Influence of background luminance on visual sensitivity during saccadic eye movements

Dependence of perceptual saccadic suppression on peri-saccadic image flow properties and luminance contrast polarity

Across saccades, perceptual detectability of brief visual stimuli is strongly diminished. We recently observed that this perceptual suppression phenomenon is jumpstarted in the retina, suggesting that the phenomenon might be significantly more visual in nature than normally acknowledged. Here, we explicitly compared saccadic suppression strength when saccades were made across a uniform image of constant luminance versus when saccades were made across image patches of different luminance, width, and trans-saccadic luminance polarity. We measured perceptual contrast thresholds of human subjects for brief peri-saccadic flashes of positive (luminance increments) or negative (luminance decrements) polarity. Thresholds were >6–7 times higher when saccades translated a luminance stripe or edge across the retina than when saccades were made over a completely uniform image patch. Critically, both background luminance and flash luminance polarity strongly modulated peri-saccadic contrast thresholds. In addition, all of these very same visual dependencies also occurred in the absence of any saccades, but with qualitatively similar rapid translations of image patches across the retina. This similarity of visual dependencies with and without saccades supports the notion that perceptual saccadic suppression may be fundamentally a visual phenomenon, which strongly motivates neurophysiological and theoretical investigations on the role of saccadic eye movement commands in modulating its properties.

Saccade suppression depends on context

Although our eyes are in constant movement, we remain unaware of the high-speed stimulation produced by the retinal displacement. Vision is drastically reduced at the time of saccades. Here, I investigated whether the reduction of the unwanted disturbance could be established through a saccade-contingent habituation to intra-saccadic displacements. In more than 100 context trials, participants were exposed either to an intra-saccadic or to a post-saccadic disturbance or to no disturbance at all. After induction of a specific context, I measured peri-saccadic suppression. Displacement discrimination thresholds of observers were high after participants were exposed to an intra-saccadic disturbance. However, after exposure to a post-saccadic disturbance or a context without any intra-saccadic stimulation, displacement discrimination improved such that observers were able to see shifts as during fixation. Saccade-contingent habituation might explain why we do not perceive trans-saccadic retinal stimulation during saccades.

Increment-threshold spectral sensitivity during saccadic eye movements in uniform visual field

We measured increment-threshold spectral sensitivity functions during saccades, immediately after saccades and during fixation. A uniform white background field which covered observer's whole visual field prevented any retinal masking effects. Visual sensitivity was degraded during saccades or after saccades. The reduction in sensitivity depended upon the wavelength of the test stimulus. The spectral sensitivity function during fixation produced a broad smooth curve in the middle and long wavelength region, while saccades caused a prominent dip around 570 nm. This finding indicates that saccadic suppression, which cannot be attributed to retinal masking, has more effect on the achromatic channel than the chromatic channel. The role of central and retinal processes dealing with the perceptual clearness and stableness across saccades will be discussed.

Cortical mechanisms for visual perception of object motion and position in space

The present review is aimed at analyzing and discussing some of the cortical mechanisms possibly involved in the perception of object motion and object localization in the visual field. A comprehensive approach to these topics would be beyond the scope of this work. The highest priority, therefore, will be given to the cortical machinery involved in these processes, while very little (or nothing at all) will be said on the possible role played by subcortical structures such as the lateral geniculate nucleus and the superior colliculus which, albeit not directly involved in perception, might contribute to it.

Suppression of contrast sensitivity during eyelid blinks

Each blink of the eyelids is associated with a concurrent suppression of vision that lasts as long as 200 msec. Saccadic eye movements are also associated with a concurrent suppression of vision. Previous studies suggested that blink and saccadic suppression may be the result of a single mechanism. Volkmann, Riggs, White and Moore [(1978) Vision Research, 18, 1193-1199] demonstrated that saccadic suppression is most evident for low spatial frequency stimuli. However, the effect of stimulus spatial frequency on blink suppression has not been evaluated. If blink suppression and saccadic suppression result from a single mechanism, then blink suppression should also exhibit its greatest effect at low spatial frequencies. The purpose of this study was to determine the effect of stimulus spatial frequency on blink suppression. The stimulus was a sine-wave grating presented at different times after the blink. Psychometric functions were produced from the data for each post-blink, stimulus onset time and a Weibull function was fit to the data to determine threshold. The magnitude and duration of blink induced contrast sensitivity suppression was found to depend on the spatial frequency of the stimulus employed (similar to saccadic suppression). This is further evidence that a single mechanism may produce both blink induced visual suppression and saccadic suppression.

Dependence of visual suppression on the amplitudes of saccades and blinks

Visual suppression accompanying voluntary saccades and eyeblinks was measured for a range of amplitudes of both. Saccade amplitudes varied from 2 to 32 degrees and blink amplitudes varied from a slight movement to a full closure of the eye. In every case, thresholds for detecting full-field luminance decrements were determined with the method of constant stimuli and a two alternative forced choice procedure. Results from three subjects show a monotonic increase in the amount of suppression produced by saccades and blinks of increasing amplitude. Data are discussed with respect to theories about the origin of visual suppression.

Perception of suprathreshold stimuli during saccadic eye movements

We have investigated the extent to which a graded series of suprathreshold stimuli are perceptually weakened by saccadic suppression. Stimuli were brief decrements of variable amplitude in the illumination of a Ganzfeld. For each of several decrements presented during fixation, we determined a matching amplitude of decrement presented during saccades. The measure of saccadic suppression was the ratio of these two amplitudes. The matches made by three observers exhibit ratios as high as 6:1 (0.8 log unit of suppression) near the threshold. Suppression decreases systematically as the strength of the stimulus increases, reaching 0.1 log unit or less for stimuli 2 log units above threshold.

Measurements of visual suppression during opening, closing and blinking of the eyes

We have previously shown that the sensitivity of human vision, as measured with a stimulus that bypasses the eyelids, is briefly impaired at the time of an eyeblink. We now find that the visual loss is almost equally extensive during eye closure if the eyes then remained closed. But little impairment occurs during eye opening when the eyes then remain open. We have previously concluded that, in blinking, visual suppression is associated with an inhibitory signal sent out by the brain. We now conclude that this signal arises primarily as a corollary to the efferent discharge that closes the eyelids.