Physical features of sensory stimuli are fixed, but sensory perception is context-dependent. The precise mechanisms that govern contextual modulation remain unknown. Here, we trained mice to switch between two contexts: passively listening to pure tones vs. performing a recognition task for the same stimuli. Two-photon imaging showed that many excitatory neurons in auditory cortex were suppressed, while some cells became more active during behavior. Whole-cell recordings showed that excitatory inputs were only modestly affected by context, but inhibition was more sensitive, with PV, SOM+, and VIP+ interneurons balancing inhibition/disinhibition within the network. Cholinergic modulation was involved in context-switching, with cholinergic axons increasing activity during behavior and directly depolarizing inhibitory cells. Network modeling captured these findings, but only when modulation coincidently drove all three interneuron subtypes, ruling out either inhibition or disinhibition alone as sole mechanism for active engagement. Parallel processing of cholinergic modulation by cortical interneurons therefore enables context-dependent behavior.

Neurons recorded in behaving animals often do not discernibly respond to sensory input and are not overtly task-modulated. These non-classically responsive neurons are difficult to interpret and are typically neglected from analysis, confounding attempts to connect neural activity to perception and behavior. Here, we describe a trial-by-trial, spike-timing-based algorithm to reveal the coding capacities of these neurons in auditory and frontal cortex of behaving rats. Classically responsive and non-classically responsive cells contained significant information about sensory stimuli and behavioral decisions. Stimulus category was more accurately represented in frontal cortex than auditory cortex, via ensembles of non-classically responsive cells coordinating the behavioral meaning of spike timings on correct but not error trials. This unbiased approach allows the contribution of all recorded neurons – particularly those without obvious task-related, trial-averaged firing rate modulation – to be assessed for behavioral relevance on single trials.

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Excitation in neural circuits must be carefully controlled by inhibition to regulate information processing and limit over-or under-excitability. During development, inhibitory and excitatory inputs in the cerebral cortex are initially mismatched but become co-tuned, although the mechanisms for balancing inhibition with excitation have remained unknown. Here we show how coordinated long-term synaptic modifications calibrate excitatory and inhibitory synaptic weights across multiple inputs. We simultaneously monitored several inputs onto mouse auditory cortical pyramidal neurons, and induced plasticity at one set of inputs by pairing presynaptic and postsynaptic activity. Changes also occurred at other inputs including heterosynaptic modifications of the largest unpaired excitatory and inhibitory inputs, computed by postsynaptic neurons over a ten-minute period. These distributed changes collectively normalized correlation across synapses, balancing inhibition with excitation. Thus the overall neuronal synaptic weight distribution is monitored on-line, with specific adjustments occurring across multiple inputs to both enhance newly-relevant stimuli while preserving excitability. One-Sentence AbstractHeterosynaptic plasticity can rapidly and specifically balance inhibition with excitation across multiple inputs onto cortical pyramidal neurons.All rights reserved. No reuse allowed without permission.(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/282012 doi: bioRxiv preprint first posted online Mar. 14, 2018; Main TextIn mature cortical networks and elsewhere throughout the adult nervous system, excitation is precisely regulated by a complex set of inhibitory circuits. GABAergic inhibition is important in many features of nervous system function, including spike generation, dendritic integration, synaptic plasticity, sleep, learning and memory, and prevention of pathological activity such as epilepsy (1-5). Consequentially, inhibitory synapses must be carefully calibrated with the relative strengths of excitatory synapses, to ensure that neurons and networks are neither hypo-nor hyperexcitable for prolonged periods. In sensory cortex, this balance between excitation and inhibition seems to be established during early postnatal development (6-11). In particular, frequency tuning curves in the primary auditory cortex (AI) tend to be initially broad or erratic; excitatory inputs mature within the first 1-2 weeks of postnatal life in rodents, but inhibitory tuning requires auditory experience over weeks 2-4 to balance excitation (7,12,13).Excitatory-inhibitory balance must also be dynamically maintained throughout life, as experience-dependent modification of excitatory synapses (e.g., occurring during and after development, learning, or conditioning) then requires corresponding changes to inhibitory inputs (7,8,14). Network simulation studies supported by experimental d...

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