Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The purpose of this thesis was to examine the functional organization of the descending tectal efferents in the Mongolian gerbil. In amphibia, such investigations have revealed that crossed outputs mediate prey orienting turns, while uncrossed efferents mediate visually elicited escape. Most previous investigations of the mammalian collicular efferents have focussed upon movements of the eyes. Although it is true that such movements are of paramount importance to animals such as primates, other types of movements should not be neglected since they form an important part of any animal's visuomotor repertoire.;In the first experiment, electrical stimulation of the superior colliculus was used to elicit both contraversive "orienting" and ipsiversive "escape" turns in normal behaving gerbils. Following this, the predorsal bundle was cut at its decussation in order to examine the impact of the loss of crossed descending tectal output on elicited behaviour. While such knifecuts abolished contraversive turns, they left ipsiversive turns intact. In many instances, electrodes that had elicited contraversive turns prior to the knifecuts elicited ipsiversive turns after knifecuts. These findings suggest that, as in the frog, the predorsal bundle mediates orienting movements whereas some other pathway mediates ipsiversive "escape" movements.;In the second experiment, the organization of tectal efferents was investigated more directly by means of behavioural experiments. Tests of visually guided orienting and visually guided escape were included. In parallel with the stimulation findings, animals with knifecuts of the predorsal bundle showed deficits in the orienting tasks, but showed only slightly diminished levels of response to visual overhead threat. Animals with small lateral tegmental lesions that interrupted the uncrossed tectal efferents, on the other hand, performed as well as shams in the orienting tasks, but did not respond to visual threat. Animals with collicular lesions had severe deficits in both types of tasks.;Past experiments have demonstrated that each of the primary retinofugal targets is specialized for the performance of particular types of visual tasks. The present findings suggest that the descending output pathways from the superior colliculus also show functional specialization for specific visuomotor abilities.



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