The Necessity of the Hippocampus for Statistical Learning

Abstract

Converging evidence points to a role for the hippocampus in statistical learning, but open questions about its necessity remain. Evidence for necessity comes from Schapiro and colleagues who report that a single patient with damage to hippocampus and broader medial temporal lobe cortex was unable to discriminate new from old sequences in several statistical learning tasks. The aim of the current study was to replicate these methods in a larger group of patients who have either damage localized to hippocampus or broader medial temporal lobe damage, to ascertain the necessity of the hippocampus in statistical learning. Patients with hippocampal damage consistently showed less learning overall compared with healthy comparison participants, consistent with an emerging consensus for hippocampal contributions to statistical learning. Interestingly, lesion size did not reliably predict performance. However, patients with hippocampal damage were not uniformly at chance and demonstrated above-chance performance in some task variants. These results suggest that hippocampus is necessary for statistical learning levels achieved by most healthy comparison participants but significant hippocampal pathology alone does not abolish such learning.

Figures

Figure 1

Magnetic resonance scans of hippocampal patients. Images are coronal slices through four points along the hippocampus from T1-weighed scans. Volume changes can be noted in the hippocampal region for Patients 1846 and 2363, and significant bilateral MTL damage including the hippocampus can be noted in Patient 1951. A = anterior; P = posterior; NC = healthy comparison brain.

Figure 2

Shape and scene stimuli (tone and syllable stimuli described in the text).

Figure 3

Experiment 1: proportion correct across four versions of the statistical learning task by patients with amnesia and healthy comparison participants. Boxplots indicate the group level IQR.

Figure 4

Experiment 2: proportion correct across four versions of the statistical learning task by patients with amnesia and healthy comparison participants. Boxplots indicate the group level IQR.

Figure 5

Experiment 3: proportion correct across three versions of the item recognition task by patients with amnesia. Boxplots indicate the group level IQR.

Figure 6

Experiment 4: proportion correct across four versions of the statistical learning task with L. S. J. triplet assignment by patients with amnesia. Boxplots indicate the group level IQR.

Figure 7

Experiment 5: proportion correct across four versions of the statistical learning task with L. S. J. pair assignment by patients with amnesia. Boxplots indicate the group level IQR.

Figure 8

Model-based estimate of by-participant random effects. Patient data across Experiments 1, 2, 4, and 5. Error bars indicate 95% confidence intervals.