Contributions of the hippocampus and the striatum to simple association and frequency-based learning

Abstract

Using fMRI and a learning paradigm, this study examined the independent contributions of the hippocampus and striatum to simple association and frequency-based learning. We scanned 10 right-handed young adult subjects using a spiral in/out sequence on a GE 3.0 T scanner during performance of the learning paradigm. The paradigm consisted of 2 cues that predicted each of 3 targets with varying probabilities. Simultaneously, we varied the frequency with which each target was presented throughout the task, independent of cue associations. Subjects had shorter response latencies to frequently occurring and highly associated target stimuli and longer response latencies to infrequent target stimuli, indicating learning. Imaging results showed increased caudate activity to infrequent relative to frequent targets and increased hippocampal activity to infrequent relative to frequent cue-target associations. This work provides evidence of different neural mechanisms underlying learning based on simple frequencies versus associations within a single paradigm.

Fig. 1

Illustrates task learning parameters, frequency of target occurrences and frequency of associations between cues and targets.

Fig. 2

The frequent target has a higher association with Cue 1 relative to Cue 2 (75% vs. 25%), providing a means of investigating response to the target based on cue–target association. The infrequent (20%) and the frequent (50%) targets are equally associated with Cue 2, allowing us to equate for context and determine regions responsive to frequency of target occurrence during learning.

Fig. 3

Illustrates task design, an ongoing stream of alternating cue and target events separated by 3 s intervals (i.e., 8 s between target events, as well as the estimated peak of the BOLD signal every 5–6 s).

Fig. 4

Mean reaction time differences observed between the frequently and infrequently presented targets. Both the Frequency of Target Occurrence and Frequency of Cue–Target Association comparisons show continued learning as the task progresses. Response times to the infrequently presented relative to the frequently presented targets increase as probabilities of occurrence and association are learned over time on task.

Fig. 5

Illustrates significant ROIs for each comparison as well as signal changes to novelty during learning.