Imaging input and output of neocortical networks in vivo

Abstract

Neural activity manifests itself as complex spatiotemporal activation patterns in cell populations. Even for local neural circuits, a comprehensive description of network activity has been impossible so far. Here we demonstrate that two-photon calcium imaging of bulk-labeled tissue permits dissection of local input and output activities in rat neocortex in vivo. Besides astroglial and neuronal calcium transients, we found spontaneous calcium signals in the neuropil that were tightly correlated to the electrocorticogram. This optical encephalogram (OEG) is shown to represent bulk calcium signals in axonal structures, thus providing a measure of local input activity. Simultaneously, output activity in local neuronal populations could be derived from action potential-evoked calcium transients with single-spike resolution. By using these OEG and spike activity measures, we characterized spontaneous activity during cortical Up states. We found that (i) spiking activity is sparse (<0.1 Hz); (ii) on average, only Ϸ10% of neurons are active during each Up state; (iii) this active subpopulation constantly changes with time; and (iv) spiking activity across the population is evenly distributed throughout the Up-state duration. Furthermore, the number of active neurons directly depended on the amplitude of the OEG, thus optically revealing an input-output function for the local network. We conclude that spontaneous activity in the neocortex is sparse and heterogeneously distributed in space and time across the neuronal population. The dissection of the various signal components in bulk-loaded tissue as demonstrated here will enable further studies of signal flow through cortical networks.bulk loading ͉ population imaging ͉ presynaptic ͉ sparse coding U nderstanding how information is represented and processed in the mammalian neocortex requires measurement not only of single-cell dynamics but also of spatiotemporal activity patterns in identified networks of neurons in vivo. So far, optical imaging of intrinsic or voltage-sensitive dye signals has revealed spatiotemporal dynamics on the scale of cortical columns but has lacked cellular resolution (1). Extracellular recording methods have enabled simultaneous measurements from multiple cells but suffer from poorly defined cell identities, lack of spatial resolution, and are incapable of resolving nonactive neurons (2). These techniques thus fall short on providing a comprehensive description of cortical microcircuits. In particular, these methods cannot monitor the activation of afferent axons that represent the input into a particular local region. Of key importance to a further understanding of information processing in the neocortex will be a method that is capable of simultaneously resolving both input and output of cortical microcircuits with single-cell and single-spike resolution.Here we apply recently developed techniques for two-photon calcium imaging of neocortical cell populations in vivo (3-5) to characterize neocortical activity during Up-and Down-state fluctua...

Delayed plasticity of inhibitory neurons in developing visual cortex

“…The phase of the fluorescence responses lagged behind the contrast modulation of the noise stimulus by 25°. The lag observed in the responses was expected because the somatic calcium signals measured with OGB-1 temporally filter the underlying spike train with an Ϸ1-s kernel (11,12). By using the stochastic noise stimulation technique we were able to obtain significant responses from nearly all cells recorded (99.2%; bootstrapped t test, P Ͻ 0.01).…”

“…We recorded the visual responses of these genetically identified inhibitory neurons together with their excitatory counterparts in vivo by imaging calcium transients in the cell bodies by using two-photon laser scanning microscopy. These somatic calcium signals have been shown to reflect cell spiking activity and not subthreshold signals (10)(11)(12). A recently developed extracellular injection technique has made it possible to load all or nearly all neurons in a 200-to 300-m region of cortex with a cell-permeant version of the calcium indicator OregonGreen BAPTA-1 (OGB-1) (13,14).…”

Conditions for Propagating Synchronous Spiking and Asynchronous Firing Rates in a Cortical Network Model

“…For the network to admit the carrying of temporal information in the form of synchronous spike volleys in embedded FFNs, the background activity must be sufficiently asynchronous and irregular at low enough rates (Ϸ5 spikes/s). Interestingly, spontaneous activity in the mammalian cortex in vivo is known to be in this regime (Abeles, 1991;Matsumura et al, 1996;Kerr et al, 2005;Waters and Helmchen, 2006).…”

“…In Figure 2a-c we show the mean firing rate, synchrony and irregularity, respectively, in the network as a function of the external input ext and the inhibition/excitation balance g. Depending on the combination of ext and g, a significant fraction of the neurons remained silent (Kerr et al, 2005), accordingly the network activity descriptors were estimated only for the spiking neurons. We found that with a small degree of heterogeneity it was possible to considerably reduce global synchrony in the network for low ext .…”

In focus in HCB

“…Although multiphoton confocal microscopy has been used successfully to image more peripheral regions of the brain (Kerr et al 2005), deeper regions remain inaccessible. Iijima et al (2017) have now developed a system consisting of (1) a single optical fiber for recording neuronal activity via eGFP expression (linked to gonadotropin releasing hormone), and (2) a newly designed animal cage whereby the floor rotates in response to head movements, thus minimizing animal movement-derived stress on the optical fiber, for monitoring and imaging via fluorescence structures deep in the brain.…”