The pretectum and visual discrimination learning in the hooded rat

In previous studies on the effects of pretectal lesions on visual discrimination performance, the lesions have either been small or their effects confounded with damage to the adjacent thalamus and the results have been inconsistent. This study compares the effects of large pretectal lesions (PRT) with lesions in lateral (TLP) and medial (MPT) posterior thalamus on simultaneous black versus white (BW), horizontal versus vertical (HV) discrimination acquisition and low frequency flicker detection. An unoperated control group treated regularly with a cycloplegic to produce mydriasis was also tested in the discrimination learning phase. PRT lesions and treatment with the cycloplegic produced mydriasis as well as an impairment on BW. Neither group was impaired on HV. Animals with MPT and TLP lesions were impaired on BW and HV and those with TLP lesions were also impaired on flicker detection. It is concluded that the pretectum is important for visual intensity discrimination learning but the effects of lesions are due to disturbances of pupillary control. The significance of this finding for the interpretation of deficits produced by lesions elsewhere in the thalamus is discussed.