Speed of elementary visual recognition operations in Parkinson's disease as measured by the mutual masking method.

Pairs of mutually different, spatially overlapping letters were exposed for recognition to groups of patients with Parkinson's disease (PD) and the age-matched control group. Stimulus onset asynchrony (SOA), medical treatment status (de novo vs. treated), and predominant symptoms (tremor vs. hypokinetic rigidity) were an other main variables. The highly significant main effects of SOA and health status demonstrated slowing of elementary visual recognition operations in Parkinson's disease; the results are based on the experimental method that requires neither fast manual responses nor tracking of the display events by saccadic eye movements. Significant interaction between the temporal order of stimulus exposure and health status showed that impairment due to PD was more pronounced for the first stimulus, including the de novo group. Qualitatively similar recognition functions in the binocular and dichoptic conditions showed that the typical pattern of results--prevalence of S2 over S1 at intermediate SOAs--cannot be attributed to retinal processes and should be originating from central processes. An earlier finding (Bachmann, 1994) that PD patients whose nonspecific thalamic nuclei were stimulated intracranially produced qualitatively unusual recognition functions that should have been the result of stimulation, rather than PD as such.