Michael L. Woodruff

The effect of a twenty-four hour intertrial interval on the acquisition of spatial discrimination by hippocampally damaged rats

Abstract Rats with either aspiration- or penicillin-induced (irritative) lesions of the hippocampus or overlying neocortex and normal rats were trained on a spatial discrimination in a T-maze after being tested for spontaneous alternation. Animals were given only one trial per day (24 hr ITI). Only the animals with aspiration lesions of the hippocampus were impaired in acquiring the spatial discrimination. Animals with either hippocampal or neocortical lesions alternated at reduced rates.

Discrimination learning in animals with lesions of hippocampus

Rats with either bilateral aspiration lesions or unilateral aspiration lesions combined with contralateral irritative lesions of the hippocampus or overlying neocortex were trained on an operant visual discrimination. Only animals with bilateral aspiration lesions of the hippocampus were impaired in the acquisition of brightness discrimination. All animals were also trained in a two-way shuttle-box avoidance task. Animals with bilateral hippocampal aspiration showed an improved acquisition rate, while animals with ventral hippocampal penicillin placements were retarded in the acquisition of this task.

Behavioral and hormonal effects of hippocampal lesions on male and female rats

Male and female rats with hippocampal lesions and controls were compared with respect to activity, body weight, performance on a DRL schedule, and extinction. Normal females acquired the DRL schedule faster than normal males. For both sexes, hippocampal lesions resulted in poor DRL performance and more responses during extinction. Male rats with hippocampal lesions were further below their target deprived weight each day than their controls; the lightest animals had the worst DRL performance. Sex-lesion interactions were found for activity in the operant chamber during DRL performance and for DRL extinction: females with hippocampal lesions were more active and made more responses during extinction than males with hippocampal lesions. Corticosterone levels were manipulated prior to DRL sessions, and resting and stress levels of corticosterone were measured at the end of the experiment. While females had higher corticosterone levels than males, brain lesions did not affect corticosterone levels, nor did hormone manipulations affect DRL performance for either sex.

Changes in focal epileptic activity produced by brainstem sections in the rabbit

Abstract The propagation of both single epileptiform spike discharges and extended after-discharges (ADs) induced by penicillin was studied in rabbits with sections made through the caudal mesencephalon at either the midcollicular-prepontine or midcollicular-postpontine level, as well as in intact animals. Penicillin was placed either on the frontal cortex, the occipital cortex or in the dorsal hippocampus. When compared to intact controls, sectioning at the midcollicular-prepontine level increased the time required for the spike discharges to develop in the primary focus in all placements and decreased the frequency of single spike discharges when penicillin was placed in the hippocampus or frontal cortex. A transection at this level increased the duration of ADs in animals with occipital foci and made the duration of ADs originating from the frontal cortex more variable. Sectioning at the midcollicular-postpontine level decreased time required for the appearance of the spike discharges in the primary focus in subjects with occipital foci, decreased the amplitude of spike discharges in subjects with occipital or hippocampal foci, decreased duration of ADs in animals with either frontal or occipital foci, but increased duration of ADs in animals with hippocampal foci. Sections at either the pre- or postpontine level decreased the percentage of secondary ADs which continued to discharge after the AD in the primary focus had stopped. These data are taken to indicate that: (1) different mechanisms underly the development of the single spike discharge and the elaboration of these discharges into an extended AD; (2) the duration of the ADs is influenced by the ongoing background electrical activity of the brain structures involved in the AD; (3) it is likely that the same pontine and medullary structures which regulate normal synchronous electrical activity of the forebrain can also influence the hypersynchronous oscillations of the AD; and (4) an intact brain-stem may be necessary for the formation of independent secondary epileptogenic foci.

Deficient go, no-go brightness discrimination in rats following hippocampal lesions

Hippocampally ablated, cortically damaged, and intact control rats were trained postoperatively on a discrete-trial, go, no-go brightness discrimination task in an operant situation. The hippocampally ablated animals were found to be deficient on acquisition primarily due to inappropriate responding on no-go (nonieinforced) trials. The deficit is consistent with other operant brightness discrimination studies, but contrasts with the facilitated go, no-go alternation performance exhibited by hippocampally damaged rats.

Impaired acquisition of a simultaneous brightness discrimination by cortically and hippocampally lesioned rats

Rats with neocortical and hippocampal lesions, as well as normal animals, were trained in a visual discrimination task. The periods of stimulus presentation indicating the “correct lever” were separated by 10-sec intertrial intervals. Both lesion groups were impaired in the acquisition of the problem, but only the animals with hippocampal damage evidenced significantly higher response rates during the intertrial intervals.

Subconvulsive epileptiform discharge and behavioral impairment

Clinical and experimental reports regarding deficits in learning and memory as a result of subclinical epileptiform discharge are reviewed. The effects of focal epileptic discharge upon behavior are compared to the effects produced by nonepileptic lesions of the same brain areas. It is suggested that whether or not epileptogenic discharge has behavioral consequences similar to ablation of the same brain region depends upon the intensity of the discharge and the extent to which the discharge spreads from the focus. Data which support the hypothesis that seizure discharge impairs retrieval of information more severely than storage are discussed. The advantages and disadvantages of several methods for producing epileptiform discharge experimentally are discussed. Finally, it is suggested that relatively long term changes in background EEG activity and cortical steady potential shifts may be the underlying physiological alterations responsible for the observed deficits, rather than simply the presence of epileptiform spike and wave discharges.

The effect of physostigmine on spontaneous alternation in infant rabbits

Beginning about 10 days after birth, infant rabbits were tested for spontaneous alternation while under single dosages of physostigmine and dl-amphetamine, as well as after saline injections or without any injection. Only under physostigmine did the animals alternate at levels which were above chance and which approximated levels found in normal adult animals of other species. This indicates that the capacity for spontaneous alternation exists in the infant rabbit, even though it is normally not expressed in behavior.

Deficient brightness discrimination acquisition produced by daily intracranial injections of penicillin in rats.

Rats with unilateral epileptiform foci produced by daily injections of penicillin into either occipital cortex or hippocampus were trained on an operant visual discrimination. Both of the experimental groups were retarded in the acquisition of the discrimination, but the rats with foci in the occipital cortex demonstrated a more severe impairment, despite the fact that secondary epileptiform discharges appeared in hippocampus and visual cortex under both conditions. As a control for the nonepileptiform properties of the penicillin, other groups of rats received intracranial injections of sodium sulfadiazine. Rats injected with this drug were not significantly different from normal controls in the percentage of correct responses emitted, despite reduced response rates. After 5–7 days of treatment this drug produced some slight seizure activity which was unrelated to performance. On Day 11 of discrimination training the rats receiving penicillin were injected with sodium sulfadiazine and the rats receiving sodium sulfadiazine were given penicillin. The rats changed from penicillin to sodium sulfadiazine showed an immediate increase in the percentage of correct responses. Rats switched from sodium sulfadiazine to penicillin demonstrated a transitory impairment in performance. These data were interpreted to indicate that epileptiform activity produced by penicillin results in a deficit in retrieval of recently formed memories.

Deficient brightness discrimination in hooded but not albino rats given neocortical ablations

Abstract Hooded and albino rats with lesions of the neucortex overlying hippocampus and normal rats of both strains were trained in an operant discrimination. Though the normal groups did not differ from one another and the neocortically lesioned albinos did not differ from the unlesioned rats, when compared to the other groups the hooded group was found to be deficient in the acquisition of the task. A greater amount of cell loss and gliosis was observed in the thalamus of the hooded rats.