Using multi-task experiments to test principles of hippocampal function

Abstract

Investigations of hippocampal functions have revealed a dizzying array of findings, from lesion-based behavioral deficits, to a diverse range of characterized neural activations, to computational models of putative functionality. Across these findings, there remains an ongoing debate about the core function of the hippocampus and the generality of its representation. Researchers have debated whether the hippocampus's primary role relates to the representation of space, the neural basis of (episodic) memory, or some more general computation that generalizes across various cognitive domains. Within these different perspectives, there is much debate about the nature of feature encodings. Here, we suggest that in order to evaluate hippocampal responses-investigating, for example, whether neuronal representations are narrowly targeted to particular tasks or if they subserve domain-general purposes-a promising research strategy may be the use of multi-task experiments, or more generally switching between multiple task contexts while recording from the same neurons in a given session. We argue that this strategy-when combined with explicitly defined theoretical motivations that guide experiment design-could be a fruitful approach to better understand how hippocampal representations support different behaviors. In doing so, we briefly review key open questions in the field, as exemplified by articles in this special issue, as well as previous work using multi-task experiments, and extrapolate to consider how this strategy could be further applied to probe fundamental questions about hippocampal function.

Keywords: cognitive map; concept cells; hippocampus; multi-task experiments; neural representation; place cells; spatial navigation.

Figure 1 -. Schematic of possible designs for multi-task experiments.

Examples show a multi-task experiment represented by a memory task (task A) and a spatial navigation task (task B) that are either administered back-to-back (A) or interleaved (B). A) Example layout of a back-to-back multi-task experiment in which subjects first complete a memory-related task, for example, viewing and responding to images of stimuli (Task A; blue). After completion of this task, subjects then immediately switch to another task, for example a space-related task, for example, navigating in a virtual/real-world environment, which could include navigating to specific locations, encountering stimuli, responding to task prompts, and receiving a reward / score based on performance accuracy (Task B; orange). The bottom raster plot indicates spikes (simulated) representing the activity on an example neuron recorded across the experiment throughout both tasks in the example session, color-coded to reflect the task to which each spike can be associated. B) Example layout of a multi-task design using the same example tasks, but in an interleaved fashion such that task A and B alternate on a block- or trial-level, with example neural activity color coded by task, with black representing non-task specific activity at the start and end of the multi-task experiment session. Note that the example tasks shown here are placeholders for any combination of task designs that can be combined. Icons from https://www.flaticon.com.