Saccadic and psychophysical discrimination of double targets.

Saccades are rapid, conjugate eye movements that shift the foveas onto peripheral targets of interest. However, the initial saccades to a peripheral pair of targets have been reported to direct the foveas between the targets, despite instructions to look at one or the other. This phenomenon, referred to as saccadic averaging, can occur for target separations of 10 degrees or more and is usually interpreted as evidence that the spatial information available to the saccadic system is of inherently poor resolution. We compared the amplitudes of initial saccades to single peripheral targets and to pairs of targets, with and without prior auditory cues that provided unambiguous information about which target represented the saccadic goal. Target duration ranged from 33 to 100 ms, followed by a mask that was intended to limit neural processing time. The results show that supplementary auditory cues reduce the extent of saccadic averaging to target separations as small as 2 degrees. Saccadic averaging is more pronounced for targets of brief duration, consistent with the degradation of psychophysical resolution that occurs for targets of short duration. We conclude that saccadic averaging results in large part because of ambiguity about which of two targets represents the saccadic goal and, to a lesser extent, because of the limited time that is available before the saccade to process target position.     

Spatio-temporal topography of saccadic overestimation of time

Rapid eye movements (saccades) induce visual misperceptions. A number of studies in recent years have investigated the spatio-temporal profiles of effects like saccadic suppression or perisaccadic mislocalization and revealed substantial functional similarities. Saccade induced chronostasis describes the subjective overestimation of stimulus duration when the stimulus onset falls within a saccade. In this study we aimed to functionally characterize saccade induced chronostasis in greater detail. Specifically we tested if chronostasis is influenced by or functionally related to saccadic suppression. In a first set of experiments, we measured the perceived duration of visual stimuli presented at different spatial positions as a function of presentation time relative to the saccade. We further compared perceived duration during saccades for isoluminant and luminant stimuli. Finally, we investigated whether or not saccade induced chronostasis is dependent on the execution of a saccade itself. We show that chronostasis occurs across the visual field with a clear spatio-temporal tuning. Furthermore, we report chronostasis during simulated saccades, indicating that spurious retinal motion induced by the saccade is a prime origin of the phenomenon.     

The relation between saccadic eye movements and electrooculogram during light and dark adaptations]

The relation between the amplitudes of saccadic eye movements and the resting potential changes detected around human eyeballs under light and dark adaptations was clarified by means of simultaneous measurements of electrooculogram (EOG) and eye movements. The authors previously reported that the potential changes are proportional to the horizontal saccadic eye movements within +/- 15 degrees (visual angle) under conditions of the room light of approximately 60 lux. There are few reliable data indicative of the presence of the above relations under light and dark adaptations. The result of the simultaneous measurements of EOG and eye movements during the light and dark adaptation at 2-minute intervals in 9 normal subjects showed that the resting potential changes were in direct proportion of the amplitudes of the saccadic eye movements at any time within +/- 20 degrees. In these experiments, the eye positions were monitored with the eye mark recorder during the eye movements. In case the eye positions excursioned from a target, the exact EOG was calculated by measuring eye positions and correcting the EOG errors.     

Contrast dependency of sacadic compression and suppression

In the occurrence of a saccadic eye movement vision becomes suppressed. Supra-threshold visual stimuli that are briefly presented at that time become perceptually compressed towards the saccade target (saccadic compression) and shifted in saccade direction (saccadic shift). We show that the strength of saccadic compression, like the strength of saccadic suppression, varies with stimulus contrast. Low contrast stimuli lead to stronger compression than high contrast stimuli. The similarity of contrast dependence and time course suggests that saccadic compression is related to saccadic suppression. Because the saccadic shift did not depend on contrast we suggest that shift and compression are different effects.     

Influence of saccades on manual reactions--a reaction time and VEP study

Experiments were performed to investigate the retardation of motor reactions, triggered immediately after saccadic eye movements, by comparing VEP latencies and manual reaction times. As visual stimuli grating patterns of different spatial frequencies were used. They were presented during continuous fixation ("resting eye" condition) as well as shortly after a saccade ("saccadic" condition) with different onset delays (25, 50, 100, 150 msec), in order to determine the influence of saccades on VEP latencies and on manual reaction times. Compared with the reaction times in the "resting eye" condition, the postsaccadic reaction times were much prolonged whereas the respective VEP latencies are almost unchanged. Thus an inhibitory interaction of different motor responses (i.e. saccadic eye movements and manual reactions in this study) at higher levels of the afferent system or at the motor control site is postulated. This result has been confirmed for stimuli of different spatial frequencies.     

Contrast sensitivity during the initiation of smooth pursuit eye movements

Eye movements challenge the perception of a stable world by inducing retinal image displacement. During saccadic eye movements visual stability is accompanied by a remapping of visual receptive fields, a compression of visual space and perceptual suppression. Here we explore whether a similar suppression changes the perception of briefly presented low contrast targets during the initiation of smooth pursuit eye movements. In a 2AFC design we investigated the contrast sensitivity for threshold-level stimuli during the initiation of smooth pursuit and during saccades. Pursuit was elicited by horizontal step-ramp and ramp stimuli. At any time from 200 ms before to 500 ms after pursuit stimulus onset, a blurred 0.3 deg wide horizontal line with low contrast just above detection threshold appeared for 10 ms either 2 deg above or below the pursuit trajectory. Observers had to pursue the moving stimulus and to indicate whether the target line appeared above or below the pursuit trajectory. In contrast to perceptual suppression effects during saccades, no pronounced suppression was found at pursuit onset for step-ramp motion. When pursuit was elicited by a ramp stimulus, pursuit initiation was accompanied by catch-up saccades, which caused saccadic suppression. Additionally, contrast sensitivity was attenuated at the time of pursuit or saccade stimulus onset. This attenuation might be due to an attentional deficit, because the stimulus required the focus of attention during the programming of the following eye movement.     

Spatial and temporal effects of spatial attention on human saccadic eye movements.

A spatial cueing paradigm was used to (a) investigate the effects of attentional orienting on spatial and temporal parameters of saccadic eye movements and (b) examine hypotheses regarding the hierarchical programming of saccade direction and amplitude. On a given trial, the subjects were presented with one of three peripheral cues: a "valid" cue provided correct information, a "neutral" cue no information and an "invalid" cue incorrect information about the location of the subsequent target (the cue was valid on about 50% of the trials). 100 or 500 msec after the cue onset (stimulus onset asynchrony, SOA), the eye movement target was presented at one of six possible locations (from -11.25 degrees to +11.25 degrees, at 3.75 degrees spacing). The results showed a significant effect of cueing on saccade latencies at the 100 msec, but no effect at the 500 msec SOA. The cueing benefits were restricted to the cued location; no other locations within the cued or uncued hemi-field were facilitated. The invalid trial latencies showed: (a) no advantage for targets on the same side as the cue relative to the opposite side, (b) no advantage for targets more proximal to the cue and (c) no advantage for targets sharing the same eccentricity as the cue. In a second experiment, subjects responded to the target by giving a simple manual response [simple reaction time (RT)] while keeping the eyes stationary. The results showed a significant cueing effect on simple RTs both at the 100 msec and, in contrast to saccadic eye movements, the 500 msec SOA. There were specific benefits for targets at the cued location relative to other targets within the same and opposite hemi-fields. Further, in contrast to saccades, there was a general advantage for targets within the same ("cued") hemi-field over targets in the opposite hemi-field. These findings suggest that, for saccadic eye movements (overt orienting), direction and amplitude are programmed holistically; whereas in covert orienting (with eyes stationary), motor parameters (e.g. for directed hand movements) are programmed in a more hierarchical fashion.     

Active and passive smooth eye movements: effects of stimulus size and location

We measured active smooth pursuit eye movements and passive smooth eye movements in the open-loop condition as subjects viewed moving stimuli of different sizes and at various retinal loci. Active movements have high gain and relatively large phase lag. Passive movements have lower gain and smaller phase lag, and occur with either foveal or eccentric stimuli. They appear to be similar or identical to optokinetic movements. Although different, active and passive movements show a similar increase in amplitude and phase lag as the size of the stimulus was increased. From these findings we suggest that: (1) The stimuli for the active movements are target position and velocity; (2) the stimulus for passive movements is target velocity; and (3) the active response to target velocity is related, in part, to the passive response and thus is related to optokinesis.     

Variability of eye movements when viewing dynamic natural scenes

How similar are the eye movement patterns of different subjects when free viewing dynamic natural scenes? We collected a large database of eye movements from 54 subjects on 18 high-resolution videos of outdoor scenes and measured their variability using the Normalized Scanpath Saliency, which we extended to the temporal domain. Even though up to about 80% of subjects looked at the same image region in some video parts, variability usually was much greater. Eye movements on natural movies were then compared with eye movements in several control conditions. "Stop-motion" movies had almost identical semantic content as the original videos but lacked continuous motion. Hollywood action movie trailers were used to probe the upper limit of eye movement coherence that can be achieved by deliberate camera work, scene cuts, etc. In a "repetitive" condition, subjects viewed the same movies ten times each over the course of 2 days. Results show several systematic differences between conditions both for general eye movement parameters such as saccade amplitude and fixation duration and for eye movement variability. Most importantly, eye movements on static images are initially driven by stimulus onset effects and later, more so than on continuous videos, by subject-specific idiosyncrasies; eye movements on Hollywood movies are significantly more coherent than those on natural movies. We conclude that the stimuli types often used in laboratory experiments, static images and professionally cut material, are not very representative of natural viewing behavior. All stimuli and gaze data are publicly available at http://www.inb.uni-luebeck.de/tools-demos/gaze.     

Saccadic compression can improve detection of Glass patterns

Around the time of saccadic eye movements, briefly presented stimuli are seen to be displaced from their real positions. The direction of the displacement is always towards the saccadic target, resulting in a compression of visual space (Nature 386 (1997) 598). To examine whether the compression may be used by the visual system to aid performance, we measured sensitivity for detecting horizontal and vertical Glass patterns around the time of saccades. Sensitivity to widely spaced horizontal Glass patterns improved slightly during the period just prior to making a horizontal saccade, while sensitivity to vertical patterns was impaired by the saccade. The results provide further evidence for compression in the direction of the saccade at saccadic onset, and show that it does not only affect the apparent position of visual stimuli, but can also improve performance on a visual task.     

The spatio-temporal tuning of the mechanisms in the control of saccadic eye movements

We compared the spatio-temporal tuning of perception to the mechanisms that drive saccadic eye movements. Detection thresholds were measured for Gabor-targets presented left or right of fixation (4 or 8deg eccentricity), at one of four spatial frequencies (1, 2, 4 or 8cpd) oscillating at one of three temporal frequencies (1, 8 or 16Hz). We then measured saccade latency to each target presented at various multiples of detection threshold. Consistent with previous research, latency decreased as a function of contrast. However, at equal detection performance, we found no systematic difference in saccadic latency and no difference in average oculometric performance (% correct saccade direction) across the different target spatio-temporal frequencies. Furthermore, position error remained fairly constant across all conditions. The results are consistent with the idea that the spatio-temporal signals used for perception are the same as those used by the mechanisms driving saccadic eye movements.     

Changing fixation in the transverse plane at eye level and Hering's law of equal innervation

Binocular eye movements were recorded photoelectrically while observers looked from one target to a second, located in a different direction and at a different distance. When the two targets were “real” with no accommodation convergence mismatch, there was (a) eye movement which included symmetrical vergence and conjugate saccades or (b) accommodative vergence which is known to occur in J. Müller's stimulus situation. Hering's law concerning equal innervation for vergence and saccadic eye movements holds best in two special situations, one in which accommodation and convergence are dissociated and the stimulus configuration is far from the observer, and another which entails the limiting case for real targets where the near target occludes the far target for one eye.     

Metrics of saccade responses to visual double stimuli: two different modes

Earlier studies using visual double stimuli along the horizontal axis have revealed a strong averaging tendency in the saccadic system. This study shows averaging also for equally eccentric double stimuli with a modest difference in direction (delta phi = 30 deg). When the difference is enlarged (delta phi = 90 deg) the response pattern becomes bistable; i.e. the eye jumps near either one or the other stimulus. This bistable response mode is reflected also in saccade amplitude when double stimuli along the horizontal axis have a large difference in eccentricity. It is concluded that the saccadic system response mode to double stimuli depends on interstimulus spacing. Furthermore, both types of response can be shown to exist with double stimuli confined to one visual half-field.     

Attenuation of perceived motion smear during vergence and pursuit tracking

When the eyes move, the images of stationary objects sweep across the retina. Despite this motion of the retinal image and the substantial integration of visual signals across time, physically stationary objects typically do not appear to be smeared during eye movements. Previous studies indicated that the extent of perceived motion smear is smaller when a stationary target is presented during pursuit or saccadic eye movements than when comparable motion of the retinal image occurs during steady fixation. In this study, we compared the extent of perceived motion smear for a stationary target during smooth pursuit and vergence eye movements with that for a physically moving target during fixation. For a target duration of 100 ms or longer, perceived motion smear is substantially less when the motion of the retinal image results from vergence or pursuit eye movements than when it results from the motion of a target during fixation. The reduced extent of perceived motion smear during eye movements compared to fixation cannot be accounted for by different spatio-temporal interactions between visual targets or by unequal attention to the moving test spot under these two types of conditions. We attribute the highly similar attenuation of perceived smear during vergence and pursuit to a comparable action of the extra-retinal signals for disjunctive and conjugate eye movements.     

Binocular eye movements during accommodative vergence

Binocular eye position was monitored by the photoelectric technique during accommodative vergence. Contrary to previous reports indicating that accommodative vergence was a uniocular phenomenon, without exception, binocular accommodative vergence movements were recorded. The total vergence amplitude in the viewing eye was reduced, on the average, by approximately 88% with respect to the vergence movement measured in the covered eye. Some saccadic eye movements that occurred during vergence movements were likewise reduced in amplitude in the viewing eye by up to 20%. Smooth eye movements were utilized to counteract the vergence movement in the viewing eye. This smooth movement alone, or in conjunction with a late saccade, returned the eye to the target and helped to maintain the retinal image of the target coincident with the foveal center for the duration of the accommodative vergence movement. Thus, there appears to be a fixation-holding mechanism which produced a general attenuation of both vergence and some saccadic movements in the viewing eye. Although this control strategy produced violations of Hering's law with respect to the magnitude of the movements in the eyes but not with respect to the direction of the movement, it was implemented in the interest of retaining the target within the sensitive foveal region.     

Idiosyncratic characteristics of saccadic eye movements when viewing different visual environments.

Eye position was recorded in different viewing conditions to assess whether the temporal and spatial characteristics of saccadic eye movements in different individuals are idiosyncratic. Our aim was to determine the degree to which oculomotor control is based on endogenous factors. A total of 15 naive subjects viewed five visual environments: (1) The absence of visual stimulation (i.e. a dark room); (2) a repetitive visual environment (i.e. simple textured patterns); (3) a complex natural scene; (4) a visual search task; and (5) reading text. Although differences in visual environment had significant effects on eye movements, idiosyncrasies were also apparent. For example, the mean fixation duration and size of an individual's saccadic eye movements when passively viewing a complex natural scene covaried significantly with those same parameters in the absence of visual stimulation and in a repetitive visual environment. In contrast, an individual's spatio-temporal characteristics of eye movements during active tasks such as reading text or visual search covaried together, but did not correlate with the pattern of eye movements detected when viewing a natural scene, simple patterns or in the dark. These idiosyncratic patterns of eye movements in normal viewing reveal an endogenous influence on oculomotor control. The independent covariance of eye movements during different visual tasks shows that saccadic eye movements during active tasks like reading or visual search differ from those engaged during the passive inspection of visual scenes.     

Orientation anisotropies in visual search revealed by noise

The human visual system is remarkably adept at finding objects of interest in cluttered visual environments, a task termed visual search. Because the human eye is highly foveated, it accomplishes this by making many discrete fixations linked by rapid eye movements called saccades. In such naturalistic tasks, we know very little about how the brain selects saccadic targets (the fixation loci). In this paper, we use a novel technique akin to psychophysical reverse correlation and stimuli that emulate the natural visual environment to measure observers' ability to locate a low-contrast target of unknown orientation. We present three main discoveries. First, we provide strong evidence for saccadic selectivity for spatial frequencies close to the target's central frequency. Second, we demonstrate that observers have distinct, idiosyncratic biases to certain orientations in saccadic programming, although there were no priors imposed on the target's orientation. These orientation biases cover a subset of the near-cardinal (horizontal/vertical) and near-oblique orientations, with orientations near vertical being the most common across observers. Further, these idiosyncratic biases were stable across time. Third, within observers, very similar biases exist for foveal target detection accuracy. These results suggest that saccadic targeting is tuned for known stimulus dimensions (here, spatial frequency) and also has some preference or default tuning for uncertain stimulus dimensions (here, orientation).     

Visual scene memory and the guidance of saccadic eye movements.

An unresolved question is how much information can be remembered from visual scenes when they are inspected by saccadic eye movements. Subjects used saccadic eye movements to scan a computer-generated scene, and afterwards, recalled as many objects as they could. Scene memory was quite good: it improved with display duration, it persisted over time long after the display was removed, and it continued to accumulate with additional viewings of the same display (Melcher, D. (2001) The persistance of memory for scenes. Nature 412, 401). The occurrence of saccadic eye movements was important to ensure good recall performance, even though subjects often recalled non-fixated objects. Inter-saccadic intervals increased with display duration, showing an influence of duration on global scanning strategy. The choice of saccadic target was predicted by a Random Selection with Distance Weighting (RSDW) model, in which the target for each saccade is selected at random from all available objects, weighted according to distance from fixation, regardless of which objects had previously been fixated. The results show that the visual memory that was reflected in the recall reports was not utilized for the immediate decision about where to look in the scene. Visual memory can be excellent, but it is not always reflected in oculomotor measures, perhaps because the cost of rapid on-line memory retrieval is too great.     

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.     

Investigation into corrective saccadic eye movements for refixation amplitudes of 10 degrees and below.

The frequencies and mean amplitudes of corrective saccadic eye movements have been measured in five subjects for refixational amplitudes of 10° and below. The results indicate a much higher percentage of undershoots than has been previously reported. This is explained on the basis of different types of experimental procedures. It is hypothesized that the frequency and mean amplitude of saccadic errors are dependent upon whether the subject responds solely to the visual stimulus or whether some extra-retinal factor such as knowledge of the target location is involved.