A cholinergic mechanism for eye fixation.

A basic question regarding the integrative properties of the central nervous system is how transient motor commands or brief sensory stimuli are able to evoke persistent neural changes, mainly in the form of a sustained tonic rate of action potentials. Examples of this persistent neural activity have been reported in prefrontal (Fuster, 1997) and entorhinal (Egorov et al., 2002) cortices, as part of the neural mechanisms involved in short-term working memory (Goldman-Rakic, 1995). Interestingly enough, the general organization of motor systems assumes the presence of bursts of short-lasting motor commands encoding movement characteristics such as velocity, duration, and amplitude, followed by a maintained tonic firing encoding the position at which the moving appendage should be maintained (Robinson, 1981; Moschovakis, 1997). Thus, persistent neural activity seems to be necessary for both behavioral (positions of fixation) and cognitive (working memory) processes.