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Ashley Rohacek, Brittany Smith, Amy Lindsey; Psychophysical Assessment of Contrast Sensitivity Functions in Surface and Hybrid Mexican Tetras. Journal of Vision 2019;19(10):159d. doi: https://doi.org/10.1167/19.10.159d.
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The Mexican tetra fish (Astyanax mexicanus) is an excellent model to study the genetics of developmental eye disorders because this single species has two distinct evolutionary forms. The surface-dwelling form is pigmented, eyed, and sees, whereas the cave-dwelling form is albino, eyeless, and blind. Hybrid progeny (surface × cavefish) display varying extents of eye/retinal development and pigmentation. Little is known about vision in hybrid Mexican tetras. To assess vision, psychophysical methods were used to determine the contrast sensitivity functions (CSFs) of surface (n=9) and hybrid (n=14) groups. Contrast threshold (T) was measured as a function of spatial frequency for 10 rotating (20rpm) black/white square-wave grating stimuli (10%–100% contrast; 0.25–4.23cpd). One fish at a time was placed in a cylindrical Plexiglas aquarium (d=20.32cm) in the center of a rotating drum (d=30.48cm) lined with one of the gratings. Optokinetic responses (saccadic eye movements of at least 5 seconds during stimulus presentation) were recorded by an observer blind to stimulus. A one-down one-up staircase procedure was used to measure T (P=0.50) for each grating. Results showed that surface fish had a typically shaped vertebrate CSF with maximal sensitivity (1/T) to a range of intermediate spatial frequencies and decreased sensitivity to low and high spatial frequencies. Hybrids had slightly reduced sensitivity at intermediate spatial frequencies and greater sensitivity at the lowest (U=4.50, p< 0.001) and highest (U=9.00, p< 0.001) spatial frequencies tested compared to surface fish. Decreased fall-off at low spatial frequencies observed in hybrids may be due to underdeveloped lateral inhibition mechanisms. Further research is necessary to explain specific mechanisms that underlie differences in vision at low and high spatial frequencies between surface and hybrid fish. These findings suggest that A. mexicanus may be a useful model to study the relationship between genetics and capacity for vision in humans with developmental eye disorders.
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