A cortical locus for overlay suppression with broadband stimuli revealed through transcranial direct current stimulation
16.437, Friday, May 10, 5:30 - 8:00 pm, Orchid Ballroom
Bruce C Hansen1, Kristin Andres1, Edward A Essock2, Daniel P Spiegel3, Benjamin Thompson3; 1Department of Psychology and Neuroscience Program, Colgate University, NY, USA, 2Department of Psychological & Brain Sciences, University of Louisville, Louisville, KY, USA, 3Department of Optometry & Vision Science, University of Auckland, Auckland, New Zealand
Human contrast sensitivity for Gabor targets is largely suppressed when superimposed on a Gabor mask of the same spatial frequency (SF) and orientation (referred to as overlay suppression, OS), with suppression being broadly tuned to orientation (~130° bandwidth) and spatial frequency (~3 octave bandwidth) (Petrov, Carandini, & McKee, 2005). Further, numerous behavioral and neurophysiological experiments have suggested that the suppressive signals leading to OS originate from the initial LGN inputs to V1. However, Kim, Haun, and Essock (2010) have reported anisotropic contrast sensitivity thresholds for differently oriented Gabor targets when masked by broadband noise. Their results suggest a much narrower orientation tuning for OS than would be expected if it originates from initial LGN inputs to V1, and imply that another cortical operation is active when overlay masks are broadband. Here, we sought to explore this notion using transcranial direct current stimulation (tDCS). Since tDCS is only effective at modulating cortical function, we hypothesized that tDCS would influence suppression for OS with broadband stimuli but not for OS with narrowband stimuli which may result from sub-cortical signals. Target stimuli consisted of 8cpd Gabor targets (subtending 0.3°) at one of three orientations (0°, 45°, or 90°). Overlay masks were either identical to the targets (narrowband masks), or consisted of broadband noise patches (20° orientation bandwidth and 2 octaves in SF). All masks were fixed at 0.21 rms contrast; with target contrast controlled by a standard 2IFC staircase procedure (stimulus duration was 150ms). Human participants performed the task foveally with tDCS (anodal and cathodal) and without (i.e., baseline). The results showed that tDCS resulted in a large anisotropic reduction in OS when the masks were broadband, but not when they were narrowband. This supports the hypothesis that OS has a cortical locus when the overlay masks are broadband in SF and orientation.