BEHAVIOR AND CNS INTEGRATION 209 



to reexamine the conclusions of earlier workers with greater accuracy and 

 much more detail. 



The predominance of olfaction in the shark telencephalon was first 

 questioned by the finding that the bulk of the nurse shark's telencephalon 

 does not receive any primary or higher order olfactory input (Ebbesson 

 1972a, Ebbesson and Heimer 1970). The possibility of visual information 

 reaching the telencephalon was immediately suggested by the previously 

 unknown presence of extensive retinal projections to the dorsolateral 

 thalamus of lemon, nurse, and tiger sharks (Ebbesson and Ramsey 1968, 

 Graeber and Ebbesson 19726). The other major recipient of retinal fibers, 

 the optic tectum, has also been shown to send a massive ascending projection 

 to this same region of the thalamus (Ebbesson 1971, 19726). When experi- 

 mental lesions are made in this dorsolateral area, degenerating axons are 

 found to project upward in a sizeable pathway to the contralateral central 

 telencephalic nucleus (Ebbesson and Schroeder 1971, Schroeder and Ebbes- 

 son 1974). 



Anatomically this thalamo-telencephalic pathway, though crossed, is 

 reminiscent of the classic geniculo-striate visual connection in mammals. 

 However, the inclusion of tectal input in the thalamic area of origin implies 

 that this tract in sharks may also relay visual information derived from the 

 tectum. In mammals, the latter function is performed by a second, separate 

 pathway connecting the tectum to the lateral posterior, or pulvinar, nucleus 

 of the thalamus and then ascending to terminate in the extra-striate visual 

 areas of the cortex. The substantial overlap between retinal and tectal ter- 

 minations in the shark thalamus suggests that perhaps the two separate as- 

 cending systems of mammals may be combined in these fish in a manner 

 closely resembling the central visual system of some common vertebrate 

 ancestor (Ebbesson 19726). 



Telencephalic Visual Evoked Potentials— Further support for telen- 

 cephalic visual function in elasmobranchs is provided by reports of evoked 

 potentials being recorded in response to electrical stimulation of the optic 

 nerves in nurse sharks (Cohen et al. 1973) and rays (Veselkin and Kovacevic 

 1973). Cohen and his colleagues obtained short-latency field potentials 

 localized in the posterior portion of the ipsilateral, central telencephalic 

 nucleus after stimulating the optic nerve with monophasic square waves 0.1 

 ms in duration (Figure 4). In confirmation of the morphological data, they 

 noted that the physiologically responsive area overlaps the area known to 

 receive fibers from the thalamic visual region. The lack of evoked potentials 

 in the contralateral telencephalon also substantiates the total recrossing of 

 the thalamo-telencephalic visual pathway after complete decussation of the 

 retinal fibers at the optic chiasm. 



Veselkin and Kovacevic (1973) examined the presence of visual evoked 

 potentials in the telencephalons of the rays Dasyatis pastinaca, Raja clauata, 

 and Torpedo ocellata and of the shark Scyllium cannicula. They also found 

 ipsilateral responses to optic nerve stimulation in all three species of rays, 

 but not in Scyllium. These responses were localized in the posterior telen- 



