Figure 2. Recording from geniculate afferents in active cortex.

a. Simultaneous recordings from single cells in LGN and local field potentials in the visual cortex. Well-isolated spikes from a single geniculate cell were used as triggers to obtain spike-triggered field potentials for each cortical channel. The time of the geniculate spike is indicated by the vertical dashed lines in the depth profiles shown in (a) and (b). The second spatial derivative of these field potentials, which is directly proportional to the current density at a point, was estimated by current-source-density analysis. The result from this spike-triggered current-source-density (STCSD) analysis is shown through the depth of the cortex as individual traces and a colorized image. b. Example of two geniculate cells that generated current sinks at the same cortical domain and had overlapping receptive fields of the same sign. The cell on the left was of Y type and the one on the right of X type. As expected from previous anatomical studies ²⁹, the Y cell had faster conduction velocity and projected higher within layer 4 than the X cell. c. 70% of the geniculate cell pairs converging at the same cortical domain had receptive fields of the same sign (n = 37, P = 0.014, Chi-square test). The frequency of cell pairs with receptive fields of the same sign is shown as a function of receptive field overlap for all cell pairs (left), cell pairs of the same type (right top), and cell pairs of different type (right bottom).