Sequential stereopsis: a simple demonstration

Most people strongly prefer to use back-and-forth eye movements in order to discriminate 3-dimensional distances among targets that are widely separated from each other in direction. This viewing strategy permits sequential stereopsis: a comparison between the foveally-seen pre-saccadic disparity of one target with post-saccadic disparity of the other. This note describes a simple and qualitatively compelling demonstration of the usefulness of sequential stereopsis, in a situation in which classical stereopsis, with steady fixation, is greatly degraded. Targets of high-spatial-frequency texture are used, with details that can be resolved foveally before and after saccades, but that are unresolvable in peripheral vision. Back-and-forth eye movements between such textured targets, separated by 8–10 deg from each other, led to estimates of threshold that averaged less than 45 sec arc disparity (corresponding to about 0.18% of viewing distance): some of the best performances ever reported for targets so widely separated.     

Attenuation of perceived motion smear during the vestibulo-ocular reflex

Previous studies indicated that less motion smear is perceived when a physically stationary target is presented during voluntary eye movements than when similar retinal-image motion occurs during steady fixation. In this study, we assessed whether the perception of motion smear is attenuated also during the involuntary vestibulo-ocular reflex (VOR). Normal observers matched the length of perceived smear in two experimental conditions that were designed to produce similar trajectories of retinal image motion. In the fixation condition, a small bright target was presented for a duration of 50-200 ms in rightward or leftward motion, while the observer remained stationary and maintained fixation. In the VOR condition, the target moved along with the observer, who underwent full-body rotation around a vertical axis in darkness. Horizontal eye movement recordings during VOR trials allowed us to calculate the velocity of retinal image motion on each VOR trial. The principal result was that the extent of perceived motion smear was significantly less during VOR than fixation trials, particularly for target durations of 100 ms or longer. These findings support the conclusion that extra-retinal signals during the involuntary VOR contribute to a reduction of perceived motion smear.     

Psychophysical measurement of eye drifts and tremor by dichoptic or monocular vernier acuity

If the two segments of a vernier target are presented to different eyes, i.e. dichoptically, thresholds are three to four times higher than with presentation to the same eye. This increase in thresholds is mainly due to uncorrelated movements of both eyes, such as tremor and drifts, that occur even under steady fixation. The psychophysically measured thresholds allow one to calculate an upper estimate for the amplitudes of uncorrelated eye movements during fixation. This estimate matches the best results from direct eye position recording, with the calculated mean amplitude of eye tremor corresponding to roughly one photoreceptor diameter. The combined amplitude of both correlated and uncorrelated eye movements was also measured by delaying one segment of the vernier relative to its partner under monocular or dichoptic conditions. Fixation proved to be relatively stable, and trained observers could sustain eye position within a few min arc.     

Eye movement perimetry in glaucoma

Present-day computerized perimetry is often inaccurate and unreliable owing to the need to maintain central fixation over long periods while repressing the normal response to presentation of peripheral stimuli. We tested a new method of perimetry that does not require prolonged central fixation. During this test eye movements were encouraged on presentation of a peripheral target. Twenty-three eyes were studied with an Octopus perimeter, with a technician monitoring eye movements. The sensitivity was 100% and the specificity 23%. The low specificity was due to the technician's inability to accurately monitor small eye movements in the central 6 degrees field. If small eye movements are monitored accurately with an eye tracker, eye movement perimetry could become an alternative method to standard perimetry.     

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.     

Changes in orientation discrimination at the time of saccadic eye movements

Perceptual performance has been known to change around the time of saccadic eye movement. In the current study, we measured the accuracy and sensitivity of orientation discrimination of bar stimuli presented during fixation and before saccadic eye movements. Human participants compared the orientations of the test and reference bar stimuli with the head erect in a two-interval forced choice task. For the targets presented during steady fixation, the accuracy and sensitivity of orientation discrimination were better near the cardinal than oblique axes, a perceptual anisotropy known as the oblique effect. For the targets presented during the 100 ms interval immediately before a saccade was executed, the anisotropy decreased mainly due to reduction in sensitivity for cardinal orientations. Directing attention to the goal location of the impending saccade emulated the saccadic effects on orientation discrimination for the targets at saccadic goal, suggesting that the saccadic effects on orientation discrimination are partly mediated by the shift of spatial attention that accompanies the saccade. These results were in line with the anti-oblique effect that perceptual judgment of motion direction along the oblique angle becomes relatively accurate for motion targets presented before saccadic eye movements [Lee, J., & Lee, C. (2005). Changes in visual motion perception before saccadic eye movements. Vision Research, 45(11), 1447-1457].     

Scanning eye movements in homonymous hemianopia documented by scanning laser ophthalmoscope retinal perimetry.

Sparing or partial recovery of visual fields in hemianopic patients is frequently difficult to document. This is because when testing large field losses, the standard automated or manual visual field testing systems have limited fixation controls. Measured visual field recovery in these cases may not be real but instead may be due to an artifact such as scanning eye movement. This article illustrates a way to separate the actual visual field sparing from scanning eye movement artifact by using perimetry testing with the scanning laser ophthalmoscope (SLO). During the SLO perimetry, the examiner has a direct and magnified view of the retinal fixation locus. This direct view allows for the added ability to monitor the fixation stability during target presentation. When eye movements larger than 1 degrees are noted, the examiner can repeat the trial. During static perimetry, our SLO records the retinal position of the fixation target at the end of the stimulus presentation and corrects scanning eye movements that occur during stimulus presentation. These special features enable us to identify when the apparent sparing of the visual field is due to the artifact of scanning. To demonstrate this, we selected the records of four hemianopic patients whose fields were examined by both standard perimetry and the SLO. We then compared the clinical visual fields with the SLO perimetry fields. One of the patients had a complete homonymous hemianopia on both the clinical perimetry and the SLO perimetry. A second patient was found by the SLO to have unstable fixation during testing. The SLO perimetry revealed that the apparent spared fields seen in standard perimetry were the result of eye scanning and not an actual enlargement of the visual field. Two other patients were confirmed by the SLO findings to have valid partial recovery of the visual field, one with and one without scanning eye movements. The advantages and limitations of SLO perimetry in analyzing hemianopic field sparing are discussed.     

What is the best fixation target? The effect of target shape on stability of fixational eye movements

People can direct their gaze at a visual target for extended periods of time. Yet, even during fixation the eyes make small, involuntary movements (e.g. tremor, drift, and microsaccades). This can be a problem during experiments that require stable fixation. The shape of a fixation target can be easily manipulated in the context of many experimental paradigms. Thus, from a purely methodological point of view, it would be good to know if there was a particular shape of a fixation target that minimizes involuntary eye movements during fixation, because this shape could then be used in experiments that require stable fixation. Based on this methodological motivation, the current experiments tested if the shape of a fixation target can be used to reduce eye movements during fixation. In two separate experiments subjects directed their gaze at a fixation target for 17 s on each trial. The shape of the fixation target varied from trial to trial and was drawn from a set of seven shapes, the use of which has been frequently reported in the literature. To determine stability of fixation we computed spatial dispersion and microsaccade rate. We found that only a target shape which looks like a combination of bulls eye and cross hair resulted in combined low dispersion and microsaccade rate. We recommend the combination of bulls eye and cross hair as fixation target shape for experiments that require stable fixation.     

Properties of anticipatory vergence responses

PURPOSE: To characterize the dynamic properties of vergence eye movements made between near and far targets that were alternately illuminated with predictable timing. METHODS: Using the magnetic search coil technique, eye movements were measured in 10 normal subjects as they shifted their point of fixation between a near green LED and a distant red laser spot, both aligned on subjects' midlines. Targets were alternately illuminated every 1.25 sec. RESULTS: All subjects showed some anticipatory responses, consisting of vergence movements that preceded target jumps, accompanied by a small saccade. Group median anticipatory interval was 191 msec. Responses preceded target motion in 83% of divergence trials, and 70% of convergence trials. The velocities of both pre- and persaccadic components of anticipatory vergence responses were greater when the near target was positioned at 20-cm compared with at 36 cm. In control experiments, in which target presentation was unpredictable, vergence movements preceded stimuli in only approximately 2% of trials; for the group, vergence responses followed target presentation after a median interval of 183 msec. To determine whether anticipatory vergence movements depended on a memory of prior stimuli, trials were run in four subjects in which oddball stimuli required a different-sized vergence movement. Most responses to oddball stimuli were not significantly different from responses to the preceding stimuli. CONCLUSIONS: Anticipatory vergence movements occur commonly in response to predictable stimulus movements in depth, but uncommonly when the timing of stimulus presentation is not predictable. The speed of anticipatory vergence movements is affected by stimulus amplitude. Properties of these movements are influenced by prior vergence responses, indicating that they depend on working memory.     

Influence of visual perception on spatial coding of saccadic eye movements and fixation

The Müller-Lyer illusion produces differences in visual length perception when lines of equal length are presented as double-arrows with inward-pointing and outward-pointing arrowheads, respectively. The aim of this study was to investigate the influence of length perception on the amplitude of saccadic eye movements and on postsaccadic fixation positions. Video recordings of eye movements were obtained from 35 healthy volunteers (age: 20-30 years) while they scanned the endpoints of Müller-Lyer targets. The targets were presented in horizontal and vertical orientations with an arrow length of 15°. Fixation periods between the saccades were adjusted at 1 s and 4 s. The final saccadic amplitudes between the endpoints of Müller-Lyer figures reflect the visually perceived length differences: lines with outward-pointing arrowheads were scanned with smaller saccades than lines with inward-pointing arrowheads. This bias in final saccadic amplitude was equal in both the horizontal and vertical target orientations as well as for 1-s and 4-s fixation periods. Postsaccadic drift after initial saccadic movements reduced the effect of the Müller-Lyer illusion depending on arrowhead and target orientation: outward-pointing horizontal arrows caused hypometric pulses, while inward-pointing vertical arrows caused hypermetric pulses. The results of this study demonstrate that spatial coding of both saccadic eye movements and fixation is strongly influenced by visual perception. While the influence of the Müller-Lyer illusion on final saccadic amplitudes did not depend on target orientation or fixation period, an additional pulse-step mismatch appeared in the horizontal (hypometria) and vertical (hypermetria) direction.     

Mislocalization of peripheral targets during fixation

To investigate the effect of the visual stimulus configuration on localization when oculomotor performance is excluded, we evaluated the errors made when subjects compare the horizontal location of two sequentially presented peripheral targets while looking at a visual or memorized fixation spot. Eye position was monitored by means of an infrared eye tracker. Significant localization errors were observed. As long as the fixation spot stayed on or off during the entire presentation time of both peripheral targets, the localization error did not depend on the presence or absence of the fixation spot. A significant change in the localization error was observed only if the fixation spot was presented together with the first peripheral target but disappeared before the presentation of the second one. The localization error did not depend on: (1) the visual asymmetry (unilateral versus bilateral target presentation); (2) the distribution of visual attention (cued versus non-cued test location); or (3) the time interval between the two targets. These results suggest that the mislocalization observed during fixation is partially due to a mismatch between egocentric and exocentric localization mechanisms.     

Vertical object motion during horizontal ocular pursuit: compensation for eye movements increases with presentation duration

Smooth pursuit eye movements change the retinal image motion of objects in the visual field. To enable an observer to perceive the motion of these objects veridically, the visual system has to compensate for the effects of the eye movements. The occurrence of the Filehne-illusion (illusory motion of a stationary object during smooth pursuit) shows that this compensation is not always perfect. The amplitude of the illusion appears to decrease with increasing presentation durations of the stationary object. In this study we investigated whether presentation duration has the same effect when an observer views a vertically moving object during horizontal pursuit. In this case, the pursuit eye movements cause the perceived motion path to be oblique instead of vertical; this error in perceived motion direction should decrease with higher presentation durations. In Experiment 1, we found that the error in perceived motion direction indeed decreased with increasing presentation duration, especially for higher pursuit velocities. The results of Experiment 2 showed that the error in perceived motion direction did not depend on the moment during pursuit at which the stimulus was presented, suggesting that the degree of compensation for eye movements is constant throughout pursuit. The results suggest that longer presentation durations cause the eye movement signal that is used by the visual system to increase more than the retinal signal.     

Voluntary selection of the target for smooth eye movement in the presence of superimposed, full-field stationary and moving stimuli

Prior work has shown that smooth eye movements in the presence of both stationary and moving stimuli are determined, at least in part, by the voluntary selection of either the stationary or the moving stimulus as the target for smooth eye movements. The effectiveness of voluntary selection in eliminating the influence of the stimuli not selected (i.e. backgrounds) on smooth eye movement is not known because prior studies used targets and backgrounds with different physical characteristics. Thus, effects of voluntary selection were confounded with the relative strength of target and background as stimuli for smooth eye movements. We measured eye movements (resolution 1') of two highly-experienced eye movement subjects with a target and background with the same physical characteristics: two, identical, full-field, superimposed patterns of randomly-positioned dots (1 dot/deg2 or 8 dots/deg2). One field was stationary and the other moved at 70.2 minarc/sec. The effect of the moving background on smooth eye movements when the stationary field was the target, and the effect of the stationary background on smooth eye movements when the moving field was the target was negligible (0-4% for one subject; 0-2% for the other). The influence of the background on smooth eye movements was affected by a six-fold reduction in the intensity of either the target or background, but effects of such intensity changes were small and different for each subject. Taken together, these results show that the effectiveness of voluntary selection in eliminating the influence of background stimuli on smooth eye movements can be virtually complete. Any observed influence of the background--however small--can be attributed to voluntary factors (e.g. subjects' failure to apply sufficient effort or attention) rather than to the operation of an involuntary mechanism that automatically integrates velocity information from target and background. The attention and effort required to ensure that voluntary selection is perfect may impair the accuracy of psychophysical judgments made about the background.     

Two stages of programming eye gaze shifts in 3-D space

Accurate saccadic and vergence eye movements towards selected visual targets are fundamental to perceive the 3-D environment. Despite this importance, shifts in eye gaze are not always perfect given that they are frequently followed by small corrective eye movements. The oculomotor system receives distinct information from various visual cues that may cause incongruity in the planning of a gaze shift. To test this idea, we analyzed eye movements in humans performing a saccade task in a 3-D setting. We show that saccades and vergence movements towards peripheral targets are guided by monocular (perceptual) cues. Approximately 200 ms after the start of fixation at the perceived target, a fixational saccade corrected the eye positions to the physical target location. Our findings suggest that shifts in eye gaze occur in two phases; a large eye movement toward the perceived target location followed by a corrective saccade that directs the eyes to the physical target location.     

Exploring the third dimension with eye movements: better than stereopsis

Eye movements are usually presumed to be irrelevant for (or detrimental to) stereoacuity. When targets of interest are not adjacent, however, better discrimination of distance can be achieved by looking back and forth between them. In order to exclude ordinary stereopsis and examine this viewing strategy in isolation, judgements of apparent equidistance have been obtained for pairs of small targets separated horizontally by the angular spacing that corresponds to the fovea-to-blind-spot distance. Precise, stereopsis-like evaluations of relative distance can be made by fixating each of those targets in turn, even if they are not simultaneously presented but are instead shown in alternation. Sequential comparisons of stimuli are thus involved in this form of distance discrimination, but direct utilization of oculomotor information (vergence) is rendered unlikely because very brief target presentation is sufficient. Hence, the evidence argues for "sequential stereopsis": comparisons of the disparities of targets, both seen foveally, before and after saccades. This interpretation makes stringent demands on oculomotor coordination during saccades, but measurements of vergence "noise" indicate that this requirement can probably be fulfilled.     

Vernier discrimination with sequentially-flashed lines: Roles of eye movements,retinal offsets and short-term memory

The two lines of a vertical vernier target were sequentially flashed (2 msec/line; 0.2–800 msec dark interval) while the subject tried to maintain an earlier position of fixation in total darkness. Vernier discrimination deteriorates with increasing dark interval: acuity threshold increases from 29″–26′; constant error generally increases. The errors are a joint result of: (1) displacements between the retinal images of the two lines that are not present in the target; these are produced by involuntary eye movements in the dark interval and (2) deterioration of memory for spatial location signalled by the first line flashed. The entire contribution of involuntary eye movements is due to its effect on retinal offset; extraretinal signals related to involuntary changes in eye position during the dark interval do not influence the discrimination at any dark interval. A systematic influence of eye-movement-produced retinal offset is measured even for the shortest dark interval of 0.2 msec (2.2 msec between centers of flashes). Memory deterioration predominates in determining both constant and variable errors with brief dark intervals; eye-movement-produced retinal offset increases in significance at longer ones. The course of memory deterioration is consistent with a random walk model. At the shortest dark interval acuity was affected by the vertical separation between the two lines of the vernier target; at longer dark intervals this influence was obscured by the eye-movement-produced increases in retinal distance and memory deterioration.     

Effects of uncertainty and target displacement on the latency of express saccades in man

Saccadic eye movements generated in response to a gap paradigm in which the fixation light spot was extinguished 200 ms prior to presentation of the target light spot showed appreciably shorter latencies than for the overlap paradigm in which the target light spot was presented 200 ms prior to extinction of the fixation light spot. When there was unpredictability in the direction of target presentation, i.e., to the left or right of the fixation light spot, the gap paradigm evoked mainly fast regular saccades of peak latency of 155 ms with relatively few express saccades which were defined as having latencies of less than 120 ms. By contrast, when the target always appeared to the right, a substantial population of express saccades with peak latency 95 ms was now generated. There was also a change in the relationship between saccadic latency and target angular displacement which covered the range 5-35 degrees . With the overlap paradigm and unpredictability of target direction, the latencies of the slow regular saccades increased markedly with target angular displacement. This was not the case with the same target direction when the latency of slow regular, fast regular, and express saccades remained constant with increasing target angular displacement. This indicates for targets appearing in the same hemifield that the ocular motor system operates with shortest latency irrespective of target angular displacement.     

Peripheral vision and oculomotor control during visual search.

The present study concerns the dynamics of multiple fixation search. We tried to gain insight into: (1) how the peripheral and foveal stimulus affect fixation duration; and (2) how fixation duration affects the peripheral target selection for saccades. We replicated the non-corroborating results of Luria and Strauss (1975) ('Eye movements during search for coded and uncoded targets', Perception and Psychophysics 17, 303-308) (saccades were selective), and Zelinsky (1996) (Using eye movements to assess the selectivity of search movements. Vision research 36(14), 2177-2187) (saccades were not selective), by manipulating the critical features for peripheral selection and discrimination separately. We found search to be more selective and efficient when the selection task was easy or when fixations were long-lasting. Remarkably, subjects did not increase their fixation durations when the peripheral selection task was more difficult. Only the discrimination task affected the fixation duration. This implies that the time available for peripheral target selection is determined mainly by the discrimination task. The results of the present experiment suggest that, besides the difficulty of the peripheral selection task, fixation duration is an important factor determining the selection of potential targets for eye movements.     

The role of eye movements in motion detection

The roles of small eye movements of fixation, and of different kinds of background in motion detection were studied. Minimum detectable displacement for a luminous line oscillating either in a blank field or in the presence of three types of background was measured under two viewing conditions: normal, when eye movements generate normal movements of the image on the retina, and stabilized when these retinal image movements were nearly eliminated. It was demonstrated that eye movements enhance motion detection for a sinusoidally moving target when the target is superimposed on a patterned background; they are detrimental when there is no background. In addition, it was found that the function relating threshold amplitude to frequency of movement is band-pass when the image is stabilized or when the bar moves on a blank field, and is more low-pass when both the background and the test target are subject to the effects of eye movements.     

Effects of fixational saccades on response timing in macaque lateral geniculate nucleus

Even during active fixation, small eye movements persist that might be expected to interfere with vision. Numerous brain mechanisms probably contribute to discounting this jitter. Changes in the timing of responses in the visual thalamus associated with fixational saccades are considered in this study. Activity of single neurons in alert monkey lateral geniculate nucleus (LGN) was recorded during fixation while pseudorandom visual noise stimuli were presented. The position of the stimulus on the display monitor was adjusted based on eye position measurements to control for changes in retinal locations due to eye movements. A method for extracting nonstationary first-order response mechanisms was applied, so that changes around the times of saccades could be observed. Saccade-related changes were seen in both amplitude and timing of geniculate responses. Amplitudes were greatly reduced around saccades. Timing was retarded slightly during a window of about 200 ms around saccades. That is, responses became more sustained. These effects were found in both parvocellular and magnocellular neurons. Timing changes in LGN might play a role in maintaining cortical responses to visual stimuli in the presence of eye movements, compensating for the spatial shifts caused by saccades via these shifts in timing.