Biomedical subjects

Heidi Hofer

Publications and source records attributed to Heidi Hofer.

5 recordsLinked to original sources

PubMed2006

Accommodation with higher-order monochromatic aberrations corrected with adaptive optics.

Higher-order monochromatic aberrations in the human eye cause a difference in the appearance of stimuli at distances nearer and farther from best focus that could serve as a signed error signal for accommodation. We explored whether higher-order monochromatic aberrations affect the accommodative response to 0.5 D step changes in vergence in experiments in which these aberrations were either present as they normally are or removed with adaptive optics. Of six subjects, one could not accommodate at all for steps in either condition. One subject clearly required higher-order aberrations to accommodate at all. The remaining four subjects could accommodate in the correct direction even when higher-order aberrations were removed. No subjects improved their accommodation when higher-order aberrations were corrected, indicating that the corresponding decrease in the depth of field of the eye did not improve the accommodative response. These results are consistent with previous findings of large individual differences in the ability to accommodate in impoverished conditions. These results suggest that at least some subjects can use monochromatic higher-order aberrations to guide accommodation. They also show that some subjects can accommodate correctly when higher-order monochromatic aberrations as well as established cues to accommodation are greatly reduced.

Accommodation, Ocular↗

PubMed2005

Organization of the human trichromatic cone mosaic.

Using high-resolution adaptive-optics imaging combined with retinal densitometry, we characterized the arrangement of short- (S), middle- (M), and long- (L) wavelength-sensitive cones in eight human foveal mosaics. As suggested by previous studies, we found males with normal color vision that varied in the ratio of L to M cones (from 1.1:1 to 16.5:1). We also found a protan carrier with an even more extreme L:M ratio (0.37:1). All subjects had nearly identical S-cone densities, indicating independence of the developmental mechanism that governs the relative numerosity of L/M and S cones. L:M cone ratio estimates were correlated highly with those obtained in the same eyes using the flicker photometric electroretinogram (ERG), although the comparison indicates that the signal from each M cone makes a larger contribution to the ERG than each L cone. Although all subjects had highly disordered arrangements of L and M cones, three subjects showed evidence for departures from a strictly random rule for assigning the L and M cone photopigments. In two retinas, these departures corresponded to local clumping of cones of like type. In a third retina, the L:M cone ratio differed significantly at two retinal locations on opposite sides of the fovea. These results suggest that the assignment of L and M pigment, although highly irregular, is not a completely random process. Surprisingly, in the protan carrier, in which X-chromosome inactivation would favor L- or M-cone clumping, there was no evidence of clumping, perhaps as a result of cone migration during foveal development.

Color Perception↗

PubMed2005

Different sensations from cones with the same photopigment.

We used adaptive optics to study color fluctuation in the appearance of tiny flashes of light. For five subjects, near threshold, monochromatic stimuli with full widths at half maximum of 1/3 arcmin were delivered throughout a patch of retina near 1 deg in which we also determined the locations of L, M, and S cones. Subjects reported a wide variety of color sensations, even for long-wavelength stimuli, and all subjects reported blue or purple sensations at wavelengths for which S cones are insensitive. Subjects with more L cones reported more red sensations, and those with more M cones tended to report more green sensations. White responses increased linearly with the asymmetry in L to M cone ratio. The diversity in the color response could not be completely explained by combined L and M cone excitation, implying that photoreceptors within the same class can elicit more than one color sensation.

Color Perception↗

PubMed2004

Functional photoreceptor loss revealed with adaptive optics: an alternate cause of color blindness.

There is enormous variation in the X-linked L/M (long/middle wavelength sensitive) gene array underlying "normal" color vision in humans. This variability has been shown to underlie individual variation in color matching behavior. Recently, red-green color blindness has also been shown to be associated with distinctly different genotypes. This has opened the possibility that there may be important phenotypic differences within classically defined groups of color blind individuals. Here, adaptive optics retinal imaging has revealed a mechanism for producing dichromatic color vision in which the expression of a mutant cone photopigment gene leads to the loss of the entire corresponding class of cone photoreceptor cells. Previously, the theory that common forms of inherited color blindness could be caused by the loss of photoreceptor cells had been discounted. We confirm that remarkably, this loss of one-third of the cones does not impair any aspect of vision other than color.

Adult↗

PubMed2003

The reflectance of single cones in the living human eye.

PURPOSE: Individual cones were imaged in the living human eye with the Rochester adaptive optics ophthalmoscope. In all eyes, there were large differences in the reflectance of different cones, even when all the photopigment was bleached. To help understand what produces this spatial variation, the investigators explored whether it is a static or a dynamic property of the cone mosaic. METHODS: Fully bleached cone images were acquired in three eyes with an adaptive optics system. Images were collected over a 10-minute period approximately every hour for 24 hours. The temporal variation in cone directionality was measured in one eye. Finally, the experimental data on the temporal variation of absorption were compared with findings in various models of reflectance. RESULTS: Cone reflectance changes over time appear to be independent from cone to cone. These temporal changes are present in all three cone classes. The spatiotemporal variation in cone reflectance is not caused by the spatiotemporal variation in the optical axes of cones. This, along with the modeling results, suggest that changes in the reflectance affect the light that passes through photopigment in the receptors rather than the stray light, and that the changes are related to the outer segment-retinal pigment epithelium (RPE) interface. CONCLUSIONS: The reflectance of individual cones is a dynamic property of the mosaic. Changes can be observed over periods of minutes as well as many hours. The cause of the variation is not known but may be related to the process of disc shedding in receptors.

Adult↗