Investigating cross-modal neuroplasticity in the profoundly deaf

Abstract

Superior visual processing in deaf individuals has been attributed to neuroplasticity, or the brain’s ability to rewire as a result of sensory deprivation. In particular, cross-modal recruitment of the auditory cortex has been associated with superior peripheral motion processing in deaf individuals. This enhancement may also be contributed to by compensatory hypertrophy, evidenced by increased responsiveness of the visual motion area (MT/MT+). However, there has been no causal evidence of a link between neuroplasticity and superior peripheral motion processing in deaf participants. Additional changes to visual motion processing identified in deaf individuals include a right visual field (RVF) advantage, due to increased recruitment of the left visual motion area (MT/MT+). Conflictingly, a left visual field (LVF) advantage has been attributed to cross-modal recruitment of right auditory cortex.

The aim of this thesis was to examine the relationship between changes to visual peripheral motion processing in profoundly deaf individuals and neuroplastic changes to cortical recruitment. Random dot kinetograms (RDK) were presented randomly to either visual field, 12° from fixation. These stimuli have been shown to elicit evidence of both cross-modal plasticity in the right auditory cortex (STG), and compensatory hypertrophy in left MT/MT+. Deaf and hearing participants performed a motion direction discrimination task while transcranial magnetic stimulation (TMS) was applied to the right STG and left MT/MT+. If there is a causal relationship between superior peripheral motion processing and cross-modal recruitment of right STG, then TMS should disrupt performance in deaf compared to hearing participants. In addition, if there is a causal relationship between compensatory hypertrophy of left MT/MT+ and superior motion processing in deaf individuals, then TMS should cause differential levels of disruption across deaf and hearing participants. Performance on the task was also assessed as a function of visual field.

Results were inconsistent with expectations. There was no effect of TMS, which does not provide any evidence of neuroplasticity in right STG or left MT/MT+ in deaf participants. The stimulation parameters may have been insufficient to disrupt task performance. There was also no evidence of superior motion direction discrimination performance or a visual field advantage in deaf participants, which is not suggestive of neuroplasticity. Extent of sensory deprivation, sign language acquisition, and task demands are thought to mediate evidence for neuroplasticity and superior visual performance in deaf individuals. Therefore, the results of the current study and previous research are evaluated on these three variables. The lack of evidence for neuroplastic changes in deaf individuals as a result of sensory deprivation or signing experience are consistent with the inconsistencies in the field to date.