The neural circuits that link the bas ganglia with the cerebral cortex are critically involved in the generation and control of temporal sequences of responses that are guided by an internal timing system that includes pacemaker, memory, and decision states. The input of the basal ganglia (that is the caudate and putamen) receive substantial projections from diverse regions of the cerebral cortex including motor, sensory, prefrontal, and limbic cortical areas. The outp nuclei of the basal ganglia (that is, the globus palladiums and the substantial Nigeria pars reticulate send their axons to the thalamus and, by this route, project back upon the cortex. Thus, a major aspect of basal ganglia circuitry is its participation in multiple open and closed loops wi the cerebral cortex. Our understanding of the organization of basal ganglia loops with the cerebral cortex has evolved considerably over the last 20 years. It is now believed that both cognitive and motor impairments result from damage to the basal ganglia, either studied with animal models or in the naturally occurring conditions of Parkinson's disease, Huntington's disease, and various types of focal lesions that occu as a result of stroke or accident. Attempts to separate the cognitive and motor components these various conditions has been hindered because it has been difficult to develop tests that were selectively sensitive to the types of dysfunctions presented by these subcortical dementias. Behavioral paradigms designed to study interval-timing mechanisms in and analogous fashion with both animals and humans have now been developed. Scalar Timing Theory is a mathematical model that describes formal properties of the analysis has successfully accounted for an extremely high percentage of the variance in behavior of both rats and humans. It has also led to the validation of a set of experimental procedures that can be used to separate the different psychological processes involved in interval timing. A battery of peak-interval timing procedures have been developed to evaluate basic temporal integration processes, temporal memory, and divided attention when auditory and visual stimuli are presented sequentially or simultaneously. By combining these behavioral procedures with the micro injection of series of selective gultamiergic receptor agonists or with placement of axon sparing lesions into specific nuclei of the basal ganglia an attempt will be made to map the neural circuitry involved in interval timing is represented by the ganglia thalamocortical loops.
