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The Neuroscientist Comments

It is well established, in the classical neurophysiological literature, that synaptic excitation and inhibition are integrated, at the level of single neurons, so as to shape neuronal output. Within the cerebral cortex, the opposing forces of synaptic excitation and inhibition, which can be represented as an E/I ratio, affect functions as diverse as feature selectivity and gain. Over time, individual cortical pyramidal neurons tend to show stable E/I ratios even though the overall level of cortical activity fluctuates. This presumably reflects a situation in which, when excitation increases, there is a proportional increase in inhibition, as a result of enhanced recruitment of inhibitory neurons; this phenomenon has been referred to as “excitation–inhibition balance.” However, there is a design challenge here: inhibitory neurons give rise to axons that branch profusely, establishing apparently indiscriminate contact with many neighboring pyramidal cells. as a result, the distribution of E/I ratio across pyramidal neurons is hard to predict. In a recent article, Xue and others (2014) asked whether inhibition is homogenously distributed, or individually matched to the different levels of excitation on distinct pyramidal cells. They report that pyramidal cells within layer II/III of the primary visual cortex of the mouse each receive inhibition that is matched so as to be proportional to excitation in that cell. They show that this matched inhibition is mediated by parvalbumin-expressing inhibitory neurons, but not somatostatin expressing ones, and they argue that it arises from the independent adjustment of synapses originating from single parvalbuminexpressing neurons that target different pyramidal neurons. Interestingly, they report that this matching of inhibition to excitation is disrupted by perturbations of pyramidal cell activity, suggesting that it is activity dependent. The striking result of this study, that is, equalization of E/I ratios across pyramidal neurons within the primary visual cortex, is interpreted by the authors as revealing an unexpected degree of order in the distribution of excitatory and inhibitory synaptic strengths that is stabilized both in time and in space. Whether this principle applies to other parts of the nervous system is not yet clear.