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Visual evoked potentials were recorded from the occipital scalp of two anaesthetized Lynx (Lynx europea) in response to alternating gratings of various spatial frequencies and contrasts. The visual acuity of the Lynx was found to be around 5-6 c/deg, i.e. very close to the visual acuity of the cat and by far inferior to human acuity.
Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.
Hubel and Wiesel (1962; Journal of Physiology, London, 160, 106-154) introduced the classification of cortical neurons as simple and complex on the basis of four tests of their receptive field structure. These tests are partly subjective and no one of them unequivocally places neurons into distinct classes. A simple, objective classification criterion based on the form of the response to drifting sinusoidal gratings has been used by several laboratories, although it has been criticized by others. We review published and unpublished evidence which indicates that this simple and objective criterion reliability divides neurons of the striate cortex in both cats and monkeys into two groups that correspond closely to the classically-described simple and complex classes.
A programmable digital video pattern generator controlled by a microprocessor is described as a part of any commercial or specific visual evoked potential analysis system. The generator handles all kinds of patterns (uniform, orthogonal and radial) with all known presentation modes (onset, offset and reversal) and resolution 640 x 200 square pixels. The frame rate of the generator is adjusted to be compatible to either low cost 50 Hz video monitor or expensive ones of higher frame rate. The operation parameters defined by software are: pattern selection, trigger mode, field selection and stimulation rate. The development of software applications for special research demands is desirable because of the control of the generator by a microprocessor.
We investigated whether the process of texture segregation can be allocated to a specific visual cortical area. We designed a stimulus to reveal the presence of a mechanism, which is specifically sensitive to a checkerboard, that is solely defined by textures segregating due to orientation differences of the constituting line segments. We recorded evoked potentials to this stimulus in man and awake monkey. A difference component, signalling texture segregation sensitivity, could be recorded from both types of subjects. Its presence depended on the spatial extent of the textures, in a manner correlating with the perceptibility of the checkerboard. This difference response could be localized in primary visual cortex by means of equivalent dipole estimations.
A modification of the visual evoked potential (VEP) technique, first employed by Braddick, Wattam-Bell and Atkinson [(1986) Nature, London, 320, 617-619] was used to estimate the orientation selectivity of 3-month-old infants. The orientation-selective VEP was recorded in response to various changes in the orientation of a square-wave grating. The magnitude (the square root of the power of the FFT) at the frequency of orientation change was assumed to represent the response to the change in grating orientation. Orientation sensitivity was then estimated by plotting the magnitude of the FFT at the frequency of orientation change as a function of the log of the orientation change in degrees. For each individual, the data were fit by nonlinear regression and threshold was defined as the largest orientation angle for which the magnitude of the FFT was zero. The results suggest that the orientation selectivity of 3-month-old infants (1.33 deg) is similar to that of adults (1.13 deg) tested with the same stimulus parameters (1 c/deg, 9 Hz). However, when adults are tested with stimulus parameters selected to optimize their VEP response (4 c/deg, 18 Hz) instead of those which optimize the infant's response, the orientation discrimination of adults improves by a factor of 2 (0.53 deg). The results obtained from adults under optimum stimulus conditions (4 c/deg, 18 Hz) approach the estimates reported in the literature for static stimuli.
The selectivity of cortical neurons remains invariant with contrast, even though the contrast-response function saturates. Both the invariance and the saturation might be due to a contrast-gain control mechanism. To test this hypothesis, a drifting grafting was used to measure the contrast-response function, while a counterphase grating was simultaneously presented at the null position of the receptive field (where it evokes no response at any contrast). When the contrast of the counterphase grating increased, the contrast-response function shifted primarily to the right. This result is consistent with the hypothesis that there is a fast-acting gain-control mechanism which effectively scales the input contrast by the average local contrast.
The present study examined how the response characteristics of the visual evoked potential (VEP) varied during the course of trials using a sinusoidal grating stimulus that reversed contrast in a square-wave manner. To accomplish this, amplitude and phase values were derived in short segments during the course of continuous stimulation for three subjects. When stimulus spatial frequencies of 0.77 or 1.55 c/deg were used, VEP amplitude remained at a stable value throughout the trial. At 3.1 c/deg, 6-12 sec were required for VEP amplitude to increase to a stable value, which was on average 204% greater than the value noted during the first few seconds of the trial. At 6.2 and 12.4 c/deg, VEP amplitude changes were more complex, first increasing and then decreasing substantially, to levels that were on average 63.8% and 38% of the peak reached earlier in the trial. In all cases, VEP phase decreased during the trial. The magnitude of this decrease ranged up to 50 deg, corresponding to an approx. 10.5 msec delay for the 6.65 Hz stimulation rate used. Prior exposure to an adapting grating diminished the changes in VEP amplitude and advanced the phase changes. Therefore, these changes appear to represent a form of contrast adaptation that is restricted to responses to high spatial frequencies. In addition, the present results provide evidence against a fundamental assumption of signal averaging--that an invariant stimulus will evoke an invariant response.
Dynamic random dot fusion, stereopsis and stereoacuity were evaluated in 149 healthy, fullterm infants, using both forced-choice preferential looking (FPL) and steady-state visual evoked potential (VEP) protocols. Few infants aged 2-3 months demonstrated fusion or stereopsis in either the FPL or VEP protocol; most infants aged 5 months and older demonstrated fusion and stereopsis in both protocols. Both FLP and VEP stereoacuity approached adult-level (< 60 sec) by 6-7 months of age. Both infants and adults exhibited non-monotonic VEP amplitude vs disparity functions with a step change in phase at an intermediate disparity, consistent with separate fine and coarse disparity mechanisms.
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Kittens were given visual experience through one or both eyes for two weeks around the peak of the sensitive period. Subsequently they were binocularly deprived for at least one year. This period of pattern deprivation erased completely the effects of the preceding temporary experience. Ocular dominance distribution, orientation selectivity and response quality of the cortical units resembled those obtained from kittens which are contour-deprived throughout their early postnatal development. This suggests that the effect of visual experience is not to engrave irreversibly certain features of the early visual world, but to adapt the cortex continuously and in an integrative fashion to features which are prominent throughout the sensitive period.
Six subjects suffering from idiopathic central serous retinopathy were examined during the acute phase using a battery of neurophysiological tests. The records (using skin electrodes) included the electro-oculogram (EOG), the white light electroretinogram (ERG) during various stages of dark and light adaptation, and the flash and pattern reversal visual evoked potentials. Results obtained from the affected eye were compared with those from the healthy eye. The values of the healthy eye were not significantly different from our normal control group. The EOGs of the affected eyes were not significantly different from those of the healthy eyes. The 'a' wave of the ERG during light adaptation was significantly (P less than 0.05) smaller in the affected eye. The amplitudes of the other components and the latencies of all components were not affected. The latency of the PR-VEP of the affected eye in all subjects was prolonged in comparison with the unaffected eye. In 3 subjects the latency was prolonged by more than 2 S.D.s compared with our control group. The amplitude of the PR-VEP in 5 of our 6 subjects was slightly but significantly (P less than 0.02) smaller in the affected eye. There was no correlation between amplitude and latency changes.
It has been proposed that latencies of some components of the pattern-reversal visual evoked potential (PRVEP) are shorter in women than in men because of differences in levels of circulating sex steroids. Pregnancy is a time when serum levels of oestrogen and progestogen are considerably greater than in the non-pregnant state. Whole and half field PRVEP latencies and amplitudes have been compared in 16 pregnant and 38 healthy non-pregnant women. The mean P100 latencies for all responses were shorter in the pregnant women, with statistically significant differences for the left eye whole field latency (P < 0.05) and the left eye right and left half field latencies (P < 0.005 and P < 0.05, respectively) and the right eye right half field latency (P < 0.05). The latencies in women in the pregnant group showed a negative correlation with gestation, which reached statistical significance for the REWF (r = -0.55, P < 0.05). These observed differences in PRVEP latencies in pregnant and non-pregnant women and the association between latency and gestation are likely to be due to differences in circulating sex steroids, and this effect may be the principal reason for latency differences between the sexes.
The effects of reading disability (RD) and attention deficit disorder (ADD) on event-related potentials (ERPs) were investigated in 52 boys between 8 and 12 years of age. There were four groups: 25 children without RD divided into two groups (17 without ADD and 8 with ADD) and 27 children with RD divided into two groups (11 without ADD and 16 with ADD). The children participated in three tasks ("games") designed to assess selective attention to the color black (vs white), to any of two letters (vs. two patterns), and to any of 12 letters (vs. 12 patterns). ERPs were recorded over the left and right occipital, central, and frontal hemispheres. Children with RD as compared to those without RD were characterized by the following ERPs: (1) a smaller amplitude positive wave at about 240 msec (P240) over the left central hemisphere: (2) a smaller amplitude positive wave at about 500 msec (P500) over the right central hemisphere: (3) a larger P500 over the right central hemisphere: (3) a larger P500 over both left and right occipital hemispheres: and (4) smaller within-subject-and-condition variability of ERP waveform. These effects did not differ significantly for the three games and, therefore, were not specific to letter processing per se. The effects of RD did not interact with the effects of ADD for the most part. The ERPs clearly indicated these two disorders, in part, involve different underlying brain processes.
Pattern-onset visual-evoked potentials (VEPs) were recorded from rats and humans in order to perform cross-species comparison of neuronal functional properties reflected by the early VEP components. The spatial frequency of a sinusoidal test grating was varied in Experiment 1. For both species, amplitude of the first positive VEP component was larger at low spatial frequency and decreased as spatial frequency increased. The immediately succeeding negative component was small at low spatial frequency and was of maximal amplitude at moderate spatial frequency. The effects of stationary pattern adaptation on these components were investigated in Experiment 2. Subjects viewed either a blank field or the test grating prior to recording VEPs. For both species, adaptation had no effect on the positive component but strongly attenuated the negative component. Experiment 3, in which only humans were tested, indicated that the negative component was of cortical origin. Only cortical neurons are known to be orientation selective, and the effect of adaptation diminished as the orientation difference between the adaptation and test gratings increased. These results suggest that the early positive and negative components arise from parallel visual pathways, and that the rat components may reflect visual processes qualitatively similar to those of humans.
In their commentary, Rossion, Curran, and Gauthier (Rossion, B., Curran, T., Gauthier, I. (2002). A defense of the subordinate-level expertise account for the N170 component. Cognition, 85, 197-202) (RC&G) raise a series of arguments against the domain-specificity account for the N170 face-effect (Carmel, D., & Bentin, S. (2002). Domain specificity versus expertise: factors influencing distinct processing of faces. Cognition, 83, 1-29). This effect consists of a large difference (always significant) observed in the amplitude of a negative component peaking at the lower posterior-temporal sites in response to human faces relative to many other stimulus categories. As an alternative to domain specificity, RC&G advocate a "subordinate-level expertise" account, by which the N170 effect can be obtained for any type of stimulus for the individual identification of which the perceiver is an expert. While considering some of their arguments well taken and interesting, we believe that, overall, RC&G's interpretation of our current data (as well as some of theirs) and of our position ignores several important aspects and, therefore, their critique is not persuasive.
The N170 is an event-related potential component associated with extrastriate visual mechanisms involved in detecting human faces and leading to their characteristic structural encoding. Consequently the N170 discriminates the processing of stimuli providing physiognomic information from the processing of other, similarly complex, visual patterns. We have used this effect to explore the top-down influence of newly acquired experience on the initial visual categorization of physiognomic stimuli. Schematic faces having the inner components mislocated did not elicit an N170 effect in naive participants. However, after normally configured versions of the schematic faces were exposed, suggesting what the ambiguous patterns represented, the same patterns elicited an N170 effect as conspicuous as that elicited by regular schematic faces. This priming effect, which changed the visual processing of distorted faces in extrastriate regions, could not be explained by post-perceptual decisions. Although accounts based on changes in focal attention, or within-vision constraints could not be categorically rejected, this outcome might suggest cognitive penetrability of early visual categorization.
With the use of the simple motor reaction time (RT), we examined the contribution of the hemiretinas to the timing of the pattern reversal visual evoked potential (PVEP). RTs were obtained to the reversal of a progressively dimmer check falling either on the central, nasal, temporal, superior or inferior retina. Results were correlated with the peak time of the P1 wave evoked to the reversal of the entire checkerboard stimulus to the same luminance conditions as that used to generate the RT measurements. Our results clearly indicate that the time taken to trigger a manual response depends not only on the luminance of the stimulus, but also on where it falls on the retina. Comparing the above data with the peak time of the PVEP obtained to the reversal of the entire screen indicates that at brighter luminance the peak time of the P1 wave correlates with that measured from the most central retina, while at dimmer intensities it correlates with the mathematical average of all the retinal locations tested.