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/° 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/° as the cutoff spatial frequency;
the spacing between rows of single and double cones predicted 6.7
c/°. 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/°
with a minimum contrast threshold of 0.03 and a cutoff at about 5
c/°, 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/° 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/°) and medium (1.0 c/°) spatial frequencies had returned
to normal. At a high spatial frequency (2.93 c/°) recovery was
much slower, and complete only in one fish.
Are Contrast Sensitivity Functions Impaired in Insulin Dependent Diabetics Without Diabetic Retinopathy?
