Showing papers by "Werner X. Schneider published in 1999"

Visual-spatial working memory, attention, and scene representation: a neuro-cognitive theory.

Abstract: This paper addresses the issue of how visual-spatial working memory, attention, and scene representation are related. The first section introduces a modified two-stage conception of visual-spatial processing. “Stage one” refers to low-level visual-spatial processing and computes in parallel for the currently available retinal information “object candidates,” here called “visual-spatial units.” An attentional process called “unit selection” allows access to stage two for one of these units at a time. Stage two contains high-level visual-spatial information that can be used for goal-directions (e.g., verbal report, grasping). It consists of three parallel processing streams. First, the currently selected unit is recognized; second, a spatial-motor program for the selected unit is computed; and third, an “object file” is set up for the selected unit. An object file contains temporary episodic representations of detailed high-level visual-spatial attributes of an “object” plus an “index.” An index acts as a pointer and is bound via temporary connections to the attributes of the file.

The Subjective Direction of Gaze Shifts Long Before the Saccade

Abstract: Subjects in eye movement experiments sometimes report that they have moved their eyes to some location before their eyes have actually moved (Deubel and Schneider, 1996). We investigated this by presenting a brief test stimulus at various points in time after directing subjects to make a saccadic eye movement to a peripheral cue. The subjects had to report where they were looking when the test stimulus was presented. We found that visual stimuli presented at the saccade target location as early as 250 ms before sac-cade onset were reported as occurring after the saccade. In a second experiment subjects performed, intentionally, a saccade to a static cue. Also under this condition, subjects reported to look at the future saccade target location long before the saccade actually occurred. The data show that subjects are unaware of the time when they make even a large saccade, and that they have no explicit knowledge of the retinal position of stimuli. Rather, they mistake movements of visual attention for movements of the eyes.

Visuomotor mental rotation of saccade direction

Abstract: This investigation studied the latencies of saccadic eye movements that were directed away from a target by a variable angular distance, which was given by instruction. Such a movement presumably requires an intentional, visuomotor mental rotation of the saccade vector, resulting in prolonged reaction times. From a study on the control of directed hand movements, it has been hypothesized that all visuomotor and visual mental rotation tasks share a common processing stage. We tested this hypothesis with a saccade task in which subjects shifted their gaze either towards (0°, pro-saccade), or 30, 60, 90, 120, 150, or 180° (anti-saccade) away from a randomly cued position on an imaginary clock face. With four different cueing conditions, latencies increased monotonically with required gaze shift from 0–150°, thus exhibiting a mental rotation latency pattern. However, we also found anti-saccades faster than 150° gaze shift and slower rotation speeds with peripheral cues than with central cues. Together with the overall shallower latency increase compared with previous findings with mental rotation tasks, these results cast doubt on the notion of a common, central processing mechanism for the different types of tasks.

Object Recognition and Goal-Directed Eye or Hand Movements are Coupled by Visual Attention

Abstract: A dual-task paradigm required the preparation of a goal-directed movement (hand or eye) concurrently with a letter discrimination task. In the first experiment a hand movement to a location on a virtual circle was required and indicated by a central cue. Simultaneously, the ability to discriminate between the symbols “E” and “3”, presented tachioscopically with various delays on a circular position within surrounding distractors, was taken as a measure of selective perceptual performance. The location of discrimination target remained constant within blocks and was known to the subjects. In the second experiment a saccadic eye movement was required instead of a pointing movement. The data in both experiments clearly demonstrate that discrimination performance is superior when the discrimination target location (DT) is identical to the location of the movement target (MT). When DT and MT refer to different objects, performance deteriorates drastically. We conclude that it is not possible to maintain attention on a stimulus for the purpose of discrimination while preparing a movement to a spatially separate object. This holds, in a quantitatively similar way, for both saccades and manual pointing.

‘Death is not the same always and everywhere’ – sociocultural aspects of brain death and the legislation of organ transplantation

Abstract: In all modern industrialised countries with advanced medical systems (USA/Canada, Europe, Asia) organ transplantation is now among the standard repertoire of available treatments, even though the closely related concept of brain death has repeatedly and persistently been the subject of more or less controversial discussion, depending on the national context and cultural traditions. Taking a cultural-sociological interpretation of the concepts of dying and death as its starting point, this article precisely states the sociological problem of brain death as a question of the demarcations, discursively generated and conveyed, between a person who is ‘still alive’ and one who is ‘already dead’, together with the associated, possibly changing cultural concepts of ‘life’ and ‘death’, extending to the empirically virtually unexplored interaction with social practice with regard to the dying and the dead. The analysis of the public discussion in Germany in the course of the consideration of the Transplant...