Modification of neuronal discharge along the ascending tectofugal pathway during visual conditioning.

Visually conditioned heart-rate change in the pigeon has been developed as vertebrate model system for analysis of associative learning. The visual pathways transmitting the conditioned stimulus information were identified, and neurophysiological analyses during conditioning were then undertaken to determine if these pathways behave merely as input lines or undergo training-induced modification. After finding that the retinal output is invariant with training, we investigated the central visual pathways, beginning with the tectofugal pathway. During conditioning single neurons in the nucleus rotundus and ectostriatum, the thalamic and telencephalic relays of the tectofugal pathway, showed enhancement of their phasic light-evoked responses. In contrast, the initial phasic responses attenuated during non-associative training. The rate at which these discharge modifications developed paralleled the development of the behavioral response. Thus, the tectofugal pathway shows plasticity during conditioning and does not behave merely as an input channel for the conditioned stimulus.