Abstract
Vicarious trial-and-errors (VTEs) are back-and-forth movements of the head exhibited by rodents and other animals when faced with a decision. These behaviors have recently been associated with prospective sweeps of hippocampal place cell firing, and thus may reflect a rodent model of deliberative decision-making. The aim of the current study was to test whether the hippocampus is essential for VTEs in a spatial memory task and in a simple visual discrimination (VD) task. We found that lesions of the hippocampus with ibotenic acid produced a significant impairment in the accuracy of choices in a serial spatial reversal (SR) task. In terms of VTEs, whereas sham-lesioned animals engaged in more VTE behavior prior to identifying the location of the reward as opposed to repeated trials after it had been located, the lesioned animals failed to show this difference. In contrast, damage to the hippocampus had no effect on acquisition of a VD or on the VTEs seen in this task. For both lesion and sham-lesion animals, adding an additional choice to the VD increased the number of VTEs and decreased the accuracy of choices. Together, these results suggest that the hippocampus may be specifically involved in VTE behavior during spatial decision making.
Highlights
When a rat makes a decision at a choice point on a maze, it often exhibits side-to-side movements of the head as it looks down each of the alternative routes
We found that rats with hippocampus damage were impaired in performing a serial spatial reversal (SR) task, and did not show an elevated number of vicarious trial-and-error (VTE) before finding where the food was located in each session, as seen in control animals
SUMMARY The current study makes three contributions. It shows that selective lesions of the hippocampus, damaging on average 81.7% of the dorsal hippocampus and 37.8% of the ventral hippocampus, do not abolish the capacity to make VTE responses, even in a task where these lesions are sufficient to produce a performance impairment
Summary
When a rat makes a decision at a choice point on a maze, it often exhibits side-to-side movements of the head as it looks down each of the alternative routes. In one experiment, Johnson and Redish (2007) associated VTE behavior with anticipatory firing of hippocampal place cells during a spatial task They recorded CA3 place cell ensembles in the dorsal hippocampus as rats navigated a multiple T-maze and found that, at the highest cost T-junction, CA3 representations tended to fire in sequence ahead of the rat along the alternative routes, transiently representing future possible routes. These hippocampal sweeps occurred when the rat was displaying VTE behavior, suggesting that VTE behavior may be associated with hippocampal representations of the future. Recent work has shown that patients with hippocampus damage show fewer “spontaneous revisitations”— looks back to previously viewed stimuli in a computer-based memory task—a behavior that is a potential human analog of VTEs (Voss et al, 2011)
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