Application of Hick's law of response speed in Alzheimer and Parkinson diseases.

Reaction time in normal subjects is known to increase in a log-linear fashion relative to the number of alternative choices. However, this relationship (formalized as "Hick's law") has received limited investigation in populations with neurological cognitive impairment. The present study used timed sorting of standard playing cards to test Hick's law for 20 young control subjects, and 20 each of age-matched elderly subjects with Alzheimer disease, Parkinson disease, and no cognitive abnormalities. Although Parkinson patients were slowest in the simple tasks of dealing out the cards and sorting by color, Alzheimer patients showed the greatest slowing for the more cognitively complex conditions of sorting by suit and rank of the cards. The performance of all four groups followed Hick's law in displaying a significant linear relationship between response time and log2 of the number of choices. These findings suggest that, although limitations of information-processing speed in Alzheimer and Parkinson disease affected choice response time, there may be sparing of fundamental cognitive organization in these disorders.