Reaction time in ambulant multiple sclerosis patients. Part II. Influence of task complexity.

Effects of task variables on reaction time (RT) were investigated in 39 multiple sclerosis (MS) patients and 25 matched control subjects, based on the premise that RT is sensitive to (subtle) changes of cognitive efficiency. The hypothesis to be tested held that inefficient cognitive processing in MS would be reflected in differences between RT patterns of MS and control subjects, and in inordinate RT increase in MS patients with increase of task complexity. No difference was found between MS patients and controls, or between MS subgroups, in (1) response to time uncertainty of stimulus presentations, (2) RT behaviour over stimulus presentations, and (3) RT change from simple to more complex conditions. No evidence of slowed cognitive processing was found in this sample of outpatients with relatively mild and stable MS. It is emphasised that executive and cognitive slowness have to be disentangled in research on MS and on so-called 'subcortical dementia', of which mental slowing is a key concept.