(25/2544) The oculomotor gap effect without a foveal fixation point.

Turning off a fixation point prior to or coincident with the appearance of a visual target reduces the latency of saccades to that target. We investigated this 'gap effect' when subjects fixated a central point or the center of a square formed by four points that were 4, 2 or 1 degree eccentric from the square's center. The fixation anchor vanished 200 ms prior to the appearance of a saccadic target in a Gap condition, coincident with the target's appearance in a 0-Gap condition, or remained on in an Overlap condition. Saccadic reaction time was reduced in the Gap relative to 0-Gap condition irrespective of the type of fixation anchor. However, saccadic reaction time was not reduced in the 0-Gap relative to Overlap condition when the points forming the square had eccentricities of 2 or 4 degrees. Results are interpreted in terms of a partial mediation of the gap effect by fixation cells in the rostral pole of the superior colliculus.  (+info)

(26/2544) Spatial attention and latencies of saccadic eye movements.

Recent theories of visual attention, such as the oculomotor readiness theory of Klein (1980) (Does oculomotor readiness mediate cognitive control of the visual attention. In: R. Nickerson, Attention and performance, Hillsdale: Erlbaum), the premotor theory of Rizzolati (1983) (Mechanisms of selective attention in mammals. In: J.P. Ewart, R.R. Capranica, D.J. Ingle, Advances in vertebrate Neuroethology (pp. 261-297). New York: Plenum) and the sequential attention theory of Henderson (1992) (Visual attention and eye movement control during reading and scene perception. In K. Rayner, Eye movements and visual cognition (260-283). New York: Springer-Verlag), propose a link between shifts in spatial attention and the generation of saccadic eye movements. In this paper we show that a winner-take-all model of spatial attention, combined with a simple model for the link between attention and eye movements, can account for the variation in saccadic latency observed in many oculomotor phenomena. These phenomena include the gap effect (Saslow M.G. (1967). Effects of components of displacement-step stimuli upon latency for saccadic eye movement. Journal of the Optical Society of America, 57, 1024-1029), the effect of target jumps on saccadic latency (Becker W. & Jurgens R. (1979). An analysis of the saccadic system by means of double step stimuli. Vision Research, 19, 967-983), the increase of saccadic latency as target eccentricity drops (Kalesnykas R.P. & Hallett P.E. (1994). Retinal eccentricity and the latency of eye saccades. Vision Research, 34, 517-531), and the modulation of saccadic accuracy using target predictability and saccadic latency (Coeffe C. & O'Regan J.K. (1987). Reducing the influence of non-target stimuli on saccade accuracy: predictability and latency effects. Vision Research, 27 (2), 227-240).  (+info)

(27/2544) Eye movements of rhesus monkeys directed towards imaginary targets.

Is the presence of foveal stimulation a necessary prerequisite for rhesus monkeys to perform visually guided eye movements? To answer this question, we trained two rhesus monkeys to direct their eyes towards imaginary targets defined by extrafoveal cues. Independent of the type of target, real or imaginary, the trajectory of target movement determined the type of eye movement produced: steps in target position resulted in saccades and ramps in target position resulted in smooth pursuit eye movements. There was a tendency for the latency of saccades as well as pursuit onset latency to be delayed in the case of an imaginary target in comparison to the real target. The initial eye acceleration during smooth pursuit initiation elicited by an imaginary target decreased in comparison to the acceleration elicited by a real target. The steady-state pursuit gain was quite similar during pursuit of an imaginary or a real target. Our results strengthen the notion that pursuit is not exclusively a foveal function.  (+info)

(28/2544) Saccades require focal attention and are facilitated by a short-term memory system.

We performed two sets of experiments in which observers were instructed to make saccades to an odd colored target embedded in an array of distractors. First, we found that when the colors of the target and distractors switched unpredictably from trial to trial (the mixed condition), saccadic latencies decreased with increasing numbers of distractors. In contrast, saccadic latencies were independent of the number of distractors when the color of the target and distractors remained the same on each trial (the blocked condition). This pattern of results mirrors visual search tasks in which focal rather than distributed attention is required (Bravo, M.J., Nakayama, K. (1992). The role of attention in different visual search tasks. Perception and Psychophysics, 51, 465-472.). Second, we found that saccades to an odd target were made more quickly and accurately when the target was the same color as on previous trials than when it changed color. This priming of the target color accumulates across five to seven trials over a period of approximately 30 s. A similar priming effect has been previously shown for the deployment of focal attention (Maljkovic, V., Nakayama, K. (1994). Priming of popout: III. Role of features. Memory and Cognition, 22(6), 657-672.). Thus, we show a close congruence between the pattern of saccadic eye movement latencies and the deployment of focal attention. This supports the view that (1) the execution of saccades requires focal as opposed to distributed attention and that (2) this focal attention is guided by a short term memory system which facilitates the rapid refixation of gaze to recently foveated targets.  (+info)

(29/2544) 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.  (+info)

(30/2544) Saccade selection in visual search: evidence for spatial frequency specific between-item interactions.

We present two experiments in which subjects were required to make a saccade to a target amongst distractors. Targets were oriented Gabor patches. Analysis of errors, when subjects fail to make a saccade to the target, showed two interesting features. First, most error saccades were directed towards a distractor and not to the blank space between distractors. This suggests that although the location of the target may not be encoded correctly, the locations of the items in the display are encoded. Second, when the display items were all of the same spatial frequency, a long-range effect occurred whereby the likelihood of an error saccade in a specific direction decreased systematically as the distance from the target increases. This systematic influence of the target location extended over practically the whole display. The long-range effect appeared whenever all display items had the same spatial frequency and showed little dependence on the spatial frequency of the display items. However, when the items had different spatial frequencies the long-range effects were absent.  (+info)

(31/2544) Behavioral evidence for visual perception of 3-dimensional surface structures in monkeys.

Human subjects perceive two crossing bars, one in front of the other, when shown a cross with disparity added to its horizontal limbs, and they also perceive neon-color spreading when shown a stereoscopic Redies-Spillmann figure. It has thus been hypothesized that the human visual system follows the principle of generic image sampling in reconstructing 3-dimensional (3-D) surface structures. Here we examine whether monkeys also perceive these surface structures. The results indicate that monkeys, like humans, perceive two crossing bars and neon-color spreading and suggest that the principle of generic image sampling may also be applied to visual perception in monkeys.  (+info)

(32/2544) Influence of head position on the spatial representation of acoustic targets.

Sound localization in humans relies on binaural differences (azimuth cues) and monaural spectral shape information (elevation cues) and is therefore the result of a neural computational process. Despite the fact that these acoustic cues are referenced with respect to the head, accurate eye movements can be generated to sounds in complete darkness. This ability necessitates the use of eye position information. So far, however, sound localization has been investigated mainly with a fixed head position, usually straight ahead. Yet the auditory system may rely on head motor information to maintain a stable and spatially accurate representation of acoustic targets in the presence of head movements. We therefore studied the influence of changes in eye-head position on auditory-guided orienting behavior of human subjects. In the first experiment, we used a visual-auditory double-step paradigm. Subjects made saccadic gaze shifts in total darkness toward brief broadband sounds presented before an intervening eye-head movement that was evoked by an earlier visual target. The data show that the preceding displacements of both eye and head are fully accounted for, resulting in spatially accurate responses. This suggests that auditory target information may be transformed into a spatial (or body-centered) frame of reference. To further investigate this possibility, we exploited the unique property of the auditory system that sound elevation is extracted independently from pinna-related spectral cues. In the absence of such cues, accurate elevation detection is not possible, even when head movements are made. This is shown in a second experiment where pure tones were localized at a fixed elevation that depended on the tone frequency rather than on the actual target elevation, both under head-fixed and -free conditions. To test, in a third experiment, whether the perceived elevation of tones relies on a head- or space-fixed target representation, eye movements were elicited toward pure tones while subjects kept their head in different vertical positions. It appeared that each tone was localized at a fixed, frequency-dependent elevation in space that shifted to a limited extent with changes in head elevation. Hence information about head position is used under static conditions too. Interestingly, the influence of head position also depended on the tone frequency. Thus tone-evoked ocular saccades typically showed a partial compensation for changes in static head position, whereas noise-evoked eye-head saccades fully compensated for intervening changes in eye-head position. We propose that the auditory localization system combines the acoustic input with head-position information to encode targets in a spatial (or body-centered) frame of reference. In this way, accurate orienting responses may be programmed despite intervening eye-head movements. A conceptual model, based on the tonotopic organization of the auditory system, is presented that may account for our findings.  (+info)