Rapid Recording of Spatial Frequency Sweep Pattern Visual Evoked Cortical Potentials Using a Lock-In Amplifier

Abstract

Abstract: : Purpose: The classical technique of flashed or pattern reversal visual evoked cortical potentials is relatively slow because averaging procedures are necessary. We have tested whether a lock-in amplifier can be used for rapid recording and immediate assessments of visual evoked cortical potentials. Methods: A square-wave-modulated checkerboard pattern of 80% contrast and 39 cd/m² mean luminance was presented on a cathode-ray tube (CRT) monitor as the stimulus. The spatial frequency sweep technique was employed. A large number of patterns of different spatial frequencies were changed in succession without interruption on the CRT. For steady-state VECPs, the reversal rate of the pattern was set at 12 reversals/sec. VECPs were recorded by a scalp electrode placed 10% above the inion, while the indifferent electrode was attached to the earlobe. Potential were fed to a lock-in amplifier which locks-in to the frequency of a reference signal and measures the input signal at that frequency. The information about response amplitude and phase can be obtained using synchronous recording. Results: A lock-in amplifier and the spatial frequency sweep technique allowed the rapid measurement of the spatial-tuning function of VECP. A peak in the amplitude of the pattern VECP was observed with intermediate checks, and the amplitudes elicited by larger or smaller checks decreased. Conclusions: VECP recordings with a lock-in amplifier provides a rapid and useful method of VECP assessment of several aspects of pattern vision. The speed and reliability of assessing pattern vision by sweeping spatial frequencies, as well as other stimulus dimensions, are of great value for both clinical and research purposes.

Keywords: electrophysiology: clinical • visual cortex • pattern vision