pattern detection and the two-dimensional fourier transform: flickering checkerboards and chromatic mechanisms. Vision Res., 1976, 16: 277-287. Kulikowski, J. Visual evoked potentials as a measure of visibility. In: J.E. Desmedt (Ed.), Visual Evoked Potentials in Man: New Developments. Clarendon Press, Oxford, 1977: 168-183. Lehmkuhle, S.W. and Fox, R. On measuring interocular transfer. Vision Res., 1976, 16: 428-430. May, J.G., Cullen, J.K., Moskowitz-Cook, A. and Siegfried, J.B. Effects of meridional variation on steady-state visual evoked potentials. Vision Res., 1979, 19: 1395-1401. May, J.G., Kratz, K.E. and Banta, A.R. Visual evoked potentials elicited by selective spatial fourier components. In: R.H. Nodar and C. Barber (Eds.), Evoked potentials. II. The Second International Evoked Potentials Symposium. Butterworth, Boston, MA, 1984: 102-113. Millodot, M. and Newton, I. VEP measurement of the amplitude of accommodation. Brit. J. Ophthal., 1981, 65: 294-298. Parker, D.M. and Salzen, E.A. Latency changes in the human visual evoked response to sinusoidal gratings. Vision Res., 1977, 17: 1201-1204. Rentschler, I. and Spinelli, D. Accuracy of evoked potential refractometry using bar gratings. Acta ophthal. (Kbh.), 1978, 56: 67-74. Sokol, S. and Dobson, V. Pattern reversal visually evoked potentials in infants. Invest. Ophthal. Vis. Sci., 1976, 15: 58-62. Sokol, S. and Jones, K. Implicit time of pattern evoked potentials in infants: an index of maturation of spatial vision. Vision Res., 1979, 19: 747-755. Sokol, S. and Moskowitz, A. Effect of retinal blur on the peak latency of the pattern evoked potential. Vision Res., 1981, 21: 1279-1286. Spekreijse, H., Van der Tweel, L.H. and Zuidema, T. Contrast evoked responses in man. Vision Res., 1973, 13: 1577-1601.

References Adachi-Usami, E. Comparison of contrast thresholds of large bars and checks measured by VECPs and psychophysically as a function of defocusing. Albrecht v. Graefes Arch. Ophthal., 1979, 2 1 2 : 1 - 9 American Electroencephalographic Society Report of the Committee on Evoked Potentials. G. Chatrian (Chairperson), W. Goff (Vice-Chairperson). Recommended Standards for Clinical Evoked Potential Studies. 1979-1982. Bodis-Wollner, I. and Hendley, C. On the separability of two mechanism involved in the detection of grating patterns in humans. J. Physiol. (Lond.), 1979, 291: 251-263. Breitmeyer, B.G. Simple reaction time as a measure of the temporal response properties of transient and sustained channels. Vision Res., 1975, 15: 1411-1412. Camisa, J., Mylin, L. and Bodis-Wollner, I. The effect of stimulus orientation on the visual evoked potential in multiple sclerosis. Ann. Neurol., 1981, 10: 532-539. Campbell, F.W. and Green, D.G. Optical and retinal factors affecting visual resolution. J. Physiol. (Lond.), 1965, 181: 576-593. Campbell, F.W. and Maffei, L. Electrophysiological evidence for the existence of orientation and size detectors in the human visual system. J. Physiol. (Lond.), 1970, 207: 635-652. Campbell, F.W. and Robson, J.G. Application of Fourier analysis to the visibility of gratings. J. Physiol. (Lond.), 1968, 197: 551-566. Cighnek, L. The EEG response (evoked potential) to light stimulus in man. Electroenceph. clin. Neuropbysiol., 1961, 13: 165-172. Collins, D.W.K., Carroll, W.M. and Black, J.L. Effect of