Concept formation as a computational cognitive process

Abstract

Hippocampus and entorhinal cortex form cognitive maps that represent relations among memories within a multidimensional space. While these relational maps have long been proposed to contribute to episodic memory, recent work suggests that they also support concept formation by representing relevant features for discriminating among related concepts. Cognitive maps may be refined by medial prefrontal cortex, which selects dimensions to represent based on their behavioral relevance. Hippocampal pattern completion, which is critical for retrieval of episodic memories, may also contribute to generalization of existing concepts to new exemplars. Navigation within hippocampal cognitive maps, which is guided by grid coding in entorhinal cortex, may contribute to imagination through recombination of event elements or concept features.

Keywords: category learning; cognitive map; episodic memory; imagination.

Figure 1.

(A) Participants learned about separate hierarchies of fictional people along two dimensions, popularity and competence. After learning, although these two dimensions were never relevant at the same time, neural representations in hippocampus (HC), entorhinal cortex (ERC), and mPFC reflected the two-dimensional Euclidean distance between items, suggesting that the individual item dimensions were represented within a unified space. Adapted from [18]. (B) Grid cells, which are thought to provide a metric for navigating cognitive maps, fire at points along a hexagonal grid within a space. Grid cell responses are predicted to result in greater activity during movement along preferred angles. Participants learned to navigate a two-dimensional feature space to move to goal locations. Grid-like activity was observed in entorhinal cortex, mPFC, and posterior cingulate cortex (PCC). Adapted from [2]. (C) Participants learned a series of different categorization problems using the same stimuli that required taking into account one stimulus dimension (low complexity) or two dimensions (medium complexity). Item representations in hippocampus reflect attentional weighting of relevant dimensions of different concept learning problems. In a one-dimensional problem, the relevant stimulus dimension is weighted more heavily. In a two-dimensional problem, both relevant dimensions are weighted highly relative to an irrelevant dimension. Adapted from [3]. (D) Through learning, activity patterns in mPFC formed lower-dimensional, compressed representations which facilitated focusing on only the relevant dimension(s) for each problem. The mPFC representations formed during learning of low- and medium-complexity problems showed greater evidence of compression compared to the high-complexity problem that requires combining information from three dimensions. Adapted from [43].

Figure 2.

Cognitive maps form organized representations of conceptually similar events. (A) Participants studied initial (AB) and overlapping (BC) pairs that shared a common object and were instructed to learn each pair. They studied 12 triads with the same structure. (B) Brain activity elicited by each item was measured using fMRI before and after the study phase. There was no particular organization before the study phase, but after study neural representations in medial temporal lobe and frontoparietal areas reflected the common structure across events in the form of consistent representational geometry of the distinction between A and C items. (C) The reliability of neural organization across triads in parahippocampal cortex, lateral parietal cortex, and hippocampus predicted performance on an inference test where participants had to select the correct A item that had been indirectly associated with a cue C item. These results suggest that retrieval may be guided by navigation within an organized cognitive map using a vector-based retrieval mechanism. Adapted from [29].

Figure 3.

(A) Hippocampus is thought to bring together information about contexts (e.g., an office, a beach, and a college campus) and information about items such as people. Recent evidence suggests this factorization of situations into combinations of items and context facilitates flexible recombination of elements to facilitate imagination or reconstruction of specific episodes (here, imagining a new episode involving James at the college campus). (B) Within a context, such as the context of selecting a new vehicle, hippocampus may represent relevant item dimensions such as cargo capacity and passenger capacity. Representation of relevant feature dimensions may support a number of operations, including categorization (e.g., identifying a new vehicle as a truck) and reconstruction of features of a specific remembered item (e.g., if the vehicle was a truck it must have had a truck bed). Multidimensional spaces in hippocampus may also support exploration of new concepts through flexible recombination of features; for example, a potential vehicle located at the point between sedans and SUVs could be predicted to have a different set of properties from known categories.