Short-Term Odor Memory: Effects of Posterior Transection of the Lateral Olfactory Tract in the Rat

Abstract

Thanos, P. K., B. M. Slotnick. Short-Term Odor Memory: Effects of Posterior Transection of the Lateral Olfactory Tract in the Rat. Physiol Behav 61(6) 903–906, 1997.—Rats were trained on a series of novel 2-odor discrimination problems before and after combined unilateral bulbectomy and posterior transection of the contralateral lateral olfactory tract. In postoperative tests, experimental rats performed as well as controls when a short intertrial interval (30 seconds) was used but, in contrast to controls, failed to learn a 2-odor discrimination when the intertrial interval was 10 minutes. When tested on a reversal task, controls showed memory for original learning by making many errors while experimental rats quickly acquired the task. The results suggest that lateral olfactory tract afferents to posterior olfactory cortex may play a significant role in short-term memory for odors.

Introduction

Because rats readily learn both simple odor detection and discrimination tasks and more complex problems including learning sets 4, 8, 12, 13, 15, 16, 17, paired associated learning [1] and delayed matching- and non-matching-to-sample 5, 9, the rodent olfactory system provides a useful model for a neurobiological analysis of cognitive functions [14]. Rats also display excellent short-term memory for odors. Thus, Lovelace and Slotnick [3] found that intertrial interval delays for up to 30 min had little effect in how rapidly novel discrimination problems were learned.

The anatomical pathways mediating these learning and memory functions are not known. Slotnick and Risser [18] failed to find deficits in long-term memory for preoperatively learned odor discriminations in rats with posterior transection of the lateral olfactory tract (LOT) or with lesions of the mediodorsal thalamic nucleus. However, interruption of both of these pathways produced marked deficits in retention and, thus, the storage or recall of long-term odor memory may involve both the olfactory thalamocortical system and olfactory projections to the limbic system (amygdala and entorhinal cortex).

With regard to short-term odor memory, Staubli, Ivy and Lynch [23] found that rats with lesions of the entorhinal cortex performed well on an olfactory discrimination task when a short intertrial interval (ITI) was used but poorly when the ITI was 3–10 min. When tested on an olfactory discrimination reversal task, experimental rats made fewer errors than controls presumably because they had little or no memory for the initial discrimination task. In a subsequent study, Staubli, Fraser, Kessler and Lynch [22] reported that rats with entorhinal lesions showed good retention of preoperatively learned odor tasks but poor retention when tested on those acquired postoperatively. Because the entorhinal cortex provides a major source of afferents to the hippocampus, a structure that has been widely implicated in mediation of short-term memory, Staubli et al. 22, 23 suggested that olfactory deafferentation of the hippocampus by entorhinal lesions produces rapid forgetting of newly acquired olfactory information. In support of this conclusion Otto and Eichenbaum [9] reported that rats with perirhinal and entorhinal cortical lesions were impaired on a delayed nonmatching-to-sample task when the delay was 30–60 s but not when a short delay (3 s) was used.

While the results of Staubli et al. 22, 23 and Otto and Eichenbaum [9] are in agreement with many studies implicating the hippocampus in memory function (e.g., [21]), recent reports suggest that it is damage to rhinal cortical regions (including the perirhinal, entorhinal and parahippocampal cortices) that is responsible for memory loss observed from large hippocampal lesions 7, 24, 25. These regions have reciprocal connections with neocortical areas subserving all sensory modalities [6] and they or components of them may be the structures critical for mediating short-term memory. Because lesions in the Staubli et al. 22, 23 and Otto and Eichenbaum [9] studies damaged these rhinal cortical areas, it is uncertain whether the effects obtained reflect a general disruption of short-term memory function or one specific to olfaction. To examine this question the present experiment used a modification of the Staubli et al. [23] procedures to assess short-term memory function in rats with lesions that deafferent posterior olfactory cortical areas (including the entorhinal cortex) from olfactory input but otherwise leave these cortical areas intact.