The relative zero potential level of the ERP was defined with a mean amplitude of EEG with a pretrigger period of 50ms. The amplitudes of the P300 components were measured as the relative potential from the above mentioned zero point to the peak. The P300 components are clearly shown in target letter counting trials. This tendency of P300 amplitudes was observed in all subjects. One is exogenous potentials and the other is endogenous potentials.
Exogenous potentials reflect obligatory responses evoked by physical features of the stimulus.
It is well known that the amplitude of the P300 component is associated with the subjective probability of appearance of relevant stimuli, task difficulty, selective attention and so on. In particular, these characteristics can be used to detect the binary decision of the subjects without any physical reactions. The P300 component has potentials as an effective communication tool for establishing a novel human interface for the locked-in syndrome patients. The present experiment demonstrated that the P300 components can be a potential tool to detect alternative or binary decisions in the mind such as A or B, Yes or No, and so on. In this study, a considerable amount of time was required to detect the subjects decision, at least about 200s.