The importance of the rat hippocampus for learning the structure of visual arrays

Abstract

It has been assumed that the integrity of the rodent hippocampus is required for learning the spatial distribution of visual elements in an array. Formally assessing this assumption is, however, far from straightforward as standard tests are amenable to alternative strategies. In order to provide a stringent test of this ability rats were trained on three concurrent visual discriminations in a water tank in which the stimuli in each pair of discriminations contained exactly the same elements but they differed in their spatial arrangement e.g. A|B vs. its mirror image B/A. Such 'structural' discriminations are a specific subtype of 'configural' or 'nonlinear' tasks. Following acquisition half of the rats received hippocampal lesions and all rats were retrained on the structural discriminations. Hippocampal lesions impaired the ability to relearn these 'structural' discriminations. In contrast, two other groups of rats with similar hippocampal lesions showed no impairment on relearning two non-structural, configural discriminations: transverse patterning and biconditional learning. All three tasks used the same apparatus, the same stimulus elements, and similar training regimes. Superior performance by the rats with hippocampal lesions during a generalization decrement probe showed that hippocampal lesions had diminished sensitivity to 'structural' features on the biconditional task. While the rat hippocampus need not be required for all configural learning, it is important for the special case when the spatial arrangements of the elements are critical. This ability may be a prerequisite for the creation of mental snapshots, which underlie episodic memory.

Structural learning difficulties implicate altered hippocampal functioning in adults with autism spectrum disorder.

“…Moreover, we predicted that this lower performance would not be attributable to difficulties in executive functions, which would have implicated abnormalities in frontalhippocampal interactions (Sanderson et al, 2006). These predictions were confirmed by the significantly lower performance in ASD compared to TD individuals over the five blocks of…”

“…The materials and procedures were adapted from Sanderson et al (2006) and similar rodent studies (e.g., Aggleton et al, 2007) and involved minimal verbal instructions. Black and white images (see Figure 1) were presented on a touch-sensitive 12-inch laptop-screen.…”

“…Similarly, Structural Learning is involved when we learn that certain behaviours in one pragmatic context facilitate social affiliation with certain individuals whilst the same behaviours in a different context or in relation to different individuals can lead to confrontation. If atypical hippocampal function plays an important role in the presentation of ASD, it follows that Structural Learning should pose particular difficulties, whereas other forms of configural learning that pose similar processing demands but do not necessitate hippocampally mediated processing of spatial-temporal information, should be preserved (Aggleton et al, 2007(Aggleton et al, , 2009Sanderson et al, 2006). For example, Biconditional Discrimination also requires the processing of specific relations among the elements of a stimulus but unlike in Structural Learning, the spatial or temporal configuration of these relations is irrelevant.…”

“…Based on the literature reviewed above, the first aim of the present study was to adapt the task by Sanderson et al (2006) to test the prediction that ASD would be characterised by specific impairments in Structural Learning, but not Biconditional Discrimination, or Transverse Patterning. In addition, we expected better performance in novel test trials (repaired for Structural Learning and mirror images for Biconditional Discrimination) and higher ratio scores for ASD compared to TD participants.…”

“…To test the prediction that specifically Structural Learning, but not Biconditional Discrimination or Transverse Patterning is impaired in ASD, the current study adopts a paradigm by Sanderson et al (2006), who examined the consequences of hippocampal lesions on these three types of configural learning in rats. In their experiment, independent groups of rats first learned to discriminate between pairs of stimuli that were constructed to specifically probe each of the three learning processes.…”