Interocular transfer in a marsupial: the brush-tailed possum (Trichosurus vulpecula)

The corpus callosum has been implicated as the major commissure mediating the interhemispheric transfer of visual information in placental mammals. As marsupials lack a corpus callosum, they may be incapable of such transfer. This study was undertaken to test this hypothesis. Three brush-tailed possums (Trichosurus vulpecula) underwent midsaggital transection of the optic chiasm to ensure that retinal input was restricted to the ipsilateral side of the brain. Three other possums had sham operations to control for visual or learning impairments that might be caused by any other aspects of the surgery. All six animals were monocularly occluded, and conditioned to respond to the positive of two visual stimuli. When a criterion of greater than or equal to 18/20 correct was attained for two consecutive blocks of 20 trials, the trained eye was occluded, and the stimuli were presented to the untrained eye. Conditioning was continued until the criterion of learning was achieved with this eye. The stimulus contingencies were then reversed, so that the previously negative stimulus became the positive one, and the conditioning procedure was repeated. Black/white and horizontal/vertical visual discriminations were used. Both the experimental and control groups exhibited high levels of interocular transfer on all tasks. It is concluded that T. vulpecula possesses a pathway capable of mediating the interhemispheric transfer of visual information.