Effects of aspiration lesions of hippocampus or overlying neocortex on concurrent and configural object discriminations in rats

The effects of hippocampal and area parahippocampalis lesions on the processing and retention of serial-order behavior, autoshaping, and spatial behavior in pigeons

We examined the role of the avian hippocampus and area parahippocampalis in serial-order behavior and a variety of other tasks known to be sensitive to hippocampal damage in mammals. Damage to the hippocampus and area parahippocampalis caused impairments in autoshaping and performance on an analogue of a radial-arm maze task, but had no effect on acquisition of 2-item, 3-item, and 4-item serial-order lists. Additionally, the lesions had no effect on the retention of 3-items lists, or on the ability to perform novel derived lists composed of elements from lists they had previously learned. The impairments in autoshaping and spatial behavior are consistent with the findings in mammals. The absence of impairments on the serial-order task may also be consistent once one considers that damage to the hippocampus in mammals seems to affect more internally-organized rather than externally-organized serial-order tasks. Together, the findings support the view that the avian hippocampal complex serves a function very similar to the mammalian hippocampus, a finding that is interesting given that the architecture of the avian hippocampus differs dramatically from that of the mammalian hippocampus.

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. © 2011 Wiley Periodicals, Inc.

Fornix transection impairs learning of randomly changing object discriminations

The hippocampus has a well established role in spatial memory, but increasing evidence points to a role in nonspatial aspects of memory. To investigate such a role, six macaque monkeys received a bilateral transection of the fornix to disconnect subcortical inputs and outputs of the hippocampus. An additional six macaque monkeys constituted an unoperated control group. To test the involvement of the hippocampus in nonspatial aspects of memory, both groups were trained postoperatively on four concurrent visual object discrimination problems, each problem having one rewarded object and one unrewarded. After acquisition to criterion of these discriminations, the monkeys learned five subsequent stages of discriminations using the same objects. In each of these stages, both the pairings of objects one with another, and the reward assignments for the objects, were randomly reassigned. In the initial acquisition stage, control and fornix animals were equally proficient in learning the discriminations. In the five reassigned stages, however, monkeys with fornix transection made on average three times as many errors as the controls in learning the discriminations. This impairment was noted even in trials where the reward assignments from the previous stage were maintained in the new stage. These findings are consistent with other recent evidence for a role beyond the spatial domain for the fornix in monkeys.

Decrease in non-selective, non-sustained attention induced by a chronic visceral inflammatory state as a new pain evaluation in rats

The aim of this study was to develop a new behavioral pain test based on the evaluation of cognitive capacity impairments in rats with colitis and to determine the impact of different acute analgesic treatments. Colitis was induced in rats by an enema containing 2,4,6-trinitrobenzen sulfonic acid. Visual non-selective, non-sustained attentional level was assessed by a new behavioral testing procedure. Animals were familiarized on three consecutive days with an open field containing four small, similar, familiar objects. On the day of testing, one of the objects was randomly replaced by a new one. Attentional level was determined by the ability of the rat to perceive this small modification to its familiar environment. The effect of morphine, acetaminophen, aspirin or ibuprofen treatment was assessed on testing day and compared with that observed during a Von Frey test to assess referred tactile hypersensitivity of the skin of the lower back. Rats with colitis had decreased attentional level but no change in their locomotor activity, interest in the environment or memory encoding. Morphine (1 mg/kg, s.c. and 10 microg/rat, i.t.) and acetaminophen (200 mg/kg, p.o.) had a beneficial effect on attentional level and on referred tactile hypersensitivity. Testing for the latter showed that aspirin and ibuprofen (400 mg/kg, p.o.) were ineffective. The decrease in visual non-selective, non-sustained attention induced by chronic inflammatory painful state can be relieved by effective analgesic treatments. This finding could lead to the development of a new behavioral test to assess spontaneous pain in chronic painful subjects.

Hippocampal lesions and discrimination performance of mice in the radial maze: sparing or impairment depending on the representational demands of the task

The effects of ibotenate hippocampal lesions on discrimination performance in an eight-arm radial maze were investigated in mice, using a three-stage paradigm in which the only parameter that varied among stages was the way the arms were presented. In the initial learning phase (stage 1), animals learned the valence or reward contingency associated with six (three positive and three negative) adjacent arms of the maze using a successive (go/no-go) discrimination procedure. In the first test phase (stage 2), the six arms were grouped into three pairs, so that on each trial, the subject was faced with a choice between two adjacent arms of opposite valence (concurrent two-choice discrimination). In the second test phase (stage 3), the subject was faced with all six arms simultaneously (six-choice discrimination). Hippocampal-lesioned mice acquired the initial learning phase at a near-normal rate but behaved as if they had learned nothing when challenged with the two-choice discriminations at stage 2. In contrast, they behaved normally when confronted with the six-choice discrimination at stage 3. Detailed examination of within- and between-stage performance suggests that hippocampal-lesioned mice perform as intact mice when presentation of the discriminanda encourages the storage and use of separate representations (i.e., in initial learning and six-choice discrimination testing), but that they fail in test situations that involve explicit comparisons between such separate representations (two-choice discriminations), hence requiring the use of relational representations.

Conditional discriminations based on external and internal cues in rats with cytotoxic hippocampal lesions

Septal-hippocampal system lesions, mostly using aspiration techniques, have been reported to impair performance of conditional tasks. Rats with axon-sparing cytotoxic hippocampal lesions were therefore tested in a range of instrumental conditional paradigms. They did not differ from controls in their ability to choose the appropriate object in a conditional object discrimination cued by internal state (hunger or thirst) or on performance of conditional visuospatial object discriminations. Acquisition of a conditional visuospatial discrimination with black and white boxes as stimuli was also unimpaired. In contrast, lesioned rats were profoundly impaired on an open T-maze task when cued by either their internal state (reference memory task) or their previous response (working memory task). The results indicate that perception or use of spatial cues, rather than conditional responding per se, is impaired by cytotoxic hippocampal lesions.

Individual differences in the functional neuroanatomy of verbal discrimination learning revealed by positron emission tomography

Why do some people have better memory abilities than others? This issue has been of long-standing interest to scientists and lay people. However, using purely behavioral methods, psychologists have made little progress in illuminating it. Now that functional brain imaging techniques have become available to study mind/brain relations, there is a new promise of understanding individual differences in learning and memory in terms of corresponding differences in brain activity. In this paper, we will present a positron emission tomography (PET) study designed to examine individual differences in learning and memory abilities. The basic assumption is that different patterns of brain activity serve as strong predictors of memory performance. Two specific questions were addressed in this study: (i) Can PET illuminate the relations between memory processes and their neuroanatomical correlates among individual learners and rememberers? and (ii) if so, how are these relations affected by the stage of practice on a given memory task? Our PET study examined individual differences in the neuroanatomical correlates of multi-trial verbal discrimination learning in 16 young healthy subjects. The results identified patterns of brain regions in which blood flow correlated with subjects' retrieval performance. However, these regions did not correlate with performance during all learning trials. Instead, a gradual shift was observed from one pattern of brain regions to another over the course of learning. These results suggest that individual differences in memory performance are related to differences in neural activity within specific brain circuits.

Is the avian hippocampus a functional homologue of the mammalian hippocampus?

The effects of hippocampal lesions on the processing and retention of visual and spatial information in birds and mammals is reviewed. Both birds and mammals with damage to the hippocampus are severely impaired on a variety of spatial tasks, such as navigation, maze learning, and the retention of spatial information. In contrast, both birds and mammals with damage to the hippocampus are not impaired on a variety of visual tasks, such as delayed matching-to-sample, concurrent discrimination, or retention of a visual discrimination. In addition, both birds and mammals with hippocampal damage display impairments in the acquisition of an autoshaped response, as well as alterations in response suppression. These findings suggest that the avian hippocampus is a functional homologue of the mammalian hippocampus, and that in both birds and mammals the hippocampus is important for the processing and retention of spatial, rather than purely visual information.

Knowing which and knowing what: a potential mouse model for age-related human declarative memory decline

The present study was built on the original report of Eichenbaum et al. [Eichenbaum, H., Fagan, A., Mathews, P. & Cohen, N.J. (1988), Behav. Neurosci., 102, 3531-3542] on the contrasting effects of fornix lesion in different versions of an odour-guided discrimination task in rats, and attempted to extend this into a mouse model for the preferential loss of declarative memory seen in human senescence. Each of the two experiments reported here consisted of a two-stage paradigm, with an initial learning phase followed by a test phase. The information acquired in the first stage was identical in both experiments, i.e. the valence or reward contingency associated with six (three positive and three negative) arms of a radial maze. The only parameter which was varied between Experiment A and B, and also between the two successive stages within each experiment, was the way of presenting the arms to the mice, i.e. either in pairs (simultaneous discriminations) or one at a time (successive go : no-go discrimination). Performance in the first stage demonstrated that our aged mice were impaired in learning concurrent simultaneous discriminations but not successive go/no-go discrimination, thereby resembling that reported in rats with hippocampal damage. Most importantly, our present set of data supports the conclusion that two forms of memory expression for the same piece of acquired experience can be assessed in the same subjects by manipulating the way of presenting two arms that were previously experienced separately. These two forms of memory expressions are differentially affected in aged mice, thereby demonstrating the highly selective and specific deleterious effect of ageing.

Visual discrimination learning under switching procedure in visually deprived cats

Previous studies have shown that fine visual discrimination learning is severely impaired in cats binocularly deprived in the early period of life (BD cats) and also somewhat in control cats reared with open eyes in the limited laboratory environment (C cats) compared with cats reared in a normal rural environment (N cats). It was concluded that visual deprivation impairs perceptual learning. In the present study discriminative stimuli were dissimilar and so the task was perceptually easy, but using a switching procedure made it associatively difficult. In regular trials a gate with a grating pattern was positive and a blank gate negative, whereas in switching trials the meaning of the gates was reversed. The switching stimulus was intermittent light in some stages of training and intermittent tone in others. Learning was severely impaired in BD cats and somewhat in C cats and the deficit was similar under visual and auditory switching. Thus, early visual deprivation impairs associative learning. The impairment probably includes associations between switching stimulus and instrumental responses and configural associations between switching stimulus and discriminative stimuli.

The effect of excitotoxic hippocampal lesions on simple and conditional discrimination learning in the rat

The effect of excitotoxic lesions of the hippocampus on acquisition and reversal of simple and conditional tasks was investigated using a Y-maze. Hippocampal-lesioned rats were severely impaired on acquisition and reversal of a conditional visuo-spatial task (where different pairs of visually distinctive choice arms indicated whether a left or right arm choice was correct on that trial) and were unable to acquire a visuo-visual conditional discrimination (where the appearance of the start arm indicated which of the visually distinctive choice arms was correct irrespective of their left/right position). They were not impaired on acquisition or reversal of a simple spatial left/right discrimination task (where all arms had the same visual appearance) nor on acquisition of a visual discrimination (where the correct, visually distinctive, choice arm varied in its left/right position). Hippocampal-lesioned rats were, however, impaired on reversal of this visual discrimination task and on acquisition and reversal of another visual discrimination task in which the visually distinctive choice arms were less different from each other than in the first version of this task. The degree of impairment in the lesioned rats was related to task difficulty for the sham-operated rats and was not specific to tasks requiring spatial choices, visual discrimination or conditional responding. The impairment on conditional tasks was greater than the impairment on those non-conditional tasks which happened to be matched for task difficulty for the sham-operated rats, suggesting that the conditional demand may target the function of the hippocampus rather closely. Statistically worse than chance performance by hippocampal-lesioned (and sham-operated) rats at the beginning of reversal testing, which was given 24 h after achieving criterion on acquisition of that task, indicated that hippocampal-lesioned rats simultaneously exhibited good memory but impaired learning for the type of information required for those tasks.