Robert L. Yolton

Visual sensitivity and color vision in ground squirrels

Abstract Several measures of visual capacity were obtained from ground squirrels ( Citellus mexicanus and Citellus tridecemlineatus ) in discrimination tests. These measures included: (a) spectral sensitivity functions for conditions of achromatic and chromatic adaptation, (b) dark-adaptation functions, (c) tests for a neutral point, (d) wavelength discrimination, and (e) chromaticity confusion lines. The all-cone character of vision in these animals is expressed in the lack of any shift in relative spectral sensitivity as achromatic adaptation state is altered and also in the rapid and relatively small increase in sensitivity during dark adaptation. More than one process was found to underlie the spectral sensitivity function. These squirrels have color vision of the dichromatic variety with a well-defined neutral point. Their wavelengthdiscriminating abilities are also characteristically dichromatic. Measurements of chromaticityconfusion loci suggest that the dichromacy of these squirrels is closest to that shown by protanopic humans. No differences were found between these two species of ground squirrel on any of the measures of visual capacity.

Center-surround balance in receptive fields of cells in the lateral geniculate nucleus

Abstract Among the receptive fields of cells in the lateral geniculate nucleus (LGN) there is wide variability in the relative prominence (balance) of the responses produced by center and by surround stimulation. A procedure for assessing balance is described and the results from a survey of balance for cells of the LGN of the squirrel monkey are reported. The results of this survey include the following: (a) about 60 per cent of the cells tested required the presence of a contrast spot imaged onto the center of the receptive field before a response to illumination of the surround could be obtained; (b) under a wide variety of stimulus conditions, a substantial number of LGN cells showed no direct response to surround stimulation and, thus, gave a near maximal response to diffuse-light stimulation; (c) a very few cells showed a surround-dominated response; (d) differences were found in the relative distributions of center-surround balance between those cells having excitatory and those having inhibitory centers; (e) the relationship between diameter of receptive field center and center-surround balance was found to differ for cells having excitatory and those having inhibitory centers; (f) for approximately one-third of the units examined, center-surround balance could be altered by raising the background light level—that alteration resulted in either an increase or a decrease in the dominance of the center component.

Preretinal absorbance in sciurid eyes.

Spectrophotometric measurements of preretinal absorbance were made on eyes from prairie dogs, western gray squirrels, four species of ground squirrels, and a southern flying squirrel. Results indicate that lenses from all species except the flying squirrel absorb significantly in the spectral region from 520 to 440 nanometers. At 440 nanometers, relative preretinal absorbance was greatest for the prairie dogs (1.31) followed in order by the ground squirrels (1.12 to 1.04), the gray squirrels (0.54), and the flying squirrel, which had an essentially transparent lens and cornea (0.08).

Myopia Associated with Hyperbaric Oxygen Therapy

Background. Hyperbaric oxygen therapy (HBOT) involves exposing patients to elevated oxygen pressures and concentrations for the treatment of soft tissue degenerative diseases. As side effects, some patients experience cataract development and/or a significant increase in myopia. Subjects. Eight subjects participated in this project. All were receiving HBOT for health problems. Methods. For all subjects, acuities and refractive error corrections were measured before and after HBOT sessions at the beginning, midpoint, and end of therapy. For four subjects, A-scan ultrasound and corneal curvature measurements were also made before and after selected HBOT sessions. In addition, susceptibility to free radical damage and indicators of actual damage were measured by blood analyses. During the first half of HBOT therapy, four subjects took placebo tablets; during the second half, antioxidant supplements were given. Results. No subject showed significant acuity or refractive error changes produced by single HBOT sessions. Two of the 8 subjects showed significant increases in myopia over the 20 session course of HBOT. No corresponding changes were found in the ultrasound, corneal curvature, or blood analysis data. Discussion. Theoretically, the increases in myopia could have been caused by lenticular refractive index changes resulting from oxidative damage to lens proteins. However, the biochemical assays used in this project did not predict which subjects were susceptible to increased myopia, nor did they demonstrate elevated levels of free radical activity in a subject who did experience an increase in myopia.

Some characteristics of the eye and the electroretinogram of the prairie dog

Abstract Examination of the eye of the prairie dog ( Cynomys ludovicianus ) shows that: the retina contains only one morphologically distinct receptor type (cones); the degree of convergence from receptors to ganglion cells is generally small throughout the retina and differs for various retinal locations; and the lenscornea system in this eye shows very substantial absorbance of short-wavelength light. Electroretinograms (ERG) recorded from the prairie dog eye have a waveform similar to that seen in other all-cone eyes. Spectral sensitivity functions derived from the ERG for both b- and d-wave components peak at 520–540 nm. These two components have similar, but not identical, spectral sensitivity.