Abstract

According to theories of selective attention, animals learn to ignore irrelevant stimuli depending on the correlation of a stimulus with a reinforcement. An attentional process by which animals learn to ignore an irrelevant stimulus is an independent mechanism, not just a consequence of any increase in attention to relevant stimuli. There is evidence that mesolimbic serotonergic system plays a role in this attentional process. In the present experiment, rats with lesions of median raphe nuclei were compared to non-lesioned normal rats for the ability to ignore irrelevant stimuli. Forty experimentally naive rats were used. Three days prior to training, animals were randomly assigned to lesioned or non-lesioned groups. Animals in each of the two groups were further subdivided into an expermental and a control group. An irrelevant stimulus, a light, which was not correlated with a reinforcement, was presented for the experimental, and not for the control groups. Turning consistently left or right in a Y maze was reinforced with foods. For the experimental groups, the light was presented randomly in one of the three arms of the Y maze. Each daily training session was composed of 20 trials. All animals were run until 17 correct responses of 20 trials were reached or for a maximum of 200 trials. The control gorups were given 20 testing trials on the day after they had met the learning criterion. The random presentations of the light were introduced in the testing session. Results showed that presentation of the irrelevant light stimulus significantly retarded Y maze learning in the median raphe lesioned. They were more distracted to the light than the non-lesioned. It is interpreted that the ability to ignore irrelevant stimuli is disrupted in animals with median raphe lesions.