Visual Contrast Detection Cannot be Predicted from Surrogate Measures of Retinal Ganglion Cell Number and Sampling...

Purpose: To establish whether a clinically-exploitable relationship exists between surrogate measures of retinal ganglion cell (RGC) number and functional sampling density and visual contrast sensitivity in healthy young eyes. Methods: Psychometric functions for contrast detection were measured at 9° eccentricity in superior and inferior visual field from 20 healthy adults (age 23-43, median 26 years). Functions were compared to corresponding localised regions of retinal nerve fibre layer (RNFL) thickness measured by optical coherence tomography, a surrogate of RGC number, and to grating resolution acuity, a psychophysical surrogate of RGC sampling density. Correlations between psychometric function parameters and RGC surrogates were measured by Spearman's rank correlation. Results: All measures exhibited a two- to four-fold variation in our sample. Despite this, correlations between measures were weak. Correlations between psychometric function parameters (threshold, spread) and RNFL thickness ranged in magnitude from 0.05 to 0.19 (p=0.43 to 0.85). Grating resolution was sampling-limited for 16 of 20 participants in superior visual field, and for 12 of 20 participants in inferior visual field. Correlations between psychometric function parameters and grating resolution acuities ranged in magnitude from 0.05 to 0.36 (p=0.12 to 0.85) when all data were considered, and from 0.06 to 0.36 (p=0.26 to 0.87) when only sampling-limited data were considered. Conclusions: Despite considerable variation in both psychometric functions for contrast detection and surrogate measures of RGC number and sampling density among healthy eyes, relationships between these measures are weak. These relationships are unlikely to be exploitable for improving clinical tests in healthy populations.