Perirhinal cortex lesions impair simultaneous but not serial feature-positive discrimination learning

Perirhinal and Postrhinal Damage Have Different Consequences on Attention as Assessed in the Five-Choice Serial Reaction Time Task

The perirhinal (PER) and postrhinal (POR) cortices, structures in the medial temporal lobe, are implicated in learning and memory. The PER is understood to process object information and the POR to process spatial or contextual information. Whether the medial temporal lobe is dedicated to memory, however, is under debate. In this study, we addressed the hypothesis that the PER and POR are also involved in non-mnemonic cognitive functions. Rats with PER or POR damage and SHAM surgical controls were shaped, trained, and tested on the five-choice serial reaction time (5CSRT) task, which assesses attention and executive function. Rats with PER damage were impaired in acquiring the task and at asymptote, although processing information about objects was not relevant to the task. When confronted with attentional challenges, rats with PER damage showed a pattern consistent with decreased attentional capacity, increased response errors, and increased impulsive behavior. Rats with POR damage showed intact acquisition and normal asymptotic performance. They also exhibited faster latencies in the absence of speed accuracy trade-off suggesting enhanced response readiness. We suggest this increased response readiness results from decreased automatic monitoring of the local environment, which might normally compete with response readiness. Our findings are consistent with a role for PER in controlled attention and a role for POR in stimulus-driven attention providing evidence that the PER and POR cortices have functions that go beyond memory for objects and memory for scenes and contexts, respectively. These findings provide new evidence for functional specialization in the medial temporal lobe.

Perirhinal cortex supports tactual discrimination tasks with increasing levels of complexity: Retrograde effect

Recent studies have suggested that the perirhinal cortex (Prh) supports representations of feature conjunctions in the visual modality during the acquisition/encoding of complex discriminations. To extend this idea to other sensory modalities and to another stage of the discrimination process, we studied the effect of Prh lesions on the expression of a series of tactual discrimination tasks learned preoperatively. These tasks differed from one another in the degree of feature overlap of the stimuli and in the difficulty of the task. During pre- and post-operative testing phases, rats had to discriminate among 3 stimuli simultaneously exposed in 3 arms of a 4-arm plus-shaped maze. Prh-damaged rats showed a profound impairment in the expression of tactual discrimination tasks when the stimuli had a high or intermediate degree of feature ambiguity, but not when they had a low degree of ambiguity (experiments 1a-1c). In order to experimentally dissociate between subregions within the medial temporal lobe, experiment 2 was conducted to show that hippocampal lesions did not cause any impairment in task expression even when the stimuli had a high degree of feature ambiguity. When the tactual discrimination tasks used simple/individual nonoverlapping features of the stimuli (size), Prh lesions did not affect the expression of these discriminations despite the high level of difficulty of these tasks (experiments 3a and 3b). These findings suggest that, in the somatosensory modality, the Prh plays an essential role in the processing of complex stimuli with overlapping features but not in simple tactual discriminations. Furthermore, the Prh is necessary not just during acquisition but also during expression/performance of the discrimination task.

Change in hippocampal theta activity during behavioral inhibition for a stimulus having an overlapping element

It is believed that a decline in hippocampal theta power is induced by response inhibition for a conflict stimulus having an overlapping element. This study used a simultaneous feature positive (simul FP: A-, AX+) task and a serial FP (A-, X→A+) task. In these tasks, the compound and single stimuli have an overlapping element, and rats are required to exhibit response inhibition for the single stimulus A. We examined hippocampal theta activity during simul FP (A-, AX+), serial FP (A-, X→A+), and simple discrimination (SD; A-, X+) tasks and revealed that the transient decrease in hippocampal theta power occurred during response inhibition for the single stimulus A in simul FP tasks, which provides evidence that a transient decline in hippocampal theta power is induced by behavioral inhibition of conflict stimuli having an overlapping element. Thus, we concluded that the transient decline in hippocampal theta power was induced by behavioral inhibition for the conflict stimulus having an overlapping element.

Hippocampal state-dependent behavioral reflex to an identical sensory input in rats

We examined the local field potential of the hippocampus to monitor brain states during a conditional discrimination task, in order to elucidate the relationship between ongoing brain states and a conditioned motor reflex. Five 10-week-old Wistar/ST male rats underwent a serial feature positive conditional discrimination task in eyeblink conditioning using a preceding light stimulus as a conditional cue for reinforced trials. In this task, a 2-s light stimulus signaled that the following 350-ms tone (conditioned stimulus) was reinforced with a co-terminating 100-ms periorbital electrical shock. The interval between the end of conditional cue and the onset of the conditioned stimulus was 4±1 s. The conditioned stimulus was not reinforced when the light was not presented. Animals successfully utilized the light stimulus as a conditional cue to drive differential responses to the identical conditioned stimulus. We found that presentation of the conditional cue elicited hippocampal theta oscillations, which persisted during the interval of conditional cue and the conditioned stimulus. Moreover, expression of the conditioned response to the tone (conditioned stimulus) was correlated with the appearance of theta oscillations immediately before the conditioned stimulus. These data support hippocampal involvement in the network underlying a conditional discrimination task in eyeblink conditioning. They also suggest that the preceding hippocampal activity can determine information processing of the tone stimulus in the cerebellum and its associated circuits.

The effects of combined perirhinal and postrhinal damage on complex discrimination tasks

Rats with combined lesions of the perirhinal (PER) and postrhinal (POR) cortices were trained on a complex discrimination in the simultaneous feature-positive and feature-negative discrimination task. In this task, a panel light (L) paired with an auditory stimulus determined whether a tone (T) or white noise (N) would be rewarded (+) or not rewarded (-). Thus, the light feature determined whether the target auditory stimuli were rewarded or not. In each session, trial types were LT+, T-, N+, and LN-. We had hypothesized that damage to the target regions would impair performance on this task. Acquisition was altered in the lesioned rats, but not in the predicted direction. Instead, lesioned rats exhibited significantly enhanced acquisition of the discrimination. Manipulation of intertrial intervals indicated that reduction of proactive interference did not explain the enhancement. Lesioned rats were not different from controls on a multiple-cued interval timing task, providing evidence that the enhancement does not extend to all types of discriminations and is not due to a deficit in timing. Other research shows that rats with PER lesions are impaired on similar tasks, thus the enhancement is likely due to the effects of POR damage. Normally in this task, context is thought to accrue inhibitory control over other cues. Without this inhibitory control, animals might be expected to learn the task more efficiently. Our conclusion is that deficits in processing contextual information underlie the enhanced acquisition observed in rats with combined PER and POR damage on this complex discrimination task.

Essential role of the perirhinal cortex in complex tactual discrimination tasks in rats

We designed a battery of tactual discrimination tasks to study whether rats with perirhinal cortex (Prh) lesions had any deficit in resolving complex/ambiguous tactual tasks in the dark. Animals had to discriminate among 3 stimuli simultaneously exposed in 3 arms of a 4-arm plus-shaped maze. Rats with Prh lesions showed a profound impairment in a texture discrimination learning task when the stimuli had a high or intermediate degree of feature ambiguity (experiments 1a and 1b), but not when they had a low degree of feature ambiguity (experiment. 1c). Hippocampal lesions, however, did not cause any impairment in task acquisition even when the stimuli had a high degree of feature ambiguity (experiment 2). Experiments 3a, 3b, and 4 showed that perirhinal and control rats performed the task similarly when the animals had to discriminate on the basis of simple/individual, nonoverlapping features of the stimuli (size) with different levels of difficulty. Finally, to isolate the task's memory functions from its perceptual functions, a reversal learning task revealed a profound deficit in the initial learning phase, but unimpaired learning in the reversal phase with identical stimuli (experiment 5). The findings suggest that the Prh plays an essential role in somatosensory perceptual functions.

Damage to posterior parietal cortex impairs two forms of relational learning

The posterior parietal cortex (PPC) is a component of a major cortico-hippocampal circuit that is involved in relational learning, yet the specific contribution of PPC to hippocampal-dependent learning is unresolved. To address this, two experiments were carried out to test the effects of PPC damage on tasks that involve forming associations between multiple sensory stimuli. In Experiment 1, sham or electrolytic lesions of the PPC were made before rats were tested on a three-phase sensory preconditioning task. During the first phase, half of the training trials consisted of pairings of an auditory stimulus followed by a light. During the other trials, a second auditory stimulus was presented alone. In the next phase of training, the same light was paired with food, but no auditory stimuli were presented. During the final phase of the procedure both auditory stimuli were presented in the absence of reinforcement during a single test session. As is typically observed during the test session, control rats exhibited greater conditioned responding to the auditory cue that was previously paired with light compared to the unpaired cue. In contrast, PPC-lesioned rats responded equally to both auditory cues. In Experiment 2, PPC-lesioned and control rats were trained in a compound feature negative discrimination task consisting of reinforced presentations of a tone-alone and non-reinforced simultaneous presentations of a light-tone compound stimulus. Control rats but not rats with damage to the PPC successfully learned the discrimination. Collectively, these results support the idea that the PPC contributes to relational learning involving multimodal sensory stimuli, perhaps by regulating the attentional processing of conditioned stimuli.

Involvement of retrosplenial cortex in forming associations between multiple sensory stimuli

The retrosplenial cortex (RSP) is highly interconnected with medial temporal lobe structures, yet relatively little is known about its specific contributions to learning and memory. One possibility is that RSP is involved in forming associations between multiple sensory stimuli. Indeed, damage to RSP disrupts learning about spatial or contextual cues and also impairs learning about co-occurring conditioned stimuli (CSs). Two experiments were conducted to test this notion more rigorously. In Experiment 1, rats were trained in a serial feature negative discrimination task consisting of reinforced presentations of a tone alone and nonreinforced serial presentations of a light followed by the tone. Thus, in contrast to prior studies, this paradigm involved serial presentation of conditioned stimuli (CS), rather than simultaneous presentation. Rats with damage to RSP failed to acquire the discrimination, indicating that RSP is required for forming associations between sensory stimuli regardless of whether they occur serially or simultaneously. In Experiment 2, a sensory preconditioning task was used to determine if RSP was necessary for forming associations between stimuli even in the absence of reinforcement. During the first phase of this procedure, one auditory stimulus was paired with a light while a second auditory stimulus was presented alone. In the next phase of training, the same light was paired with food. During the final phase of the procedure both auditory stimuli were presented alone during a single session. Control, but not RSP-lesioned rats, exhibited more food cup behavior following presentation of the auditory cue that was previously paired with light compared with the unpaired auditory stimulus, indicating that a stimulus-stimulus association was formed during the first phase of training. These results support the idea that RSP has a fundamental role in forming associations between environmental stimuli.

Cortical efferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

We investigated the cortical efferents of the parahippocampal region by placing injections of the anterograde tracers, Phaseolus vulgaris-leuccoagglutinin, and biotinylated dextran amine, throughout the perirhinal (PER), postrhinal (POR), and entorhinal cortices of the rat brain. The resulting density of labeled fibers was evaluated in 25 subregions of the piriform, frontal, insular, temporal, cingulate, parietal, and occipital areas. The locations of labeled terminal fibers differed substantially depending on whether the location of the injection site was in PER area 35, PER area 36, POR, or the lateral or the medial entorhinal (LEA and MEA). The differences were greater for sensory regions. For example, the POR efferents preferentially target visual and spatial regions, whereas the PER efferents target all sensory modalities. The cortical efferents of each region largely reciprocate the cortical afferents, though the degree of reciprocity varied across originating and target regions. The laminar pattern of terminal fibers was consistent with the notion that the efferents are feedback projections. The density and amount of labeled fibers also differed substantially depending on the regional location of injection sites. PER area 36 and POR give rise to a greater number of heavy projections, followed by PER area 35. LEA also gives rise to widespread cortical efferents, arising mainly from a narrow band of cortex adjacent to the PER. In contrast, the remainder of the LEA and the MEA provides only weak efferents to cortical regions. Prior work has shown that nonspatial and spatial information is transmitted to the hippocampus via the PER-LEA and POR-MEA pathways, respectively. Our findings suggest that the return projections follow the same pathways, though perhaps with less segregration.

Suppression to visual, auditory, and gustatory stimuli habituates normally in rats with excitotoxic lesions of the perirhinal cortex

In 3 habituation experiments, rats with excitotoxic lesions of the perirhinal cortex were found to be indistinguishable from control rats. Two of the habituation experiments examined the habituation of suppression of responding on an appetitive, instrumental baseline. One of those experiments used stimuli selected from the visual modality (lights), the other used auditory stimuli. The third experiment examined habituation of suppression of novel-flavored water consumption. In contrast to the null results on the habituation experiments, the perirhinal lesions disrupted transfer performance on a configural, visual discrimination, indicating the behavioral effectiveness of the lesions. Implications for comparator theories of habituation are considered, and it is concluded that others' demonstrations of the sensitivity of object recognition to perirhinal cortex damage is not the result of standard habituation.

Cerebellar inactivation impairs cross modal savings of eyeblink conditioning

Eyeblink conditioning using a conditioned stimulus (CS) from one sensory modality (e.g., an auditory CS) is greatly enhanced when the subject is previously trained with a CS from a different sensory modality (e.g., a visual CS). The enhanced acquisition to the second modality CS results from cross modal savings. The current study was designed to examine the role of the cerebellum in establishing cross modal savings in eyeblink conditioning with rats. In the first experiment rats were given paired or unpaired presentations with a CS (tone or light) and an unconditioned stimulus. All rats were then given paired training with a different modality CS. Only rats given paired training showed cross modal savings to the second modality CS. Experiment 2 showed that cerebellar inactivation during initial acquisition to the first modality CS completely prevented savings when training was switched to the second modality CS. Experiment 3 showed that cerebellar inactivation during initial cross modal training also prevented savings to the second modality stimulus. These results indicate that the cerebellum plays an essential role in establishing cross modal savings of eyeblink conditioning.

Metabolic mapping of rat forebrain and midbrain during delay and trace eyeblink conditioning

While the essential neural circuitry for delay eyeblink conditioning has been largely identified, much of the neural circuitry for trace conditioning has yet to be determined. The major difference between delay and trace conditioning is a time gap between the presentation of the conditioned stimulus (CS) and the unconditioned stimulus (US) during trace conditioning. It is this time gap, which accounts for the additional memory component and may require extra neural structures, including hippocampus and prefrontal cortex. A metabolic marker of energy use, radioactively labeled glucose analog, was used to compare differences in glucose analog uptake between delay, trace, and unpaired experimental groups (rats, Long-Evans), to identify possible new areas of involvement within forebrain and midbrain. Here, we identify increased 2-DG uptake for the delay group compared to the unpaired group in various areas including: the medial geniculate nuclei (MGN), the amygdala, cingulate cortex, auditory cortex, medial dorsal thalamus, and frontal cortices. For the trace group, compared to the unpaired group, there was an increase in 2-DG uptake for the medial orbital frontal cortex and the medial MGN. The trace group also exhibited more increases lateralized to the right hemisphere, opposite to the side of US delivery, in various areas including: CA1, subiculum, presubiculum, perirhinal cortex, ventral and dorsal MGN, and the basolateral and central amygdala. While some of these areas have been identified as important for delay or trace conditioning, some new structures have been identified such as the orbital frontal cortex for both delay and trace groups.

Visual perception and memory: a new view of medial temporal lobe function in primates and rodents

The prevailing view of medial temporal lobe (MTL) function has two principal elements: first, that the MTL subserves memory but not perception, and second, that the many anatomically distinctive parts of the MTL function together in the service of declarative memory. Recent neuropsychological studies have, however, challenged both opinions. First, studies in rodents, nonhuman primates, and humans suggest that the perirhinal cortex represents information about objects for both mnemonic and perceptual purposes. Second, the idea that MTL components contribute to declarative memory in similar ways has also been contradicted. Whereas the perirhinal cortex plays an essential role in familiarity-based object recognition, the hippocampus contributes little, if at all, to this function. In both primates and rodents, the hippocampus contributes to the memory and perception of places and paths, whereas the perirhinal cortex does so for objects and the contents of scenes.

Perirhinal cortex lesions impair feature-negative discrimination

The role of the perirhinal cortex in inhibitory eyeblink conditioning was examined. In Experiment 1, rats were given lesions of the perirhinal cortex or control surgery and subsequently trained with a feature-negative discrimination procedure followed by summation and retardation tests for conditioned inhibition. Perirhinal cortex lesions impaired, but did not prevent acquisition of feature-negative discrimination. Results from the summation test showed that rats with perirhinal cortex lesions could not generalize feature-negative discrimination to a new stimulus. There were no group differences during the retardation test. Experiment 2 showed that lesions of the perirhinal cortex did not impair simple excitatory conditioning. Experiment 3 showed that perirhinal cortex lesions had no effect on acquisition of a simple tone-light discrimination. The results suggest that the perirhinal cortex plays a role in eyeblink conditioning when using discrimination procedures involving overlapping stimuli.