An experimental study of lateral displacement of ganglion cells (GCs) from foveal cones in six human retinas is reported. At 406-675 microm in length, as measured in radially oriented cross-sections, Henle fibers are substantially longer than previously reported. However, a new theoretical model indicates that the discrepancies in these reports are mainly due to meridional differences. The model takes into account the effects of optical degradation and peripheral ON/OFF asymmetry and predicts a central GC:cone ratio of 2.24:1. It provides estimates of cumulative counts and GC receptive field density at 0 degrees -30 degrees along the principal meridians of the visual field.

The non-linear projection of the retinal image has significant implications in both clinical ophthalmology and physiological optics, and was therefore considered by several (Prechtel and Wesley, I970; Mandell and St Helen, I97I) were considered in an attempt to formulate an adequate specification for a simplified schematic eye, and this was found for the most part to approximate to the earlier paraxial data of Cowan (I928) which was utilized in a wide-angle model by Stine (1934). It appeared necessary, however, to make some fundamental modifications. Accumulating data on corneal topography has supported the suggestion of Helmholtz (i 866) that the shape of the cornea is closely simulated by an ellipsoid. Such a surface can be specified in terms of an apical radius and an 'e' value, or eccentricity. The data of Prechtel and Wesley (1970), based on 150 eyes, show a clear unimodal tendency of this parameter which is further supported by the more detailed findings of Mandell and St Helen (I97I) on eight human eyes. The peak value from these papers was taken as e = o-, while retaining the apical radius of 7-8 mm. which results in the ellipsoid defined by the simple equation: x= l243 36-2y'It was also considered necessary to modify the data on the margin of the retina. This was located in accordance with the anatomical values quoted by Duke-Elder and Wybar (I96I ) on limbal distance of the ora serrata and the junction of the pars caeca with the functional retina. The resultant vertical and horizontal asymmetry of the retinal margin is shown in the Table, and is apparent also in the diagram of the optical geometry shown in Fig. i. The slope of the ellipse at any one point could be Address for reprints: N. Drasdo,

No abstract

The visual correlates of dyslexia are the subject of controversy, and much evidence suggests that they may include some aspects of binocular and accommodative function. These factors were investigated in 43 control and 39 dyslexic children, who were matched for age, sex and performance intelligence quotient. The dyslexic group exhibited significantly lower positive and negative vergence reserves, and vergence instability when the eyes were dissociated at near. Their amplitudes of accommodation also were significantly reduced. However, other measures including dissociated and associated heterophoria and accommodative lag and facility were similar in both groups. The stability of motor ocular dominance, as assessed with a modified Dunlop test, was similar in both groups. The results of a simulated reading visual search task suggested that the vergence and accommodative dysfunction were not a major cause of the dyslexia. Further analyses, using reading-age matched groups, suggested that these ocular motor correlates were not attributable to the better reading performance in the control group. The most likely remaining explanation is that they are, in most cases, non-causal correlates of the dyslexia.

With disc appearance used for the classification of open-angle glaucoma and ocular hypertension, significant electrophysiological losses were found in both conditions. The modest correlation between the structural and electrophysiological measures suggests that these assess different aspects of the pathologic process; electrophysiology can be used to quantify retinal ganglion cell dysfunction that occurs before cell death.