Transactions 29th European Strabismological Association Meeting – de Faber (ed) © 2005 European Strabismological Association, ISBN 04 1537 211 9

Effects of early and late onset strabismic amblyopia on magnocellular and parvocellular visual function

John J Sloper, Alison R Davis, Magella M Neveu & Chris R Hogg

Moorfields Eye Hospital, City Road, London, U.K.

Michael J Morgan & Graham E Holder

City University, London, U.K.

ABSTRACT: Contrast sensitivities were measured to an achromatic 0.8 c.p.d. grating with rapid onset to measure magnocellular sensitivity and to a red-green isoluminant grating of 3.2 c.p.d. and a slow onset to assess parvocellular function. Fifteen early and fourteen late onset strabismic amblyopes with similar ranges of visual acuity were studied and the data compared with those from 15 age-matched normal subjects.

Both luminance and colour CS were significantly lower in the amblyopic than fellow eyes of all amblyopes. For luminance CS this difference was due both to an increased CS in the fellow eye and a reduced CS in the amblyopic eye. Colour CS was greatly reduced in both the amblyopic and fellow eyes of early and late onset strabismic amblyopes compared to normal. The reduction of colour CS compared to luminance CS was significantly greater in the amblyopic eyes of late onset than early onset amblyopes.

1INTRODUCTION

Evidence from primate studies has shown that monocular visual deprivation has different effects on the anatomy of the magnocellular (M) and parvocellular (P) pathways depending on age of onset of the deprivation (Headon, Sloper, Hiorns and Powell 1985; Sloper 1993). In human strabismic amblyopes, pattern appearance VEPs and contrast sensitivity to a 3.2 c.p.d. achromatic grating show marked differences in the changes found between strabismic amblyopes with an onset before or after 18 months of age indicating the presence of two distinctly different periods of developmental sensitivity in man (Davis, Sloper, Neveu, Hogg, Morgan and Holder 2003). The present study has used luminance and colour contrast sensitivity (CS) measurements to see whether adult human strabismic amblyopes show differences in the function of their M and P pathways and whether these differ with age of onset.

2METHODS

Contrast sensitivities were measured with a staircase method using an achromatic 0.8 c.p.d. grating with rapid onset to measure magnocellular sensitivity and a red-green isoluminant grating of 3.2 c.p.d. and a slow onset to assess parvocellular function. Fifteen early and fourteen late onset strabismic amblyopes with similar ranges of visual acuity were studied and the data compared to those from 15 age-matched normal control subjects.

3RESULTS

Both luminance and colour CS were significantly lower in the amblyopic than fellow eyes of all amblyopes (Figs 1 & 2; both P 0.01: paired t-tests). For luminance CS this difference was due

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Figure 1. Luminance contrast sensitivities. Error bars 1 s.d.

Figure 2. Colour contrast sensitivities. Error bars 1 s.d.

both to an increased CS in the fellow eye and a reduced CS in the amblyopic eye. Colour CS was reduced in both the amblyopic and fellow eyes of early and late onset strabismic amblyopes compared to normal (all P 0.001 unpaired t-tests).

The reduction of colour CS compared to luminance CS was quantified by calculating the ratio:

This ratio approaches one as colour CS declines relative to Luminance CS. The mean ratio for normal subjects was 0.78. This was increased in the amblyopic eyes of early and late onset groups (Fig. 3; both P 0.001; unpaired t-test c.f. normal) and in the fellow eyes of both groups (Fig. 3; both P 0.05; unpaired t-test c.f. normal). The difference between amblyopic and fellow eyes was significant for both and early and late onset groups (both P 0.05; paired t-test).

The ratio was significantly greater in the amblyopic eyes of the late onset than early onset group indicating greater relative depression of parvocellular function in the late onset group (Fig. 3, P 0.05; unpaired t-test).

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Figure 3. Ratios of magnocellular to parvocellular contrast sensitivity (see text). Error bars 1 s.d.

4DISCUSSION

Amblyopic eyes of adult strabismic amblyopes show reduced contrast sensitivity to both a parvocellular and magnocellular biased stimulus when compared to the sensitivity of the fellow eye. However, when compared to normal subjects, both amblyopic and fellow eyes show a depression of parvocellular sensitivity compared to normal, with more reduction in the amblyopic eye. The difference between amblyopic and fellow eyes in sensitivity to a magnocellular stimulus is as much due to an increase in sensitivity of the fellow eye as to reduced sensitivity of the amblyopic eye. Thus in both amblyopic and fellow eyes there is a reduction in parvocellular sensitivity relative to magnocellular sensitivity. This was confirmed by calculating a ratio of parvocellular to magnocellular sensitivity.

There is an interesting parallel between this and the relative reduction of parvocellular to magnocellular cell size seen for both deprived and undeprived LGN cells following long-term visual deprivation in non-human primates (Headon, Sloper, Hiorns and Powell 1985)

The difference in sensitivity between magno and parvocellular pathways is more marked in the amblyopes with an age of onset after 18 months of age. This is in keeping with the evidence for differences between the anatomical effects of early and late onset visual deprivation in primates (Headon, Sloper, Hiorns and Powell 1985; Sloper 1993) and the electrophysiological and psychophysical differences between early and late onset human strabismic amblyopes (Davis, Sloper, Neveu, Hogg, Morgan and Holder 2003)

REFERENCES

Davis, A.R., Sloper, J.J., Neveu, M.M., Hogg, C.R., Morgan, M.J. & Holder, G.E. (2003). Electrophysiological and psychophysical Differences bcetween Earlyand Late-Onset Strabismic Amblyopia. Investigative Ophthalmology and Visual Science, 44: 610–617.

Headon, M.P., Sloper, J.J., Hiorns, R.W. & Powell, T.P.S. (1985). Effects of Monocular Closure at Different Ages on Deprived and Undeprived Cells in the Primate Lateral Geniculate Nucleus. Developmental Brain Research, 18: 57–78.

Sloper, J.J. (1993). Edridge-Green Lecture. Competition and Cooperation in Visual Development. Eye, 7: 319–331.

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