Electrophysiological and neuroethological investigations of the teleostean optic tectum




McDonald, Craig G.

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Although colour-opponent neurons appear to subserve colour vision, precisely how these cells encode hue is still not clear. Single-unit, extracellular recordings from the rainbow trout optic tectum were made in order to examine the possible role of action potential timing in coding chromatic stimuli. I found that colour-opponent units can exhibit differences in response latency which are a function of wavelength and response sign, with the Off response exhibiting the shorter response latency. I also found that units often responded with spike bursts characterized by early and late spikes separated by a silent period, with the relative proportion of early and late spikes varying as a function of wavelength. This type of discharge pattern appears to be a result of inhibitory, colour-opponent processes. I suggest that complete inhibition of early spikes may be the mechanism underlying the observed latency differences. These findings suggest a role for action potential patterning in coding chromatic stimuli. To further explore chromatic processing in the trout tectum, I recorded tectal evoked potentials (TEPs) from the tectal surface. I found that TEP waveforms show distinct variation as a function of wavelength. In addition, my findings indicate that the On and Off channels of the tectum each possess distinctly different wavelength dependent properties. Middle wavelength stimulation typically evoked a waveform similar to that reported for another anamniote vertebrate, the toad. For both the On and Off response, this waveform was comprised of two negative waves, N1 and N2, which were interrupted by a positive wave, P2. The N2 wave was followed by a final positive wave, P3. Principal components analysis revealed that the N2/P3 wave sequence of the On response became significantly more pronounced as a function of increasing wavelength. In contrast, the N2/P3 wave sequence was most pronounced at middle wavelengths for the Off response. The N1 wave was relatively invariant with respect to wavelength. Should colour-opponent tectal units provide a substantive contribution to the TEP, it is probable that its wave length-dependent properties indicate underlying neural processes that facilitate colour discrimination. Despite considerable study of the visual behaviour of fish, neurophysiological studies that examine the nature of perception of realistic ethologically relevant stimuli are lacking. The second goal of this dissertation, therefore, was to investigate how an ethological stimulus, the agonistic display of Betta splendens, is perceived by conspecifics. To this end, I made multi-unit recordings from the optic tectum of Betta splendens while they viewed the agonistic display of conspecifics. I have found that the discharge pattern of tectal units is strongly modulated by dynamic movement associated with agonistic display. Moreover, the tectum appears to be attuned to a feature of the display that is known to be of ethological significance, the onset of full display. It is hoped that the work presented here will encourage future neurophysiological investigators to utilize natural stimuli to explore visual sensation in animals.



Rainbow trout, Physiology, Visual pathways, Optic nerve