Fig. 2.

Target brain state and two-dimensional experiment parameter space for both studies. (a) Based on a previous study (Braga et al., 2013) we identified two target brain regions: bilateral lateral occipital cortex (red) and bilateral superior temporal cortex (blue) that strongly activate for complex visual (e.g., naturalistic movie) or auditory stimuli (e.g., speech), respectively. The two tested target brain states of interest were: (1) maximized occipital cortex activity with minimum superior temporal cortex activity and (2) maximized superior temporal cortex with minimized occipital cortex activity. (b) Parameter space of Study 1 with 10 × 10 (100) possible combinations composed of auditory and visual stimuli of varying complexity. The optimization algorithm traversed through the two-dimensional parameter space in order the find the most optimal audio–visual stimulus combination. Based on previous work (Braga et al., 2013), the hypothesized optimal stimulus combination for evoking target brain state (1) is the most complex visual stimulus in combination with no auditory input (red square). The reverse stimulus combination (complex auditory, no visual input) was hypothesized to be optimal for target brain state (2) (blue square). (c) The larger and more challenging parameter space of Study 2 involving 19 × 19 (361) possible combinations. Here, stimuli were only optimized for target brain state (1). The hypothesized optimal stimulus combination is now, due to mirroring of the axes, located in the center of the grid (red square).