The photoreceptors in atypical achromatopsia.

Abstract

SUMMARY1. The receptoral mechanisms underlying the vision of two atypical achromats of the complete variety were studied with standard psychophysical procedures.2. Under scotopic conditions the spectral sensitivity of each achromat was well described by the CIE (Commission Internationale de l'Eclairage) scotopic sensitivity function and the recovery of sensitivity after a retinal bleach showed characteristic duplex behaviour with the time constant of recovery of the slower phase matching that of normal rod vision for both foveal and peripheral stimulation.3. Their spectral sensitivity was measured under conditions of chromatic adaptation in order to reveal any residual middle or long wavelength cone activity. Only one photopic spectral response was found and this was adequately described by the spectral sensitivity function of Stiles' H3 mechanism of normal vision.4. Increment threshold measurements as a function of background intensity revealed a double-branched function in the fovea. The lower branch was found to have the spectral sensitivity of the rods; the upper branch that of Stiles' n3 mechanism. Stiles-Crawford measurements of directional sensitivity confirmed that the branch with the rhodopsin action spectrum had the directional sensitivity of rods and that the branch with the action spectrum of rI3 had the directional sensitivity of cones.5. These was no evidence for hue discrimination under photopic conditions. Regions of apparently normal performance on hue discrimination tests on more careful examination could be explained by luminosity judgements mediated by short wavelength-absorbing receptors.6. We reject the notion of there being rhodopsin-filled cones in the fovea of these subjects. The foveal and peripheral vision of each of these achromats can be adequately described in terms of the participation of only two types of receptor, namely normally functional rods under scotopic conditions and normally functioning short wavelength-absorbing cones under photopic conditions. They are therefore functional blue mono-cone monochromats, an explanation which was originally proposed by Blackwell & Blackwell (1957) over thirty years ago.

An extended 15 Hz ERG protocol (2): data of normal subjects and patients with achromatopsia, CSNB1, and CSNB2

“…Therefore, we argue that there is no difference between the 15 Hz ERGs of an achromat and normal subjects for low flash strengths and that the primary and secondary rod pathways function normally. In the literature, the functioning of the rods in achromatic patients is generally described as normal [32][33][34][35][36], although recently, abnormalities in rod driven ERGs have also been reported [37,38]. Therefore, it would be interesting to measure 15 Hz ERGs in more patients with achromatopsia.…”

Human photopic vision with only short wavelength cones: post‐receptoral properties.

“…For subject S. B. fixation was central whereas for P. S. it was vertically misaligned by 2 deg (see Fig. I of Hess et al 1989).…”

Abnormal retinotopic representations in human visual cortex revealed by fMRI

“…The rod photoreceptors, however, remain unaected. Rod achromats have visual function that closely resembles that of a normal under very low (scotopic) light levels (e.g., Hess, Mullen, Sharpe, & Zrenner, 1989;Sharpe, Fach, & Nordby, 1988). Rod achromats have poor visual acuity, principally because of the lack of functioning photoreceptors in the fovea where cones are tightly packed to sample the retinal image at high resolution.…”