In sensory cortex regions, neurons are tuned to specific stimulus features. For example, in the visual cortex, many neurons fire predominantly in response to moving objects of a preferred orientation. However, the characteristics of the synaptic input that cortical neurons receive to generate their output firing pattern remain unclear. Here we report a novel approach for the visualization and functional mapping of sensory inputs to the dendrites of cortical neurons in vivo. By combining high-speed two-photon imaging with electrophysiological recordings, we identify local subthreshold calcium signals that correspond to orientation-specific synaptic inputs. We find that even inputs that share the same orientation preference are widely distributed throughout the dendritic tree. At the same time, inputs of different orientation preference are interspersed, so that adjacent dendritic segments are tuned to distinct orientations. Thus, orientation-tuned neurons can compute their characteristic firing pattern by integrating spatially distributed synaptic inputs coding for multiple stimulus orientations.

The individual functional properties and spatial arrangement of afferent synaptic inputs on dendrites have a critical role in the processing of information by neurons in the mammalian brain. Although recent work has identified visually-evoked local dendritic calcium signals in the rodent visual cortex, sensory-evoked signalling on the level of dendritic spines, corresponding to individual afferent excitatory synapses, remains unexplored. Here we used a new variant of high-resolution two-photon imaging to detect sensory-evoked calcium transients in single dendritic spines of mouse cortical neurons in vivo. Calcium signals evoked by sound stimulation required the activation of NMDA (N-methyl-D-aspartate) receptors. Active spines are widely distributed on basal and apical dendrites and pure-tone stimulation at different frequencies revealed both narrowly and widely tuned spines. Notably, spines tuned for different frequencies were highly interspersed on the same dendrites: even neighbouring spines were mostly tuned to different frequencies. Thus, our results demonstrate that NMDA-receptor-dependent single-spine synaptic inputs to the same dendrite are highly heterogeneous. Furthermore, our study opens the way for in vivo mapping of functionally defined afferent sensory inputs with single-synapse resolution.

Cortical responses to sensory stimuli are modulated by behavioral state. In the primary visual cortex (V1), visual responses of pyramidal neurons increase during locomotion. This response gain was suggested to be mediated through inhibitory neurons, resulting in the disinhibition of pyramidal neurons. Using in vivo two-photon calcium imaging in layers 2/3 and 4 in mouse V1, we reveal that locomotion increases the activity of vasoactive intestinal peptide (VIP), somatostatin (SST) and parvalbumin (PV)-positive interneurons during visual stimulation, challenging the disinhibition model. In darkness, while most VIP and PV neurons remained locomotion responsive, SST and excitatory neurons were largely non-responsive. Context-dependent locomotion responses were found in each cell type, with the highest proportion among SST neurons. These findings establish that modulation of neuronal activity by locomotion is context-dependent and contest the generality of a disinhibitory circuit for gain control of sensory responses by behavioral state.DOI: http://dx.doi.org/10.7554/eLife.14985.001

Eye-opening represents a turning point in the function of the visual cortex. Before eye-opening, the visual cortex is largely devoid of sensory inputs and neuronal activities are generated intrinsically. After eye-opening, the cortex starts to integrate visual information. Here we used in vivo two-photon calcium imaging to explore the developmental changes of the mouse visual cortex by analyzing the ongoing spontaneous activity. We found that before eye-opening, the activity of layer 2/3 neurons consists predominantly of slow wave oscillations. These waves were first detected at postnatal day 8 (P8). Their initial very low frequency (0.01 Hz) gradually increased during development to Ϸ0.5 Hz in adults. Before eye-opening, a large fraction of neurons (>75%) was active during each wave. One day after eye-opening, this dense mode of recruitment changed to a sparse mode with only 36% of active neurons per wave. This was followed by a progressive decrease during the following weeks, reaching 12% of active neurons per wave in adults. The possible role of visual experience for this process of sparsification was investigated by analyzing darkreared mice. We found that sparsification also occurred in these mice, but that the switch from a dense to a sparse activity pattern was delayed by 3-4 days as compared with normally-reared mice. These results reveal a modulatory contribution of visual experience during the first days after eye-opening, but an overall dominating role of intrinsic factors. We propose that the transformation in network activity from dense to sparse is a prerequisite for the changed cortical function at eye-opening.calcium waves ͉ cortical development ͉ mouse ͉ two-photon imaging ͉ up-down states

EMBO reports VOL 13 | NO 8 | 2012 699 review review Dendritic spines arise as small protrusions from the dendritic shaft of various types of neuron and receive inputs from excitatory axons. Ever since dendritic spines were first described in the nineteenth century, questions about their function have spawned many hypotheses. In this review, we introduce understanding of the structural and biochemical properties of dendritic spines with emphasis on components studied with imaging methods. We then explore advances in in vivo imaging methods that are allowing spine activity to be studied in living tissue, from super-resolution techniques to calcium imaging. Finally, we review studies on spine structure and function in vivo. These new results shed light on the development, integration properties and plasticity of spines.