Variability not ability: another basis for performance decrements in neglect

Abstract

Objective: To determine reaction time (RT) and its variability, as a function of horizontal spatial position, in subjects with neglect.

Background: In neglect, performance is frequently reported a as mean and a decreased ability to perform the task inferred by comparison to control groups. Few studies have examined how consistency and optimal performance relate to spatial neglect.

Methods: Ten subjects with brain damage, five with and five without spatial neglect, were assessed on a RT task. Subjects responded by pushing a computer key to the onset of a white square appearing on a black screen. The locations of stimuli were randomly varied along the horizontal meridian.

Results: For three of five neglect subjects, optimal RT showed no or little relation to horizontal location. Four of five neglect subjects demonstrated an increased variability in RT that correlated with spatial position and which was not present in our brain damaged subjects without neglect. The relationship was not an artifact of left sided stimuli, in general, being processed differently. For the two neglect subjects with the most trials, a significant correlation between RT variability and spatial position existed for left-sided trials alone. Increased variability was not a consequence of simply looking left proportionately less often, nor could a model of multiple compensatory systems operating in parallel explain the enhanced variability. Neither hemianopsia alone nor brain damage per se could account for the spatial modulation of RT variability.

Conclusions: That neglect subjects perform the RT task normally on some trials, even in their 'neglected field', challenges the notion that neglect must reflect an irreparably damaged cognitive system. Performance decrements in neglect can reflect an inability to consistently detect and respond. Evaluating optimal performance and variability of performance can indicate if a capacity has been lost absolutely or merely degraded such that normal performance cannot be sustained.