A comparison of the effects of superior collicular ablation in infant and adult rats.

Rats sustaining damage to the superior colliculus in infancy or adulthood and sham-operated controls were tested on two tasks requiring visual orienting to 'unexpected' and 'expected' stimuli, respectively. Experiment 1 measured the distracting effect of novel visual and auditory stimuli presented while the animals traversed a familiar runway. Control rats oriented to the distracting stimulus and had lengthened running times on the trials where it was presented. Rats with lesions of the superior colliculus were less distracted by the appearance of unexpected stimuli than sham operated controls but the severity of this effect depended on modality, and on age at operation. Experiment 2 examined the same animals' ability to detect and respond to a small and expected light that appeared at randomly determined positions around the perimeter of a chamber while the rat was at the centre. Filmed records and analyses of response latencies and errors showed that at low stimulus intensities, the active exploration shown particularly by infant-operated animals resulted in performance that was superior to that of adult-operated animals and, at least in terms of error rate, sham-operated controls. It is concluded that unusual behavioural strategies can explain all the differences between the animals operated at different ages. There was no evidence that novel projections from the retina were responsible.