Saccadic suppression in the monkey

Three rhesus monkeys were trained to make a behavioral response to a short duration flash of light presented during the eye movements of optokinetic nystagmus. This behavioral testing demonstrated a visual threshold elevation of 0.5–0.8 log units from 25 msec before until 50 msec after onset of the fast phase of optokinetic nystagmus in the monkey, similar to the phenomenon of saccadic suppression in the human. Following behavioral testing, chronically implanted electrodes in the striate visual cortex of these monkeys measured the visual evoked response during suppression of the monkey's behavioral response. The cortical response to light was decreased during the fast phase of optokinetic nystagmus, but this reduction in visual cortical response was not specifically related to the decrease in the monkey's perception of the light.     

Saccadic suppression of displacement in face of saccade adaptation

Saccades challenge visual perception since they induce large shifts of the image on the retina. Nevertheless, we perceive the outer world as being stable. The saccadic system also can rapidly adapt to changes in the environment (saccadic adaptation). In such case, a dissociation is introduced between a driving visual signal (the original saccade target) and a motor output (the adapted saccade vector). The question arises, how saccadic adaptation interferes with perceptual visual stability. In order to answer this question, we engaged human subjects in a saccade adaptation paradigm and interspersed trials in which the saccade target was displaced perisaccadically to a random position. In these trials subjects had to report on their perception of displacements of the saccade target. Subjects were tested in two conditions. In the ‘blank’ condition, the saccade target was briefly blanked after the end of the saccade. In the ‘no-blank’ condition the target was permanently visible. Confirming previous findings, the visual system was rather insensitive to displacements of the saccade target in an unadapted state, an effect termed saccadic suppression of displacement (SSD). In all adaptation conditions, we found spatial perception to correlate with the adaptive changes in saccade landing site. In contrast, small changes in saccade amplitude that occurred on a trial by trial basis did not correlate with perception. In the ‘no-blank’ condition we observed a prominent increase in suppression strength during backward adaptation. We discuss our findings in the context of existing theories on transsaccadic perceptual stability and its neural basis.     

Saccadic suppression of displacement: influence of postsaccadic exposure duration and of saccadic stimulus elimination

The threshold for detection of stimulus displacement, which is normally raised in the presence of voluntary saccades relative to its value during steady fixation ("saccadic suppression of displacement"), decreases from 50x to 25x its value during steady fixation when the duration of the second display is experimentally increased from 33 to 461 msec; further increase of the duration of the second display has no additional effect. It might be expected that this improvement in sensitivity to displacement is a consequence of the elimination from perception of the visible smear corresponding to the saccadic stimulus by the action of metacontrast from the postsaccadic stimulus. That this is not so is shown by the fact that the improvement in displacement sensitivity with increased postsaccadic exposure duration is unaffected by experimental elimination of the retinal stimulus normally present during the saccade, even under conditions for which the saccadic stimulus is normally visible and appears smeared. The results demonstrate that the basis for saccadic suppression of displacement lies in the transient saccade-related modification of extraretinal eye position information.     

Apparent motion can survive binocular rivalry suppression

For short-range motion, observers dichoptically viewed a random-dot cinematogram and a rival target. Upon keypress, the first frame of the cinematogram was replaced by the second frame. Observers judged the direction of motion, which was governed by the initial position of the central region. Performance was well above chance during both dominance and suppression. For long-range motion, observers rated the motion produced by sequentially flashing two small spots, with the first spot contained within a rivalrous region. Suppression reduced but did not prevent perception of this motion. Presenting the second motion frame to both eyes weakened both forms of motion.     

Contour suppression during stroboscopic motion and metacontrast

Findings of visual contour masking obtained when two stationary and spatially separated stimuli are presented briefly and successively in time indicate that the contour masking typically observed while viewing a stimulus in real movement also occurs while viewing a stimulus in stroboscopic movement. Additional results indicate that the loss of detailed contour information attending stroboscopic movement may contribute to. though not constitute, the contour suppression effects observed in metacontrast.     

Reduced surround suppression in monocular motion perception

Motion discrimination of large stimuli is impaired at high contrast and short durations. This psychophysical result has been linked with the center-surround suppression found in neurons of area MT. Recent physiology results have shown that most frontoparallel MT cells respond more strongly to binocular than to monocular stimulation. Here we measured the surround suppression strength under binocular and monocular viewing. Thirty-nine participants took part in two experiments: (a) where the nonstimulated eye viewed a blank field of the same luminance (n = 8) and (b) where it was occluded with a patch (n = 31). In both experiments, we measured duration thresholds for small (1 deg diameter) and large (7 deg) drifting gratings of 1 cpd with 85% contrast. For each subject, a Motion Suppression Index (MSI) was computed by subtracting the duration thresholds in logarithmic units of the large minus the small stimulus. Results were similar in both experiments. Combining the MSI of both experiments, we found that the strength of suppression for binocular condition (MSI_binocular = 0.249 ± 0.126 log₁₀ (ms)) is 1.79 times higher than under monocular viewing (MSI_monocular = 0.139 ± 0.137 log₁₀ (ms)). This increase is too high to be explained by the higher perceived contrast of binocular stimuli and offers a new way of testing whether MT neurons account for surround suppression. Potentially, differences in surround suppression reported in clinical populations may reflect altered binocular processing.     

Adaptation to invisible motion results in low-level but not high-level aftereffects

After prolonged exposure to moving stimuli, illusory motion is perceived in stimuli that do not contain consistent motion, a phenomenon termed the motion aftereffect (MAE). In this study, we tested MAEs under binocular suppression that renders the motion adaptor invisible for the entire adaptation period. We developed a variant of the continuous flash suppression method to reliably suppress target motion stimuli for durations longer than several tens of seconds. Here, we ask whether motion systems are functional in the absence of perception by measuring the MAE, a question difficult to address using binocular rivalry that accompanies a switch of percept between visible and invisible. Results show that both the MAEs with static and dynamic tests are attenuated with an invisible adaptor when the adaptor and the test stimulus are presented to the same eye. In contrast, when the test pattern was presented to the other eye, the dynamic MAE was observed in invisible adaptor conditions. These results indicate that low-level adaptation survives under total binocular suppression, a finding predicted by previous studies. In contrast, disappearance of interocular transfer in the dynamic MAE suggests that a high-level motion detector does not operate when the motion adaptor is rendered invisible.     

Binocular rivalry suppression disrupts recovery from motion adaptation.

The motion aftereffect (MAE) lasts longer when the test period does not immediately follow adaptation, a phenomenon called storage. Does storage of the MAE occur if the test target is present but rendered phenomenally invisible owing to the presence of a rival target presented to the other eye during the storage period? Our experiment addressed this question. Following adaptation to a drifting grating, an intervening period preceded testing with a stationary grating. During this period, the adapted eye either viewed the test target immediately or was occluded, and the unadapted eye either viewed a high-contrast rival target or was occluded. Thus four conditions were employed. The duration of the residual MAE was found to be longer for the rivalry condition (grating and rival target viewed) than for the normal MAE condition (grating viewed), and comparable to that in the stored MAE condition (both eyes occluded). Thus, the MAE is stored when the test target is rendered invisible due to binocular rivalry, indicating that a suppressed target is ineffective at promoting decay of the MAE. So while suppression does not prevent information about the adapting grating from reaching the site of generation of the MAE (Lehmkuhle & Fox, 1975), it can prevent information about the test target from reaching the site of the stored MAE. Current models attribute the MAE to reduced responsiveness of direction-selective cortical neurons (Sutherland, 1961; Barlow & Hill, 1963). Thus, storage should reflect a differential return of these adapted cells to preadapted response levels, dependent on postadaptation stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of binocular rivalry suppression on the motion aftereffect.

The relative loci within the visual system of the site of the motion aftereffect (MAE) and the site of binocular rivalry suppression was inferred by measuring the magnitude of the MAE when the inducing motion was phenomenally suppressed for > 50 per cent of the inspection period. The MAE magnitude was a function of the duration of physical impingement of the inducing stimulus; the state of suppression exerted no effect, thereby implying that the site of suppression does not occur before the site of the MAE. This result, together with other data, is interpreted to mean that the site of suppression is cortical.     

Saccadic localization of occluded targets

Saccadic eye movements are able to localize spatially-extended targets, including patterns of random dots and simple shapes, with a high degree of precision [McGowan, Kowler, Sharma & Chubb (1998). Vision Research, 38, 895-909; Melcher & Kowler (1999). Vision Research, 39, 2929-2946]. This paper investigates the representations of object shape that guide saccades. We studied saccadic localization of partially-occluded triangles (two or three vertices removed) to find out whether saccades have access to a representation of the full shape, despite the missing portions. Targets were configured so that they could be seen either as triangles, which were partially occluded by polygons, or as fragments in front of the same polygons. Subjects tried to saccade to the inferred full triangle and a discrimination paradigm was used to evaluate their success. Occlusion cues were ineffective in that saccades directed to the occluded triangles landed near the center of the visible fragment, even when it was configured as a triangle behind occluders. Removing the occluders and leaving only three segments of the triangle (vertices removed) helped somewhat, but performance never resembled that achieved with either a full triangle or a 3-dot configuration. We conclude that the saccadic system is insensitive to at least some cues that can be used to infer the shape of objects. For occluded targets, the representation used by saccades may be closer to the configuration of the retinal image.     

Saccadic presentation of a moving target

Observers viewed a rightward moving dot (CRO Trace) during rightward and diagonal eye movements. It was found that: (1) with horizontal saccades, when the trace moved at the same velocity as the eyes it appeared as a stationary dot located at the left edge of the trace's path (indicating the absence of an extraretinal movement signal); and (2) whether the perceived target was aligned with the left or right edge of the trace's path (rightward eye movement condition) and whether it appeared to move upward or downward from the actual trace path (diagonal eye movement condition) depended upon whether the velocity of the trace was greater than or less than that of the eyes. Results were discussed in terms of ex-afference and re-afference and the interpretation that one makes of such stimuli.     

Saccadic eye movements in myasthenia gravis

The peak velocities of horizontal saccades were measured in patients with myasthenia gravis (MG) to determine whether they can differentiate MG from other causes of ophthalmoplegia. Eye movements were recorded with electrooculography (EOG) or infrared scleral reflection (IR) in 42 patients with MG, 26 patients with sixth cranial nerve palsy (CNP), 19 patients with chronic progressive external ophthalmoplegia (PEO) and 28 normal subjects. Despite limitation of ductions in MG, the group means of velocities of 10 deg saccades recorded with IR were similar in MG and normal subjects. With EOG, small but statistically significant decreases in mean velocities of 10, 20 and 30 deg saccades were found in MG, compared to those in normal subjects. Twenty-one to 28% of MG patients had velocities outside of the normal range (outliers). In contrast, the group means in CNP and PEO were markedly lower than those in MG and normal subjects. The frequencies of outliers were 89 to 100% in CNP and 88 to 100% in PEO. Measurement of saccadic velocities can be helpful in differentiating MG from other causes of ophthalmoplegia.     

Sound can enhance the suppression of visual target detection in apparent motion trajectory

Detection performance is impaired for a visual target presented in an apparent motion (AM) trajectory, and this AM interference weakens when orientation information is inconsistent between the target and AM stimuli. These indicate that the target is perceptually suppressed by internal object representations of AM stimuli established along the AM trajectory. Here, we showed that transient sounds presented together with AM stimuli could enhance the magnitude of AM interference. Furthermore, this auditory effect attenuated when frequencies of the sounds were inconsistent during AM. We also confirmed that the sounds wholly elevated the magnitude of AM interference irrespective of the inconsistency in orientation information between the target and AM stimuli when the saliency of the sounds was maintained. These results suggest that sounds can contribute to the robust establishment and spatiotemporal maintenance of the internal object representation of an AM stimulus.