Electrophysiological and behavioural studies of vision in the pigeon

Abstract

The literature concerning the eye and the visual pathways in the pigeon is reviewed (Chapter l).An electrophysiological investigation of the forebrain of the pigeon using the method of evoked potentials revealed four main areas within this structure in which large amplitude responses were found. These areas were the hyperstriatum, neostriatum caudale, paleostriatum augmentatum and an area of tissue surrounding the ectostriatum. Comparison of the latencies of these potentials with the latencies of potentials found within the mesencephalon and diencephalon excludes the nucleus rotundus as being involved in the relay of these potentials to the forebrain. Previous work by other authors has suggested that this nucleus is a major source of telencephalic visual afferents. Only one area within the forebrain produced potentials which are compatible as regards latency with being relayed by the nucleus rotundus. This area is the dorsolateral corticoid area. These results are discussed and compared to, previous work (Chapter 2).An investigation of the effects of destruction of the centrifugal pathway to the retina, by lesioning the isthmo-optic nucleus or sectioning the optic tract, on the pigeodb electroretinogram has revealed no evidence of this pathway being involved in the control of this potential. Evidence from two birds suggests that previous reports of centrifugal effects on the electroretinogram of the pigeon can be attributed to metabolic abnormalities occurring within the retina as a consequence of optic tract section (Chapter 5). Destruction of the hyperstriatum of the pigeon revealed no evidence of profound effects on visual discrimination tasks. Birds lesioned in this area show increased choice times on a compound colour-brightness discrimination but no increase in trials to criterion. No effects were apparent on pattern discrimination contrary to previous reports of such effects in the literature (Chapter 4). Destruction of the hyperstriatum or the neostriatum caudale revealed no profound disturbance of the animals ability to analyse visual stimuli in terms of orientation, brightness or size, when the performance of these animals was compared to controls using the method of stimulus generalisation. The gradients of the generalisation slopes to size, but not those to orientation or brightness, were significantly flatter in the experimental groups than in the control group (Chapter 5). The implication of the findings concerning the visual projections to the hyperstriatum and neostriatum caudale and the effects of lesions to these areas on our understanding of the function of the pigeons forebrain are discussed (Chapter 6).