Source geometry and dynamics of the visual evoked potential.

A method is developed for the analysis of the steady-state visual evoked potential (VEP) recorded at an array of electrodes. The VEP is assumed to result from the sum of a number of independent mechanisms. Each mechanism proves to have a fixed intracerebral source (independent of stimulus paradigm) and dynamics which depend on the stimulus. The steps in the analysis are: Fourier analysis of the responses and retention of the first 4 even harmonics; factor analysis in the frequency domain; an invariant representation ('the invariant fingerprint') of the geometric information contained in the factor analysis; and interpretation of the invariant fingerprint in terms of a simple biophysical model. This analysis is applied to responses elicited by a contrast-reversing checkerboard. The dependence of the midline VEP wave form on reversal frequency, check size, and area of stimulation (upper, lower, and full-field) reflects a change in the dynamics of the generators, rather than a change in their geometry.

Light-dark adaptation of bacteriorhodopsin in triton-treated purple membrane.

Solubilization of purple membrane with Triton X-100 yields Triton micelles containing bacteriorhodopsin monomers. The absorption maximum of dark-adapted solubilized bacteriorhodopsin is blue-shifted to 549 nm. Light adaption increases the absorbance by 4% and shifts the absorption maximum to 553 nm, i.e., the extent of light adaptation is considerably less than in intact purple membrane. Extraction of dark-adapted bacteriorhodopsin in Triton yields a 13-cis- to all-trans-retinal ratio of 58 : 42 which changes after light adaptation to 38 : 62. It has been shown by Sperling et al. (Sperling, W., Carl, P., Rafferty, Ch.N. and Dencher, N.A. (1977) Biophys. Struct. Mech. 3, 79-94) that light adaptation in intact purple membrane occurs through a branching of the 13-cis photoreaction cycle, so that part of the pigment during each cycle crosses over into the all-trans photoreaction cycle. We explain the decreased extent of light adaptation in solubilized bacteriorhodopsin by assuming a significant back reaction from the all-trans to the 13-cis cycle. This assumption predicts a wavelength dependence of the extent of light adaptation, which is born out by experiment.