Lock-in techniques for the swept stimulus evoked potential.

We discuss the use of synchronous-demodulation (lock-in) techniques for evoked potential retrieval. Application to electronically swept visual displays is emphasized. These techniques permit a visual threshold to be specified in 20 s, but their application to visual assessment requires careful consideration of several problems, notably alleged delay in the instrumentation, specification of the baseline response level, and the nature of EEG interference. In addition, since stimulus waveform information is lost in all lock-in methods, questions concerning what activity is contributing to the measured response must be answered. A technique addressing these issues and combining phase-sensitive detection and vector computation is presented.