SOME LONG-TERM EFFECTS OF TEMPORAL LOBE RESECTIONS ON AUDITORY AND VISUAL DISCRIMINATION IN MONKEYS

Different effects of lesions to auditory core and belt cortex on auditory recognition in dogs

Auditory recognition memory, in contrast to memory in other modalities, is not affected by damage to the perihinal cortex, and its neural basis remains unknown. In an attempt to elucidate this problem, we investigated the role of canine auditory core and belt areas in auditory recognition. Either core or posterior belt areas were surgically removed. The core and belt regions were defined on the basis of response properties and thalamocortical connectivity established in previous studies. The animals were tested on auditory delayed matching to sample (DMS, a recognition memory task) using complex, trial-unique auditory stimuli. Both core and belt lesions impaired auditory recognition, however, the underlying deficit was different. Lesions to the core areas impaired auditory localization abilities. Lesions to the posterior belt areas did not affect this component of the recognition task, but affected auditory quality discrimination and/or recognition. The deficit following the posterior belt lesion did not increase with retention delay, suggesting that auditory belt areas do not constitute a substrate for auditory recognition memory. Their main function appears to be processing of complex sound patterns, including immediate recognition.

Developmental study of the hippocampal formation in rhesus monkeys (Macaca mulatta): I. Early ablations spare discrimination learning but not recognition memory

In clinical cases of amnesia that followed bilateral excisions of medial temporal lobe structures, with some perceptual learning abilities intact, damage to the hippocampus was presumed to be the critical factor. The authors' search for an animal model of amnesia, based on ablations aimed at the hippocampal formation in infant rhesus monkeys, provides support for this view. Ablations of the hippocampal formation in 2-month-old infants tested shortly after recovery from surgery resulted in a deficit on a recognition memory task but left intact the ability to learn the concurrent object discrimination task, even though the latter task was administered with the use of a massed practice procedure. Thus, early damage, unlike that at 2 years of age or later, allowed the authors to dissociate associative learning with repeated trials, independent of hippocampal functions and recognition memory that depends on the integrity of the hippocampus.

Amnesia for Complex Naturalistic Scenes and for Objects Following Fornix Transection in the Rhesus Monkey

Rhesus monkeys (Macaca mulatta) were trained to discriminate among many complex naturalistic scenes. The scenes were still frames from a cinema film. They were presented as the discriminative stimuli in a concurrent discrimination learning task in which each discriminative stimulus was presented on one trial each day. Learning of this task was severely impaired by fornix transection. The same animals were also deficient in a similar concurrent discrimination learning task, with each discriminative stimulus presented on one trial each day, but with objects, not complex scenes, as the stimulus material. The impairment in object discrimination learning in the present experiment is attributable to an interaction of object discrimination learning with scene discrimination learning, and can be understood as an effect of interference in long-term memory. In contrast to these impairments in long-term memory, a test of within-session learning of complex scenes, in which the average interval between successive presentations of the same stimulus was < 3 min, was performed without significant impairment by the fornix-transected animals. These results show that long-term memory for complex naturalistic scenes reveals analogues in the monkey of human episodic memory and its impairment in amnesia.

The effects of superior temporal cortex lesions on the processing and retention of auditory information in monkeys (Cebus apella)

Three monkeys with extensive preoperative training on visual and auditory memory tasks (delayed matching-to-sample), an auditory pattern-discrimination task, and a visual serial-order task, received bilateral lesions of the superior temporal (ST) cortex in two stages, with testing after each lesion. Unilateral ST cortex lesions resulted in only moderate auditory memory impairments, whereas bilateral ST cortex lesions resulted in severe auditory memory impairments. The bilateral ST cortex lesions also resulted in severe impairments in the ability to relearn the auditory pattern-discrimination task. In contrast to the auditory impairments, neither unilateral nor bilateral ST cortex lesions had any effect whatsoever on either visual memory or visual serial-order behavior. These findings indicate that the ST cortex plays a role in auditory processing and retention similar to that played by the inferior temporal cortex for visual processing and retention.

Scene-Specific Memory for Objects: A Model of Episodic Memory Impairment in Monkeys with Fornix Transection

A series of five experiments investigated the relationship between object memory and scene memory in normal and fornix-transected monkeys. An algorithm created formally defined background and objects on a large visual display; the disposition of some particular objects in particular places in a particular background constitutes a formally defined scene. The animals learned four types of discrimination problem: (1) object-in-place discrimination learning, in which the correct (rewarded) response was to a particular object that always occupied the same place in a particular unique background, (2) place discrimination learning, in which the correct response was to a particular place in a unique background, with no distinctive object at that place, (3) object discrimination learning in unique backgrounds, in which the correct response was to a particular object that could occupy one or the other of two possible places in a unique background, and (4) object discrimination learning in varying backgrounds, in which the correct response was to a particular object that could appear at any place in any background. The severest impairment produced by fornix transection was in object-in-place learning. Fornix transection did not impair object discrimination learning in varying backgrounds. The results from the other two types of learning task showed intermediate severity of impairment in the fornix-transected animals. The idea that fornix transection in the monkey impairs spatial memory but leaves object memory intact is thus shown to be an oversimplification. The impairments of object memory in the present experiments are analogous to the impairments of episodic memory seen in human amnesic patients.

An analysis of the correlation of lesion size, localization and behavioral effects in 283 published studies of cortical and subcortical lesions in old-world monkeys

The present article evaluates the quality and magnitude of the effects of lesion size and location and their interaction, on the behavioral performance of old world monkeys by a quantitative comparison of 283 published studies. The results indicate that lesion size alone is a poor predictor of the behavioral performance of monkeys, as opposed to Lashley's work in rats. Lesion location is a reliable predictor of the behavioral performance for brain regions thought to be primarily involved in a specific behavior; however, similar behavioral effects, although less reliable, can be observed for many different lesion loci, suggesting a specialized and a holistic brain functioning to be working at the same time. Some lesion loci are, in sharp contrast to current hypotheses about functional localization in the brain, not associated with impairments, but with significant improvements of a specific behavior. For such lesion loci the correlation of lesion size and behavioral performance may yield significant positive relationships (that is, increasing behavioral improvement with increasing lesion size); these relationships are contrasted by the significant negative relationships obtained for lesions of brain regions thought to be primarily involved in a given behavior. Thus, the lesion size may be a good predictor of the behavioral performance, depending on the lesion location and on the behavior under measurement. The behaviors analysed in this study were discrimination or delayed reaction or delayed matching-to-sample. The former two behaviors involve habit-like learning and are thought to be mediated by corticostriate functional pathways in the brain and the latter behavior implies the learning of single events, being thought to be mediated by corticolimbic functional pathways in the brain. Improved performances were observed for habit-like behaviors after lesions of brain regions (lateral frontal, premotor/motor, parietal, inferotemporal cortex, amygdala and fornix) being not primarily involved in a given behavior but possibly being able to inhibit the corticostriate pathways. Interestingly, lesions of subareas of the neostriatum were found to cause impairments in habit-like behaviors presumably being processed via these subareas (e.g. head of the caudate nucleus and delayed reaction), but to cause significant improvements in other behaviors (e.g. head of the caudate nucleus and visual discrimination). Thus, it may be concluded that diverse systems of functionally interconnected brain regions may maintain reciprocal inhibitions, with the result that a lesion within one system not only leads to a loss of one behavior, but in addition leads to a modification, may be a facilitation, of another behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

Auditory association cortex lesions impair auditory short-term memory in monkeys

Monkeys that were trained to perform auditory and visual short-term memory tasks (delayed matching-to-sample) received lesions of the auditory association cortex in the superior temporal gyrus. Although visual memory was completely unaffected by the lesions, auditory memory was severely impaired. Despite this impairment, all monkeys could discriminate sounds closer in frequency than those used in the auditory memory task. This result suggests that the superior temporal cortex plays a role in auditory processing and retention similar to the role the inferior temporal cortex plays in visual processing and retention.

Effects of sodium amylobarbitone on memory processes in the pigeon

An experiment was carried out in the pigeon to assess the effects of sodium amylobarbitone (SAB) on memory, using a delayed pair comparison task. Performance was disrupted at all delays by doses of 10 and 20 mg/kg SAB and it was concluded that this effect was not due to perceptual impairment or stereotyped responding, which the drug might have caused, since performance on a visual discrimination task was unaffected by the same doses of SAB. Two further experiments assessed proactive and retroactive effects by varying the inter-trial interval and by introducing an irrelevant orange stimulus at various stages of the delay interval. Although these manipulations disrupted performance, there was no evidence to suggest that the drug potentiated their effects in a specific manner. The results are discussed in terms of an encoding hypothesis, and some reference is made to the possible neuropharmacological basis of action of this drug.

The effects of chlordiazepoxide on a delayed pair comparison task in pigeons

Five pigeons were successfully taught a variation of the Konorski delayed pair comparison task, using a red and a green stimulus. The bird was required to respond to the left or right hand key, depending on whether the 2 successively presented centre key stimuli were the same (left) or different (right). The delay intervals ranged from zero to 9 s, and stable performance decrements (with increasing delay) were obtained. Chlordiazepoxide which, it has been argued, produces behavioural changes similar to those observed after hippocampal lesions was found to disrupt performance at doses greater than 4 mg/kg (i.m.). All delays (including zero) were affected. Some interpretations of this result in terms of encoding and attention are discussed, and some reference made to the possible neuropharmacological basis of this drug action.