The artefact of retinoscopy in the rat and rabbit eye has its origin at the retina/vitreous interface rather than in longitudinal chromatic aberration

Voltage-gated sodium (Na_v) channels are known to augment cone bipolar cell light responses, increasing the electroretinogram (ERG) b-wave in response to stimulus strengths above the cone threshold. However previous in vivo studies on a number of animal models have not found a role for Na_v channels in augmenting the b-wave in scotopic conditions below the cone threshold. We recorded ERGs from mice and rats using a series of TTX concentrations and tested retinal output to ensure complete Na_v channel block. We found that TTX concentrations sufficient to completely suppress retinal output caused large (∼40%) decrease in the scotopic electroretinogram (ERG) response to high stimulus strengths (1.0 log cd s/m²). In addition the b-wave was reduced by ∼20% even at stimulus strengths that should predominately excite the rod pathway (−2.2 log cd s/m²). Modulating stimulus strength and background luminance showed that Na_v channel contribution to the b-wave is strongest in mesopic conditions with low strength stimuli. Blocking GABA_c receptors indicted that Na_v channels predominately contribute to the b-wave by supporting GABA_c input to rod bipolar cells in addition to directly amplifying the light response of cone ON bipolar cells. We also determined that saturating levels of TTX reduced the rat b-wave below cone threshold. Na_v channels increase the ERG b-wave in both rod and cone bipolar cell-dominated circuits. In circuits involving rod bipolar cells the effect is mediated indirectly via GABAergic inhibitory cells, while Na_v channels directly located on cone bipolar cells amplify light responses in the cone pathways.

Young animals compensate for defocus imposed by positive or negative spectacle lenses by adjusting the elongation rate of their vitreous chambers, thus matching the length of the eye with the focal length of the eye’s optics combined with the spectacle lenses. The ability to compensate for either negative or positive lenses could rely on the ability to distinguish between myopic and hyperopic blur, or it could rely on the fact that positive lenses would bring nearby objects into focus, thereby reducing the amount of blur, whereas negative lenses would not. This study asks whether eyes emmetropize using the magnitude of blur or the sign of blur as a directional cue.

We fitted chick eyes with positive lenses while imposing a substantial amount of blur, either (a) by having them wear lenses only when restrained in the center of a cylinder, the walls of which were beyond their far-point or (b) by having them wear mild diffusers over positive lenses. We found good refractive compensation in both situations in a large number of birds. Furthermore, we found that mild diffusers worn on top of positive lenses differentially affected the two ocular components of refractive compensation: there was less choroidal thickening, but more inhibition of ocular elongation, compared to wearing positive lenses alone. These findings argue both that the eye can discern the sign of the blur and that choroidal and ocular-elongation components of the refractive compensation do not respond identically to visual inputs.

Chicks were raised in a low-ceiling environment to find out if their eye growth could compensate for locally imposed hyperopic refractive errors. These chicks became selectively more myopic in the upper visual field than chicks raised in a high-ceiling environment. The vitreous chamber in the low-ceiling birds showed a selective elongation in the ventral region that was not seen in the eyes of the high-ceiling birds. This morphological difference was small, but probably adequate to account for the additional myopia in the low-ceiling birds. These results are consistent with the idea of a visually mediated growth mechanism regulating the local refractive state across the entire visual field so that it matches the customary viewing conditions. Such a mechanism might account for the finding of Fitzke, Hayes, Hodos, Holden and Low (Journal of Physiology, London, 369, 33–44, 1985) that the refractive errors in the lower field are exactly appropriate for focusing the image of the ground on the retina.

The wide-angle optical quality of the cane toad eye was measured using a single-pass intraocular optical fibre microprobe, a double-pass projection method and high magnification ophthalmoscopy to photograph individual photoreceptors. The cane toad eye is a wide angle optical device with a horizontal field of nearly 200 deg and a large relative aperture (caf/1). Its image quality, which is poor compared to diffraction-limited performance, descreases relatively little towards the periphery. Rough matching was found between image quality and the potential resolution of the rod array, although undersampling occurs for small pupils. Undersampling probably also occurs for the cone and ganglion cell arrays. The relative rotational symmetry of the topographic distribution of image quality precludes direct matching to the topography of the visual streak at the ganglion cell layer although it remains to be determined whether it is matched to the receptor distribution.

Chicks deprived of form-vision in the lateral part of their visual fields become severely myopic largely because of elongation of the vitreous chamber. The myopia mostly affects the visually deprived nasal retina; the nondeprived temporal retina is unaffected. These changes occur most rapidly early in life, being evident then after only 3 days of visual restriction. The susceptibility declines with age, being proportional to the rate of increase of axial length. Recovery from this myopia occurs if the visual restriction is removed during the first 6 weeks of life, as a result of the cessation of elongation of the vitreous chamber. The rate of recovery is directly related to the degree of myopia and inversely related to age. The pattern of changes in refractive status and variability argue for the probable existence of an active mechanism regulating eye growth in a manner dependent on refractive error, thereby producing emmetropization.

A new technique, Infrared Photoretinoscopy, has been employed for recording natural accommodation in the chicken. The illumination of the pupil by the fundus reflection of infrared light provided by high output light emitting diodes (I.R. LED's) was monitored on a video screen. The defocus of the eye could be calculated by evaluating the fraction of the pupil which was illuminated. It was found that: (a) in the chick the full range of accommodation (about 17D) is present in the first day after hatching, (b) accommodation acts completely independently in both eyes, (c) the “near pupillary response” is weaker in younger chicks, (d) the pupil constriction in response to light starts at higher luminance in the younger chicks (e) the developmental decrease of the f/number is not sufficient to explain the change of the pupil reaction to light. Problems resulting from the use of drugs in order to measure the refractive state using normal retinoscopy are discussed.