Rats with lesions of the hippocampus are impaired on the delayed nonmatching-to-sample task

Spatial memory and choice behavior in the radial arm maze after fornix transection

In the radial arm maze task, it is well established that performance of rats with hippocampal damage is severely impaired on the place version, which relies heavily, if not exclusively, on spatial information. However, very little is known about the effects of hippocampal damage on actual choice behavior. To address this issue, sham-operated (SH) and fornix-transected (FX) rats were trained and tested on the place task in the eight-arm radial maze. The following measures were recorded: the frequency of re-entry errors, the number of choices separating repeated visits to the same arm, the latency to arm re-entry, the distribution and targets of microchoices defined as orientations toward an arm or entries in the proximal portion of an arm [Brown, M.F., 1992. Does a cognitive map guide choices in the radial arm maze? J. Exp. Psychol., Anim. Behav. Processes 18, 56-66]. These measures were used as indexes of performance, within-trial retroactive intrusion, memory trace decay and choice behavior, respectively. As generally observed in the literature, the frequency of errors was higher in rats of the FX group than in rats of the SH group; the impairment persisted even after the training criterion was reached. The analysis of latency to arm re-entry and of the number of choices separating re-entries suggested that this impairment was the result of faster memory decay rather than retroactive interference. Both FX and SH groups exhibited a systematic pattern of microchoices, but the frequency of microchoices was higher in FX lesioned rats than in SH controls. Moreover, in lesioned animals, relatively fewer of the initial microchoices were directed toward the baited arms during training-to-criterion. Some of the results provide support to the working memory theory [Olton, D.S., Becker, J.T., Handelmann, G.E., 1979. Hippocampus, space, and memory. Behav. Brain Sci. 2, 313-365], whereas others look more consistent with the cognitive map view [O'Keefe, J., Nadel, L., 1978. The Hippocampus as a Cognitive Map. Clarendon Press, Oxford]. The discussion suggests that both theories and a distinction between prospective and retrospective memory may be required to account for the function of the hippocampal formation in memory.

Hippocampal damage and exploratory preferences in rats: memory for objects, places, and contexts

Rats have a natural tendency to spend more time exploring novel objects than familiar objects, and this preference can be used as an index of object recognition. Rats also show an exploratory preference for objects in locations where they have not previously encountered objects (an index of place memory) and for familiar objects in contexts different from those in which the objects were originally encountered (an index of context memory). In this experiment, rats with cytotoxic lesions of the hippocampal formation were tested on all three versions of the novelty-preference paradigm, with a 5-min retention interval between the familiarization and test phases. Rats with sham lesions displayed a novelty preference on all three trial types, whereas the rats with hippocampal lesions displayed a novelty preference on Object trials but did not discriminate between the objects on Place trials or Context trials. The findings indicate that hippocampal damage impairs memory for contextual or spatial aspects of an experience, whereas memory for objects that were part of the same experience are left relatively intact.

Perspectives on object-recognition memory following hippocampal damage: lessons from studies in rats

One of the routine memory abilities impaired in amnesic patients with temporal-lobe damage is object-recognition memory--the ability to discriminate the familiarity of previously encountered objects. Reproducing this impairment has played a central role in animal models of amnesia during the past two decades, and until recent years most of the emphasis was on describing how hippocampal damage could impair object recognition. Today most investigators are looking outside the hippocampus to explain the impairment. This paper reviews studies of object-recognition memory in rats with hippocampal damage produced by ablation, fornix transection, or forebrain ischemia. Some new perspectives on previous findings reinforce the conclusion that damage to the hippocampus has little if any impact on the ability to recognize objects, while damage in some areas outside the hippocampus is far more effective. The few circumstances in which hippocampal damage can impair performance on object-recognition tasks are situations where ancillary abilities are likely to play a significant role in supporting task performance. Some of the factors that contributed to the origins and persistence of the hippocampalcentric view of object-recognition are considered, including lesion confounds, failure to distinguish between impaired task performance and impairment of a memory ability, and disproportionate attention to a few lesion studies in monkeys, even though the hypothesis was tested far more times in rats, under a greater variety of conditions, and rejected on nearly every occasion.

Impaired auditory recognition memory in amnesic patients with medial temporal lobe lesions

Two tests of auditory recognition memory were given to four patients with bilateral hippocampal damage (H+) and three patients with large medial temporal lobe lesions and additional variable damage to lateral temporal cortex (MTL+). When single stimuli were presented, performance was normal across delays as long as 30 sec, presumably because information could be maintained in working memory through rehearsal. When lists of 10 stimuli were presented, performance was impaired after a 5-min delay. Patients with MTL+ lesions performed marginally worse than patients with H+ lesions, consistent with findings for recognition memory in other modalities. The findings show that auditory recognition, like recognition memory in other sensory modalities, is dependent on the medial temporal lobe.