Electrophysiological Assessment of Visual Processing After Brain Injury

The present project will investigate the contribution of subcortical and extra-striate pathways to the processing of visual information using behavioural and electrophysiological recordings. Lesions along the visual pathways result in visual deficits. In particular post-geniculate lesions within the early visual areas often result in areas of blindness in the visual field. Over two decades of investigations have shown that in the absence of systematic rehabilitation, the extent of deficits remains unchanged in chronic cases. However, both animal models and observations in humans have demonstrated the existence of some residual visual capacities although there are some discrepancies in the findings. These residual capacities are often attributed to the intact and multiple parallel processing pathways.

One approach to resolving the contribution of different pathways to the residual visual processing is to converge the behavioural testing methods in humans to those in animal studies by using forced-response procedures in which the observer is forced to make a choice between alternatives (Sahraie et al., 2006). Physiological measures of visual processing are alternative ways that can remove reliance on subjective reports. Behavioural studies have shown that high temporal frequencies are more likely to be processed within the blind fields. EEG signals have millisecond temporal resolution and can track the processing at high temporal frequencies. We propose to investigate the relationship between visual processing and EEG signals in those with damaged visual pathways. Should the EEG signals be found to correlate well with the presence or absence of visual processing obtained using behavioural techniques, we can develop more efficient and objective tools for probing the existence of residual visual processing in the blind field.

Visual attention has been reported to be a fundamental ingredient for triggering visual plasticity. We intend to combine behavioural measures with scalp recordings of steady-state Visual Evoked Potentials (SSVEPs; Norcia et al., 2015) using the frequency-tagging technique in order to investigate the relationship between residual visual processing within the blind field and visual attention. The benefits of this approach are two folds. One is that changes in EEG signals with visual attention in response to visual targets presented within the blind fields are potential biomarkers for predicting the existence of residual visual function. The second is that EEG source localisation can be used to identify the cortical sources mediating behavioural output. Although EEG source localisation does not have the spatial resolution of e.g. fMRI, this disadvantage is more than compensated for the present purposes by the high temporal resolution.

An important hypothesis for visual rehabilitation is that correct detection in the absence of awareness (blindsight type I), detection with some limited awareness (blindsight type II) and conscious vision lie on an awareness continuum rather being discrete states (Sahraie et al., 2013). Repeated visual stimulation can lead to behavioural changes leading to recovery of function often attributed to cortical plasticity. We intend to investigate the link between the presence and absence of behavioural detection and EEG signals in the context of candidate sites for visual rehabilitation. Should these exist, EEG signals can be used as biomarkers for cortical plasticity, leading to development of tools that can inform expected prognosis.