Effects of abnormal binocular vision due to amblyopia on early stages of somatosensory processing

Abstract

Purpose: Abnormal visual experience during development results in amblyopia and/or strabismus and affects neural activity in the striate and extrastriate visual areas; however, no studies to date examined the effect of amblyopia on neurophysiological processing beyond the primary visual areas. Previous work with healthy adults has shown that crossmodal stimuli can modulate neural excitability at very early stages of information processing. For example, the amplitude of the early somatosensory event-related potential (ERP), P50, is significantly enhanced when a tactile stimulus is presented concurrently with a relevant visual stimulus.¹ We hypothesised that altered neural excitability in the primary visual areas in amblyopia will alter crossmodal interactions in other brain regions.

Methods: Participants were three patients with amblyopia and five visually-normal adults. Crossmodal interaction was investigated during binocular viewing using EEG. Specifically, the P50 somatosensory ERP was recorded as participants engaged in a task which required discrimination of the amplitude of a tactile and visual stimulus. Tactile stimuli were discrete vibrations presented to the left index finger and visual stimuli were presented as a central horizontal bar on a computer screen. Participants executed a graded motor response with the right hand indicating the summation of the amplitude of the sensory inputs. Three experimental conditions were tested: unimodal (tactile only), bimodal concurrent (tactile and visual presented simultaneously), and bimodal priming (visual presented 100 ms prior to tactile).

Results: As hypothesized, patients showed no enhancement of the P50 amplitude in the bimodal conditions (unimodal: 2.97±0.40μV; bimodal concurrent: 1.86±0.84μV; bimodal priming: 1.94±0.81μV). In contrast, visually normal participants showed a significant enhancement of the P50 amplitude in the bimodal condition (unimodal: 2.07±0.33μV; bimodal concurrent: 2.46±0.34μV; bimodal priming: 3.22±0.50μV), which is consistent with the previous study¹.

Conclusions: Preliminary results indicate that early crossmodal facilitation is affected by abnormal visual experience during development. Thus, this study offers the first evidence that the consequences of amblyopia on neurophysiological processing extend beyond the primary visual areas.<br /> ¹Popovich & Staines (2014) Brain Behav 4:247-60