Peripheral Gratings Research Articles

The lateral effect of oscillation of peripheral luminance gratings: Test of various hypotheses

Foveal, psychophysical increment thresholds measured on a background of variable luminance are elevated by a peripheral grating of constant luminance oscillating with a frequency of 4 Hz. There is an optimal luminance of the background for which this reduction of threshold visibility is maximum. This finding leads to rejection of the facilitation, excitation, stray light, and “noise” hypotheses as possible explanations of the lateral effect of the oscillations. The dependency of threshold elevation on background luminance is accounted for by a formal model which allows the construction of response curves of a hypothetical threshold detecting mechanism. According to this model, the effect of a jerking, as opposed to a stationary peripheral pattern, is to decrease the sensitivity and to increase the operating luminance range of the underlying mechanism. This result is related to the analogous suppression of vision during saccadic eye movements for which a possible cortical transient-on-sustained neuron inhibition is suggested.

Nonlinear spatial summation and the contrast gain control of cat retinal ganglion cells.

1. We studied how responses to visual stimuli at spatially separated locations were combined by cat retinal ganglion cells. 2. The temporal signal which modulated the stimuli was a sum of sinusoids. Fourier analysis of the ganglion cell impulse train yielded first order responses at the modulation frequencies, and second order responses at sums and differences of the input frequencies. 3. Spatial stimuli were spots in the centre and periphery of the cell's receptive field. Four conditions of stimulation were used: centre alone, periphery alone, centre and periphery in phase, centre and periphery out of phase. 4. The effective first order response of the centre was defined as the response due to centre stimulation in the presence of periphery stimulation, but independent of the relative phases of the two regions. Likewise, the effective first order response of the periphery was defined as the response due to periphery in the presence of centre stimulation, but independent of the relative phases of the two regions. These effective responses may be calculated by addition and subtraction of the measured responses to the combined stimuli. 5. There was a consistent difference between the first order frequency kernal of the effective centre and the first order kernel of the centre alone. The amplitudes of the effective centre responses were diminished at low frequencies of modulation compared to the isolated centre responses. Also, the phase of the effective centre's response to high frequencies was advanced. Such non-linear interaction occurred in all ganglion cells, X or Y, but the effects were larger in Y cells. 6. In addition to spatially uniform stimuli in the periphery, spatial grating patterns were also used. These peripheral gratings affected the first order kernal of the centre even though the peripheral gratings produced no first order responses by themselves. 7. The temporal properties of the non-linear interaction of centre and periphery were probed by modulation in the periphery with single sinusoids. The most effective temporal frequencies for producing non-linear summation were: (a) 4-15 Hz when all the visual stimuli were spatially uniform, (b) 2-8 Hz when spatial grating patterns were used in the periphery. 8. The characteristics of non-linear spatial summation observed in these experiments are explained by the properties of the contrast gain control mechanism which we have previously postulated.

How presaccadic gratings modify postsaccadic modulation transfer function

Threshold modulations for sinusoidal gratings tachistoscopically presented on the fovea after a saccade were determined as a function of the spatial frequency of a peripheral grating at the saccade goal and compared with thresholds of the “resting eye” condition. The results show a clear enhancement for medium spatial frequency gratings (3.2 c/deg) with peripheral gratings of the same spatial frequency. Therefore an information transfer from periphery to centre induced by saccadic eye movements is suggested. Suppression of low spatial frequency gratings (0.5 c/deg) is found to be independent from additional peripheral gratings and interpreted as an effect of a central inhibitory process elicited by the saccadic motor command. Some aspects concerning saccadic suppression, refixation, and visual stability are considered.