Egocentric and Exocentric Spatial Judgements of Visual Displacement

The role of changes in ego- and exocentric spatial relationships on perceptual judgements about visual displacement was investigated in this study. Subjects were asked to indicate whether a dot in a test stimulus was displaced compared to a dot in a reference stimulus. Subjects were given explicit instructions to report displacement relative to themselves (egocentric) or relative to a circle surrounding the dot (exocentric). Four types of test stimuli were used in which object-circle (exocentric) and object-observer (egocentric) relations were systematically varied. It was found that for test stimuli that reveal conflicting ego- and exocentric spatial information, subjects performed poorly in both instruction conditions. This suggests that ego- and exocentric representations cannot be used independently and are probably interconnected.

Spatial localization in patients with unilateral posterior left or right hemisphere lesions

In this study, perceptual and visuomotor spatial localization were examined in patients with unilateral right (RH) or left (LH) hemisphere lesions and in a group of control subjects. Perceptual localization was measured with a position-discrimination task; in the visuomotor localization task, subjects had to point to a visual target. Both tasks were investigated in conditions with or without background visibility and with central and peripheral targets. In the visuomotor task, hand visibility was also manipulated. In both tasks, targets were presented in the left and right visual hemifield. The perceptual task revealed impairments for both LH and RH patients in the contra-lateral visual hemifield. RH patients also revealed slightly larger impairments in conditions without a visual background. In the visuomotor task, the LH patients were not impaired, whereas the RH patients were impaired in conditions without hand visibility and/or background visibility. Hence, our data strengthen the idea that spatial localization is not a unitary function and that perceptual and visuomotor localization can be selectively impaired. We suggest that one of the important factors distinguishing between localization impairments in RH and LH patients might be absolute versus relative localization.

Factors limiting large-scale localisation

The mechanisms mediating relative spatial localisation in the visual system are still unclear. There is a growing amount of evidence that this capability is not merely limited by the processing of the front-end visual system. Models of localisation should, therefore, include higher-level processing stages. A careful study of the sources of error in localisation tasks may further our understanding of the nature of these processes. A study is reported in which the possible role of higher-order processing in relative spatial localisation is explicitly addressed. For this purpose the error sources of threshold performance were investigated for two similar relative-spatial-localisation tasks: two-dot separation discrimination and two-dot orientation discrimination. Fovea-centered stimuli with large dot separations were used. The front-end processing for these stimuli is probably identical in both tasks. Hence, differential effects of the variation of the experimental parameters on threshold performance for both tasks may reveal the characteristics of the higher-level processing involved. The effects of dot separation, stimulus orientation, and experimental procedure (single-stimulus binary forced choice versus two-alternative forced choice) on threshold performance for both tasks are reported. The results show that thresholds for both tasks increase proportionally with dot separation. However, separation-discrimination thresholds are always significantly higher than orientation-discrimination thresholds. Thresholds for separation discrimination are independent of stimulus orientation. In contrast, orientation-discrimination thresholds show an oblique effect: thresholds are consistently lower for horizontal stimuli. Both tasks also show a different dependency of threshold behaviour on the experimental procedure. For a horizontal stimulus orientation, separation discrimination is better with an explicit (physical) reference standard, whereas orientation discrimination is better with an implicit referent. These differential effects cannot be explained by any of the known characteristics of the front-end visual system. They suggest that large-scale spatial-localisation performance is probably limited at a processing level at which spatial relations are explicitly represented.