The effects of forebrain lesions on innate and imprinted colour, brightness and shape preferences in domestic chicks

In previous reports we have found that lesions of the lateral forebrain of domestic chicks prevent or destroy imprinted preferential discrimination of objects differing in colour/ brightness and shape. However, there were non-significant departures from random responding and it is possible that some residual learning was interacting with innate colour/ brightness preferences. An initial experiment in the present study showed that visually naive chicks had a significant blue and round shape preference when 1-2 days old. Visually experienced but non-imprinted chicks retained the round shape preference but showed weakened blue choice at 5-6 days of age. Chicks imprinted with a green slab showed a significant green preference at 5-6 days but chose slab and ball shapes equally. Taken together, these findings demonstrate imprinting for both colour and shape. Two further experiments tested lateral-, anterior-, and posterior-lesion and control chicks for retention of imprinted colour/brightness (mid-green) and shape (square) separately. Only the lateral- lesion group failed to choose the stimulus object with the familiar imprinted colour/brightness and instead chose the unfamiliar one (dark blue). No lesion group differed from the controls in the imprinted shape discriminations, all choosing the unfamiliar (round) as often as the familiar one. It appeared that the lateral-lesion chicks were like naive 1-day-olds in their colour/brightness preference but like imprinted birds in their shape preference. The final experiment tested the naive colour/brightness preferences of lateral- and anterior- lesion and control chicks using green and blue coloured discs in two tests with reversal of the brightness values. Both lesion groups behaved like controls in choosing dark blue in preference to light green. With reversed brightness values the controls showed a preference for the darker green discs, the anterior-lesion birds showed a slight tendency to choose the lighter blue disc, and the lateral-lesion birds reponded equally to the two colours. Thus, choices appeared to depend on brightness but in comparison with the controls there was some reduction in the brightness effect in the lateral-lesion birds and especially so in the anterior-lesion chicks. Since anterior lesions did not seriously impair the colour/brightness imprinted preference in the previous experiments, the reversal of the imprinted preference produced by lateral forebrain lesions is probably a true learning /memory loss and is not simply due to a colour or brightness sensory impairment.