Variability of response times as a marker of diverted attention

Anderson et al. (Variability not ability: another basis for performance dec rements in neglect. Neuropsychologia 2000;38:785-796) have recently reporte d that variability of response times (RTs) progressively increases from the right to the left side in left neglect patients. Anderson et al. propose t hat this lack of consistency is an important determinant of patients' behav iour, and may result from a deficit independent of other mechanisms causing neglect. Here we suggest that an increase of variability, and not only of RTs. is to be expected when attention is exogenously biased away from the p robed location. Consequently, space-based variability can be interpreted in the framework of existing models of unilateral neglect. According to one s uch model, a basic impairment in left neglect is a bias toward rightward ex ogenous orienting of attention. As a result, left targets often fail to rap idly capture patients' attention, thus yielding slow RTs. However, since th e probability for a left target attracting attention is low but not null, r elatively fast RTs can occur on those rare occasions in which a left target does capture patients' attention. The coexistence of these relatively fast with slow RTs could be at the basis of space-based variability in neglect. Empirical support for our hypothesis comes from the results of a re-analys is for variability of cued RTs obtained in IS normal individuals and six le ft neglect patients. Cues were peripheral and non-informative, thus eliciti ng an exogenous attentional shift. For normal individuals, invalid trials y ielded less consistent response times than valid trials at short (150 ms) c ue-target interval; for neglect patients, a similar phenomenon occurred for left invalidly-cued targets, thus paralleling the disproportionate cost in RTs typically evoked by this condition in unilateral neglect. We conclude by discussing some possible determinants of gradient-shaped effects and by outlining the implications of space-based variability for current models of unilateral neglect. (C) 2001 Elsevier Science Ltd. All rights reserved.