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Omar H. Butt, Noah C. Benson, Ritobrato Datta, David H. Brainard, Geoffrey K. Aguirre; Multi-focal and phase-encoded retinotopy compared. Journal of Vision 2011;11(11):1069. doi: 10.1167/11.11.1069.
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© 2017 Association for Research in Vision and Ophthalmology.
Phase encoded (PE) retinotopic mapping stimuli are traveling wedges and rings while multi-focal (MF) stimulation divides visual space into discrete regions driven by a set of pseudorandom sequences (S Vanni, 2005. Neuroimage 27:95–105). Simulations of the BOLD fMRI system predict that MF and PE approaches should have similar statistical power, and studies have shown comparable qualitative results in area V1. How MF mapping quantitatively compares to the PE approach, however, has not been examined empirically.
We performed retinotopic mapping (RM) with BOLD fMRI (6 scans, 120 TRs, TR = 3, 3 mm voxels, at 3 Tesla) upon 10 subjects using both PE and MF methods. Quadratic residue sequences defined stimulation (5 Hz flicker) in 6 second intervals for the 48 MF sectors, and prohibited simultaneous activation of adjacent sectors. PE were coherent wedges and rings of sectors that traveled every 6 seconds. Eccentricity and polar angle values from each subject were combined within an anatomical V1 region defined by registration to a spherical atlas of cortical topology (Hinds et al 2008). We computed the correlations of each individual's map with a template derived from the data for the remaining nine subjects. We averaged these correlations as a measure of the reliability of each method.
12 minutes of PE scanning produced an average correlation for polar angle similar to 24 minutes of MF scanning (PE 0.51 ± 0.1sd vs MF 0.47 ± 0.14sd); 12 minutes of MF scanning produced significantly poorer results (0.26 ± 0.30sd), with discontinuities in the retinotopic map.
Despite having equivalent predicted statistical power, MF mapping was empirically half as powerful as PE approaches within area V1. Although adjacent sectors were never simultaneously illuminated, lateral inhibition may nonetheless contribute to this effect. More generally, MF presentations appear similar to visual noise and may be a suboptimal cortical stimulus.
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