The Novelty Effect in Recovered Hemineglect

Abstract

Left neglect patients, patients who had recovered from left neglect and control subjects performed a task of simple motor reaction times (RTs) to lateralised visual stimuli. Neglect and recovered patients were slower than controls on left-sided targets. To explore the time course of the allocation of attention across space, an analysis of responses as a function of the serial order of the trials was performed. While neglect patients' performance did not substantially change over time, recovered patients showed a stereotyped 'novelty effect', consisting of larger left/right RT differences at the beginning of the task than at the end of it. To explain this practice-related change, a trade-off is hypothesised between the process of learning the motor task and the mechanisms involved in recovery from neglect, such as the reorienting of attention toward the contralesional side following the initial ipsilesional orienting. A possible role is proposed for the prefrontal cortex as the crucial neural structure that mediates both processes.

Orienting of attention in left unilateral neglect

“…This could be the basis for the fact that voluntary control can have a relative in¯uence on exogenous orienting; for example, abrupt onsets of visual stimuli capture attention only when attention is unfocused [19]. Also consistent with these ideas is the proposal [98,157] that frontal cognitive abilities are important for recovery from neglect; this recovery is indeed more rapid in patients without injury to the right frontal cortex [158], and is related to the restoration of metabolism in the ipsiand contralesional frontal cortices [104,105,159].…”

“…The early rightward orientation of attention may be observed as a residual sign of spatial bias in patients who had recovered from left neglect [98,107,108]. Thus, to produce clinical neglect, either the initial rightward orienting bias must be present in a certain critical amount, or it must be accompanied by other component de®cits.…”

“…Such non-lateralized de®cits may be invoked to explain the fact that neglect patients are slower than normal individuals when responding to visual targets even in the ipsilesional, non-neglected space. Indeed, this ipsilesional slowing might disappear with recovery of neglect [98]. The normal timing of attentional events also seems to be disrupted in neglect for centrally presented visual stimuli.…”

Variability of response times as a marker of diverted attention

“…If neglect patients suffer from a biased orienting of attention, which concerns primarily exogenous shifts [2,11,13], then it is conceivable that visual stimuli occurring from the right to the left side be progressively less likely to capture patients' attention. This would provoke, on the one hand, an increase in detection time for left-sided stimuli as compared to right-sided ones; on the other hand, variability of RTs would be spatially affected, with occasional fast responses to left targets on those rare occasions in which they rapidly captured patients' attention 1 .…”

“…This study focused on an aspect of neglect performance, variability, that has often been encountered by researchers in the field (see, e.g. [2,11]), but has not hitherto been studied in detail. Anderson et al discounted some possible explanations of their findings, such as an effect of hemianopia, a general deficit of sustained attention, a biased ocular exploration, or an interplay between impaired and compensatory systems, and argued for the possibility that 'one mechanism for spatial neglect in some subjects is a spatially modulated effect on sustained performance completely independent of abnormalities in any other cognitive systems' (p. 794).…”