These time-domain (cross-correlation) and frequency-domain (coherence) analyses together indicate that sensory experience alters the synchrony of neuronal groups more than it detectably alters the absolute firing rates of individual cells. All other things being equal, the reduced TC synaptic connectivity

we found should decrease rather than increase L4 synchrony. Enhanced L4 synchrony suggests that experience alters an additional element of the circuit. One possibility is that the pruning of TC-L4 synapses triggers homeostatic rescaling of the strength of synapses—afferent and/or intracortical—onto an excitatory L4 neuron to maintain its normal firing rate. To check this, we removed the stimulus-induced correlation to reveal millisecond-scale neural interactions. Near-synchronous events in a “raw” cross-correlogram (Figure 4B, bottom; Figure 4E, learn more top) result from a pair of cells receiving shared common input(s) and/or being embedded in independent circuits whose activity is transiently JQ1 ic50 modulated by the same stimulus. The stimulus-induced correlation

can be estimated by shifting one of the spike trains by a stimulus trial and calculating a “shift corrector” (Figure 4E, middle). The difference of the raw correlogram and corrector is an estimate of shared input, synapses that derive from the same divergent axons. The millisecond-scale locking of such synapses produces a sharp peak in the correlogram (Figure 4E, arrow), which represents some unknown number of diverging fibers that contact both cells. Significant shared inputs occurred in 13 out

of 23 (57%) control pairs and 12 out of 26 (46%) trimmed pairs. For each of these significant pairs, we measured the strengths of shared inputs (Figure 4F). Trimming significantly increased the strengths of shared inputs (t test, p = 0.017). Enhancement of shared inputs is also visible in normalized population cross-correlograms, in which the relative sizes of the fast millisecond-scale component and slower stimulus-induced of component differ between groups (Figure S2D). These results suggest that homeostatic strengthening of corticocortical synapses and/or unpruned thalamocortical synapses may parallel or follow TC synapse loss, thereby enhancing correlated activity in L4. Because the synchrony of a neuronal population can impact the response magnitude of its downstream targets (Bruno, 2011), experience-induced changes in L4 synchrony may constitute a previously unconsidered contributor to functional plasticity in layer 2/3 (Feldman and Brecht, 2005, Fox, 2002 and Karmarkar and Dan, 2006). Changes in corticocortical connectivity have long been thought to mediate adult plasticity. Our study reveals that thalamocortical axons also remain plastic in adulthood. Simply trimming whiskers, a nondestructive alteration in sensory experience, brought about a 25% decrease in total thalamocortical arborization.