Response choice in Parkinson's disease. The effects of uncertainty and stimulus-response compatibility

Reaction time paradigms provide a set of methods for assessing aspects of the planning and execution of voluntary movements in Parkinson's disease. Attention has focused mainly on the issue of programming of responses, employing a combination of simple reaction time (SRT) and choice reaction time (CRT) paradigms. The first part of the present study replicated an earlier finding in which patients showed a disproportionate slowing in CRT compared with SRT. The main aim of the study was to investigate the possible role of response choice, a key stage prior to motor programming in this CRT deficit. Two factors were manipulated: (i) response uncertainty and (ii) stimulus-response compatibility. The patients showed a normal increase in reaction time with increasing uncertainty in the compatible conditions, and a normal response to stimulus-response compatibility in the two-choice task. However, the two groups showed qualitatively different patterns of interaction between the two experimental factors, with only the patients showing a disproportionate slowing with incompatible stimulus-response relationships in the four-choice task. The data were interpreted in terms of Hasbroucq et al.'s (1990) list-rule model of stimulus-response compatibility effects, which suggested that the patients and controls were using different strategies for dealing with incompatible stimulus-response relationships. The use of different strategies makes it impossible to determine whether or not the processing of the patients is impaired in Parkinson's disease, although further research is suggested to clarify the question. However, the present data suggest that any impairment in response choice is unlikely to contribute to the slowing in CRT in Parkinson's disease under conditions of high stimulus-response compatibility.