Acuity and contrast sensitivity of the bluegill sunfish and how they change during optic nerve regeneration

Abstract

Spatial vision was studied in the bluegill sunfish, Lepomis macrochirus (9.5-14 cm standard length) to assess the limitations imposed by the optics of the eye, the retinal receptor spacing and the retinotectal projection during regeneration. Examination of images formed by the dioptric elements of the eye showed that spatial frequencies up to 29 c/ degrees could be imaged on the retina. Cone spacing was measured in the retina of fresh, intact eyes. The spacing of rows of double cones predicted 3.4 c/degrees as the cutoff spatial frequency; the spacing between rows of single and double cones predicted 6.7 c/degrees. Contrast sensitivity functions were obtained psychophysically in normals and fish with one regenerating optic nerve. Fish were trained to orient to gratings (mean luminance = 25 cd/m2) presented to either eye. In normals, contrast sensitivity functions were similar in shape and bandwidth to those of other species, peaking at 0.4 c/degrees with a minimum contrast threshold of 0.03 and a cutoff at about 5 c/degrees, which was within the range predicted by cone spacing. Given that the optical cutoff frequency exceeds that predicted by cone spacing, it is possible that gratings could be detected by aliasing with the bluegill's regular cone mosaic. However, tests with high contrast gratings up to 15 c/ degrees found no evidence of such detection. After crushing one optic nerve in three trained sunfish, recovery of visual avoidance, dorsal light reflex and orienting to gratings, were monitored over 315 days. At 64-69 days postcrush, responses to gratings reappeared, and within 2-5 days contrast sensitivity at low (0.15 c/degrees) and medium (1.0 c/ degrees) spatial frequencies had returned to normal. At a high spatial frequency (2.93 c/degrees) recovery was much slower, and complete only in one fish.