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Abstract
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 microM 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.