COORDINATE FRAME FOR PATTERN-RECOGNITION IN UNILATERAL SPATIAL NEGLECT

The present research examines the effect of spatial (object-centered) attentional constraints on pattern recognition. Four normal subjects a nd two right-hemisphere-damaged patients with left visual neglect part icipated in the study Small, letter-like, prelearned patterns served a s stimuli. Short exposure time prevented overt scanpaths during stimul us presentation. Attention was attracted to a central (midsagittal) fi xation point by precuing this location prior to each stimulus presenta tion. Minute (up to 1.5 degrees of visual angle) rightward and leftwar d stimulus shifts caused attention to be allocated each time to a diff erent location on the object space, while remaining in a fixed central position in viewer-centered coordinates. The task was to decide which of several prelearned patterns was presented in each trial. In the no rmal subjects, best performance was achieved when the luminance centro id (LC; derived from the analysis of low-spatial frequencies in the ob ject space) of each pattern coincided with the spatial position of the precue. In contrast, the patients with neglect showed optimal recogni tion performance when precuing attracted attention to locations within the object space, to the left of the LC. The normal performance sugge sts that the LC may serve as a center of gravity for attention allocat ion during pattern recognition. This point seems to be the target loca tion where focal attention is normally directed, following a primary g lobal analysis based on the low spatial frequencies. Thus, the LC of a simple pattern may serve as the origin point for an object-centered-c oordinate-frame (OCCF), dividing it into right and left. This, in turn , serves to create a prototype description of the pattern, in its own coordinates, in memory, to be addressed during subsequent recognition tasks. The best match of the percept with the stored description may e xplain the observed advantage of allocating attention to the LC. The p erformance of the brain-damaged patients can be explained in terms of neglect operating in the OCCF.