OBJECTIVE: We evaluated whether transorbital ACS modulates BOLD activity in early visual cortex using high-resolution 7 Tesla functional magnetic resonance imaging (fMRI).
METHODS: In this feasibility study transorbital ACS in the alpha range and sham ACS was applied in a random block design in five healthy subjects for 20 min at 1 mA. Brain activation in the visual areas V1, V2 and V3 were measured using 7 Tesla fMRI-based retinotopic mapping at the time points before (baseline) and after stimulation. In addition, we collected data from one hemianopic stroke patient with visual cortex damage after ten daily sessions with 25-50 min stimulation duration.
RESULTS: In healthy subjects transorbital ACS increased the activated cortical surface area, decreased the fMRI response amplitude and increased coherence in the visual cortex, which was most prominent in the full field task. In the patient, stimulation improved contrast sensitivity in the central visual field. BOLD amplitudes and coherence values were increased in most early visual areas in both hemispheres, with the most pronounced activation detected during eccentricity testing in retinotopic mapping.
CONCLUSIONS: This feasibility study showed that transorbital ACS modifies BOLD activity to visual stimulation, which outlasts the duration of the AC stimulation. This is in line with earlier neurophysiological findings of increased power in EEG recordings and functional connectivity reorganization in patients with impaired vision. Accordingly, the larger BOLD response area after stimulation can be explained by more coherent activation and lower variability in the activation. Alternatively, increased neuronal activity can also be taken into account. Controlled trials are needed to systematically evaluate the potential of repetitive transorbital ACS to improve visual function after visual pathway stroke and to determine the cause-effect relationship between neural and BOLD activity changes.
MATERIALS AND METHODS: Fourteen HS and 14 TLE patients with age and gender matched underwent resting state functional magnetic resonance imaging (rsfMRI) scanning using a 3-Tesla MRI machine to investigate the EC and percentage of amplitude fluctuation (PerAF) involving SMG and PRE. The rsfMRI data were analysed using Statistical Parametric Mapping (SPM12) and Spectral Dynamic Causal Modelling (spDCM) from which causal models were specified, estimated and inferred.
RESULTS: Model with bidirectional connections between PRE and SMG was chosen as the winning model. The EC from PRE to SMG is positive but the EC from SMG to PRE is negative in both hemispheres and in HS and TLE. Based on the findings from the EC analysis, there is an excitatory effect shown by PRE to SMG connection indicating a dominant role of PRE over SMG in both groups.
CONCLUSION: There is important evidence showing that PRE might also have influence on areas outside resting state network and the influence changes in the presence of brain abnormality.