Adaptation mechanisms in the salmonid visual system
| dc.contributor.author | Beaudet, Luc | |
| dc.contributor.supervisor | Hawryshyn, Craig W. | |
| dc.date.accessioned | 2018-07-20T17:24:12Z | |
| dc.date.available | 2018-07-20T17:24:12Z | |
| dc.date.copyright | 1997 | en_US |
| dc.date.issued | 2018-07-20 | |
| dc.degree.department | Department of Biology | |
| dc.degree.level | Doctor of Philosophy Ph.D. | en_US |
| dc.description.abstract | Animals in general, but fish in particular, inhabit environments characterized by dynamic photic conditions that are influenced by cyclical events such as the night-day cycle, or by spatial heterogeneity in the distribution of light. Effects of these dynamic properties on the visual system are compounded in salmonid fishes by migrations that expose individuals to various types of habitats, at various stages of their ontogeny. This dissertation examines some of the adaptations that enable the retina of salmonid fishes to cope with their changes of “visual environment” caused by migration and by the night-day cycle. In the first part of this dissertation, I used a combination of optic nerve response (ONR) recordings and conventional histology of the retina to investigate the ontogeny of sensitivity to ultraviolet (UV) light in salmonid fishes. I found that the UV cone mechanism contributed mostly to the ON response of retinal ganglion cells in rainbow trout (Oncorhpichus mykiss). Furthermore, the presence of UV sensitivity in rainbow trout was associated with the presence of accessory corner cones in the retinal cone mosaic, as both UV sensitivity and these cones were absent in larger (59.5-83 5g) juveniles. These results suggest that corner cones in the salmonid retina are sensitive to UV light, and that their ontogenetic disappearance leads to the loss of UV sensitivity. The changes in the photic environment that occur when mature salmonid fishes return to their natal stream to reproduce mirror those undergone during the first migration. To determine if the accessory comer cones, lost during this first migration, reappear at the time of the return migration, I studied the structure of the photoreceptor layer in sexually mature Pacific salmonids from four species: chinook (O. tschawytscha), chum (O. keta) and coho (O. kisutch) salmon, and rainbow trout. I found accessory comer cones over a large area of the dorso-temporal retina in all four species examined, which provides support for the contention that these cones are the product of late cellular addition. I investigated possible pathways for visual information to various brain centers in rainbow trout by labelling retinal projections and torus semicircularis connections in the same individuals. Double-labelling of neuronal tracts revealed two possible indirect pathways between the retina and the torus semicircularis, through the accessory optic center of the diencephalon and the optic tectum respectively. In the second part of this dissertation, I qualitatively and quantitatively examined the effects of various levels and spectral types of ambient lighting conditions on the sensitivity and time course of multi-unit responses recorded from the optic nerve of juvenile rainbow trout. Change in threshold from the dark-adapted state to progressively brighter ambient light conditions was examined at four wavelengths (380, 430, 540 and 620 nm) and found to be linear over most of the scotopic range, with a slope around 0.8. The results also suggested that, under mesopic conditions, rods and the long-wavelength cone mechanism were active simultaneously, in their respective parts of the spectrum. Implicit time, or time-to-peak of the scotopic responses decreased with stimulus intensity following a logarithmic relationship with a slope of -0.10, suggesting that the scotopic system of trout acts as an 11-stage low-pass filter, a number similar to that inferred in cat and rat, but different from other non-mammalian vertebrates. Similarly, implicit time at threshold decreased logarithmically with background intensity for the scotopic system, with a slope of -0.09 Varying the spectral content of ambient light led to differences in sensitivity and time course of ONRs across the spectrum, suggesting physiological differences between cone mechanisms. Possible implications for the coding of visual information are briefly discussed. In conclusion I provide a qualitative model of light adaptation in the trout visual system. | en_US |
| dc.description.scholarlevel | Graduate | en_US |
| dc.identifier.uri | http://hdl.handle.net/1828/9753 | |
| dc.language | English | eng |
| dc.language.iso | en | en_US |
| dc.rights | Available to the World Wide Web | en_US |
| dc.subject | Salmonidae | en_US |
| dc.subject | Visual pathways | en_US |
| dc.subject | Retina | en_US |
| dc.title | Adaptation mechanisms in the salmonid visual system | en_US |
| dc.type | Thesis | en_US |