Visual Information Present in Infragranular Layers of Mouse Auditory Cortex

The cerebral cortex is a major hub for the convergence and integration of signals from across the sensory modalities; sensory cortices, including primary regions, are no exception. Here we show that visual stimuli influence neural firing in the auditory cortex of awake male and female mice, using multisite probes to sample single units across multiple cortical layers. We demonstrate that visual stimuli influence firing in both primary and secondary auditory cortex. We then determine the laminar location of recording sites through electrode track tracing with fluorescent dye and optogenetic identification using layer-specific markers. Spiking responses to visual stimulation occur deep in auditory cortex and are particularly prominent in layer 6. Visual modulation of firing rate occurs more frequently at areas with secondary-like auditory responses than those with primary-like responses. Auditory cortical responses to drifting visual gratings are not orientation-tuned, unlike visual cortex responses. The deepest cortical layers thus appear to be an important locus for cross-modal integration in auditory cortex.SIGNIFICANCE STATEMENT The deepest layers of the auditory cortex are often considered its most enigmatic, possessing a wide range of cell morphologies and atypical sensory responses. Here we show that, in mouse auditory cortex, these layers represent a locus of cross-modal convergence, containing many units responsive to visual stimuli. Our results suggest that this visual signal conveys the presence and timing of a stimulus rather than specifics about that stimulus, such as its orientation. These results shed light on both how and what types of cross-modal information is integrated at the earliest stages of sensory cortical processing

Visual Information Present in Infragranular Layers of Mouse Auditory Cortex

The cerebral cortex is a major hub for the convergence and integration of signals from across the sensory modalities; sensory cortices, including primary regions, are no exception. Here we show that visual stimuli influence neural firing in the auditory cortex of awake male and female mice, using multisite probes to sample single units across multiple cortical layers. We demonstrate that visual stimuli influence firing in both primary and secondary auditory cortex. We then determine the laminar location of recording sites through electrode track tracing with fluorescent dye and optogenetic identification using layer-specific markers. Spiking responses to visual stimulation occur deep in auditory cortex and are particularly prominent in layer 6. Visual modulation of firing rate occurs more frequently at areas with secondary-like auditory responses than those with primary-like responses. Auditory cortical responses to drifting visual gratings are not orientation-tuned, unlike visual cortex responses. The deepest cortical layers thus appear to be an important locus for cross-modal integration in auditory cortex.SIGNIFICANCE STATEMENT The deepest layers of the auditory cortex are often considered its most enigmatic, possessing a wide range of cell morphologies and atypical sensory responses. Here we show that, in mouse auditory cortex, these layers represent a locus of cross-modal convergence, containing many units responsive to visual stimuli. Our results suggest that this visual signal conveys the presence and timing of a stimulus rather than specifics about that stimulus, such as its orientation. These results shed light on both how and what types of cross-modal information is integrated at the earliest stages of sensory cortical processing

Cortical Interneurons Differentially Regulate the Effects of Acoustic Context

Both behavioral and neural responses to sounds are generally modified by the acoustic context in which they are encountered. As an example, in the auditory cortex, preceding sounds can powerfully suppress responses to later, spectrally similar sounds—a phenomenon called forward suppression (FWS). Whether cortical inhibitory networks shape such suppression or whether it is wholly regulated by common mechanisms such as synaptic depression or spike frequency adaptation is controversial. Here, we show that optogenetically suppressing somatostatin-positive (Sst+) interneurons weakens forward suppression, often revealing facilitation in neurons that are normally forward-suppressed. In contrast, inactivating parvalbumin-positive (Pvalb+) interneurons strengthens forward suppression and alters its frequency dependence. In a simple network model, we show that these effects can be accounted for by differences in short-term synaptic dynamics of inputs onto Pvalb+ and Sst+ interneurons. These results demonstrate separate roles for somatostatin and parvalbumin interneurons in regulating the context dependence of auditory processing

Cortical Interneurons Differentially Regulate the Effects of Acoustic Context

Both behavioral and neural responses to sounds are generally modified by the acoustic context in which they are encountered. As an example, in the auditory cortex, preceding sounds can powerfully suppress responses to later, spectrally similar sounds—a phenomenon called forward suppression (FWS). Whether cortical inhibitory networks shape such suppression or whether it is wholly regulated by common mechanisms such as synaptic depression or spike frequency adaptation is controversial. Here, we show that optogenetically suppressing somatostatin-positive (Sst+) interneurons weakens forward suppression, often revealing facilitation in neurons that are normally forward-suppressed. In contrast, inactivating parvalbumin-positive (Pvalb+) interneurons strengthens forward suppression and alters its frequency dependence. In a simple network model, we show that these effects can be accounted for by differences in short-term synaptic dynamics of inputs onto Pvalb+ and Sst+ interneurons. These results demonstrate separate roles for somatostatin and parvalbumin interneurons in regulating the context dependence of auditory processing

Inhibitory Actions Unified by Network Integration

Cortical function is regulated by a strikingly diverse array of local-circuit inhibitory neurons. We evaluated how optogenetically activating somatostatin- and parvalbumin-positive interneurons subtractively or divisively suppressed auditory cortical cells' responses to tones. In both awake and anesthetized animals, we found that activating either family of interneurons produced mixtures of divisive and subtractive effects and that simultaneously recorded neurons were often suppressed in qualitatively different ways. A simple network model shows that threshold nonlinearities can interact with network activity to transform subtractive inhibition of neurons into divisive inhibition of networks, or vice versa. Varying threshold and the strength of suppression of a model neuron could determine whether the effect of inhibition appeared divisive, subtractive, or both. We conclude that the characteristics of response inhibition specific to a single interneuron type can be "masked" by the network configuration and cellular properties of the network in which they are embedded