Selective attention modulates the nonlinear interaction between stimuli
23.505, Saturday, May 11, 8:30 am - 12:30 pm, Vista Ballroom
Yee Joon Kim1, Preeti Verghese1; 1Smith-Kettlewell Eye Research Institute
Prior studies suggest that visual attention selects objects of interest by biasing the competition in favor of attended items. However, neurophysiological studies of selective attention to one of two objects typically report aggregate responses to individual objects as well as to their interaction. To separate these two response components, we directly measured the interaction between stimuli by using high-density steady-state visual evoked potential (SSVEP) combined with cortical source localization. This technique offers a powerful approach to directly measure attentional modulation of individual stimuli (self-terms) as well as their nonlinear interaction (intermodulation terms). Observers were tested with a pair of adjacent wedge-shaped gratings flickering at two different frequencies (F1 and F2 at 7.14 and 5.56 Hz, respectively). By asking observers to attend to both stimuli or to one of them in separate conditions, we determined the attentional modulations of self-terms and intermodulation terms compared to a condition when observers attended away from the flickering gratings to two static wedges located diametrically opposite the flickering wedges. Our data show that selective attention differentially modulates self-terms at harmonics of F1 and F2 as well as intermodulation terms at the frequency F1+F2. Consistent with previous single-cell studies, the self-terms have the greatest amplitude when attention is directed to one of the two stimuli. In contrast, the intermodulation term has the greatest amplitude when observers attend to both stimuli, is smaller when they attend to a single stimulus, and insignificant when attention is directed away. A similar pattern is seen in cortical source-imaged activities in various ROIs, indicating that the intermodulation term serves as an index of attentional selection throughout the cortical hierarchy. Current study advances our understanding of processes involved in selective attention by separately tracking response components resulting from individual stimuli and from their nonlinear interaction.