Processing efficiency of the orienting and the focusing of covert attention in relation to the level of disability in Parkinson's disease

This study assessed covert visuo-spatial attentional mechanisms in Parkinson's disease (PD). Reaction times (RTs) of subjects at early and late disease stages, and of matched control subjects, were compared. The task was to respond to a stimulus in a square positioned in either the left or right hemifield. To assess the orienting of attention, the stimulus was preceded by an arrow which gave a valid (stimulus appeared in the indicated square) or an invalid (stimulus appeared in the square which had not been indicated) cue. Both hemifields were indicated in the case of neutral trials. To assess the focusing of attention, the square could be small (1 x 1 degrees ), medium-sized (2 x 2 degrees ) or large (4 x 4 degrees ). To compare voluntary and reflexive mechanisms, the cue(s) could be central or peripheral, respectively. For the orienting of attention, controls and "early" PD subjects showed greater RTs for centrally cued invalid than for neutral trials. "Late" PD subjects showed no such difference. In contrast, the pattern of results for peripherally cued stimuli was similar across all groups. With respect to focusing, "late" PD subjects showed the normal pattern of an increase of reaction time with square size, both for centrally and peripherally cued trials; however, this increase was greater than that of the control and "early" PD subjects. "Late" PD subjects showed greater reaction times for centrally than for peripherally cued trials; however, unlike controls and "early" PDs, this difference was reduced for invalid trials, and absent for trials to the small and medium-sized squares. It is concluded that Parkinson's disease compromises both endogenous and exogenous visuo-spatial functions. However, it is particularly processes which have a more endogenous component which show the greatest deterioration at later disease stages.