Mar 12

Operating characteristics of the implicit procedural learning system

Sanchez, D.J., Gobel, E.W., Reber, P.J. CNADC 16th Annual Alzheimer Day. May 6th, 2010.

The majority of actions we take to accomplish everyday activities are based around routine behaviors. The procedural learning involved in developing a routine behavior, or skill, relies on an implicit memory system preserved in patients with Alzheimer’s disease. Rooted in the basal ganglia and connected cortico-striatal circuits, this preserved learning may support cognitive rehabilitation that utilizes abilities that AD patients retain. It is therefore critical to understand the capacity and operating characteristics of this type of memory. Procedural learning can be isolated from other types of memory in a perceptual-motor sequence learning task, such as the serial interception sequence learning (SISL) task. Based on the well-studied serial reaction time (SRT) task, the SISL task requires participants to make continuous sequences of keypress responses to cues scrolling vertically up a computer screen. Participants are instructed to time their response to coincide with a cue reaching a stationary target on the opposite end of the screen. Participants are not told that the cues appear in a repeating sequence (typically 12 items long), but they do show gradual improvements in performance that reflect learning of the repeating sequence.

Three experiments are reported that use the SISL task to examine procedural learning dependent on cortico-striatal circuits. In the first experiment, participants learned to perform the SISL task more accurately after practice with a repeating sequence but exhibited little or no explicit, conscious memory of the repeating sequence. This result shows that learning in this task is independent of the explicit memory system that is impaired in Alzheimer’s disease. In a second experiment, participants learned a repeating sequence with an embedded timing structure. When presented with the same sequence of motor responses but novel inter-item timing, performance was as poor as with a novel sequence. The lack of transfer indicates that this type of procedural learning integrates both action selection and timing. In a third experiment, participants exhibited significant learning of much longer sequences than have previously been studied; up to 80 items long. The ability to learn such long sequences indicates that the cortico-striatal learning system has a greater learning capacity than has previously been appreciated. By furthering our understanding of the function and capacities of this learning system, future work may be able to develop a cognitive rehabilitation approach that builds on this preserved memory function and improves general cognitive function in memory-disordered patients.