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Visual responses in the cerebral cortex are believed to rely on the geniculate input to the primary visual cortex (V1). Indeed, V1 lesions substantially reduce visual responses throughout the cortex. Visual information enters the cortex also through the superior colliculus (SC), but the function of this input on visual responses in the cortex is less clear. SC lesions affect cortical visual responses less than V1 lesions, and no visual cortical area appears to entirely rely on SC inputs. We show that visual responses in a mouse lateral visual cortical area called the postrhinal cortex are independent of V1 and are abolished upon silencing of the SC. This area outperforms V1 in discriminating moving objects. We thus identify a collicular primary visual cortex that is independent of the geniculo-cortical pathway and is capable of motion discrimination.

The cerebral cortex constructs a coherent representation of the world by integrating distinct features of the sensory environment. Although these features are processed vertically across cortical layers, horizontal projections interconnecting neighbouring cortical domains allow these features to be processed in a context-dependent manner. Despite the wealth of physiological and psychophysical studies addressing the function of horizontal projections, how they coordinate activity among cortical domains remains poorly understood. We addressed this question by selectively activating horizontal projection neurons in mouse somatosensory cortex, and determined how the resulting spatial pattern of excitation and inhibition affects cortical activity. We found that horizontal projections suppress superficial layers while simultaneously activating deeper cortical output layers. This layer-specific modulation does not result from a spatial separation of excitation and inhibition, but from a layer-specific ratio between these two opposing conductances. Through this mechanism, cortical domains exploit horizontal projections to compete for cortical space. Brain circuits: cortical layering A common anatomical feature of sensory cortex in many species is that neurons processing similar features cluster into vertically oriented domains spanning all layers of the cortex. In addition, neurons modulate other neurons located in neighbouring domains through extensive horizontal connections. Using a combination of optogenetic activation and simultaneous electrophysiological recordings from multiple cortical layers, Hillel Adesnik and Massimo Scanziani characterize the cellular basis of these interactions. They find that horizontal projections suppress layers of cortex devoted to processing inputs, but facilitate layers devoted to outputs. This organization may enable neurons in one cortical domain to expand their functional territory. A common anatomical feature of the sensory cortex in many species is that neurons with similar features cluster into vertically orientated domains spanning all layers of the cortex. Moreover, neurons in one domain modulate neurons in neighbouring domains through horizontal connections. A combination of techniques has now been used to show that such horizontal projections suppress layers of cortex devoted to processing inputs, but facilitate layers devoted to outputs.

Using optogenetics to silence the cortex, the authors show that thalamic inputs to layer 4 V1 neurons in anesthetized mice only contribute a third of the total excitation to these cells during presentation of visual stimuli. Moreover, they find that a small offset in the center of ON and OFF receptive subfields accounts for the orientation tuning of thalamic excitation to these cells. Cortical neurons in thalamic recipient layers receive excitation from the thalamus and the cortex. The relative contribution of these two sources of excitation to sensory tuning is poorly understood. We optogenetically silenced the visual cortex of mice to isolate thalamic excitation onto layer 4 neurons during visual stimulation. Thalamic excitation contributed to a third of the total excitation and was organized in spatially offset, yet overlapping, ON and OFF receptive fields. This receptive field structure predicted the orientation tuning of thalamic excitation. Finally, both thalamic and total excitation were similarly tuned to orientation and direction and had the same temporal phase relationship to the visual stimulus. Our results indicate that tuning of thalamic excitation is unlikely to be imparted by direction- or orientation-selective thalamic neurons and that a principal role of cortical circuits is to amplify tuned thalamic excitation.

Detecting the direction of motion of an object is essential for our representation of the visual environment. The visual cortex is one of the main stages in the mammalian nervous system in which the direction of motion may be computed de novo. Experiments and theories indicate that cortical neurons respond selectively to motion direction by combining inputs that provide information about distinct spatial locations with distinct time delays. Despite the importance of this spatiotemporal offset for direction selectivity, its origin and cellular mechanisms are not fully understood. We show that approximately 80 ± 10 thalamic neurons, which respond with distinct time courses to stimuli in distinct locations, excite mouse visual cortical neurons during visual stimulation. The integration of thalamic inputs with the appropriate spatiotemporal offset provides cortical neurons with a primordial bias for direction selectivity. These data show how cortical neurons selectively combine the spatiotemporal response diversity of thalamic neurons to extract fundamental features of the visual world. Direction selectivity emerges de novo in layer 4 neurons of primary visual cortex through the convergence of synaptic inputs from thalamic neurons that respond with distinct time courses to visual stimuli in distinct locations.

Animals sense the environment through pathways that link sensory organs to the brain. In the visual system, these feedforward pathways define the classical feedforward receptive field (ffRF), the area in space in which visual stimuli excite a neuron 1 . The visual system also uses visual context—the visual scene surrounding a stimulus—to predict the content of the stimulus 2 , and accordingly, neurons have been identified that are excited by stimuli outside their ffRF 3 – 8 . However, the mechanisms that generate excitation to stimuli outside the ffRF are unclear. Here we show that feedback projections onto excitatory neurons in the mouse primary visual cortex generate a second receptive field that is driven by stimuli outside the ffRF. The stimulation of this feedback receptive field (fbRF) elicits responses that are slower and are delayed in comparison with those resulting from the stimulation of the ffRF. These responses are preferentially reduced by anaesthesia and by silencing higher visual areas. Feedback inputs from higher visual areas have scattered receptive fields relative to their putative targets in the primary visual cortex, which enables the generation of the fbRF. Neurons with fbRFs are located in cortical layers that receive strong feedback projections and are absent in the main input layer, which is consistent with a laminar processing hierarchy. The observation that large, uniform stimuli—which cover both the fbRF and the ffRF—suppress these responses indicates that the fbRF and the ffRF are mutually antagonistic. Whereas somatostatin-expressing inhibitory neurons are driven by these large stimuli, inhibitory neurons that express parvalbumin and vasoactive intestinal peptide have mutually antagonistic fbRF and ffRF, similar to excitatory neurons. Feedback projections may therefore enable neurons to use context to estimate information that is missing from the ffRF and to report differences in stimulus features across visual space, regardless of whether excitation occurs inside or outside the ffRF. By complementing the ffRF, the fbRF that we identify here could contribute to predictive processing. Feedback projections onto neurons of the mouse primary visual cortex generate a second excitatory receptive field that is driven by stimuli outside of the classical feedforward receptive field, with responses mediated by higher visual areas.

Using a combination of optogenetics, single-cell molecular profiling and paired electrophysiological recordings in the mouse visual cortex, Pfeffer and colleagues derived the connectivity matrix of three major classes of interneurons with their post-synaptic GABAergic targets. This study provides a comprehensive overview of the wiring rules of the inhibition of inhibition in the cortex. Cortical inhibitory neurons contact each other to form a network of inhibitory synaptic connections. Our knowledge of the connectivity pattern underlying this inhibitory network is, however, still incomplete. Here we describe a simple and complementary interaction scheme between three large, molecularly distinct interneuron populations in mouse visual cortex: parvalbumin-expressing interneurons strongly inhibit one another but provide little inhibition to other populations. In contrast, somatostatin-expressing interneurons avoid inhibiting one another yet strongly inhibit all other populations. Finally, vasoactive intestinal peptide–expressing interneurons preferentially inhibit somatostatin-expressing interneurons. This scheme occurs in supragranular and infragranular layers, suggesting that inhibitory networks operate similarly at the input and output of the visual cortex. Thus, as the specificity of connections between excitatory neurons forms the basis for the cortical canonical circuit, the scheme described here outlines a standard connectivity pattern among cortical inhibitory neurons.

Visually guided perceptual decisions involve the sequential activation of a hierarchy of cortical areas. It has been hypothesized that a brief time window of activity in each area is sufficient to enable the decision but direct measurements of this time window are lacking. To address this question, we develop a visual discrimination task in mice that depends on visual cortex and in which we precisely control the time window of visual cortical activity as the animal performs the task at different levels of difficulty. We show that threshold duration of activity in visual cortex enabling perceptual discrimination is between 40 and 80 milliseconds. During this time window the vast majority of neurons discriminating the stimulus fire one or no spikes and less than 16% fire more than two. This result establishes that the firing of the first visually evoked spikes in visual cortex is sufficient to enable a perceptual decision.

How intracortical recurrent circuits in mammalian sensory cortex influence dynamics of sensory representation is not understood. Previous methods could not distinguish the relative contributions of recurrent circuits and thalamic afferents to cortical dynamics. We accomplish this by optogenetically manipulating thalamus and cortex. Over the initial 40 ms of visual stimulation, excitation from recurrent circuits in visual cortex progressively increased to exceed direct thalamocortical excitation. Even when recurrent excitation exceeded thalamic excitation, upon silencing thalamus, sensory-evoked activity in cortex decayed rapidly, with a time constant of 10 ms, which is similar to a neuron's integration time window. In awake mice, this cortical decay function predicted the time-locking of cortical activity to thalamic input at frequencies <15 Hz and attenuation of the cortical response to higher frequencies. Under anesthesia, depression at thalamocortical synapses disrupted the fidelity of sensory transmission. Thus, we determine dynamics intrinsic to cortical recurrent circuits that transform afferent input in time. By optogenetically silencing thalamus, the authors show that visual cortex does not sustain a response without thalamus for more than a few tens of milliseconds. This rapid cortical activity decay predicts the temporal dynamics of sensory activity transmission between thalamus and cortex in awake animals, whereas under anesthesia, the fidelity of thalamo-cortical connection is dominated by the effect of synaptic depression.

After entering the cerebral cortex, sensory information spreads through six different horizontal neuronal layers that are interconnected by vertical axonal projections. It is believed that through these projections layers can influence each other's response to sensory stimuli, but the specific role that each layer has in cortical processing is still poorly understood. Here we show that layer six in the primary visual cortex of the mouse has a crucial role in controlling the gain of visually evoked activity in neurons of the upper layers without changing their tuning to orientation. This gain modulation results from the coordinated action of layer six intracortical projections to superficial layers and deep projections to the thalamus, with a substantial role of the intracortical circuit. This study establishes layer six as a major mediator of cortical gain modulation and suggests that it could be a node through which convergent inputs from several brain areas can regulate the earliest steps of cortical visual processing. Layer six in the mouse primary visual cortex is a major mediator of cortical gain modulation and may be a node through which convergent inputs from several brain areas can regulate the earliest steps of cortical visual processing. Visual processing stacks up The cerebral cortex, which is responsible for perception and other cognitive functions, is composed of multiple distinct layers of cells. Little is known about how individual layers function, but here, Massimo Scanziani and colleagues establish the role of a specific cortical layer in sensory processing. Using optogenetics to selectively drive or suppress layer-six neurons in the mouse visual cortex — a previously impossible manipulation — the authors show that the neurons modulate the size of the response of upper-layer neurons to visual stimuli without changing their selectivity. The authors conclude that layer six plays a part in controlling the gain of visual cortical processing by interacting with other neurons in both the cortex and the thalamus.

Vertebrates can change their behavior upon detection of visual stimuli according to the outcome their actions produce. Such goal-directed behavior involves evolutionary conserved brain structures like the striatum and optic tectum, which receive ascending visual input from the periphery. In mammals, however, these structures also receive descending visual input from visual cortex (VC), via neurons that give rise to cortico-fugal projections. The function of cortico-fugal neurons in visually guided, goal-directed behavior remains unclear. Here, we address the impact of two populations of cortico-fugal neurons in mouse VC in the learning and performance of a visual detection task. We show that the ablation of striatal projecting neurons reduces learning speed, whereas the ablation of superior colliculus projecting neurons does not impact learning but reduces detection sensitivity. This functional dissociation between distinct cortico-fugal neurons in controlling learning speed and detection sensitivity suggests an adaptive contribution of cortico-fugal pathways even in simple goal-directed behavior.