Master's Theses

Date of Award

2013

Document Type

Thesis

Department

Biomedical Engineering

First Advisor

Simon Kelly

Keywords

electrophysiology, Visual system, signal processing

Abstract

The steady-state visual evoked potential (SSVEP) is an electroencephalographic response to flickering stimuli generated in significant part by activity in primary visual cortex (V1). SSVEP signal-to-noise ratio is generally low for stimuli that are located in the visual periphery, at frequencies higher than 20 Hz, or at low contrast. Because of the typical "cruciform" geometry of V1, large stimuli tend to excite neighboring cortical regions of opposite orientation, likely resulting in electric field cancellation. In Study 1, we explored ways to exploit V1 geometry in order to boost scalp SSVEP amplitude via oscillatory summation, by manipulating flicker-phase offsets among angular segments of a large annular stimulus. We found that by dividing the annulus into standard octants, flickering upper horizontal octants with opposite temporal phase to the lower horizontal ones, and left vertical octants opposite to the right vertical ones, the normalized SSVEP power was enhanced by 202% relative to the conventional condition with no temporal phase offsets. In two further conditions we individually customized the phase-segment boundaries based on early-latency topographical shifts in pattern-pulse multifocal visual-evoked potentials (PPMVEP) derived for each of 32 equal-sized segments. Adjusting the boundaries between 8 phase-segments by visual inspection resulted in significant enhancement of normalized SSVEP power of 383%, a further significant improvement over the standard octants condition. An automatic segment-phase assignment algorithm based on the relative strength of vertically- and horizontally-oriented multifocal VEP scalp potential amplitudes produced an enhancement of 300%. In Study 2, we applied the same principle to obtain more reliable measures of visual evoked activity to obtain surround suppression measures. Here we report for the first time, a novel vii paradigm that exploits simple signal processing, sensory physiology and psychophysical evidences in order to extract a direct index of surround suppression using EEG. Surround suppression effects were tested for low and high flickering frequencies in two different configurations of a flickering stimulus (foreground, FG) on a static surrounding pattern (background, BG): foveal, where the stimulus was a unique central disc, and peripheral, where four discs were presented at symmetrical locations around the horizontal meridian. We varied FG and BG contrast combinations and also evaluated the influence of differences in spatial phase and orientation between the surrounding pattern and the foreground. Across a population of sixteen healthy subjects, we found that the foreground contrast response function was significantly suppressed in proportion with the contrast of the background, and that, like psychophysical measures, this suppression effect was greater when the background was oriented in parallel with the foreground than when it was orthogonal. Suppression effects were also greater for the peripheral stimulus condition. This is the first demonstration of a clear surround suppression effect in the visual evoked potentials of humans, and paves the way for the first definitive measurement of the relative contributions of under-inhibition and over-excitation to hyperexcitability in epilepsy.

 
 

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