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Rats use place (allocentric) or stimulus-response (egocentric) learning strategies for foraging under ethological and/or experimental conditions, proposed to be conveyed by hippocampus or neostriatum, respectively. We investigated here the effect of a reversible blockade of neostriatum on learning strategies assessed by a cross maze paradigm, comparing A x C (phenotypically similar to wild rats) versus Long-Evans rat strains. The rats were trained to reach a consistently baited-arm (west arm), starting from the same arm (south arm). The learning strategy was evaluated at days 11 and 19, when test trials were performed placing the rat in a start-box at the arm (north arm) opposite to that when starting the training, following a saline or lidocaine injection into the neostriatum. Rats entering to the baited-west arm were considered to be place learners and those entering to the unbaited-east arm were response learners. It was found that Long-Evans rats injected with saline were place learners on day 11 and response learners on day 19, but were place learners on both days when injected with lidocaine. A x C rats injected with saline were response learners on days 11 and 19, and place learners on both days when injected with lidocaine. Thus, rat strain influences the memory strategy for solving a cross maze paradigm. Long-Evans, but not A x C rats, shift from place (allocentric) to response (egocentric) learning along the training. When neostriatum was blocked by lidocaine, both rat strains elicited a place learning strategy only.

Place memory is relevant for exploration and forage behaviour. When food supply is dispersed, a win-shift has advantage over a win-stay strategy. In the Olton Octagonal Maze, the rat follows a win-shift strategy using working memory. However, in the Olton 4x4 version, the rat follows a win-stay strategy, using both working and long-term memories. It has been suggested that the neocortex is required for the resolution of tasks demanding long-term, but not for that demanding working memory alone. The role of anteromedial/posterior parietal cortex (AM/PPC) was investigated here, using a reversible lesion induced by intracerebral lidocaine infusion. Long-Evans rats were implanted with guide cannulae into the AM/PPC and trained in an Olton 4x4 maze, counting working and long-term memory errors after a delay. Then, the animals were infused with lidocaine or saline during the delay phase and tested for three days. Another series of animals, treated as before, was tested in an Olton Octagonal Maze and subjected to the same injection schedule. In the Olton 4x4 Maze, lidocaine produced a significant increase in working and long-term memory errors, compared to saline and post-lidocaine conditions. In contrast, in the Olton Octagonal Maze, lidocaine did not induce any effect on working memory errors. Thus, AM/PPC is required when both working with previous information and long-term memories are needed, but not when only working memory is required, as it happens under ethological conditions. Whenever food supply is dispersed, a win-shift strategy is preferable.

The primary visual cortex of rats is surrounded laterally (in Oc2L) and medially (in Oc2M) by several peristriate visual areas. Previous studies from our laboratory demonstrated that bilateral lesions in Oc2L result in visual pattern discrimination deficit, and in failure to solve a conditional discrimination which requires figure-background association. In contrast, neurotoxic lesions of the rostral part of Oc2M (which contains the anteromedial and anterior peristriate visual areas, collectively referred to as AM complex) result in deficits in visuospatial discrimination, and in disruptions in visual tasks involving spatial memory. The objective of this study was to behaviorally test the role of AM complex in a spatial memory task in absence of visual cues. For this purpose, we analyzed memory retention of Lashley III maze in blind rats after bilateral ibotenate lesions in AM complex, or in the primary visual cortex (V1, Oc1), to test the hypothesis that AM complex is essential for this cognitive task. The results showed a significant loss of memory retention of the maze in rats with lesions in AM complex, but not in rats with lesions in V1. Furthermore, the retention loss in rats with AM complex lesions was positively and significantly correlated with the size of the lesion. The results indicate a critical role of AM complex in spatial memory mechanisms independent on visual cues. A probable homology of rat AM complex with the posterior parietal cortex of primates is discussed.

Rodents in search of food use visual environmental signals and complex spatial strategies and do not return to previously-visited locations, known as the win-shift strategy. The solution to the Olton Octagonal Maze (OOM) involves Working Memory (WM). A modified OOM was used that allows for measuring WM and Long Term Memory (LTM). The delayed spatial win-shift task consisted of a Training and Test phase separated by a delay. Prior to the Training phase, four arms were chosen at random and blocked, and food pellets were placed in the food cups of the four remaining open arms. Each rat was allowed to retrieve the pellets from the four open arms and then return to its home cage for the delay period (either 5 or 20 min). In the Test phase all 8 lanes were open, and the bait was placed in those blocked in the previous phase. Two experimental groups of rats, Long Evans and Norvegicus, and their corresponding control groups were trained. The experimental subjects performed Training-Delay-Test. The controls were only trained in the Test phase. Revisiting an arm previously explored in the 1st Phase was considered a LTM error. Revisiting an arm in the same trial constituted a WM error. It was concluded that the experimental groups do in fact possess LTM, with differences in favor of Norvegicus. There was no difference with respect to WM errors. The Norvegicus control group changes its strategy from allocentric to egocentric, which did not occur in the Long Evans control group.

The Morris water-maze task was used to evaluate the role of the anteromedial extrastriate visual cortex in the processing of visuospatial information in rats. Six gray male rats received bilateral ibotenic acid injections targeted stereotactically to the rostral part of the anteromedial extrastriate visual cortex. These operated subjects and six other unoperated control rats were tested in the maze. Histological analysis confirmed the localization, symmetry, and depth of lesions in the rostral part of anteromedial area (AMa) in the operated subjects. In these animals, a significantly greater latency to reach the submerged platform was found (U = 0, p = 0.004). The Morris water-maze may be considered as a reference memory task. It presents a stronger demand on the use of allocentric spatial visual cues than on the use of egocentric cues for navigation. Therefore, the present data lend support to the participation of area AMa in the integration of allocentric visuospatial cues or as a link in the memory system involved in the acquisition of this task.

Rats were trained in a lateralized reaching motor task during either an 'early' (22-31 days old) or a 'late' (62-71 days old) postnatal period. The 'late' group showed significant neuronal density reduction in cortical layers II-III of the contralateral motor forelimb representation. The 'early' group evidenced a similar localized contralateral effect that persisted after a subsequent period without training. Furthermore, in this group, a bilateral overall decrease in neuronal density was found throughout the motor cortex. This bilateral experience and age-dependent effect is conceivably related to a critical period of motor cortical development. The localized reduction of neuronal density strongly indicates a morphological expression of the motor engram. Our present study supports the concept that the acquisition and retention of motor learning involves the persistence of structural changes in the brain.

To assess the role of the rat anteromedial extrastriate cortex (AM) in a visuospatial discrimination task, restricted bilateral ibotenic acid lesions were placed stereotaxically in this region. Gray rats with lesions in AM were trained in a task requiring them to discriminate the location of a light stimulus placed vertically at different elevations. Correct responses required pressing right or left levers to obtain rewards. In contrast to unoperated controls, lesioned rats failed in learning the visuospatial discrimination task. A correlation was found between the bilateral extent of the lesion in area AM and the behavioral deficit. Another group of lesioned rats was trained to discriminate brightness differences of the light stimulus but requiring the same egocentric right/left motor response. The performance of these rats was similar to that of controls. From these results we conclude that extrastriate area AM in the rat is necessary for visuospatial discrimination, but not for correct egocentric motor responses. The visuospatial functions of area AM in the rat are reminiscent of visuospatial functions ascribed to the parietal streams of extrastriate visual areas in the monkey.

The effects of a lateralized reaching task on the morphological structure of the rat motor cortex were studied during the early postweaning period. Our results show that the consistent use of one forelimb accounts for a significant decrease in the numerical density of cells and an increase in cortical thickness of the contralateral 'forelimb' motor cortex. As a consequence of the early motor training the cell distribution, which is lower rostrally than caudally in nontrained hemispheres, is reversed in the trained hemispheres. This may be interpreted as the specific motor training triggering a higher neuronal branching in the corresponding cortical region. The present findings may further the understanding of the mechanisms involved in the generation of morphological brain asymmetries.

In order to automate a simultaneous visual discrimination system, a Sutherland jumping-stand box was modified by adding two lateral alleys which connect the feeder compartment with the starting box. By means of a digital interface, this modified box was connected to an Apple IIe PC. A group of rats (n = 8) was trained. All subjects learned. Advantages relevant to the apparatus design, as well as the benefits derived from the automated procedure are discussed.

The involvement of rat anteromedial extrastriate cortex (area AM, in the anterior portion of area 18b) in the integration of visual and somatic cues was assessed behaviorally. Following restricted bilateral lesions of selected cortical regions, rats were tested on their ability to retain or relearn a conditional visuosomatic discrimination task learned prior to surgery. Two compound, visuosomatic stimuli were used: white or black associated with either one of two degrees of roughness. The use of a guided-response procedure was essential for rats to learn this difficult conditional bimodal task. None of the 6 rats with lesions aimed at area AM retained the habit postoperatively. Four of these rats were incapable of relearning the task after 3 postoperative training series, and they had either extensive lesions of area AM or relatively small, symmetric damage of anterior portions of AM. The remaining two rats with lesions in area AM were able to relearn the task in the second postoperative training series, and their lesions either were restricted to posteromedial aspects of area AM or they involved asymmetric loci in anterior area AM. In contrast to rats with lesions of area AM, rats with lesions in visual cortex in areas 17 and 18a, or in auditory cortex in area 41, were able either to retain the task or to relearn in the first postoperative training series. These results indicate that the integrity of area AM appears necessary for rats to discriminate between pairs of compound stimuli that differ in brightness and roughness. The behavioural data point to the notion that this area might be involved in the integration of these types of visual and somatic stimuli. In addition, our finding that performance was largely unimpaired following extensive lesions of lateral extrastriate area supports previous reports indicating that medial and lateral extrastriate visual areas differ in function.

In order to assess the behavioural effects of environmental stimulation at different stages of development, two groups of rats were exposed to multisensory enrichment on days 10-24 (preweaning) or 25-39 (postweaning). Both groups had four 25-min sessions per day in a large cage with a variety of stimuli, in addition to 3 min of handling before each session. The mother of the preweaning group remained in the home cage during the stimulation sessions. A third group was maintained in a social condition. Testing in a Hebb-Williams maze started when the rats of the 3 groups were 100 days old. Error, latency and running time scores were lowest in the preweaning group.

The effects of postnatal environmental stimulation on the branching patterns of cortical dendrites were measured in rats. Pups were exposed to 4 daily multisensory enrichment sessions from days 10-24, while littermates were maintained in standard conditions. At 25 days of age, the brains were stained using the Golgi-Cox-Sholl method. Camera lucida drawings were made of the basilar dendritic trees from a total of 528 layer-III occipital cortex pyramidal neurons. A highly significant increase was found in number and length of segments from order 1-5 in the neurons from the enriched subjects.

A guided-response procedure was used to train a visual pattern discrimination by rats in a modified Sutherland box. The method consisted of guiding the animal to the correct choice by means of a retractable bridge that led to reinforcers, followed by gradually removing this prompt. This method was compared to a stimulus-fading procedure, in which the initial differences between discriminative stimuli were gradually faded until they differed only with respect to the critical dimension for discrimination, and to a trial-and-error procedure. Both gradual procedures resulted in fewer errors compared to the trial-and-error procedure. The higher efficiency of the fading procedures was attributed to less aversiveness derived from performance with few errors and to the use of step-by-step requirements relative to the criterion performance.