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Florian Baumgartner, Peter U. Tse, Mark W. Greenlee; fMRI BOLD signal varies proportionally with the size of small saccades in human V1 and V2. Journal of Vision 2007;7(9):736. doi: 10.1167/7.9.736.
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The goal of this research is to determine the neural correlates in early areas of visual cortex of small visually guided saccades executed during fMRI. Methods: We monitored voluntary saccades from one eye in the spatiotemporal domain of microsaccades using a Limbus infrared eyetracker (1 kHz sampling rate), while collecting fMRI data in a mixed event-related block design (3T Siemens Allegra scanner, TR = 402 ms, 7 slices along calcarine, TE = 30 ms, flip angle = 35°, 800 volumes, n = 6 subjects, 6–8 runs per subject). In blocks with small voluntary saccades (amplitudes: 0.16, 0.38, 0.82, 1.64, and 3.28 visual degrees; 12 pseudorandom events per epoch), subjects tracked with their gaze a small point that jumped either to the left or to the right. The fixation point was projected onto a one-degree wide horizontal white band superimposed onto a high-contrast, polar grating. In separate runs, retinotopic areas were mapped using the phase-encoding method (TR = 2000 ms, TE = 30 ms, FA = 90°, 30 slices, 152 volumes) in all subjects. Results: BOLD signals transiently increased in early visual areas during epochs with saccades of all sizes, relative to fixation epochs, including those that were as small as microsaccades. In V1 and elsewhere in visual cortex, the strength of the BOLD signal varied proportionally with the size of very small visually guided saccades. These results emphasize the need to monitor fixation behavior during fMRI and suggest that small saccadic and/or fixational eye movements can evoke significant responses in primary visual cortex.
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