Systematic comparison of the effects of hippocampal and fornix-fimbria lesions on acquisition of three configural discriminations

Abstract

The effects of lesions to the hippocampal system on acquisition of three different configural tasks by rats were tested. Lesions of either the hippocampus (kainic acid/colchicine) or fornix-fimbria (radiofrequency current) were made before training. After recovery from surgery, rats were trained to discriminate between simple and compoundconfigural cues that signaled the availability or nonavailability of food when a bar was pressed. When positive cues were present, one food pellet could be earned by pressing a lever after a variable time had elapsed. The trial terminated on food delivery (variable interval 15 s). This procedure eliminates some possible alternative explanations of the results of previous experiments on configural learning. Hippocampal lesions increased rates of responding and retarded acquisition of a negative patterning task (A 1 , B 1 , AB 2 ); using a ratio measure of discrimination performance these lesions had a milder retarding effect on a biconditional discrimination (AX 1 , AY 2 , BY 1 , BX 2 ), and they had no effect on a conditional context discrimination (X: A 1 , B 2 ; Y: A 2 , B 1 ). Fornix-fimbria lesions did not affect acquisition of any of these tasks but increased rates of responding. The results suggest that several task parameters determine the involvement of the hippocampus in configural learning; however, all tasks tested can also be learned to some extent in the absence of an intact hippocampal system, presumably by other learning/memory systems that remain intact following surgery. The lack of effect of fornix-fimbria lesions on any of these tasks suggests that retrohippocampal connections with other brain areas may mediate hippocampal contributions to the learning of some configural tasks. An analysis of these results and of experiments on spatial learning situations suggests that involvement of the hippocampus is a function of the degree to which correct performance depends on a knowledge of relationships among cues in a situation.

The Role of CA3 Hippocampal NMDA Receptors in Paired Associate Learning

“…Episodic memory may involve hippocampal-dependent encoding of a conjunctive entity (or entities) derived from multiple cues that assume a different meaning or salience from that of the individual cues. However, nonlinear discrimination tasks requiring conjunctive representations may be acquired in rodents with extensive hippocampal lesions (O'Reilly and Rudy, 2001), including tasks that are formally indistinguishable from the paired associate task used in the present study (Winocur and Olds, 1978;McDonald et al, 1997;Good et al, 1998). Thus, a requirement for conjunctive encoding does not by itself incur hippocampal dependence for learning.…”

Fornix Transection Impairs Learning of Randomly Changing Object Discriminations

“…We chose to investigate the current task using fornix transection, rather than selective hippocampal lesions. Fornix transection interrupts the subcortical inputs and outputs of the hippocampus, and has previously been shown to have equivalent effects in a number of tasks to direct lesions to the hippocampus (Aggleton et al, 1992;Wible et al, 1992;Whishaw and Jarrard, 1995), although their effects are not always identical (McDonald et al, 1997). Importantly, transection of the fornix avoids damage to any of the cortical areas surrounding the hippocampus, particularly areas of rhinal cortex, and so may be a better way to produce selective hippocampal dysfunction than direct lesions.…”

Context representations, context functions, and the parahippocampal–hippocampal system

“…It is important to note that this type of arrangement almost guarantees that the animal will experience the correct inputs from the context features, the cues, the response, and the outcomes. Rats with complete damage to the hippocampal formation learn these discriminations as readily as intact animals (McDonald et al 1997;Good et al 1998;Coutureau et al 2002).…”