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A. Furuta, J. Liu, S. Nakadomari, K. Asakawa, S. Maeda, K. Maeda, B.A. Wandell; Quick Retinotopic Mapping With Composite Stimulus . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5664.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Retinotopic mapping of human visual cortex using functional magnetic resonance imaging (fMRI) is a well established method. The conventional methodology uses two separate experiments, one with an expanding ring and the other with a rotating wedge stimulus to measure eccentricity (ring) and polar angle (wedge) maps, respectively. Here we explore a composite stimulus that may be more efficient and have applications to real–time clinical examination. Methods: The composite stimulus presents the expanding ring and rotating wedge stimuli simultaneously; each stimulus is repeatedly presented at its own repetition period. We measured retinotopic maps using two composite stimuli: (1) expanding ring, 18 sec period; rotating wedge, 24 sec period. (2) expanding ring, 24 sec period; rotating wedge, 18 sec period. We compared these measurements with conventional stimuli (expanding ring or rotating wedge only) at 18 or 24 sec period. Stimuli subtended 16 deg eccentricity; the radial width of expanding ring was 4 deg and the angular width of rotating wedge was 45 deg. Experiments were done in a 3 Tesla scanner with 20 coronal slices covering the occipital lobe (voxel size: 2.5x2.5x3 mm, TR: 1.5 sec). The fMRI time course of the composite stimulus was analyzed to calculate the Fourier components at the 18 sec and 24 sec periods (signal) and at other periods (noise). Voxels that contain strong fMRI responses to the composite stimulus had high signal to noise ratios. The phase of each component measures the eccentricity or polar angle at which the stimulus most effectively drives the fMRI response. Results: Retinotopic visual areas (V1/2/3, hV4, V3A/B, etc.) responded with high signal–to–noise ratios in all experiments. The eccentricity and polar angle maps were in excellent agreement in the composite and conventional experiments. However, the amplitudes of individual Fourier components were sometimes lower when using the composite stimulus than using the conventional stimuli. Conclusions: Quick retinotopic mapping reliably measures retinotopic maps in visual cortex, obtaining results that match conventional measurements. In addition to the reliability, this quick mapping method reduces required scan time and potentially reduces head movement artifacts. Moreover, it becomes possible to examine in real–time the brain mapping in visual cortex using just a single experimental scan. These benefits are particularly valuable in clinical applications.
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