Simultaneous measurement of human brain activity using near infra-red spectroscopy, electroencephalogram and the steady state visually evoked potential

The focus of this study was the comparison of electrical and blood oxygenation data collected simultaneously from the human visual cortex in response to a steady-state visual stimulus. Electrical brain activity was obtained via the Electroencephalogram (EEG) signal recorded from a single scalp electrode. Simultaneous changes in cortical blood oxygenation were detected and recorded by use of a Continuous Wave (CW) Near-Infrared Spectroscopy (NIRS) device, located over the region of the occipital cortex. The steady-state visual stimulus used was a white visual flicker. Each flicker frequency between 3Hz and 45Hz was presented one at a time for a period of one minute, with the order of the presentation of the flicker frequencies being randomised. Data collected from five subjects were investigated (both graphically and statistically via correlation analysis) to identify and determine the strength of any relationship between the electrical and oxygenation data. NIRS blood oxygenation data was compared separately to Steady-State Visually Evoked Potential (SSVEP) and Power Spectrum data, both of which were obtained via the EEG. It was proposed that any relationship between blood oxygenation and SSVEP amplitude would be negative in nature when the flicker frequency of the visual stimulus was in the Delta, Theta and Alpha frequency ranges. Conversely, when stimulus flicker frequency was in the Beta and Gamma frequency ranges, any relationship between the two data types was expected to be positive. Similar behaviour was anticipated between the blood oxygenation data and EEG power spectrum data, when the power spectrum data was broken up into its’ component frequency ranges (i.e. possible negative relationship in the Delta, Theta and Alpha ranges and positive in the Beta and Gamma ranges). No significant relationship was found between the NIRS blood oxygenation signals and the SSVEP amplitude data. When comparing the EEG power spectrum and NIRS blood oxygenation data, significant negative correlation was seen for one subject in the Delta (0-4Hz) and Theta (4-8Hz) frequency ranges. These correlation coefficients remained significant at α = 0.0001. Even though the experimental aim of this study was achieved, it was recommended that a larger subject pool should be used in future studies, as it may help to reduce the influence that anatomical differences between subjects could have had on the experimental results. This in turn may allow more informed comment to be made as to the existence of a coupling relationship between electrical and vascular cortical activity.