Biomedical subjects

I T Hooge

Publications and source records attributed to I T Hooge.

8 recordsLinked to original sources

PubMed2001

Systematic eye movements do not account for the perception of motion during attentive tracking.

It has been suggested that attention can disambiguate stimuli that have equal motion energy in opposite directions (e.g. a counterphasing grating), such that a clear motion direction is perceived. The direction of this movement is determined by the observer and can be changed at will. Assuming that the responses of front-end motion detectors are equal for the two opponent directions, it has been proposed that the unambiguous motion perceived with attentive tracking arises from an independent mechanism that monitors the shifts of attention directed to the moving feature of interest. However, while perceiving motion under attentive tracking conditions, observers often report a strong impression that they are making eye movements. In this study, we investigated whether systematic eye movements are present during attentive tracking and, as a result, could be responsible for the subjective experience of movement. We had observers track an object in smooth motion, apparent motion and ambiguous motion, either with eye movements or with attention. The results show that there are negligible eye movements during attentive tracking, which are neither systematic nor correlated with the stimulus. Given that neither eye movements nor retinal image motion can account for subjectively perceived motion, as well as the absence of any other plausible explanation, we find it tempting evidence for an earlier suggestion that the percept of movement must arise from a specialized mechanism.

Attention↗

PubMed2001

Perceiving slant about a horizontal axis from stereopsis.

Rotating a surface about a horizontal axis alters the retinal horizontal-shear disparities. Opposed torsional eye movements (cyclovergence) also change horizontal shear. If there were no compensation for the horizontal disparities created by cyclovergence, slant estimates would be erroneous. We asked whether compensation for cyclovergence occurs, and, if it does, whether it occurs by use of an extraretinal cyclovergence signal, by use of vertical-shear disparities, or by use of both signals. In four experiments, we found that compensation is nearly veridical when vertical-shear disparities are available and easily measured. When they are not available or easily measured, no compensation occurs. Thus, the visual system does not seem to use an extraretinal cyclovergence signal in stereoscopic slant estimation. We also looked for evidence of an extraretinal cyclovergence signal in a visual direction task and found none. We calculated the statistical reliabilities of slant-from-disparity and slant-from-texture estimates and found that the more reliable of the two means of estimation varies significantly with distance and slant. Finally, we examined how slant about a horizontal axis might be estimated when the eyes look eccentrically.

Convergence, Ocular↗

PubMed2000

Inhibition of saccade return (ISR): spatio-temporal properties of saccade programming.

Tasks such as reading or visual search consist of series of saccades. We have investigated to what extent saccades that are made within a series of self-paced movements are influenced by preceding movements. The present paper concerns an analysis of the duration of the fixations preceding saccades. We tested human subjects in a paradigm where they had to fixate two to four targets in a fixed order as fast as they could. We found that fixations before so-called 'return saccades' (saccades returning to the previously fixated position) are considerably longer (up to 40%) than other fixations. This phenomenon, which we call 'Inhibition of Saccade Return' (ISR), is present when return and regular saccades are mixed in one trial, and seems to be reset after each saccade. ISR is strongest at the previously fixated target, and decreases gradually from there. The radius of the area where ISR is found is about 4 degrees. The relation between ISR and 'Inhibition of Return' of spatial attention [Posner & Cohen, 1984] is discussed, as well as the neurophysiological basis of ISR.

Adult↗

PubMed2000

Visually evoked cyclovergence and extended listing's law.

Cyclovergence is a simultaneously occurring cyclorotation of the two eyes in opposite directions. Cyclovergence can be elicited visually by opposite cyclorotation of the two eyes' images. It also can occur in conjunction with horizontal vergence and vertical version in a stereotyped manner as described by the extended Listing's law (or L2). We manipulated L2-related and visually evoked cyclovergence independently, using stereoscopic images of three-dimensional (3D) scenes. During pursuit in the midsagittal plane, cyclovergence followed L2. The amount of L2-related cyclovergence during pursuit varied between subjects. Each pursuit trial was repeated three times. Two of the three trials had additional image rotation to visually evoke cyclovergence. We could separate the L2-related and visual components of cyclovergence by subtraction of the cyclovergence response in matched trials that differed only in the image rotation that was applied during pursuit. This indicates that visual and L2-related contributions to cyclovergence add linearly, suggesting the presence of two independent systems. Visually evoked cyclovergence gains were characteristic for a given subject, little affected by visual stimulus parameters, and usually low (0.1-0.5) when a static target was fixated. Gain and phase lag of the visually evoked cyclovergence during vertical pursuit was comparable with that during fixation of a static target. The binocular orientations are in better agreement to orientations predicted by L2 then would be predicted by nulling of the cyclodisparities. On the basis of our results, we suggest that visually driven and L2-related cyclovergence are independent of each other and superimpose linearly.

Adult↗

PubMed1999

Visual search of heading direction.

When we move along we frequently look around. How quickly and accurately can we gaze in the direction of heading? We studied the temporal aspects of heading perception in expanding and contracting patterns simulating self-motion. Center of flow (CF) eccentricity was 15 degrees. Subjects had to indicate the CF by making a saccade to it. A temporal constraint on the response time was introduced, because stimuli were presented briefly (1 s). On average, subjects needed two saccades to find the CF. Initial saccades covered about 50-60% of the distance between the fixation point and the CF. Subjects underestimated the eccentricity of the CF. The systematic radial error ranged from -2.4 degrees to -4.9 degrees. The systematic tangential error was small (about 0.5 degree). The variable radial error ranged from 2.7 degrees to 4.6 degrees. We found a relation between saccade onset time and saccade endpoint error. Saccade endpoint error decreased with increasing saccade onset time, suggesting that saccades were often fired before the heading processing had been completed. From the saccade onset times, saccade endpoint errors and an estimate for the saccadic dead time (interval prior to the saccade during which modification is impossible 70 ms), we estimated the heading processing time (HPT 0.43 s). In three out of four subjects, HPT was longer for trials simulating backward movement than for trials simulating forward movement. For each saccade we determined whether it reduced the distance error. The second saccade reduced the error more effectively per time unit than the initial saccade. On the basis of this finding, we suggest that visual processing that occurs during the saccadic dead time of the first saccade is used in the preparation of the second saccade.

Adult↗

PubMed1999

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.

Adult↗

PubMed1998

Adjustment of fixation duration in visual search.

To investigate whether fixation durations are adjusted to the duration of a foveal analysis task, we designed a search task in which each stimulus element yielded information about the position of the target. We asked subjects to look for the target by making eye movements in the direction indicated by each stimulus element. We explicitly asked the subjects to make the eye movements in the correct direction, but they did not always do this. They made only 65-80% of the eye movements in directions indicated by the stimulus elements. From these results we conclude that fixation durations are not solely determined by the immediate visual stimulus and that subjects encounter difficulties when trying to increase fixation durations to values that would enable them to direct saccades accurately. In a second experiment we shortened the presentation time in order to provide an incentive for the subjects to speed up search. Shortening the presentation time did not affect fixation duration. Therefore, we suggest that fixation duration is controlled by a mechanism that uses estimations of the foveal analysis time of previous fixated stimulus elements.

Adult↗

PubMed1996

Control of fixation duration in a simple search task.

To obtain insight into the control of fixation duration during visual search, we had 4 subjects perform simple search tasks in which we systematically varied the discriminability of the target. The experiment was carried out under two conditions. Under the first condition (blocked), the discriminability of the target was kept constant during a session. Under the second condition (mixed), the discriminability of the target varied per trial. Under the blocked condition, fixation duration increased with decreasing discriminability. For 2 subjects, we found much shorter fixation durations in difficult trials with the mixed condition than in difficult trials with the blocked condition. Overall, the subjects fixated the target, continued to search, and then went back to the target in 5%-55% of the correct trials. In these trials, the result of the analysis of the foveal target was not used for preparing the next saccade. The results support a preprogramming model of the control of fixation duration. In a simple search task, control of fixation duration appears to be indirect.

Adult↗