Differences between ON and OFF cortical function in patients with amblyopia

Abstract

"Purpose: Evidence from psychophysical studies indicate that amblyopia affects ON more than OFF visual pathways. This prospective study aims to directly measure the effect of amblyopia on cortical responses driven by ON and OFF pathways with visual evoked potentials (VEPs).

Methods: Adults (18-65 years) with amblyopia (strabismic, anisometropic, mixed, or deprivational) and control subjects with binocularly normal-corrected vision were recruited. All subjects were screened with ATS-EDTRS best-corrected visual acuity (BCVA) and Randot Preschool stereoacuity. To be eligible, amblyopes must have an interocular difference of ≥2 logMAR (logarithmic minimum angle of resolution) lines, and controls must have BCVA within 1 logMAR line in both eyes and a stereoacuity of ≤100 arcsec. VEP was recorded with a portable lightweight headset (Wearable Sensing Inc.) that sampled the visual cortex with 9 dry electrodes. The visual stimuli presented were checkerboards with half checks equal to the background and half darker or lighter than the background (100% and 50% contrast, viewed through right eye, left eye, or both eyes, 900 trials). Eye fixation was monitored with an eye tracker (Eyelink 1000). The reliability of cortical responses was quantified with a correlation index that selected the 20 stimulus trials generating the strongest responses and measured the average of all possible correlations between trial pairs.

Results: We tested eight amblyopic subjects and two controls. The average amplitude of the cortical responses was marginally stronger for the AE than FE, but the difference was not statistically significant (23.00 ± 4.65 vs. 22.68 ± 5.62 microV, p > 0.05, Wilcoxon test). Differences in response amplitude between FE and AE were weaker for light than dark stimuli but also did not reach significance (-0.38 ± 4.81 vs. -0.26 ± 7.22 microV, p > 0.05, Wilcoxon test). The mean correlation index was larger in the FE than the AE, but this difference was not statistically significant (0.43 ± 0.20 vs. 0.40 ± 0.22, p > 0.05, Wilcoxon test). The FE-AE differences in mean correlation index were larger for light than dark stimuli, but again, the differences did not reach significance (0.09 ± 0.20 vs. -0.03 ± 0.22, p > 0.05, Wilcoxon Test). Within-subject comparisons in select individuals revealed variable VEP patterns. Control subjects showed no interocular or ON/OFF differences, while amblyopic participants displayed mixed results. Some were consistent with the hypothesis of reduced ON pathway strength in the AE, whereas others showed the opposite or no difference at all.

Conclusion: While some amblyopes may show ON/OFF pathway asymmetries, these effects are not consistently observable across all cases. Variability is inherent with VEP and may limit the ability to detect ON/OFF pathway differences at the group level, emphasizing the need for individualized analysis in amblyopia research."