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Daniel Berman, Xiangrui Li, Zhong-Lin Lu, Dirk Walther; Increasing efficiency of fMRI retinotopic mapping using Maximum Length Sequences. Journal of Vision 2013;13(9):42. doi: https://doi.org/10.1167/13.9.42.
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Visual cortex is organized in a series of retinotopic maps. Functional magnetic resonance imaging (fMRI) is routinely used to establish the correspondence between locations in the visual field and the activity of specific pieces of cortical tissue in response to visual stimulation of these locations. The standard stimulus for this type of mapping consists of a rotating wedge and an expanding ring, establishing the polar angle and eccentricity of each voxel’s preferred location. This stimulus is conceptually simple and ensures orthogonality of the time courses of stimulation at each visual field location. However, the wedge/ring technique does not make optimal use of scan time. Depending on the specifics of the design, each portion of visual cortex is being stimulated for only about one quarter or less of the total scan time. Here we demonstrate a new, more efficient way of mapping spatial receptive fields of voxels in early visual areas. The stimulation of each segment of visual field, defined by polar angle and eccentricity, is controlled by a Maximum Length Sequence (MLS) of ones and zeros. Since an MLS is essentially orthogonal to itself when shifted in time (autocorrelation zero everywhere except at t=0), we use shifted versions of the same MLS for each visual field location. This procedure ensures orthogonality of the stimulation sequences while stimulating each location for half of the total scan time on average. In computer simulations we have found a significant increase in efficiency, which is reflected in higher accuracy of the receptive field mapping when using the same scan time as for the wedge/ring stimulus, or in a decrease of the scan time required to achieve the same accuracy. In fMRI experiments we compare the results of both types of stimuli within subjects, thus verifying the predictions of our computer simulations.
Meeting abstract presented at VSS 2013
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