Integration of evoked responses in supragranular cortex studied with optical recordings in vivo

Complex representations in sensory cortices rely on the integration of inputs that overlap temporally and spatially, particularly in supragranular layers, yet the spatiotemporal dynamics of this synaptic integration are largely unknown. The rodent somatosensory system offers an excellent opportunity to study these dynamics because of the overlapping functional representations of single-whisker inputs. We recorded responses in mouse primary somatosensory (barrel) cortex to single and paired whisker deflections using high-speed voltage-sensitive dye imaging. Responses to paired deflections at intervals of 0 and 10 ms summed sublinearly, producing a single transient smaller in amplitude than the sum of the component responses. At longer intervals of 50 and 100 ms, the response to the second deflection was reduced in amplitude and limited spatially relative to control. Between 100 and 200 ms, the response to the second deflection recovered and often showed areas of facilitation. With increasing interstimulus interval from 50 to 200 ms, recovery of the second response occurred from the second stimulated whisker's barrel column outward. In contrast to results with paired-whisker stimulation, when a whisker deflection was preceded by a weak electrical stimulus applied to the neighboring cortex, the summation of evoked responses was predominantly linear at all intervals tested. Thus under our conditions, the linearity of response summation in cortex was not predicted by the amplitudes of the component responses on a column-by-column basis, but rather by the timing and nature of the inputs.