Orientation invariance of shape recognition in forebrain-lesioned pigeons.

Pigeons were trained to perform a visual orientation invariance task consisting of shape matching-to-sample or oddity-from-sample discriminations where the comparison forms differed in orientation from the sample forms, and the odd comparison forms were always a mirror image of the sample. They then received lesions affecting the visual projection area within the anterior hyperstriatum or the dorsal neostriatum, a control area with no known visual function. Both groups of birds evinced minor transient postoperative deficits of similar magnitude during the shape recognition task under orientation invariance conditions when the habitual training forms were used. When novel forms were introduced, the performance of hyperstriatal pigeons was significantly worse than that of the neostriatal pigeons, but still well above chance. The introduction of a delay between the offset of the sample and the onset of the habitual comparison stimuli did not yield any differential effect. It is concluded that orientation invariance of pattern recognition performance of birds, in contrast to that in mammals, is probably a midbrain, optic tectum function.