Abstract
Purpose :
Previous studies have implied that sight restoration may result in reduction in (tactile-visual) cross-modal plasticity in primary visual cortex (V1), a potential impediment to recovery. However, using tactile-evoked V1 responses has not been shown to be an effective biomarker for plasticity, due to the extensive inter-subject variability reported in absolute activation baseline. In this work, we propose a novel index for measuring the tactile-visual plasticity in sight restoration that may more effectively quantify cross-modal plastic changes.
Methods :
We recruited 2 blind retinitis pigmentosa subjects, implanted with Argus II retinal prosthesis per an approved IRB at the University of Michigan: P1 (male) and P2 (female), both implanted in the left eye and right-handed while received the implant 4 and 5 years ago, respectively. We collected functional MRI data from the subjects during right-hand tactile tasks. Activations were calculated in each subject’s native space in the identified regions of interest (ROIs) for V1 and left primary somatosensory cortex (S1) in terms of strength (mean z statistics) and extent (%activated voxels in the ROI, FDR<0.05). Our proposed indices were calculated as the ratio of the strength of activation in S1 to V1 (S1 strength:V1 strength) and the ratio of extent of activation in S1 to V1 (S1 extent:V1 extent).
Results :
Our results indicated that the S1 strength:V1 strength and S1 extent:V1 extent were higher in P1 (2.7±0.8 and 9.7%±12.5%) with larger average device usage (1 hour/day) compared with P2 (2.1±0.7 and 2.5%±1.2%) with less usage (2 hours/week). This cross-modal effect could not be uncovered by merely relying on the absolute strength/extent of activation in V1 for P1 (1.4±0.4 and 22.5% ±17.6%) compared with P2 (1.7±0.3 and 40.0% ±18.5%).
Conclusions :
We hypothesize that our proposed S1:V1 index more accurately quantifies differences in strength and extent of activation by taking into account the relative contribution of V1 and S1 in processing a tactile task in each subject. We also propose that the extent of device usage, and not necessarily the time-since-implantation, cited in prior studies, is an effective factor for reversing cross-modal changes. Data from larger numbers of subjects with this rare condition are needed to further investigate our proposition.
This is a 2021 ARVO Annual Meeting abstract.