Neural circuitry for rat recognition memory

Abstract

Information concerning the roles of different brain regions in recognition memory processes is reviewed. The review concentrates on findings from spontaneous recognition memory tasks performed by rats, including memory for single objects, locations, object-location associations and temporal order. Particular emphasis is given to the potential roles of different regions in the circuit of interacting structures involving the perirhinal cortex, hippocampus, medial prefrontal cortex and medial dorsal thalamus in recognition memory for the association of objects and places. It is concluded that while all structures in this circuit play roles critical to such memory, these roles can potentially be differentiated and differences in the underlying synaptic and biochemical processes involved in each region are beginning to be uncovered.

Keywords: Hippocampus; Medial prefrontal cortex; Neural circuit; Object recognition memory; Perirhinal cortex.

Fig. 1

Diagram of the four object recognition memory tasks. (A) Novel object preference task, (B) object location task, (C) object-in-place task, (D) temporal order task.

Fig. 2

Schematic diagram of the main pathways underlying object recognition memory that involves multiple items and their contextual associations or the temporal order in which items are encountered.

Fig. 3

(A) Diagram of the temporal order memory task including the timing of the drug infusions. Illustrated is the square arena containing the stimulus objects. In sample phase 1 two identical objects are presented and following an inter-sample interval two different objects are presented in sample phase 2. Following a retention delay, an object from sample phase 1 and an object from sample phase 2 are presented in the test phase. To examine drug effects on encoding, drug infusions were given prior to sample phase 2. To examine effects on retrieval infusions were given prior to the test phase. (B) The effect of AMPA receptor blockade on temporal order memory. Illustrated for each group is the mean (+SEM) discrimination ratio. **P < 0.01, ***P < 0.001, difference between groups. Bilateral infusion of CNQX into the perirhinal cortex (PRH) or medial prefrontal cortex (PL/IL) given before the sample phase 2 or before the test phase. (C) The effect of unilateral drug infusions into the perirhinal cortex (PRH) and medial prefrontal cortex (PL/IL) in opposite hemispheres, prior to sample phase 2 on temporal order memory. Unilateral drug infusions into the PRH, represented by grey shading, disrupts encoding of the object presented in sample phase 2 (S2) so that the S2 object is represented as a novel object. The object information is sent to the un-infused PL/IL. In the opposite hemisphere the un-infused PRH encodes the objects presented in sample phase 1 (S1) and sample phase 2 (S2) as familiar, and may also encode the relative recency of the object presentation, i.e. that the S1 object is an ‘old’ object, while the S2 object has been encountered relatively recently. This object information is sent to the infused PL/IL, but here the order information cannot be expressed correctly as processing has been disrupted by the drug infusion. The bilateral disruption within the temporal order memory circuit results in impaired discrimination.