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R Shapley

Publications and source records attributed to R Shapley.

15 recordsLinked to original sources
PubMed1988

Brightness induction by local contrast and the spatial dependence of assimilation.

Two mechanisms of brightness perception (1) brightness induction by local contrast and (2) assimilation, were examined for a variety of visual stimuli. Local contrast is the primary determinant of brightness perception, making objects appear brighter on a background of lower luminance and darker on a background of greater luminance. Assimilation is the opposite effect, whereby objects on a brighter (but not necessarily more luminant) background appear brighter or on a dark background appear darker. We have compared the relative strength of the two effects using stimuli which permit them to be studied separately. Brightness induction by local contrast is quantitatively stronger in all situations. Further, the strength of assimilation is strongly dependent on spatial parameters in the visual scene. These results are shown to be true both for simple visual stimuli as well as for complicated Mondrian-like patterns. The Retinex theory of brightness perception predicts that the two effects are equal. Our results show a range of relative strengths (assimilation vs brightness induction due to contrast) from 0.59 to 0.63 at 5' down to 0.34 at 43'.

Humans↗
PubMed1986

Hyperacuity in cat retinal ganglion cells.

Cat X retinal ganglion cells that can resolve sine gratings of only 2.5 cycles per degree can nevertheless respond reliably to displacements of a grating of approximately 1 minute of arc. This is a form of hyperacuity comparable in magnitude to that seen in human vision. A theoretical analysis of this form of hyperacuity reveals it to be a result of the high gain and low noise of ganglion cells. The hyperacuity expected for the best retinal ganglion cells is substantially better than that observed in behavioral experiments. Thus the brain, rather than improving on the retinal signal-to-noise ratio by pooling signals from many ganglion cells, is unable to make use of all the hyperacuity information present in single ganglion cell responses.

Animals↗
PubMed1986

The importance of contrast for the activity of single neurons, the VEP and perception.

The brightness of a visually perceived object is mainly determined by the average local contrast around the border between object and background. This fact is demonstrated here with several examples of equiluminant objects on nonuniformly luminant backgrounds. Even in Mondrian-like patterns resembling those used by Land and McCann (1971), equiluminant objects may appear to be of unequal brightness. This result does not agree with predictions of the Retinex Theory. The importance of contrast in vision is also suggested by neurophysiological findings, both classical and recent, that reveal the dependence of visual responses on contrast over most of the visual operating range of mean illumination. The dependence on contrast appears to be the result of retinal gain control mechanisms and is not due to center-surround interaction in the receptive field. We have discovered parallel neural channels with high and low contrast gain in the monkey's visual pathway by means of single unit techniques. Visual evoked potential measurements suggest that similar visual pathways, and with high and low contrast-sensitivity, exist in man and monkey.

Adaptation, Ocular↗
PubMed1985

Contrast and assimilation in the perception of brightness.

The rapid estimation of the brightness of objects is one of the nervous system's major visual tasks. Exactly how the eye and brain perform this basic task is still not understood. Two mechanisms that contribute to human perception of the brightness of objects have been identified previously: (i) the visual response to physical contrast and (ii) assimilation. Use of a unique visual display device allowed us to measure the relative importance of these two mechanisms. The present results reveal that assimilation is about half as effective as physical contrast in determining the apparent brightness of objects. These results imply that previous theories of vision--for instance, the retinex theory--will have to be revised; the importance of physical contrast must be weighted more strongly.

Acculturation↗
PubMed1979

Spatial properties of X and Y cells in the lateral geniculate nucleus of the cat and conduction veolcities of their inputs.

Visual neurons in the lateral geniculate nucleus (LGN) of the cat may be separated into distinct X and Y classes based on a test of the linearity of spatial summation. Y cells produce nonlinear responses especially when the visual stimulus is a fine spatial grating. X cells exhibit mainly linear summation properties. X cells respond mainly at the fundamental modulation frequency of a contrast reversal grating while Y cells respond at the fundamental and at the second harmonic of the modulation frequency. The spatial resolution of X cells' fundamental responses and Y cells' second harmonic responses is about the same, and both are two to eight times higher than the spatial resolution of the Y cells' fundamental response. The conduction velocity of the Y optic tract afferents is greater than that of the velocity of the X afferents. However, the LGN latencies of the responses of the two classes of cells to optic chiasm stimulation overlap considerably.

Animals↗
PubMed1979

Deficits in the visual evoked potentials of cats as a result of visual deprivation.

Visual evoked potentials in response to contrast reversal of grating patterns were used as a measure of visual function in normal and visually deprived cats. In cats which had been dark reared from birth (BD cats) there was a characteristic change in VEP waveform from normal, for both eyes and for all spatial frequencies of testing stimulus. In cats which had one eye sutured from the age of one week (MD cats), the VEP from the deprived eye was smaller for contrast reversal of coarse patterns. Kittens given only restricted periods of monocular exposure gave VEPs which resembled the pathological responses of the BD cats. However, the amplitudes of response were larger for the more experienced eye at higher spatial frequency. This work reinforces the idea that two factors govern cortical development: competition and experience. Lack of sufficient visual experience leads to severe intracortical pathology.

Aging↗
PubMed1971

Effects of lateral inhibition on fluctuations of the impulse rate.

Inhibition from neighboring eccentric cells has an effect on the variability of firing of a given eccentric cell. The reduction in the average impulse rate which is caused by inhibition decreases the variance of the impulse rate. However, this reduction of the average rate increases the coefficient of variation of the impulse rate. Inhibitory synaptic noise should add to the low frequency portion of the variance spectrum of the impulse rate. This occurs because of the slow time course of inhibitory synaptic potentials. As a consequence, inhibition decreases the signal-to-noise ratio for low frequency modulated stimuli.

Animals↗