Spatio-temporal structure of multi-focal MEG potentials shows evidence of striate global/local signalling.
26.304, Saturday, May 11, 2:45 - 6:45 pm, Royal Ballroom 6-8
David Crewther1, Alyse Brown1, Laila Hugrass1; 1Centre for Human Psychopharmacology, FLSS, Swinburne University of Technology
Nonlinearities of the VEP, with signatures for magnocellular and parvocellular processing have provided useful insights into early cortical neural processing in normals (1) and autism spectrum (2). Signatures have been based on contrast response function, saturation and latency of response peaks. We extended these studies to magnetoencephalography (MEG). Five participants (young adult, 4 female, 1 male) observed a projected dartboard comprising 9 unstructured patches each executing pseudo-random binary m-sequences (VPixx). Four minute recordings on the 306 sensor Elekta Triux system were made at Michelson contrasts of 24% and 95% (60 Hz frame rate). Wiener kernel analysis (foveal patch) of the first-order K1 and the first two slices of the second-order kernel (K2 .1 & K2 .2) were characterized by an initial response latency of <50ms with major peaks with latencies of 95 ms (K1) 85 ms (K2.1) while the second slice (K2.2) demonstrated peaks rather more delayed in latency, similar to the electrical VEP. The early peak of the first slice K2.1 was already easily measurable with 24% contrast and did not grow at higher contrasts, indicative of the likely magnocellular origin. In a second experiment, a diamond illusion stimulus (3) was supposed over the randomly flashing central stimulus patch and participants reported global percepts (diamond shape oscillating horizontally) and local percepts (4 ungrouped moving lines) via pressing different buttons. Mean difference (Global Local) magnetic evoked fields (MEF) showed little difference for times of <100ms, and a distinct peak with latency around 180ms centred over occipital cortex sensors. Thus perceptual rivalry exerts effects on the occipital MEF. References: (1) Klistorner A, et al (1997), Vis Res 37(15): 2161-9; (2) Sutherland A & Crewther DP, Brain 133: 2089-2097; (3) Fang F et al (2008), JVis 8(7):2, 29