Heart rate conditioning in the pigeon immobilized with alpha-bungarotoxin

A second ascending visual pathway from the optic tectum to the telencephalon in the pigeon (Columba livia)

Previous studies in the pigeon (Karten and Revzin: Brain Res. 2:368-377, '66; Karten and Hodos: J. Comp. Neurol. 140:35-52, '70) have described an ascending tectofugal visual pathway from the optic tectum to the ectostriatum by way of the nucleus rotundus of the thalamus. This present study used anterograde autoradiographic and retrograde horseradish peroxidase pathway-tracing techniques to investigate another ascending tectofugal pathway in the pigeon. Injections of 3H-proline/leucine confirmed a previous report that the optic tectum projects to the nucleus dorsolateralis posterior of the thalamus (DLP). This projection is predominantly ipsilateral and is confined to a large-celled caudal region of the nucleus (DLPc); the rostral region of the nucleus (DLPr) is not tectorecipient. Injections of horseradish peroxidase in DLPc labeled cells predominantly ipsilaterally in layers 8-15 of the optic tectum. Injections of 3H-proline/leucine placed in the DLPc labeled a discrete region of the ipsilateral telencephalon. Similar injections of DLPr labeled a contiguous, but more rostral, region of the neostriatum intermedium. Nissl- and silver-stained material indicated that the region in which DLP terminates is cytoarchitecturally distinct from ventromedial ectostriatal core and belt. Injections of horseradish peroxidase at various locations in the neostriatal DLP terminal field demonstrated a rostrocaudal ordering of the DLP projection upon the neostriatum intermedium. Single-unit recording demonstrated that cells in DLPc respond to whole-field illumination at the same latency as cells in the nucleus rotundus, indicating that the tecto-DLPc-neostriatal pathway transmits visual information to the telencephalon. We suggest that comparable pathways may exist in both reptiles and mammals.

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.

Invariance of retinal output during visual learning

Visually conditioned heart-rate change in the pigeon has been developed as a vertebrate model system for the cellular analysis of associative learning. This development included identifying the visual pathways that transmit the conditioned stimulus information. That, in turn, established a foundation for neurophysiological analyses during conditioning to determine whether these pathways behave merely as input lines or undergo training-induced modification. We began this analysis at the visual periphery, the retina. By recording the activity of single optic tract fibers over the acquisition of the conditioned response it was demonstrated that neither the maintained nor CS-evoked activity of retinal ganglion cells are modified during non-associative or associative paradigms. Thus, the data (a) describe the temporal properties of the CS-evoked retinal response, (b) exclude various possibilities that might have modified this response during learning, and (c) establish a firm foundation for cellular neurophysiological analysis of central visual structures involved in transmitting the CS information.

Modification of the discharge of vagal cardiac neurons during learned heart rate change

Visually conditioned heart rate change in the pigeon has been developed as a vertebrate model system for the cellular neurophysiological analysis of associative learning. In previous studies of the "final common path," it was shown that both the vagal and sympathetic cardiac innervations contribute to this response. The present experiments indicate that, prior to any behavioral training, the visual stimulus elicits a small decrease in the discharge of vagal cardiac neurons. During conditioning, this stimulus evokes a progressively greater decrease in discharge that parallels the acquisition of the conditioned cardioacceleration. In contrast, nonassociative control animals show habituation of the initial decrease in discharge. These data confirm the involvement of the vagal cardiac innervation in conditioned heart rate change, indicate that the vagal innervation acts synergistically with the sympathetic to produce cardioacceleration, and suggest that a short-latency pathway mediates the conditioned response.