Functional neocortical microcircuitry demonstrated with intrinsic signal optical imaging in vitro

Intrinsic signal optical imaging was used to record the changes in light transmittance evoked by electrical stimulation in slices prepared from sensorimotor cortex of young adult rats. The spatial characteristics of the optical signal evoked by stimulation of layer II/III, IV, V, or VI were clearly different. Layer IV and V stimulation elicited a radially-oriented region of increased light transmittance which was 'hourglass' shaped: its tangential extent was greatest in layers II/III and layer V, and least in layer IV. Layer VI stimulation also elicited a radially-oriented signal but the tangential extent of this signal was the same across layers II-VI - that is, it was column-shaped. Upper layer stimulation produced a signal whose tangential extent was much greater in the upper layers than its radial extent to the deeper layers. The spatial form of the stimulus-evoked intrinsic signal was not dependent on the cytoarchitectonic area in which it was elicited. The tangential and radial distribution of the signal evoked by stimulation of different layers appears to reflect the connectivity of cortex, particularly the horizontal connectivity present in layers II/III, V, and VI, and the interlaminar connections that exist between layers II/III and V and from layers VI to IV. The spatial characteristics of the intrinsic signal were independent of the strength of stimulation used. The idea that inhibitory mechanisms restrict the tangential extent of the signal was evaluated in experiments in which the intrinsic signal was recorded before and after the addition of 10 μM bicuculline methiodide. In all slices studied in this way (n = 12), bicuculline methiodide drastically increased the tangential extent of the signal. In 4/12 slices, the tangential spread of the signal was asymmetric with respect to the stimulus site. Asymmetric spread of the signal occurred for both layer V and layer VI stimulation and, in 2/4 of those cases, could be attributed to a cytoarchitectonic border whose presence appeared to restrict the spread of the signal across the border. Although increasing stimulation strength did not change the spatial characteristics of the radially-oriented signal evoked by layer V or VI stimulation, at maximal stimulus intensity the signal evoked from these layers was often accompanied by a band of decreased light transmittance in the most superficial layers (layers I and II). It is concluded that in vitro intrinsic optical signal imaging allows one to image a response attributable to activation of local subsets of cortical connections. In addition, the opposite effects of high-intensity deep layer stimulation on the superficial layers vs layers III-VI of the same column raise the possibility that the most superficial layers may respond differently to repetitive input drive than the rest of the cortical column.