Method for detection of changes in the EEG induced by the presence of sensory stimuli

The onset and offset of sensory stimuli evoke transient changes in the electroencephalogram (EEG) that can be detected by linear and/or nonlinear analysis. However, there is presently no systematic procedure to quantify the brain-electrical-activity correlate of the presence of a stimulus (as opposed to its onset evoked potential). We describe a method for detecting a stimulus-related change in brain electrical activity that persists while the stimulus is present (presence effect). The method, which is based on phase-space embedding of the EEG time series followed by quantitative analysis of the recurrence plot of the embedded signal, was used to demonstrate the occurrence of a presence effect in separate groups of human subjects exposed to sound, a magnetic field, and light. Any form of law-governed dynamical activity induced in the EEG can be detected, particularly activity that is nonlinearly related to the stimulus. Salient mathematical features of the method were reproduced in a model EEG system containing known nonlinear determinism.