Localization of visual targets during optokinetic eye movements

We investigated localization of brief visual targets during reflexive eye movements (optokinetic nystagmus). Subjects mislocalized these targets in the direction of the slow eye movement. This error decreased shortly before a saccade and temporarily increased afterwards. The pattern of mislocalization differs markedly from mislocalization during voluntary eye movements in the presence of visual references, but (spatially) resembles mislocalization during voluntary eye movements in darkness. Because neither reflexive eye movements nor voluntary eye movements in darkness have explicit (visual) goals, these data support the view that visual goals support perceptual stability as an important link between pre- and post-saccadic scenes.     

Sensorimotor transformation during eye movements to remembered visual targets.

For eye movements made to visual targets, the brain must transform the retinotopic coordinate frame of the visual system to that of the oculomotor plant. Ideally, responses should exactly match target demands. However, during eye movements to remembered targets, responses are spatially distorted. The transformation does not retain accurate retinotopic registration, having both constant and variable components of error. Generally, the constant pattern of distortion appears as a hypermetria for upward saccades and a hypometria for downward movements. Most of the error accumulates during the first 800 msec of memory-contingent delay. The results are interpreted with respect to theories of how spatial information may be coded and transformed.     

Vergence eye movements and visual suppression

Visual sensitivity was measured during vergence eye movements in order to determine whether a suppression of vision similar to that associated with saccades is also present during vergence. Suppression was evaluated psychophysically by determining sensitivity to briefly presented, full-field decrements of light in a Ganzfeld. Subjects were rougly 0.5 log unit less sensitive when stimuli were presented at the beginning of a 2-3 deg convergent or divergent eye movement, than during steady fixation. Thus, the concept of saccadic suppression must be broadened to include visual suppression that also accompanies nonsaccadic eye movements. These results support the hypothesis that vision is affected by signals that accompany initiation of oculomotor activity.     

Saccadic eye movements to flashed targets.

A target presented as a flash in darkness, before, during or after a saccade, elicits a subsequent goal-directed saccade of normal amplitude and appropriate latency. In a flashed target variation of the Wheeless paradigm, “cancellation time” is not observed in circumstances where the first target is believed to be ineffective. Latency is approximately the same whether target steps are synchronized to saccades or not. Little or no processing for a primary saccade occurs before the prior primary saccade.     

Characteristics of smooth eye movements with stabilized targets

Eye movements of two subjects were recorded with a Double Purkinje Image Tracker while they pursued horizontal triangle or ramp stimuli (1, 2, 4 or 8°/sec, p—p 8°). Subjects then attempted to imitate their smooth pursuit eye movement patterns with an electronically-stabilized target or with an afterimage. Next, they attempted to reset the velocity of the stimulus to their memory of the velocity they had previously pursued. The smooth pursuit eye movements of both subjects were very similar. Their attempts to imitate this pattern of eye movements with a stabilized target were only partially successful and subject to large, qualitative individual differences. These differences did not arise from faulty memories of the nature of the pursuit stimuli. Similar results were obtained on the vertical meridian, with a lighted background, and with the stabilized target in eccentric retinal positions. We conclude that stabilization techniques are of dubious value in elucidating properties of the human smooth pursuit subsystem.     

Saccadic eye movements evoked by electrical stimulation of the cat's visual cortex

Eye movements were recorded with the scleral search coil method while striate cortex (area 17) was stimulated in alert cats with their heads fixed. Regardless of where stimulation was applied in the retinotopic map, eye position at the onset of stimulation strongly affected the amplitudes of evoked saccades, but had much less influence on their directions. Application of long stimulus trains evoked repeated saccades at all sites tested. Highly convergent or goal-directed saccades were not observed. Cortically evoked saccades appeared to habituate with repeated stimulation and had higher thresholds and longer latencies that those reported for saccades evoked from the superior colliculus. The directions of cortically evoked saccades generally agreed with those predicted from the retinotopic coordinates of the stimulus sites, but saccade amplitudes were usually lower than expected. It is suggested that these findings are consistent with certain characteristics of eye-head coordination in the cat's normal visual orienting behavior. The results are difficult to reconcile with the hypothesis that goal-directed saccades are a normal response to targets outside the cat's oculomotor range.     

Torsional eye movements and constancy of the visual field

The eye movements during lateral head tilting were investigated. For recording the eye movements we used Yarbus' method of the suction device. It is found that the smooth tilts of the head to the shoulder cause two kinds of the eye movements. The first is a slow rotation of the eye in the direction opposite to one of the head. During this phase the attitude of the eye with reference to the gravity vertical is not altering. The second kind of the eye movement is a saccadic rotation about the visual axis in the direction of the head tilt. The speed of these saccades is about 200°/sec. The sizes of the saccades are in the 0.5–8° range.The two described types of eye rotations correspond to the mechanisms of the orientation constancy in the visual system. In the slow phase the constancy is provided by keeping the eye attitude unchanged. But during the fast phase the saccades change the eye orientation in space and the action of a neuronal mechanism is supposed. This mechanism is to transform the retinae afferentation some proper way.From this point of view the results of the near electrophisiological studies of the constant elements in the visual cortex of unaesthetized cats are cleared.     

Global visual processing for saccadic eye movements

Four experiments are reported in which saccadic eye movements are examined when the eye moves to targets in peripheral vision which consist of two discrete stimuli. It is found that under a variety of conditions, the saccade amplitude is such that the saccade lands at an intermediate position between the stimuli. This result has been termed the global effect and is interpreted as an influence of the global target configuration on the saccade amplitude. It is suggested that this phenomenon may be explicable in terms of activity in an ensemble of cells with large receptive fields. The experiments demonstrate the global effect in the situations of rapid automatic tracking, scanning for target detail and comparison of target configurations. The effect depends in a systematic quantitative manner on the properties of the visual stimuli. This may be loosely described by saying the saccade is directed to the centre of gravity of the target configuration. The saccades are however in general directed closer to the near target than predicted by the geometric centre of gravity. Although the effect appears in a similar form in all the conditions tested, minor differences do occur. It is also shown that the effect shows a dependence on the latency of the saccade, being most pronounced for saccades with short latencfes. It is suggested that this may be a consequence of the dynamics of visual information processing.     

Suppression of visual phosphenes during saccadic eye movements

A loss of visual sensitivity is shown to accompany a saccadic eye movement even under conditions of total darkness. Optical factors are eliminated by the use of electrically produced phosphenes, rather than flashes of light, to test the changes in visual threshold. Separate experiments matching the brightness of electrical phosphenes to real lights allow the threshold changes to be expressed in terms of real light. The results point to the conclusion that a substantial portion of saccadic suppression is neural, rather than optical in its origin. The time course of the suppression is consistent with electrophysiological studies in cat and monkey in which inhibition of nerve impulses occurs over a period that includes, but is considerably longer than the duration of the eye movement. No correlation has yet been shown, however, between the occurrence of saccadic suppression and identifiable features of the human visually evoked occipital responses. Saccades of a given amplitude are found to be slower in total darkness.     

Expansion of visual space after saccadic eye movements

Human subjects reported the perceived two-dimensional location of a visual target that was briefly presented after a saccade in the absence of visual references. Consistent with previous studies, immediately after horizontal saccades, there was a salient horizontal component in mislocalization in the direction opposite to the saccade. However, the horizontal component in mislocalization was not constant and was larger for targets presented further into the visual field contraversive to the saccade. For the same horizontal saccades, the vertical component in mislocalization was also obvious, and it was larger for targets located further away from the saccade trajectory. The saccadic effects resulted in an overall pattern of mislocalization that could be best described as a two-dimensional expansion of visual space. The point of expansive origin was not associated with the saccade goal, but was shifted from the saccade goal in the direction of the saccade. These results suggest that spatial information processing at the time of saccades reflects topographic interactions between neural activations from saccade execution and the visual target. The configuration of mislocalized positions of single point stimuli along a line was not comparable to the pattern of non-veridical motion perception described by Park, Lee & Lee (2001), indicating that spatial mislocalization and non-veridical motion perception after saccades are independent phenomena.     

Changes in visual motion perception before saccadic eye movements

Execution of a saccadic eye movement influences subsequent motion perception [Park, J., Lee, J., & Lee, C. (2001). Non-veridical visual motion perception immediately after saccades. Vision Research, 41, 3751-3761]. In the current study, we determined the pattern of perceptual changes for visual motion presented before saccades. The accuracy of judging the direction of a moving target was variable depending on the direction of target motion. Based on the pattern of judgment errors, the direction associated with no error, or DNE, could be defined. When a moving target was seen by stationary eyes, the DNE was roughly vertical, and the perceptual judgment for adjacent directions was biased away from the vertical direction. When the same visual motion was seen before horizontal saccades, the DNE shifted in the direction of the impending saccade, and the perceptual judgment of adjacent directions was shifted away from the new DNE, thus, shifting the perceived direction of the vertical in the direction opposite to the saccade. These changes improved the accuracy of direction judgment for visual motion in the visual field ipsiversive to impending saccades. In addition to shift of the DNE, perceptual judgment for oblique directions became near veridical before saccades, which we call the anti-oblique effect. These results suggest that motion perception is dynamically and anisotropically modulated at the time of saccades, and the DNE shift may be a part of processes dynamically reallocating computational resources, improving perceptual performance in advance for sensory events to be acquired by impending saccades.     

Smooth anticipatory eye movements alter the memorized position of flashed targets

Briefly flashed visual stimuli presented during smooth object- or self-motion are systematically mislocalized. This phenomenon is called the "flash-lag effect" (Nijhawan, 1994). All previous studies had one common characteristic, the subject's sense of motion. Here we asked whether motion perception is a necessary condition for the flash-lag effect to occur. In our first experiment, we briefly flashed a target during smooth anticipatory eye movements in darkness and subjects had to orient their gaze toward the perceived flash position. Subjects reported to have no sense of eye motion during anticipatory movements. In our second experiment, subjects had to adjust a cursor on the perceived position of the flash. As a result, we show that gaze orientation reflects the actual perceived flash position. Furthermore, a flash-lag effect is present despite the absence of motion perception. Moreover, the time course of gaze orientation shows that the flash-lag effect appeared immediately after the egocentric to allocentric reference frame transformation.     

Directionalization of visual targets during involuntary eye movement

Perceived visual direction with respect to one's self (egocentric direction) depends upon the retinal location of a target's image (its oculocentric direction) and concurrent information about the position of the eyes in the head. Information about eye position is presumably obtained from efference copy signals. However, in order to explain illusory target motion that can occur during reflexive eye movements (e.g., post-rotary nystagmus), these signals have been suggested to accompany only voluntary oculomotor responses. In the experiment reported here, manual pointing was used to assess the perceived direction of targets flashed during optokinetic afternystagmus, an involuntary movement of the eyes that occurs in darkness following optokinetic stimulation. Egocentric directionalization was essentially veridical, indicating that accurate eye position information is available during optokinetic afternystagmus. A model is proposed that accounts for illusory target motion during involuntary oculomotor responses by the cancellation of efference copy information about these eye movements with signals of oppositely directed head motion.     

Scene context guides eye movements during visual search

How does scene context guide search behavior to likely target locations? We had observers search for scene-constrained and scene-unconstrained targets, and found that scene-constrained targets were detected faster and with fewer eye movements. Observers also directed more initial saccades to target-consistent scene regions and devoted more time to searching these regions. However, final checking fixations on target-inconsistent regions were common in target-absent trials, suggesting that scene context does not strictly confine search to likely target locations. We interpret these data as evidence for a rapid top-down biasing of search behavior by scene context to the target-consistent regions of a scene.     

Short-term predictive changes in the dynamics of disparity vergence eye movements

Repetitive stimulation of the disparity vergence system to large convergent step stimuli has been shown to increase the dynamics of subsequent responses to smaller step stimuli. Here we show that decreases in the dynamics of both disparity convergence and divergence eye movements can be induced using a frequently occurring small amplitude conditioning stimulus to modify responses to a larger, occasionally presented test stimulus. In one experiment, a simple conditioning stimulus consisting of repetitive 1 degrees step stimuli was used to modify the dynamic vergence response to an occasional 4 degrees step test stimulus. An experimental trial consisted of three phases: baseline, conditioning, and recovery. The baseline and recovery phases used only the 4 degrees test stimuli. The dynamic characteristics of the responses to test stimuli were quantified by measuring the magnitude of the peak velocity. A statistically significant change was observed between the dynamics of conditioned responses compared to baseline and recovery responses indicting modification by the conditioning stimuli. During recovery, the response dynamics returned to levels near baseline levels showing that the decrease in response dynamics was caused by the conditioning stimulus, not fatigue. Another experiment showed that the response dynamics to large stimuli could be decreased whereas the dynamics of small stimuli could be increased by the same intermediate conditioning stimulus. Other experiments suggest that the modifications are due to a predictive mechanism. The results indicate that the dynamics of disparity vergence eye movements are malleable and depend to some extent on the amplitude of preceding stimuli.