The role of local, distal, and global information in latent spatial learning

Elemental and Configural Associative Learning in Spatial Tasks: Could Zebrafish be Used to Advance Our Knowledge?

Spatial learning and memory have been studied for several decades. Analyses of these processes pose fundamental scientific questions but are also relevant from a biomedical perspective. The cellular, synaptic and molecular mechanisms underlying spatial learning have been intensively investigated, yet the behavioral mechanisms/strategies in a spatial task still pose unanswered questions. Spatial learning relies upon configural information about cues in the environment. However, each of these cues can also independently form part of an elemental association with the specific spatial position, and thus spatial tasks may be solved using elemental (single CS and US association) learning. Here, we first briefly review what we know about configural learning from studies with rodents. Subsequently, we discuss the pros and cons of employing a relatively novel laboratory organism, the zebrafish in such studies, providing some examples of methods with which both elemental and configural learning may be explored with this species. Last, we speculate about future research directions focusing on how zebrafish may advance our knowledge. We argue that zebrafish strikes a reasonable compromise between system complexity and practical simplicity and that adding this species to the studies with laboratory rodents will allow us to gain a better understanding of both the evolution of and the mechanisms underlying spatial learning. We conclude that zebrafish research will enhance the translational relevance of our findings.

Age-related preference for geometric spatial cues during real-world navigation

Ageing effects on spatial navigation are characterized mainly in terms of impaired allocentric strategies. However, an alternative hypothesis is that navigation difficulties in aged people are associated with deficits in processing and encoding spatial cues. We tested this hypothesis by studying how geometry and landmark cues control navigation in young and older adults in a real, ecological environment. Recordings of body and gaze dynamics revealed a preference for geometry-based navigation in older adults, and for landmark-based navigation in younger ones. While cue processing was associated with specific fixation patterns, advanced age manifested itself in a longer reorientation time, reflecting an unbalanced exploration-exploitation trade-off in scanning policies. Moreover, a battery of tests revealed a specific cognitive deficit in older adults with geometric preference. These results suggest that allocentric strategy deficits in ageing can result from difficulties related to landmark coding, and predict recovery of allocentric strategies in geometrically polarized environments.

Perirhinal cortex lesions that impair object recognition memory spare landmark discriminations

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Loss of perirhinal cortex spares mirror-imaged landmark discriminations.

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Perirhinal cortex lesions do not disrupt latent spatial learning.

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Further underlines dissociation between perirhinal and hippocampal function.

Rats with lesions in the perirhinal cortex and their control group learnt to discriminate between mirror-imaged visual landmarks to find a submerged platform in a watermaze. Rats initially learnt this discrimination passively, in that they were repeatedly placed on the platform in one corner of a square watermaze with walls of different appearance, prior to swimming to that same location for the first time in a subsequent probe trial. Perirhinal cortex lesions spared this passively learnt ability, despite the common visual elements shared by the guiding landmarks. These results challenge models of perirhinal function that emphasise its role in solving discriminations between stimuli with ambiguous or overlapping features, while underlining how this cortical region is often not required for spatial processes that involve the hippocampus.

The fallacy of placing confidence in confidence intervals

Interval estimates - estimates of parameters that include an allowance for sampling uncertainty - have long been touted as a key component of statistical analyses. There are several kinds of interval estimates, but the most popular are confidence intervals (CIs): intervals that contain the true parameter value in some known proportion of repeated samples, on average. The width of confidence intervals is thought to index the precision of an estimate; CIs are thought to be a guide to which parameter values are plausible or reasonable; and the confidence coefficient of the interval (e.g., 95 %) is thought to index the plausibility that the true parameter is included in the interval. We show in a number of examples that CIs do not necessarily have any of these properties, and can lead to unjustified or arbitrary inferences. For this reason, we caution against relying upon confidence interval theory to justify interval estimates, and suggest that other theories of interval estimation should be used instead.

Evidence for concrete but not abstract representation of length during spatial learning in rats

In 4 experiments, rats had to discriminate between the lengths of 2 objects of the same color, black or white, before a test trial with the same objects but of opposite color. The experiments took place in a pool from which rats had to escape by swimming to 1 of 2 submerged platforms. For Experiments 1 and 2, the platforms were situated near the centers of panels of 1 length, but not another, that were pasted onto the gray walls of a square arena. The acquired preference for the correct length was eliminated by changing the color of the panels. In Experiment 3, the platforms were situated near the middle of the long walls of a rectangular pool, and in Experiment 4 they were situated in 1 pair of diagonally opposite corners of the same pool. Changing the color of the walls markedly disrupted the effects of the original training in both experiments. The results indicate that rats represent the length of objects not by their abstract, geometric attributes but in a more concrete fashion such as by a mental snapshot or by the amount of color stimulation they provide.

The effect of retrosplenial cortex lesions in rats on incidental and active spatial learning

The study examined the importance of the retrosplenial cortex for the incidental learning of the spatial arrangement of distinctive features within a scene. In a modified Morris water-maze, rats spontaneously learnt the location of an escape platform prior to swimming to that location. For this, rats were repeatedly placed on a submerged platform in one corner of either a rectangular (Experiment 1) or square (Experiments 2, 3) pool with walls of different appearance. The rats were then released in the center of the pool for their first test trial. In Experiment 1, the correct corner and its diagonally opposite partner (also correct) were specified by the geometric properties of the pool. Rats with retrosplenial lesions took longer to first reach a correct corner, subsequently showing an attenuated preference for the correct corners. A reduced preference for the correct corner was also found in Experiment 2, when platform location was determined by the juxtaposition of highly salient visual cues (black vs. white walls). In Experiment 3, less salient visual cues (striped vs. white walls) led to a robust lesion impairment, as the retrosplenial lesioned rats showed no preference for the correct corner. When subsequently trained actively to swim to the correct corner over successive trials, retrosplenial lesions spared performance on all three discriminations. The findings not only reveal the importance of the retrosplenial cortex for processing various classes of visuospatial information but also highlight a broader role in the incidental learning of the features of a spatial array, consistent with the translation of scene information.

The impact of fornix lesions in rats on spatial learning tasks sensitive to anterior thalamic and hippocampal damage

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Fornix damage mildly impair spatial biconditional and passive place learning tasks.

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Fornix lesions impair spatial go/no-go and alternation problems.

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Fornix lesions impair tests making flexible demands on spatial memory.

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Fornix connections are not always required for learning fixed spatial responses.

The present study sought to understand how the hippocampus and anterior thalamic nuclei are conjointly required for spatial learning by examining the impact of cutting a major tract (the fornix) that interconnects these two sites. The initial experiments examined the consequences of fornix lesions in rats on spatial biconditional discrimination learning. The rationale arose from previous findings showing that fornix lesions spare the learning of spatial biconditional tasks, despite the same task being highly sensitive to both hippocampal and anterior thalamic nuclei lesions. In the present study, fornix lesions only delayed acquisition of the spatial biconditional task, pointing to additional contributions from non-fornical routes linking the hippocampus with the anterior thalamic nuclei. The same fornix lesions spared the learning of an analogous nonspatial biconditional task that used local contextual cues. Subsequent tests, including T-maze place alternation, place learning in a cross-maze, and a go/no-go place discrimination, highlighted the impact of fornix lesions when distal spatial information is used flexibly to guide behaviour. The final experiment examined the ability to learn incidentally the spatial features of a square water-maze that had differently patterned walls. Fornix lesions disrupted performance but did not stop the rats from distinguishing the various corners of the maze. Overall, the results indicate that interconnections between the hippocampus and anterior thalamus, via the fornix, help to resolve problems with flexible spatial and temporal cues, but the results also signal the importance of additional, non-fornical contributions to hippocampal-anterior thalamic spatial processing, particularly for problems with more stable spatial solutions.

The role of the hippocampus in passive and active spatial learning

Rats with lesions of the hippocampus or sham lesions were required in four experiments to escape from a square swimming pool by finding a submerged platform. Experiments 1 and 2 commenced with passive training in which rats were repeatedly placed on the platform in one corner—the correct corner—of a pool with distinctive walls. A test trial then revealed a strong preference for the correct corner in the sham but not the hippocampal group. Subsequent active training of being required to swim to the platform resulted in both groups acquiring a preference for the correct corner in the two experiments. In Experiments 3 and 4, rats were required to solve a discrimination between different panels pasted to the walls of the pool, by swimming to the middle of a correct panel. Hippocampal lesions prevented a discrimination being formed between panels of different lengths (Experiment 3), but not between panels showing lines of different orientations (Experiment 4); rats with sham lesions mastered both problems. It is suggested that an intact hippocampus is necessary for the formation of stimulus-goal associations that permit successful passive spatial leaning. It is further suggested that an intact hippocampus is not necessary for the formation of stimulus-response associations, except when they involve information about length or distance. © 2014 The Authors. Hippocampus Published by Wiley Periodicals, Inc.

The impact of anterior thalamic lesions on active and passive spatial learning in stimulus controlled environments: geometric cues and pattern arrangement

The anterior thalamic nuclei are vital for many spatial tasks. To determine more precisely their role, the present study modified the conventional Morris watermaze task. In each of 3 experiments, rats were repeatedly placed on a submerged platform in 1 corner (the 'correct' corner) of either a rectangular pool (Experiment 1) or a square pool with walls of different appearances (Experiments 2 and 3). The rats were then released into the pool for a first test trial in the absence of the platform. In Experiment 1, normal rats distinguished the 2 sets of corners in the rectangular pool by their geometric properties, preferring the correct corner and its diagonally opposite partner. Anterior thalamic lesions severely impaired this discrimination. In Experiments 2 and 3, normal rats typically swam directly to the correct corner of the square pool on the first test trial. Rats with anterior thalamic lesions, however, often failed to initially select the correct corner, taking more time to reach that location. Nevertheless, the lesioned rats still showed a subsequent preference for the correct corner. The same lesioned rats also showed no deficits in Experiments 2 and 3 when subsequently trained to swim to the correct corner over repeated trials. The findings show how the anterior thalamic nuclei contribute to multiple aspects of spatial processing. These thalamic nuclei may be required to distinguish relative dimensions (Experiment 1) as well as translate the appearance of spatial cues when viewed for the first time from different perspectives (Experiments 2, 3).