A great deal of research points to the idea that there are different forms of memory which depend on different brain systems. Explicit memory, which is memory for facts and events that you are consciously aware of appears to depend on brain circuitry including medial temporal lobes structures. Implicit memory, on the other hand, refers to information learned without awareness, which does not seem to depend on this brain system. In the proposed project, we plan to study habit learning, which is a form of Implicit learning. Habit learning refers to the gradual learning of associations across trials. Some research has suggested that the basal ganglia play a role in this type of learning. However, habit learning is not well understood in terms of the precise neural circuitry underlying it or its behavioral characteristics. By using animal models and studies with patients with basal ganglia disorders I plan to address both of these questions. In one habit learning task, we train rats to find food in parts of a maze that are signaled by a visual cue. We plan to make lesions in the caudate nucleus (part of the basal ganglia) at different times after training to see whether memories are stored in this structure or if it is only involved early in training with memories eventually stored elsewhere. We will also investigate the role of the nucleus accumbens in habit learning, a structure with significant interconnections with the basal ganglia that has been implicated in reinforcement. The work on humans will focus on patients with Parkinson's disease, which results in basal ganglia dysfunction. We will use a human habit learning task that I have developed in which different cues are probabilistically associated with different outcomes. Because the associations are probabilistic, subjects learn them implicitly and feel as if they are guessing. Patients with Parkinson's disease are impaired at this task, and I will test whether surgical treatments used to treat this disease have any effect on performance. I will also develop additional tasks to tap into this system, and examine the behavioral characteristics of these tasks, such as their dependence on attention. I will compare patients with Parkinson's disease and Alzheimer's disease on habit learning and explicit memory tasks. Because Alzheimer's disease mostly spares the basal ganglia, I will examine if habit learning can proceed normally in those patients. I will also look at basal ganglia activity using functional MRI during performance of implicit learning tasks to help define the characteristics of basal ganglia dependent learning. The proposed project also has a large educational component, in that I am continuing to develop a cognitive neuroscience major in our Psychology department to give students integrated training in the study of the nervous system and in cognitive science. Understanding the functions of basal ganglia has practical significance because this system is affected in a number of human disease states including Huntington's disease and Parkinson's disease. Although these diseases are generally thought of as involving motor function, they also involve cognitive deficits and mood changes. Implicit learning problems may have an impact on these deficits, and the depression associated with these diseases may in part stem from difficulties in using implicitly learned social information. Learning how the basal ganglia circuitry participates in learning will help us to better treat patients with these diseases.
