Figure 1. Brain state-triggered target presentation: General approach and application.

A: Neurophysiology experiments often involve target stimuli presented at constant or random intervals (‘block’ or randomized design, black vertical lines), and address stimulus interactions with ongoing brain states (blue schematic timecourse) using offline analyses. To more efficiently assess the influence of ongoing brain activity on behaviorally relevant stimuli, we propose presentation of stimuli triggered online by particular brain states of interest (green lines). B: We tested this approach using an auditory detection task. Subjects were presented with 5 Hz trains of tones (black rectangles) to the right ear (at 350 Hz) and left ear (at 1300 Hz, 100 ms delayed onset). Subjects were cued to attend to the right or left ear and to detect slight increases in intensity of a deviant tone (green rectangle) at the cued ear. Detected deviant tones were reported after the stimulus train ended, visual feedback was provided, and deviant tone intensity threshold was updated to maintain 50% detection performance for both attended ears. C: Electroencephalographic voltage potentials (EEG) were recorded from 29 electrodes on a whole-head EEG cap (Brainproducts.com) and sent to a computer for online analysis of responses to sounds occurring in the previous 1.25 s (i.e. sliding window of six left/right tone pairs). Emergence of a ‘brain state’ corresponding to strong directed neural bias towards stimuli at the cued or non-cued ear triggered immediate presentation of a deviant tone at the cued ear.