Cortical response selectivity derives from strength in numbers of synapses

Single neocortical neurons are driven by populations of excitatory inputs, forming the basis of neuronal selectivity to features of sensory input. Excitatory connections are thought to mature during development through activity-dependent Hebbian plasticity(1), whereby similarity between presynaptic and postsynaptic activity selectively strengthens some synapses and weakens others(2). Evidence in support of this process ranges from measurements of synaptic ultrastructure to in vitro and in vivo physiology and imaging studies(3,4,5,6,7,8). These corroborating lines of evidence lead to the prediction that a small number of strong synaptic inputs drive neuronal selectivity, while weak synaptic inputs are less correlated with the somatic output and modulate activity overall(6,7). Supporting evidence from cortical circuits, however, has been limited to measurements of neighboring, connected cell pairs, raising the question of whether this prediction holds for a broad range of synapses converging onto cortical neurons. Here we measure the strengths of functionally characterized excitatory inputs contacting single pyramidal neurons in ferret primary visual cortex (V1) by combining in vivo two-photon synaptic imaging and post hoc electron microscopy (EM). Using EM reconstruction of individual synapses as a metric of strength, we find no evidence that strong synapses play a predominant role in the selectivity of cortical neuron responses to visual stimuli. Instead, selectivity appears to arise from the total number of synapses activated by different stimuli. Moreover, spatial clustering of co-active inputs appears reserved for weaker synapses, enhancing the contribution of weak synapses to somatic responses. Our results challenge the role of Hebbian mechanisms in shaping neuronal selectivity in cortical circuits, and suggest that selectivity reflects the co-activation of large populations of presynaptic neurons with similar properties and a mixture of strengths.