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Alexandre Reynaud, Simon Clavagnier, Robert Hess; Texture-modulation channels for spatial frequency and orientation. Journal of Vision 2012;12(9):888. doi: 10.1167/12.9.888.
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Second-order orientation-modulated stimuli are thought to be processed by a two stages mechanism : the first, processing the carrier in striate cortex and the second, processing the modulation in extra-striate cortex. Much is known about the spatial properties of the mechanisms underlying carrier detection however little is known about the properties of the mechanisms involved in modulation detection and integration of carrier-based information. Here we propose to investigate the properties of spatial channels underlying this detection of second-order texture-modulations. For this purpose, we chose to use a discrimination at detection paradigm, implemented by a two-by-two alternative forced choice (2x2AFC) protocol. This protocol allows a normalized indicator of the performance, thus comparable across sessions and subjects and a perfect discrimination test. We show that the mechanisms underlying modulation detection are tuned for relative modulator spatial frequency and orientation, namely channels of about 1-2 octaves for spatial frequency and 30° for orientation. This tuning is not substantially different from that previously described for carrier mechanisms. Furthermore, for orientation, these stimuli exhibit an oblique effect which could be modulated by carrier orientation, suggesting facilitative interactions between first and second stage processes. To conclude, these observations suggest sub-mechanisms tuned for modulation orientation that are similar in their tuning to that previously reported for 1st order, luminance-defined stimuli. They also support a model of two mixed processing stages, one for carrier and one for modulator and highlights the need to better understand the types of interactions that occur between 1st and 2nd order processes, probably at different stages in ventral and dorsal pathways. In a near future, we will investigate these physiological aspect by modulating cortical excitability with repetitive transcranial magnetic stimulation (rTMS).
Meeting abstract presented at VSS 2012
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