Visual discrimination learning in the water maze: An adjunct to spatially dependent cognitive tasks

D. Harbaran, L. Robinson and G. Riedel

School of Medical Sciences, University of Aberdeen, Aberdeen, Scotland, UK

Learning and memory experiments using rodents rely heavily on normal processing of visual cues. When investigating pharmacological compounds that interfere with the cognitive abilities, it is paramount to exclude drug-induced effects on the visual system, as they may confound the results and lead to false positive interpretations. Here we describe a training procedure enabling visual acuity to be tested in the water maze. Lister-hooded rats and C57BL6/J mice were trained to associate the submerged platform with a 1 cm black and white vertical striped cue card to 80% correct responses criterion. The animals visual acuity was tested by juxtaposing the correct cue card (1 cm) to a cue card with either 1, 2, 3, 4, 5 or 10 cm wide stripes. Both rodents can learn to readily discriminate between 1 and 2 cm wide striations. The muscarinic receptor antagonist, scopolamine hydrobromide, at a dose of 0.2 mg/kg (ip.) was then shown to impair rats on a spatial reference memory paradigm in the water maze; however, there were no deficits in visual acuity. Two mg/kg scopolamine severely impaired rats in differentiating between 1, and 2, 3 and 4 cm wide striations. In mice, visual acuity was compromised up to 3 cm wide striations in the presence of 2 mg/kg scopolamine. We deduced that the difference in scopolamine effects on visual acuity between the two rodents was the alteration in search strategy adopted by mice as revealed through detailed analysis of the swim paths throughout different zones of the water maze. To confirm that these results were dependent on visual processing, and not on other sensory cues or gross motor impairments, rats, implanted with cannulae targeting the lateral geniculate nucleus, were infused with 0.4 ug/ul muscimol prior to testing in our visual acuity task. Percentage correct responses in these rats were impaired at all striations; however, the effect was reversible as the rats attained signi.cant improvement when infused with vehicle. These data suggest that it is possible to dissociate drug-induced effects on memory from changes in sensory perception, and that our task is solely dependent on visual processing.

Paper presented at Measuring Behavior 2005 , 5^th International Conference on Methods and Techniques in Behavioral Research, 30 August - 2 September 2005, Wageningen, The Netherlands.

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