Spatial summation of perimetric stimuli across the visual field in anisometropic and strabismic amblyopia

Abstract

 

Purpose

 

To investigate differences in the area of complete spatial summation (Ricco’s area) for standard perimetric stimuli, between amblyopic eyes and their fellow non-amblyopic eye, and to compare any such inter-ocular differences to those in observers with normal binocular vision.

 

Methods

 

Achromatic contrast detection thresholds were measured with circular incremental stimuli of different area (Goldmann I-V; 0.02°-1.7° diameter) at 12 visual field locations (9°, 15° and 21° eccentricity; 4 locations each) in observers with strabismic amblyopia (n=2; age: 19, 23 years), anisometropic amblyopia (n=3; age:18, 19, 20 years), and normal binocular vision (n=5; mean age: 25.8 years, range: 20, 31). Background luminance was 10cd/m². Experiments were performed on an Octopus 900 perimeter (Haag Streit, Koeniz, Switzerland), with the Open Perimetry Interface. Thresholds were averaged by eccentricity and three spatial summation curves were constructed for each eye. Ricco’s area was estimated from each by two-phase regression analysis. Estimates were compared between amblyopic and fellow non-amblyopic eyes at each eccentricity. Inter-ocular differences at each eccentricity were compared between observers with and without amblyopia.

 

Results

 

Ricco’s area was significantly larger (mean: 0.3 log deg²) in amblyopic eyes than in fellow non-amblyopic eyes at all eccentricities (p=0.01, paired t-test, Holm-Bonferroni correction). Inter-ocular differences in control subjects (mean: 0.03 log deg²) were not statistically significant (p>0.05, paired t-test, Holm-Bonferroni correction). Compared with control eyes, mean Ricco’s area was larger in amblyopic eyes but smaller in non-amblyopic eyes (Figure).

 

Conclusions

 

Given that changes in receptive fields of retinal ganglion cells in amblyopia are not marked, our findings likely point to a difference in representation of each eye at the cortical level. Greater convergence of signals from retinal ganglion cells in amblyopic eyes to thinner ocular dominance (OD) columns in V1, and less convergence from those in the fellow non-amblyopic eye to thicker OD columns, may explain these results. These findings also support the concept that the physiological basis for Ricco’s area reflects processing along the entire visual pathway, and is not confined to the retina.  

 

Mean Ricco’s area as a function of eccentricity in amblyopic, non-amblyopic and control eyes. (Error bars: SEM)