Responses of hippocampal formation neurons in the monkey related to delayed spatial response and object-place memory tasks

The memory for where in the environment a particular visual stimulus has been seen is one of the types of memory relatively specifically impaired by hippocampal damage in primates including man. In order to investigate what processing might be performed by the hippocampus related to this type of memory, the activity of hippocampal neurons was recorded while monkeys performed an object-place memory task. In this task, the monkey was shown a sample stimulus in one position on a video screen, there was a delay of 2 s, and then the same or a different stimulus was shown in the same or in a different position. The monkey remembered the sample and its position, and if both matched the delayed stimulus, he licked to obtain fruit juice. Of the 600 neurons analysed in this task, 3.8% responded differently for the different spatial positions, with some of these responding differentially during the sample presentation, some in the delay period, and some in the match period. Thus some hippocampal neurons respond differently for stimuli shown in different positions in space, and some respond differently when the monkey is remembering different positions in space. In addition some of the neurons responded to a combination of object and place information, in that they responded only to a novel object in a particular place. These neuronal responses were not due to any response being made or prepared by the monkey, for information about which behavioral response was required was not available until the match stimulus was shown. This is the first demonstration that some hippocampal neurons in the primate have activity related to the spatial position of stimuli. The activity of these neurons was also measured in a delayed spatial response task, in which the monkey was shown a stimulus in one position, and, after a 2 s delay when two identical stimuli were shown, had to reach to touch the stimulus which was in the position in which it had previously been seen. It was found that the majority of the neurons which responded in the object-place memory task did not respond in the delayed response task. Instead, a different population of neurons (5.7% of the total) responded in the delayed spatial response task, with differential left-right responses in the sample, delay, or match periods.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differential neuronal responsiveness in primate perirhinal cortex and hippocampal formation during performance of a conditional visual discrimination task

Neuronal responses in the hippocampal formation, including the entorhinal cortex, have been compared with those in the inferior temporal cortex, including the perirhinal cortex, during performance by monkeys of a visual conditional discrimination task. In the task, the arrangement of three geometric shapes determined the correctness of either a left or right behavioural response according to a conditional rule. Neurons that responded differently to different types of trial were common (50% of the visually responsive neurons) in the entorhinal cortex, perirhinal cortex and area TE of the inferior temporal cortex, but significantly less common in the hippocampus (13%). This differential incidence suggests a more important role for the rhinal cortices and area TE than for the hippocampus in this task. Based on the neuronal responses, arguments are advanced that the animals probably solved the task by a strategy that did not require spatial or hippocampal processing. Thus, of the differential responses, those that would allow the animals to solve the task by using a conditional rule and so avoid spatial processing were twice as common (37%) as those allowing solution to be by selection of a particular spatially directed response to each arrangement of shapes (19%). Moreover, the differential latencies of responses that allowed the task to be solved by a conditional rule were shorter (< approximately 165 ms), and hence processing was faster, than those that provided information about particular individual types of trial ( approximately 195 ms). Even so, hippocampal responsiveness in the conditional task was differentially enhanced when compared with that during a recognition memory task, and the neuronal responses potentially allow the animal to employ a second, alternative strategy that might be expected to depend on hippocampal processing.     

Complimentary roles of the hippocampus and retrosplenial cortex in behavioral context discrimination

Complex cognitive functions, such as learning and memory, arise from the interaction of multiple brain regions that comprise functional circuits and different components of these circuits make unique contributions to learning. The hippocampus and the retrosplenial cortex (RSC) are anatomically interconnected and both regions are involved in learning and memory. Previous studies indicate that the hippocampus exhibits unique firing patterns for different contexts and that RSC neurons selectively respond to cues that predict reinforcement or the need for a behavioral response, suggesting a hippocampal role in encoding contexts and an RSC role in encoding behaviorally significant cues. To test this, we simultaneously recorded hippocampal and RSC neuronal activity as rats learned to discriminate two behavioral contexts. The rats learned to approach the east arm of a plus maze for reward during the first half of each session and to approach the west arm during the second half. The "go east" and "go west" conditions constitute distinct behavioral contexts, which were cued by the reward location. Neurons in both regions developed highly context-specific responses as subjects learned to discriminate the contexts, but the response patterns differed in the two brain regions. Consistent with a context processing role, hippocampal neurons developed context-specific responses to a variety of task stimuli and events. In contrast, RSC neurons only developed context-specific responses to the reward location, which served as the context identifying cue. These results suggest that the hippocampus and RSC play distinct, but complimentary roles in mediating context appropriate memories and behaviors.     

Chemotherapy disrupts learning,neurogenesis and theta activity in the adult brain

Chemotherapy, especially if prolonged, disrupts attention, working memory and speed of processing in humans. Most cancer drugs that cross the blood–brain barrier also decrease adult neurogenesis. Because new neurons are generated in the hippocampus, this decrease may contribute to the deficits in working memory and related thought processes. The neurophysiological mechanisms that underlie these deficits are generally unknown. A possible mediator is hippocampal oscillatory activity within the theta range (3–12 Hz). Theta activity predicts and promotes efficient learning in healthy animals and humans. Here, we hypothesised that chemotherapy disrupts learning via decreases in hippocampal adult neurogenesis and theta activity. Temozolomide was administered to adult male Sprague–Dawley rats in a cyclic manner for several weeks. Treatment was followed by training with different types of eyeblink classical conditioning, a form of associative learning. Chemotherapy reduced both neurogenesis and endogenous theta activity, as well as disrupted learning and related theta‐band responses to the conditioned stimulus. The detrimental effects of temozolomide only occurred after several weeks of treatment, and only on a task that requires the association of events across a temporal gap and not during training with temporally overlapping stimuli. Chemotherapy did not disrupt the memory for previously learned associations, a memory independent of (new neurons in) the hippocampus. In conclusion, prolonged systemic chemotherapy is associated with a decrease in hippocampal adult neurogenesis and theta activity that may explain the selective deficits in processes of learning that describe the ‘chemobrain’.     

Spatial responsiveness of monkey hippocampal neurons to various visual and auditory stimuli.

To investigate involvement of the hippocampal formation in spatial information processing, activity of neurons in the hippocampal formation of the conscious monkey was recorded during presentation of various visual and auditory stimuli from several directions around the monkey. Of 1,047 neurons recorded, 106 (10.1%) responded to some stimuli from one or more directions. Of these 106 neurons with directionally differentiating responsiveness, 49 responded to visual stimulation, 35 to auditory stimulation, and 22 to both. Among 81 neurons, each tested with more than 10 different stimuli, one type responded independent of the nature of the stimulus (nonselective, n = 39), and responses of the other type depended on the nature of the stimulus (selective, n = 42). To investigate effects of change in spatial relations between test stimuli and background stimuli fixed on the monkey or fixed in the environment, 59 of 106 neurons were tested while the experimental apparatus holding the stimulus was moved relative to the monkey. Of these 59 neurons, 36 changed their responsiveness; 7 maintained the magnitude of their responses but changed the response direction with the movement of the apparatus, 5 changed direction regardless of the movement, and 24 did not change direction, but decreased or extinguished responses from the preferred direction. Thirty-two of 106 neurons were also tested by rotating the monkey. The directionally differentiating responsiveness of 11 neurons followed the monkey (egocentric neurons), that of 9 remained in place in the environment (allocentric neurons), and responses of 12 were reversibly extinguished when the monkey was rotated. The results suggest that these hippocampal neurons may be involved in identification of relations among various kinds of stimuli in different spatial frameworks (egocentric or allocentric) and this identification may be developed from multiple sensory modalities.     

Learning and memory is reflected in the responses of reinforcement-related neurons in the primate basal forebrain

Certain basal forebrain neurons encode the learned reinforcement value of objects: they respond differentially to visual stimuli that signal availability of fruit juice (positively reinforcing) or saline (negatively reinforcing) obtained by lick responses in visual discrimination tasks. In this report we describe the rapid, learning-related changes in the responses of these neurons during the acquisition and reversal of the reinforcement contingency of a visual discrimination reversal task. The same neurons also responded differentially to novel and familiar stimuli in 2 recognition memory tasks, in which monkeys applied the learned rule that lick responses to novel stimuli elicited saline and responses to familiar stimuli elicited juice. These differential responses to novel and familiar stimuli thus reflected the reinforcement value of the stimuli. A single presentation of a novel or a familiar stimulus was sufficient to elicit a differential response which was maintained even when the stimulus had not been seen recently. The maintenance of the differential response indicates that these neurons are influenced by a durable memory for the stimuli, estimated to be 30 trials on average. These differential neurons were recorded in the substantia innominata, the diagonal band of Broca, and a periventricular region of the basal forebrain. The responses of the reinforcement-related neurons in these 3 regions were similar in most respects. These results support the conclusion that basal forebrain neurons respond to sensory stimuli that, through learning of different contingencies, signal the availability of reinforcement. We suggest that the properties of learning and memory reflected in these neuronal responses are due to afferent pathways from ventromedial regions of the prefrontal and temporal cortices and the amygdala, and that the responses of these neurons provide an enabling mechanism that facilitates the operation of diverse cortical regions in which specific sensory, motor, or mnemonic functions take place.     

Hippocampal neurons in the monkey with activity related to the place in which a stimulus is shown

In order to analyze the functions of the hippocampus in the primate, and to advance the understanding of amnesia, the activity of 994 single hippocampal neurons in the monkey was analyzed during the performance of a task known to be affected by hippocampal damage in which both an object, and its position in space, must be remembered. The serial multiple object-place memory task required a memory for the position on a video monitor in which a given object had appeared previously. It was found that 9.3% of neurons recorded in the hippocampus and hippocampal gyrus had spatial fields in this and related tasks, in that they responded whenever there was a stimulus in some but not in other positions on the screen. We found that 2.4% of the neurons responded to a combination of spatial information and information about the object seen, in that they responded more the first time a particular object was seen in any position. Six of these neurons were found that showed this combination even more clearly, in that, for example, they responded only to some positions and only if it was the first time that a particular stimulus had appeared there. It is concluded that there are neurons in the primate hippocampus which (1) respond to position in space and (2) in some cases combine information about stimuli and their position in space, responding to a stimulus only the first time it is seen in a position in space, for example. Thus, not only is spatial information processed by the primate hippocampus, but it can be combined with information about which stimuli have been seen before. The ability of the hippocampus to form such conjunctions may be an important property for its role in memory.     

Environment-spatial conditional learning in rats with selective lesions of medial septal cholinergic neurons

Cholinergic medial septal neurons may regulate several aspects of hippocampal function, including place field stability and spatial working memory. Monkeys with damage to septal cholinergic neurons are impaired in visual-spatial conditional learning tasks; however, this candidate function of septal cholinergic neurons has not been studied extensively in the rat. In the present study, rats with selective lesions of cholinergic neurons in the medial septum and vertical limb of the diagonal band of Broca (MS/VDB), made with 192 IgG-saporin, were tested on a conditional associative learning task. In this task, which we term "environment-spatial" conditional learning, the correct location of a spatial response depended on the array of local environmental cues. MS/VDB-lesioned rats were impaired when the two parts of the conditional problem were presented concurrently, but not when one environment had been learned before the full conditional problem was presented. Our findings suggest that cholinergic MS/VDB neurons participate in some aspects of conditional associative learning in rats. They may also shed light on the involvement of cholinergic projections to the hippocampus in modulating and remodeling hippocampal spatial representations.     

Neuronal Sianallina of Information Imoortant to Visual Recognition-Memory in Rat Rhinal aid Neighbouring Cortices

This study was conducted to discover whether the rat cortex contains neurons that signal information concerning the previous occurrence of stimuli, as has been found in the primate. Recordings of the activity of 396 single neurons were made while unanaesthetized rats were shown objects. The effects on neuronal responsiveness of stimulus repetition and of the relative familiarity of the stimuli were sought. The areas sampled were the rhinal (entorhinal and perirhinal) cortex, area TE of the temporal cortex, the lateral occipital cortex and the hippocampal formation. The response to the first presentations of objects was significantly different from that to their second presentations for 63 (34%) of the 185 responsive neurons; for 39 of the neurons the response was smaller when the stimulus was repeated, whereas for 24 it was larger. The incidence of decremental responses was higher in the non-hippocampal cortex than in the hippocampal formation, while the incidence of incremental responses was higher in the hippocampal formation than other cortical areas. The response to unfamiliar objects was significantly different from that to highly familiar objects for 15 (22%) of 67 responsive neurons so tested; for 12 of the neurons the response was smaller when the stimulus was repeated, and for three it was larger; most of these neurons were found in area TE. The responses of ten familiarity neurons varied significantly with the relative familiarity of the stimuli but not with stimulus repetition; the responses of seven recency neurons varied significantly upon stimulus repetition but not with the relative familiarity of the stimuli. Thus information concerning stimulus repetition and familiarity is separably encoded at the single neuron level in the rat cortex. The results demonstrate that in the rat cortex as in the monkey cortex there are neurons that signal information concerning the prior occurrence of stimuli; such information is of importance to recognition memory, working memory and priming memory.     

The Effects of Lesions to the Mammillary Region and the Hippocampus on Conditional Associative Learning by Rats

Rats with extensive lesions to the mammillary body region, the hippocampus, or rats which had received a control operation were trained postoperatively on two visuo-spatial conditional associative learning tasks in which they had to learn to associate spatial cues with particular visual/auditory stimuli. The animals were subsequently trained on a spatial working memory task, the eight-arm radial maze. Rats with lesions to the mammillary body region were able to acquire the conditional associative learning tasks at a rate comparable to that of operated control animals, whereas those with hippocampal lesions were not. By contrast, rats with a lesion of the mammillary body region or the hippocampus were significantly impaired in comparison with the operated control animals in the radial maze. The findings suggest that lesions to the mammillary body region impair spatial working memory without affecting the capacity to associate particular exteroceptive cues with spatial locations.     

Neural correlates of memory retrieval in the prefrontal cortex

Working memory includes short-term representations of information that were recently experienced or retrieved from long-term representations of sensory stimuli. Evidence is presented here that working memory activates the same dorsolateral prefrontal cortex neurons that: (a) maintained recently perceived visual stimuli; and (b) retrieved visual stimuli from long-term memory (LTM). Single neuron activity was recorded in the dorsolateral prefrontal cortex while trained monkeys discriminated between two orientated lines shown sequentially, separated by a fixed interstimulus interval. This visual task required the monkey to compare the orientation of the second line with the memory trace of the first and to decide the relative orientation of the second. When the behavioural task required the monkey to maintain in working memory a first stimulus that continually changed from trial to trial, the discharge in these cells was related to the parameters--the orientation--of the memorized item. Then, what the monkey had to recall from memory was manipulated by switching to another task in which the first stimulus was not shown, and had to be retrieved from LTM. The discharge rates of the same neurons also varied depending on the parameters of the memorized stimuli, and their response was progressively delayed as the monkey performed the task. These results suggest that working memory activates dorsolateral prefrontal cortex neurons that maintain parametrical visual information in short-term and LTM, and that the contents of working memory cannot be limited to what has recently happened in the sensory environment.     

Responses of striatal neurons in the behaving monkey. 1. Head of the caudate nucleus

The activity of 394 neurons in the head of the caudate nucleus and the most anterior part of the putamen was analyzed in 3 behaving rhesus monkeys in order to analyze the functions of this part of the striatum. Of these neurons, 64.2% responded in the tests used in relation to, for example, environmental events, movements made by the monkey, the performance of a visual discrimination, or during feeding. However, only relatively small proportions of these neurons had responses which were unconditionally related to visual (9.6%), auditory (3.5%), or gustatory (0.5%) stimuli, or to movements (4.1%). Instead, the majority of the responsive neurons had activity in relation to stimuli or movements which was conditional, in that the responses occurred in only some test situations, and were often dependent on the performance of a task by the monkeys. Thus, it was found in the visual discrimination task that 14.5% of the neurons responded during a 0.5 sec tone/light cue period which signalled the start of each trial; 31.1% responded in the period in which the discriminative visual stimuli were shown, with 24.3% of these responding more to either the visual stimulus which signified food reward or to that which signified punishment; and 6.2% responded in relation to lick responses. Yet these neurons typically did not respond in relation to the cue stimuli, to the visual stimuli, or to movements, when these occurred independently of the task, or when performance of the task was prevented. Comparably, of the neurons tested during feeding, 25.8% responded when the food was seen by the monkey, 6.2% when he tasted it, and 22.4% during a cue given by the experimenter that a food or non-food object was about to be presented. However, only few of these neurons had responses to the same stimuli presented in different situations.It is concluded that many neurons in the head of the caudate nucleus and the most anterior part of the putamen respond in relation to events which are used as cues to prepare for the performance of tasks, including feeding, in which movements must be initiated. Other neurons respond in relation to the stimuli used and the movements made in these tasks. However, the majority of these neurons do not have unconditional sensory or motor responses. It is therefore suggested that the anterior neostriatum contains neuronal mechanisms which are important in the process by which environmental cues are used in the preparation of behavioral responses, and in the initiation of particular behavioral responses made in particular situations to particular environmental stimuli. Deficits in the initiation of movements following damage to striatal pathways may arise in part because of interference with these functions of the anterior neostriatum.     

The effects of selective hippocampal damage on tests of oddity in rhesus macaques

The oddity task (e.g., A-, A-, B+) is classified as a conjunctive or relational task in which accurate performance depends upon learning to attend to stimulus relationships, not stimulus identity, and has no retention component as stimuli are presented simultaneously. It has been suggested that the hippocampus may play a particular role in learning this type of task in humans and animals. To test this, we trained adult rhesus macaques with selective neurotoxic damage to the hippocampal formation on their ability to learn and apply an oddity rule. The results suggest that the monkeys were able to adapt simple strategies to solve variations of the oddity task, however as the opportunity for such strategies was reduced, monkeys with hippocampal damage were increasingly impaired.     

Functional contributions and interactions between the human hippocampus and subregions of the striatum during arbitrary associative learning and memory

The hippocampus and striatum are thought to have different functional roles in learning and memory. It is unknown under what experimental conditions their contributions are dissimilar or converge, and the extent to which they interact over the course of learning. In order to evaluate both the functional contributions of as well as the interactions between the human hippocampus and striatum, the present study used high‐resolution functional magnetic resonance imaging (fMRI) and variations of a conditional visuomotor associative learning task that either taxed arbitrary associative learning (Experiment 1) or stimulus‐response learning (Experiment 2). In the first experiment, we observed changes in activity in the hippocampus and anterior caudate that reflect differences between the two regions consistent with distinct computational principles. In the second experiment, we observed activity in the putamen that reflected content specific representations during the learning of arbitrary conditional visuomotor associations. In both experiments, the hippocampus and ventral striatum demonstrated dynamic functional coupling during the learning of new arbitrary associations, but not during retrieval of well‐learned arbitrary associations using control variants of the tasks that did not preferentially tax one system versus the other. These findings suggest that both the hippocampus and subregions of the dorsal striatum contribute uniquely to the learning of arbitrary associations while the hippocampus and ventral striatum interact over the course of learning. © 2015 Wiley Periodicals, Inc.     

Modulation during learning of the responses of neurons in the lateral hypothalamus to the sight of food

Recordings were made from single neurons in the lateral hypothalamus and substantia innominata of the rhesus and squirrel monkey during feeding. A population of these neurons which altered their firing rates while the monkeys looked at food but not at nonfood objects was investigated. Because the responses of these neurons must have been affected by the previous experience of the animals, the activity of the neurons was measured during tasks in which the monkeys learned whether or not objects which they saw were associated with food. During visual discrimination tests these neurons came to respond when the monkey saw one stimulus associated with food (e.g., a black syringe from which the animal was fed glucose), but not when the monkey saw a different stimulus which was not associated with food (e.g., a white syringe from which the animal was offered saline). During extinction tests these units ceased to respond when the monkey saw a visual stimulus such as a peanut if the peanut was repeatedly not given to the monkey to eat. The learning or extinction behavior approximately paralleled the response of the neurons.The findings that the neurons in the lateral hypothalamus and substantia innominata respond when a monkey is shown food only if he is hungry, and as shown here, if as a result of learning the visual stimulus signifies food, provide information on a part of the brain which may be involved in feeding. The findings are consistent with other data which suggest that the responses of these neurons are involved in the autonomic and/or behavioral reactions of the animal to the sight of food.     

Hippocampal encoding of non-spatial trace conditioning.

Trace eyeblink classical conditioning is a non-spatial learning paradigm that requires an intact hippocampus. This task is hippocampus-dependent because the auditory tone conditioned stimulus (CS) is temporally separated from the corneal airpuff unconditioned stimulus (US) by a 500-ms trace interval. Our laboratory has performed a series of neurophysiological experiments that have examined the activity of pyramidal cells in the CA1 area of the hippocampus during trace eyeblink conditioning. We have found that the non-spatial stimuli involved in this paradigm are encoded in the hippocampus in a logical order that is necessary for their association and the subsequent expression of behavioral learning. Although there were many profiles of single neurons responding to the CS-US trial during training, the majority of the neurons showed an increase in activity to the airpuff-US. Prior to learning, it appears that hippocampal cells and ensembles of cells were preferentially attending to the stimulus with immediate behavioral importance, the US. Hippocampal cells then began to respond to the associated neutral stimulus, the CS. Shortly thereafter, animals began to show increases in the behavioral expression of CRs. In some experiments, hippocampal neurons from aged animals exhibited impairments in the encoding of CS and US information. These aged animals were not able to associate these stimuli and acquire trace eyeblink CRs. Our findings along with the findings of other spatial learning studies, suggest that the hippocampus is involved in encoding information about discontiguous sets of stimuli, either spatial or nonspatial, especially early in the learning process.     

Mechanisms of Visual Perceptual Learning in Macaque Visual Cortex

The neural mechanisms underlying behavioral improvement in the detection or discrimination of visual stimuli following learning are still ill understood. Studies in nonhuman primates have shown relatively small and, across studies, variable effects of fine discrimination learning in primary visual cortex when tested outside the context of the learned task. At later stages, such as extrastriate area V4, extensive practice in fine discrimination produces more consistent effects upon responses and neural tuning. In V1 and V4, the effects of learning were most prominent in those neurons that can contribute the most reliable information about the trained stimuli. I suggest that, depending on the particulars of the task demands, neurons at various stages of stimulus and task processing can change their tuning and responses, so that execution of the task will produce a higher frequency of reward. I speculate that the sort of changes that will occur depend on the task and on stimulus analysis requirements, and they may vary from changes in bottom‐up stimulus processing/tuning within early visual areas or more efficient readout of early visual areas to top‐down driven changes in response properties of these areas.     

Functional dissociation between fornix and hippocampus in spatial conditional learning

Do lesions of the fornix or the hippocampus impair the performance of spatial conditional associative learning tasks, and to what extent does damage to these brain structures result in comparable deficits in this type of spatial behavior? The available evidence is not clear. In the present study, rats with lesions of the fornix, hippocampus, and normal control animals were trained on two spatial–visual conditional learning tasks in which they had to form arbitrary associations between visual stimuli and the context in which these stimuli were embedded. In one condition, rats were required to choose stimulus X in place A and stimulus Y in place B, and there was no overlap in the contents of the two scenes. In the other condition, the animal approached the same scene from two different directions and had to select stimulus X when the scene was viewed from perspective A and to select stimulus Y when the scene was viewed from perspective B. Rats with fornix transection were able to learn both conditional tasks at a rate comparable to that of normal control animals, but rats with hippocampal damage were severely impaired under both conditions. The findings extend the range of tasks known to be sensitive to damage of the hippocampus. In addition, the results argue that the fornix is not necessary for the acquisition of certain spatial conditional learning tasks and that this brain structure cannot be used as an indicator of hippocampal dysfunction under all learning situations. © 2007 Wiley‐Liss, Inc.     

Hippocampal lesions can enhance discrimination learning despite normal sensitivity to interference from incidental information

Spatial properties of stimuli are sometimes encoded even when incidental to the demands of a particular learning task. Incidental encoding of spatial information may interfere with learning by (i) causing a failure to generalize learning between trials in which a cue is presented in different spatial locations and (ii) adding common spatial features to stimuli that predict different outcomes. Hippocampal lesions have been found to facilitate acquisition of certain tasks. This facilitation may occur because hippocampal lesions impair incidental encoding of spatial information that interferes with learning. To test this prediction mice with lesions of the hippocampus were trained on appetitive simple simultaneous discrimination tasks using inserts in the goal arms of a T-maze. It was found that hippocampal lesioned mice were facilitated at learning the discriminations, but they were sensitive to changes in spatial information in a manner that was similar to control mice. In a second experiment it was found that both control and hippocampal lesioned mice showed equivalent incidental encoding of egocentric spatial properties of the inserts, but both groups did not encode the allocentric information. These results demonstrate that mice show incidental encoding of egocentric spatial information that decreases the ability to solve simultaneous discrimination tasks. The normal egocentric spatial encoding in hippocampal lesioned mice contradicts theories of hippocampal function that suggest that the hippocampus is necessary for incidental learning per se, or is required for modulating stimulus representations based on the relevancy of information. The facilitated learning suggests that the hippocampal lesions can enhance learning of the same qualitative information as acquired by control mice.     

Electrophysiological evidence for reduced latent inhibition in schizophrenic patients

The present study examined latent inhibition (LI) effects in 17 acute and 16 partially remitted schizophrenic patients, and in 20 healthy controls, by measuring manual response latencies and event-related potentials (ERPs) during an association learning task. ERPs were recorded to elucidate the role of attention in the LI effect. Subjects performed a go/no-go task with an auditory conditional stimulus predicting a visual go command. Half of the subjects in each diagnostic group were pre-exposed to the conditional stimulus which had been used as an irrelevant distractor in a preceding discrimination task. Independent of diagnostic group membership, pre-exposed subjects showed slower manual responses to go stimuli than non-pre-exposed subjects, reflecting a robust LI effect. The N100 wave after the conditional stimuli, however, showed a differential pattern: pre-exposure increased N100 amplitudes in acute schizophrenics, whereas pre-exposed control subjects showed a trend for decreased N100. The amplitude of the contingent negative variation (CNV) was unaffected by pre-exposure. The ERP results suggest that acute schizophrenics have a deficit in learned inattention to irrelevant stimuli. However, the intact LI effect in schizophrenics at the motor speed level shows that human LI is a complex phenomenon depending on the tasks and measures used.