Werner X. Schneider

Postsaccadic target blanking prevents saccadic suppression of image displacement

Displacement of a visual target during a saccadic eye movement is normally detected only at a high threshold, implying that high-quality information about target position is not stored in the nervous system across the saccade. We show that blanking the target for 50-300 msec after a saccade restores sensitivity to the displacement. With blanking, subjects reliably detect displacements as small as 0.33 deg across 6 deg eye movements, with correspondingly steep psychophysical functions. Performance with blanking in a fixation control is inferior, evidence for a saccadic enhancement of sensitivity to image displacement. If blanking is delayed so that the target is visible immediately after the saccade in its displaced position, performance declines to non-blanking levels. Blanking the target before the saccade, and restoring it during the saccade, yields a similar but weaker effect. We interpret these results with a model in which the visual system searches for the postsaccadic goal target within a restricted spatiotemporal window. If it is not found, the assumption of stationarity of the world is broken and the system makes use of other information such as extraretinal signals for calibrating location.

VAM: A neuro-cognitive model for visual attention control of segmentation, object recognition, and space-based motor action

Abstract This paper introduces a new neuro-cognitive Visual Attention Model, called VAM. It is a model of visual attention control of segmentation, object recognition, and space-based motor action. VAM is concerned with two main functions of visual attention-that is “selection-for-object-recognition” and “selection-for-space-based-motor-action”. The attentional control processes that perform these two functions restructure the results of stimulus-driven and local perceptual grouping and segregation processes, the “visual chunks”, in such a way that one visual chunk is globally segmented and implemented as an “object token”. This attentional segmentation solves the “inter- and intra-object-binding problem”. It can be controlled by higher-level visual modules of the what-pathway (e.g. V4/IT) and/or the where-pathway (e.g. PPC) that contain relatively invariant “type-level” information (e.g. an alphabet of shape primitives, colors with constancy, locations for space-based motor actions). What-based attention...

Selective Dorsal and Ventral Processing: Evidence for a Common Attentional Mechanism in Reaching and Perception

The primate visual system can be divided into a ventral stream for perception and recognition and a dorsal stream for computing spatial information for motor action. How are selection mechanisms in both processing streams coordinated? We recently demonstrated that selection-for-perception in the ventral stream (usually termed “visual attention”) and saccade target selection in the dorsal stream are tightly coupled (Deubel & Schneider, 1996). Here we investigate whether such coupling also holds for the preparation of manual reaching movements. A dual-task paradigm required the preparation of a reaching movement to a cued item in a letter string. Simultaneously, the ability to discriminate between the symbols “E” and “∃” presented tachistoscopically within the surrounding distractors was taken as a measure of perceptual performance. The data demonstrate thatdiscrimination performance is superior when the discrimination stimulus is also the target for manual aiming; when the discrimination stimulus and point...

Immediate post-saccadic information mediates space constancy

We recently demonstrated that the perceived stability of a visual target that is displaced during a saccade critically depends on whether the target is present immediately when the saccade ends; blanking a target during and just after a saccade makes its intra-saccadic displacement more visible (Deubel et al. Vis Res 1996;36:985-996). Here, we investigate the interaction of visual context and blanking. Subjects saw a saccade target and an equal-sized distractor. During a saccade one or the other was displaced left or right. At the same time, one of the objects could be blanked briefly. Subjects reported whether the target or the distractor had jumped. The object that was blanked was more often seen as jumping (Experiment 1), regardless of which object really jumped, implying that continuously visible objects are preferentially perceived as stable. When both objects were blanked, longer blanking led to better accuracy at identifying which had jumped during a saccade. When one object was jumped and the other, stationary object was blanked (Experiment 2), the blanked object was mistakenly seen as jumping until the jump covered 50% or more of the saccade amplitude. In Experiment 3 a large continuously present texture underwent an undetected jump during a saccade, biasing judgments of simultaneous jumps of a blanked target. The results demonstrate that space constancy in normal situations is dominated by the assumption that a continuously present pattern is stable--this pattern becomes the spatial reference for the post-saccadic recalibration of perceptual space.

Transsaccadic memory of position and form

Why and how people perceive the visual world as continuous and stable, despite the gross changes of its retinal projection that occur with each saccade, is one of the classic problems in perception. In the present paper, we argue that an important factor of visual stability and transsaccadic perception is formed by the reafferent visual information, i.e., the visual display that is present when the eyes land. After a review of some of the relevant theoretical, behavioural and physiological research on space constancy, saccadic suppression and transsaccadic memory, three experiments are presented. In a first experiment, we study the effect of an extended horizontal bar covering the target area for a short period after the saccade on saccadic suppression of image displacement. The results show that the bar acts just like a temporary blanking of the saccade target, leading to a strong reduction of saccadic suppression. In the second experiment, we show that any object that is present immediately after the saccade can establish a spatial reference, even if it is dissimilar to the saccade target. In a third experiment we study, with a similar approach, the effect of blanking and postsaccadic information on transsaccadic integration of form information. The data demonstrate that a localized postsaccadic object tends to replace the content of transsaccadic memory.

Usability of a theory of visual attention (TVA) for parameter-based measurement of attention I: Evidence from normal subjects

The present study investigated the usability of whole and partial report of briefly displayed letter arrays as a diagnostic tool for the assessment of attentional functions. The tool is based on Bundesens (1990, 1998, 2002; Bundesen et al., 2005) theory of visual attention (TVA), which assumes four separable attentional components: processing speed, working memory storage capacity, spatial distribution of attention, and top-down control. A number of studies (Duncan et al., 1999; Habekost & Bundesen, 2003; Peers et al., 2005) have already demonstrated the clinical relevance of these parameters. The present study was designed to examine whether (a) a shortened procedure bears sufficient accuracy and reliability, (b) whether the procedures reveal attentional constructs with clinical relevance, and (c) whether the mathematically independent parameters are also empirically independent. In a sample of 35 young healthy subjects, we found high intraparameter correlations between full- and short-length tests and sufficient internal consistencies as measured via a bootstrapping method. The clinical relevance of the TVA parameters was demonstrated by significant correlations with established clinical tests measuring similar constructs. The empirical independence of the four TVA parameters is suggested by nonsignificant or, in the case of processing speed and working memory storage capacity, only modest correlations between the parameter values.

Delayed saccades, but not delayed manual aiming movements, require visual attention shifts.

Several studies have shown that during the preparation of a goal‐directed movement, perceptual selection (i.e., visual attention) and action selection (the selection of the movement target) are closely coupled. Here, we study attentional selection in situations in which delayed saccadic eye movements and delayed manual movements are prepared. A dual‐task paradigm was used which combined the movement preparation with a perceptual discrimination task. The results demonstrate a fundamental difference between the preparation of saccades and of manual reaching. For delayed saccades, attention is pinned to the saccade target until the onset of the response. This does not hold for manual reaching, however. Although fast reaching movements require attention, reaches delayed more than 300 ms after movement cue onset can be already performed “off‐line” that is, attention can be withdrawn from the movement target.

Where to Look Next? Combining Static and Dynamic Proto-objects in a TVA-based Model of Visual Attention

To decide “Where to look next ?” is a central function of the attention system of humans, animals and robots. Control of attention depends on three factors, that is, low-level static and dynamic visual features of the environment (bottom-up), medium-level visual features of proto-objects and the task (top-down). We present a novel integrated computational model that includes all these factors in a coherent architecture based on findings and constraints from the primate visual system. The model combines spatially inhomogeneous processing of static features, spatio-temporal motion features and task-dependent priority control in the form of the first computational implementation of saliency computation as specified by the “Theory of Visual Attention” (TVA, [7]). Importantly, static and dynamic processing streams are fused at the level of visual proto-objects, that is, ellipsoidal visual units that have the additional medium-level features of position, size, shape and orientation of the principal axis. Proto-objects serve as input to the TVA process that combines top-down and bottom-up information for computing attentional priorities so that relatively complex search tasks can be implemented. To this end, separately computed static and dynamic proto-objects are filtered and subsequently merged into one combined map of proto-objects. For each proto-object, attentional priorities in the form of attentional weights are computed according to TVA. The target of the next saccade is the center of gravity of the proto-object with the highest weight according to the task. We illustrate the approach by applying it to several real world image sequences and show that it is robust to parameter variations.

Visual Attention and Saccadic Eye Movements: Evidence for Obligatory and Selective Spatial Coupling

Abstract The spatial interaction of visual attention and saccadic eye movements was investigated in a task that combines stimulus-elicited saccades with letter discrimination. Subjects had to saccade to locations within horizontal letter strings left or right from a central fixation cross. The performance in discriminating between the letters “E” and “mirror-E” presented tachistoscopically within surrounding distractors before the saccade was taken as a measure of attentional orienting. The data showed that discrimination performance is best when discrimination target and saccade target referred to the same object. The results strongly argue for an obligatory coupling of saccade programming and visual attention allocation to one common object. Further, the discrimination performance was better when the saccade is directed to a target located more foveally of the discrimination target than when the saccade target was located more peripherally. Similar asymmetries have been found in lateral masking studies.

Attentional selection during preparation of prehension movements

In two experiments coupling between dorsal attentional selection for action and ventral attentional selection for perception during preparation of prehension movements was examined. In a dual-task paradigm subjects had to grasp an X-shaped object with either the left or the right hands thumb and index finger. Simultaneously a discrimination task was used to measure visual attention prior to the execution of the prehension movements: Mask items transiently changed into distractors or discrimination targets. There was exactly one discrimination target per trial, which appeared at one of the four branch ends of the object. In Experiment 1 target position varied randomly while in Experiment 2 it was constant and known to subjects in each block of trials. In both experiments discrimination performance was significantly better for discrimination target positions at to-be-grasped branch ends than for not-to-be-grasped branch ends. We conclude that during preparation of prehension movements visual attention is largely confined to those parts of an object that will be grasped.