Modulation of sound perception by attention, learning and emotion : recent advances on cortical sound coding

We then turned our attention to recent advances in the neural basis of sound discrimination. Work carried out on the responses of neurons in layers L2/L3 studied by calcium imaging in anesthetized mice (Bathellier B, Ushakova L. and Rumpel S., Neuron, 2012) concluded the existence of a limited repertoire of responses for a given population of neurons. Sound discrimination would lie in the global response of the cortex, which would leverage the combination of distinct modes of response to generate a stimulus-specific global response. This work also revealed the existence of a non-linear mode of operation in the auditory cortex: the responses of neurons in a field that responds to two sounds with distinct modes show that, when responding to a mixture of these two sounds, the response modes are either of one type or another, and are never mixed. All in all, a discriminative representation of sounds emerges in the L2/L3 layer of the auditory cortex only at a global level, in which the functional units are made up of a few hundred neurons. A very small fraction of these functional units account for a large proportion of discrimination for a given pair of sounds.

A notable advance concerning the organization and functioning of neural microcircuits, the fruit of very recently published work, has been discussed (Cossell L., Mrsic-Flogel T.D., Nature, 2015). The strength of synaptic connections between neurons determines how they influence each other's responses. The amplitude of excitatory responses between pairs of cortical neurons varies by two orders of magnitude, but only a small number of synaptic connections are very strong. This heterogeneity of synaptic connection strengths is observed in various cortical areas, but its contribution to information processing in local microcircuits is not known. To address this question, the intensity of the response of neurons in layers L2/L3 of primary visual cortex V1 to various visual stimuli was studied, using two-photon calcium imaging, in anesthetized mice. The imaged region was marked and then, by electrophysiological recordings on cortex slices (patch-clampen whole-cell configuration), the strength of synaptic connections between imaged neurons was measured by the amplitude of their excitatory postsynaptic potentials (EPSPs). Ultra-fast imaging identified pairs of neurons whose responses show a strong temporal correlation. Their distribution reveals the existence of a small population of neuron pairs whose activity is highly correlated. PPSEs with the highest amplitude are associated with these neurons. This result is in line with the hypothesis formulated by Hebb in 1949 (Hebb D.O., 1949), according to which the correlation of neuron activity increases the strength of their synaptic contacts. The results also show that half of the synaptic strength comes from the activity of the 7% of neuron pairs whose response to a stimulus is most strongly correlated. Conversely, weak synaptic connections are those of neurons whose responses are not correlated. Strong, reciprocal synaptic connections often occur between neurons engaged in the same functions. This simplifies the apparent complexity of neural networks. The establishment of strong synaptic connections could amplify the response to specific stimuli in cortical microcircuits. The authors propose that weak synaptic connections, on the other hand, ensure the response plasticity of neural networks. This is an incentive to rethink the way we explore brain microcircuitry.