Mark A. Berkley

Grating visibility as a function of orientation and retinal eccentricity

Abstract Acuity was measured with high contrast gratings in three orientations at several retinal eccentricities. Acuity for obliquely oriented gratings was poorer than for vertical or horizontal gratings but this difference disappeared with eccentric viewing. The precise locus of disappearance varied from 8° to 18° between subjects. A similar reduction of the orientation effect can be achieved with target contrast reduction which supports the view that the magnitude of the orientation preference (oblique effect) is inversely related to the size of receptive fields participating in detection of the grating.

The orientation selectivity of single neurons in cat striate cortex

SummaryThe sensitivity to stimulus orientation — orientation selectivity — of simple and complex neurons in cat striate cortex was studied quantitatively. Orientation selectivity was found to be related to receptive field size — neurons with larger receptive fields being less sensitive to stimulus orientation than those with smaller receptive fields. Simple cells were more orientationally selective than complex cells. The orientation selectivity of simple cells was only weakly related to their receptive field geometry (i.e., receptive field width/length ratio), but simple cells with narrow, elongated excitatory receptive fields are more orientationally selective than those with squarer, excitatory receptive fields. These results indicate that orientation selectivity cannot be accounted for solely by the geometry of the excitatory zones and suggest that inhibitory influences sharpen the tuning of simple striate neurons for stimulus orientation.

Grating resolution and refraction in the cat estimated from evoked cerebral potentials

Abstract The amplitude of summed cerebral potentials recorded from anesthetized cats in response to phase-reversing grating stimuli was found to be affected by degree of focus and grating size. Refraction was accurately corrected when the lens producing the shortest latency, largest amplitude response was used. A linear relation between amplitude of the response and log grating frequency was found which permitted extrapolation to the zero response point. This procedure yielded threshold grating frequency estimates that agree closely with behavioral acuity data and the resolution limits imposed by the optics of the cat eye.

Visual acuity and the near point of accommodation in cats.

Abstract The near point of accommodation was determined in four cats by measuring visual acuity at viewing distances varying from 125 to 12 cm. The near point of accommodation was taken as the closest viewing distance which just produced a decrement in grating acuity. The values obtained ranged from 25 to 36 cm and are in good agreement with physiological and optical estimates of the near point of accommodation.

Apparent contrast as a function of modulation depth and spatial frequency: A comparison between perceptual and electrophysiological measures

Abstract The contrast sensitivity function describing the interrelated contrast and spatial response characteristics of the visual system was determined for sine-wave gratings. Three spatial frequencies were then selected for psychophysical scaling of apparent contrast using an intermodal matching technique. The perceptual contrast curves were to a fair approximation power functions of the physical contrast of the striped target. Power transformations as a function of spatial fequency were observed, i.e. with decreasing sensitivity the exponents of the apparent contrast functions increased. A reanalysis of evoked response data published by Campbell and Maffei confirmed these observations.

Visual Discriminations in the Cat

For all their popularity as pets, cats have been conspicuously neglected as experimental animals by behavioral scientists. Yet there are many advantages and even strong reasons for using the cat as a behavioral animal. He is of a convenient size, is easy to maintain, and generally speaking, has a reasonable disposition so that he can be easily handled in the laboratory. Perhaps a more important reason is that a great deal of research has been done by physiologists and anatomists on his nervous system.

Evoked potential estimates of the time course of adaptation and recovery to counterphase gratings

Scalp-recorded evoked potentials (VEP) were sequentially sampled in humans during adaptation to and recovery from prolonged viewing of counterphase sinusoidal grating targets. The sum of the power at the first and second harmonics of the Fourier-transformed VEP components was found to decrease during adaptation and increase during recovery. Time constants (T) for the adaptation and recovery processes as estimated from exponential functions ranged from 2.9 to 19 sec, varying non-monotonically with the spatial frequency and contrast of the stimulus. The observed T values are shorter than those reported in psychophysical studies of adaptation but overlap estimates derived from single cell studies. An unexpected finding was the occurrence of a 3-6 sec delay in the appearance of the maximum VEP response after the onset of the adaptation stimulus. The delay occurred in all subjects and at all spatial frequencies when moderate to high adapting contrasts (e.g. greater than 0.2) were used. The data support a feature-selective, multi-channel lateral inhibitory model of spatial vision and suggest the presence of tonic inhibition between the channels.

Temporal modulation sensitivity of the cat — I : Behavioral measures

Abstract Critical fusion frequency (CFF) and a temporal modulation sensitivity function (TMSF) were determined for three cats using behavioral testing procedures. CFFs ranged from 40 to 55 Hz. The sensitivity functions showed the classic high frequency attenuation but had some unusual features below 15 Hz. Comparisons with the sensitivity function of humans derived with the same stimulus indicated that at the same average luminance and modulation depth, cats were capable of detecting higher rates of flicker. This difference is smaller, but not eliminated, when targets are equated for retinal illumination.

Contrast sensitivity for vertically and obliquely oriented gratings as a function of grating area

Contrast sensitivity for both vertically and obliquely oriented gratings increased with increases in stimulus area to an asymptotic value. Sensitivity grew more slowly for oblique gratings and reached an asymptotic value at a larger area than for vertical gratings. For equal areas, oblique gratings always yielded poorer sensitivity. The results suggest a larger spatial summation area for obliquely oriented gratings.

Evoked potentials elicited by brief vernier offsets: Estimating vernier thresholds and properties of the neural substrate

The characteristics of the neural generator producing an evoked potential in response to the brief presentation of a vernier offset was investigated in three experiments. In the first study, averaged evoked potentials (EPs) recorded in response to a single vernier offset stimulus consisting of a horizontal line which changed from colinearity to noncolinearity for 100 msec every 1.5 sec were compared to responses elicited by other vernier configurations consisting of: stimuli with multiple offsets; stimuli presented in different orientations; targets with different offset features; and with the simple displacement of a colinear line. The results showed that a single vernier offset elicited a robust response if the offset was located in the central zone (1 degree) of the target. Other features of the target configuration were unimportant. Displacement of a colinear line over the same range without an offset evoked little, if any, response. In the second study, EPs were recorded in response to a single offset target which varied in magnitude from 21 to 82 sec of visual angle on different trials. The latency and amplitude of the EP response varied systematically with the amplitude of the vernier offset. Plots of EP amplitude against log of the offset magnitude were linear over the range of offsets employed. Straight lines fitted to the data and extrapolated to zero amplitude provided estimates of vernier threshold. These estimates agreed closely with psychophysical measures taken with the same targets and confirm the initial observations by Levi et al. (1983). In the third experiment, irrelevant contours were added to the vernier target in various spatial and temporal configurations. The addition of stationary, contiguous contours to the vernier target reduced the amplitude of the EP response when the contours were within 4-8 min of the offset, producing progressively less EP attenuation with increasing distance from the offset. However, brief presentation of the irrelevant contour (e.g. a single line passing through the offset) with an onset asynchrony relative to the vernier offset stimulus appropriate to assure simultaneity of the line-elicited EP and the offset-elicited EP yielded an enhanced response, i.e. to two responses added algebraically. The long latency of the offset evoked response and the summation results of the EPs generated by an offset and by a briefly presented linear contour suggests independence of the neural generators producing the response to these two targets.