A systematic study of visual extinction. Between- and within-field deficits of attention in hemispatial neglect.

Mechanisms of visual extinction were investigated in four patients with right hemisphere damage using a partial report paradigm. Different shapes (star or triangle) were displayed in one, two or four possible locations so that double simultaneous stimuli occurred either across the two hemifields or within the same hemifield. Patients attended either to the location (right, left or both), number (one, two or four) or shape (no, one or two stars among the shapes presented) of stimuli in three separate experiments using the same displays and exposure duration. Reporting the location (Experiment 1) produced marked contralesional extinction, although reaction time was delayed compared with unilateral right trials, indicating unconscious processing. Reaction time was also delayed on correct bilateral and unilateral left trials. In contrast, enumerating stimuli (Experiment 2) caused no significant contralesional extinction on bilateral displays and reaction time was similar on bilateral and unilateral right trials, suggesting that information from both fields was grouped in a single numerable percept in this task. However, patients often detected only one of two stimuli within the left field. Whereas similarity of shapes improved localization and did not affect enumeration, identifying stars among shapes (Experiment 3) revealed a severe inability to detect two similar targets between hemifields as well as within each of the hemifields. Distracting triangles were generally less detrimental to the perception of a concurrent target on either side, but slowed the reaction time regardless of whether they were in the same or the opposite field. Relative difficulty in ignoring distractors correlated with neglect severity on a cancellation task, and was most prominent in one patient with a large amount of frontal damage. These findings suggest that (i) allocation of attention to identical stimuli can be modulated by task demand; (ii) enumerating a small set of items across fields may not require attending to individual stimuli but relies on preattentive subitizing ability, as found in normal subjects; (iii) location information may be critical for attentional mechanisms subserved by the parietal cortex and pathological competition for awareness in extinction; (iv) extinction entails a bilateral deficit in attending to two concurrent similar targets when their features must be identified; and (v) the relevance of the stimuli can modulate the distribution of attention, possibly through frontal top-down control. These findings are consistent with recent neurophysiological evidence of parietal and frontal attentional influences on ventral visual pathways.