INTERPRETATION OF HIGH-RESOLUTION CURRENT SOURCE DENSITY PROFILES - ASIMULATION OF SUBLAMINAR CONTRIBUTIONS TO THE VISUAL-EVOKED POTENTIAL

Current source density (CSD) analysis provides an index of the locatio n, direction, and density of transmembrane currents that arise with sy nchronous activation of neural tissue and that generate an evoked pote ntial profile in the extracellular medium. In neocortex and other lami nated structures, a simplified, one-dimensional CSD analysis can be co mputed by differentiation of voltages sampled at discrete points in a linear array. One-dimensional CSD analysis is a practical and accurate method for defining both regional activity patterns and neural genera tors of surface-recorded evoked and event-related potentials. In compu ting the CSD, common practices of differentiating across spatial grids of 200 mum or more and use of spatial smoothing routines help to redu ce noise, but severely limit the spatial resolution available to the a nalysis. High-resolution CSD procedures (i.e., 3 point differentiation using a spatial grid of 100 mum or less) are more suited to identific ation of processes within individual cortical laminae or sublaminae, b ut can magnify the contributions of computational artifacts. Despite t he inclusion of independent indices of cellular activity (e.g., multiu nit activity), both high- and low-resolution analyses may indicate cur rent source and sink configurations for which there is more than one p lausible physiological interpretation. In the present study we examine d the resolving capacity and pitfalls of common CSD procedures using s imulated ensembles of current dipoles. These were positioned and orien ted to model the depolarization of lamina 4C stellate cells and thalam ocortical afferents in macaque striate cortex. Empirically, the surfac e N40 appears in association with a CSD configuration which includes c urrent sinks within the thalamorecipient (stellate) subdivisions of la mina 4C and a large current source extending considerably below 4C. Di pole ensemble contributions to the CSD profile were computed and compa red to physiological data from this region. Small asymmetries in activ ation of model stellate laminae were sufficient to produce substantial open field contributions. However, the best fit with empirical CSD pr ofile was found when the simulation included contributions from thalam ocortical axons, along with both open and closed field contributions f rom dual stellate cell sublaminae. High-resolution CSD profiles were s hown to be interpretable when computational artifacts characteristic o f closed and open fields were identified using a series of differentia tion grids.