Thomas L. Bennett

Hippocampectomy and the attenuation of blocking

Rats with dorsal and ventral hippocampal lesions and sham-operated controls were tested in two experiments using Kamins [(1968). In M. R. Jones (Ed.), “Miami Symposium on the Prediction of Behavior.” Miami: University of Miami Press; (1969). In B. Campbell and R. Church (Eds.), “Punishment and Aversive Behavior.” New York: Appleton-Century-Crofts] two-stage blocking paradigm. In Experiment 1 hippocampal and sham rats conditioned to a compound conditioned stimulus (CS) (tone + light) suppressed on nonreinforced test trials to either element. In contrast, hippocampal and sham rats trained to tone or to light prior to conditioning to the compound CS performed differentially when tested to the redundant cue: Conditioning was blocked for shams, while blocking was severely attenuated with hippocampal rats. In Experiment 2 two parameters known to affect learning rate were manipulated to determine whether a general or specific deficit was responsible for the unblocking in hippocampal rats. Neither manipulation produced blocking in hippocampal rats suggesting that the attenuation of blocking observed in hippocampal rats was due to an impairment in attention rather than to a general learning deficit. The results indicate hippocampectomy produces an inability to exclude irrelevant or redundant information from the stimulus flux.

The Electrical Activity of the Hippocampus and Processes of Attention

Since the rediscovery of hippocampal θ activity in unanesthetized animals by Green and Arduini in 1954, determination of the significance of this bioelectrical pattern has become one of the most challenging and perplexing problems in psychobiological research. This slow, synchronous, high-amplitude pattern is only one of the major EEG rhythms of the hippocampus. The other major hippocampal pattern which may be easily observed in the chronically implanted, awake animal is a fast, irregular, low-amplitude pattern which is similar in appearance to the arousal pattern of the neocortex. I will discuss the possible significance of this pattern later, but the emphasis of the chapter will be to describe our investigations attempting to define the behavioral correlates of the θ response and the neccessity of this EEG rhythm for behaviors with which θ is normally correlated. An excellent analysis of the neural pathways mediating the θ and desynchronized patterns appears in an article by Anchel and Lindsley (1972).

Hippocampal theta activity and the attention component of discrimination learning

To assess the possibility that the occurrence of hippocampal theta during learning reflects the role of this structure in mediating attention or orienting responses, hippocampal electrical activity was monitored while chronically implanted cats learned and were shifted from a CRF to a cued DRL schedule in an operant conditioning apparatus. The end of the DRL was signaled by the onset of a panel of lights, and it was hypothesized that if theta is a correlate of attention to environmental responses, then as learning progressed and the animals attended to this relevant environmental cue, the incidence of theta between bar-presses should increase. Bennett and Gottfried (1970) previously reported that learning and performance of a noncued DRL are accompanied by hippocampal desynchronization. The results supported the hypothesis, and in conjunction with results previously reported by Bennett and Gottfried the present findings support the notion that the critical difference between those tasks in which learning and performance are correlated with theta versus those accompanied by hippocampal desynchronization is whether the animal can master the problem by attending to environmental cues. Some broader implications of these results for theories of hippocampal functioning are discussed.

The Neuropsychology of Epilepsy

Epilepsy is a nervous system disturbance that abruptly interferes with ongoing behavior, perception, movement, consciousness, or other brain functions. Individual attacks are called seizures, and when the problem is persistent it is called either a seizure disorder or epilepsy. Seizures are relatively common among infants, children, and adolescents. Probably 8 of every 1000 children experience some sort of seizure activity, even if it is only a single occurrence of a febrile seizure (Lechtenberg, 1984). Occasionally, a seizure disorder will disappear as a child matures, but in a majority of cases, childhood epilepsy persists into adulthood, and in about 80% of adults with epilepsy this condition developed when they were children. At least 5 of every 1000 adults in the United States have epilepsy (Lechtenberg, 1984). The various types of epilepsy were described in Chapter 21 of this volume. The purpose of this chapter is to describe the neuropsychology of epilepsy, and the emphasis will be to discuss the emotional/behavioral and cognitive concomitants of epilepsy. We use the term concomitants to underscore the fact that epilepsy is a complex phenomenon, and the behavioral and cognitive events associated with it are the product of a complex interaction among neurological, medication, and psychosocial variables (Hermann & Whitman, 1986).

The effects of centrally blocking hippocampal theta activity on learning and retention

Although hippocampal theta has been correlated with several possible functions of the hippocampus in mediating behavior, the necessity of this bioelectric response for the occurrence of these behaviors has not been substantiated. The present research investigated this issue by examining the effects of blocking hippocampal theta activity on learning and retention of a simultaneous brightness discrimination, a task whose performance is normally accompanied by theta on 90–95% of the approach trials. Our previous research suggested that the occurrence of theta in this situation is related to functions performed by the hippocampus in mediating specific processes of attention. Theta blocking was accomplished by injecting 2 μ liters of scopolamine hydrobromide (10 μg/μliter) into the medial septal nucleus of cats via a chronically implanted cannula. This procedure resulted in theta occurring on only 6–8% of the approach responses as compared to the normal 92–93% of the trials in the nondrugged state. Animals were administered the drug centrally either during original learning or during retention test sessions. It was found that neither the rate of original task learning nor the amount retained were affected by theta blocking. Thus, theta is not necessary for the occurrence, during learning, of behaviors with which it is normally correlated, and hippocampal theta is clearly not required for either the laying down of memory traces or for the subsequent recall of information from memory as some researchers have contended. The significance of the hippocampal theta pattern as an index of hippocampal functioning is also discussed. EEG and lesion literature are reviewed which suggest that when theta is present, the hippocampus is in a relatively inactive or inhibited state. It is proposed that differences in the frequency of theta in certain behavior situations reflect different degrees of inhibition of the hippocampus.

Differential performance of hippocampally ablated rats on nondiscriminated and discriminated DRL schedules

Two experiments compared the performance of hippocampectomized, neodecorticated, and normal rats on DRL 10, 20, or 30 sec schedules of reinforcement either by using conventional schedules or by modifying the schedule with a visual cue which signaled the end of the required interresponse time (IRT). Rats with hippocampal ablations showed typical decrements in performance under the undiscriminated conditions, but the magnitude of the deficit relative to controls did not increase with longer IRTs. Although initial differences were found between hippocampectomized S s and controls on the discriminated schedules in Expt 2, the terminal performance of the groups was similar; the rate of achieving asymptotic levels varied inversely with schedule length. These findings are (a) inconsistent with a response-inhibition hypothesis of hippocampal functioning, and (b) suggest that the behavioral impairment of hippocampally ablated S s reflects a deficit in the processing of information from response-produced stimuli.

Neuropsychological evaluation in rehabilitation planning and evaluation of functional skills.

This paper focuses on the benefits of neuropsychological assessment for evaluating a brain-injured persons functional abilities and rehabilitation needs. The importance of early assessment in addressing these goals is discussed. Strengths and weaknesses of neuropsychological assessment for predicting a persons ability to participate in normal activities of daily living are considered. Test data taken alone often lack ecological validity, but nevertheless, specific test findings can be used to predict functional skills. The ecological validity of neuropsychological testing can be extended by observing the patients approach to tasks in the assessment environment and by observing the patient in his or her normal activities. Evaluation information obtained by allied rehabilitation professionals, including occupational and speech and language therapies, can be integrated with neuropsychological test data to both extend the generalizability of our findings and to validate hypotheses generated from our test data. The use of neuropsychological test data in planning a cognitive rehabilitation program is also considered.

Species differences in the behavior correlates of hippocampal RSA

The possibility that differences between species in the behaviors associated with hippocampal RSA were responsible for theoretical conflicts regarding the significance of this bioelectric response was examined. Specifically, a species associated with the voluntary movement theory learned a task associated with the attention theory regarding hippocampal RSA. Electrical activity from the dorsal and ventral hippocampus was monitored while rats learned and were shifted from a CRF to either a cued- or noncued-DRL schedule of reinforcement. Previous research had shown that cued-DRL training increased the incidence of dorsal RSA from the cat as a result of increased attention to the environmental cue that was guiding behavior. Since the same amount of voluntary movement is required of both cued- and noncued-DRL performance, it was predicted that rats performing on cued-DRL, like cats, would show an increase in the incidence of RSA as learning progressed. The results of both visual and computer derived power spectral-density analyses supported the hypothesis that dorsal RSA is associated with attention to environmental cues. Since rats behaviorally orient differently to stimuli than do cats, it was suggested that differences in the behavior correlates of dorsal RSA between species can be accounted for by a consideration of the orienting behavior of the species being studied.

Performance of Hippocampectomized rats on discontinuous negatively correlated reward

Hippocampally lesioned, cortically lesioned and unoperated rats received 180 trials in an alley runway during which running speed was uncorrelated with reward. The animals were then shifted to discontinuous negatively correlated reinforcement (DNC). Although the groups did not differ appreciably in their asymptotic running speeds during the uncorrelated condition, reliable differences emerged among the groups after the shift to the DNC condition. In the latter situation, the hippocampal rats demonstrated an inability to master the DNC condition, a finding which conforms to results obtained with DRL by other investigators. Application of an extraneous stimulus on test trials produced a decline in running speed for all groups.

Effects of scopolamine on hippocampal theta and correlated discrimination performance

Abstract Adey and his associates have postulated that theta reflects the active involvement of the hippocampus in the processing, storage and recall of information. This theory implies that if the appearance of theta is experimentally blocked, then behavior based on the recall of previously stored information will be adversely affected. The present research examined this implication. Theta blocking was accomplished by intraperitoneal administration of different dosages of scopolamine hydrobromide, and the effects of such theta blocking on performance of a previously mastered Adey-type discrimination task were assessed. Analysis of the results indicated that all doses of scopolamine suppressed hippocampal theta; however only the two highest doses of scopolamine resulted in a decrement of discrimination task performance. Under these stronger dosages, performance still remained at a high level indicating that discrimination capacities were not severely disrupted. The findings are interpreted as being inconsistent with Adeys interpretation of the significance of the hippocampal theta rhythm.