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Scientific Understanding of Consciousness |
Cortical Microcircuits can perform Pattern Completion
Science 12 Aug 2016: Vol. 353, Issue 6300, pp. 691-694 Imprinting and Recalling Cortical Ensembles Luis Carrillo-Reid, et.al. NeuroTechnology Center, Department of Biological Sciences, Columbia University, New York, NY 10027, USA. [paraphrase] Neuronal ensembles are coactive groups of neurons that may represent building blocks of cortical circuits. These ensembles could be formed by Hebbian plasticity, whereby synapses between coactive neurons are strengthened. Here we report that repetitive activation with two-photon optogenetics of neuronal populations from ensembles in the visual cortex of awake mice builds neuronal ensembles that recur spontaneously after being imprinted and do not disrupt preexisting ones. Moreover, imprinted ensembles can be recalled by single- cell stimulation and remain coactive on consecutive days. Our results demonstrate the persistent reconfiguration of cortical circuits by two-photon optogenetics into neuronal ensembles that can perform pattern completion. Cortical ensembles are groups of coactive neurons evoked by sensory stimuli or motor behaviors and likely constitute emergent building blocks of cortical function. In the absence of external inputs, ongoing cortical ensembles resemble sensory evoked ones, as if the cortex has an imprinted representation of the world, implemented by groups of neurons with strong synaptic connectivity. Ensembles could result from Hebbian plasticity, whereby the connectivity between coactive neurons becomes strengthened. Optogenetic studies in which all expressing neurons and their axons are simultaneously photostimulated have demonstrated Hebbian plasticity. However, the artificial generation of cortical ensembles with single-cell resolution has so far been experimentally difficult. To do so, we used simultaneous two-photon calcium imaging and two-photon photostimulation in the primary visual cortex of head-fixed mice running on a treadmill. GCaMP6s signals of layer 2/3 neurons were imaged through a reinforced thinned-skull window, whereas C1V1-expressing neurons were optogenetically stimulated with a second two-photon laser. Two-photon photostimulation of a neuronal population evoked calcium transients reliably in a specific subset of neurons. In vivo electrophysiological recordings demonstrated that population photostimulation evoked action potential bursts independently of the spatial location of the neurons. Neurons responding to direct photostimulation were differentiated from other active neurons and from photostimulation light artifacts by their different temporal responses. This enabled us to distinguish photostimulated cells from those that became active because of the effects of photostimulation on the circuit. Repeated optogenetic stimulation reliably recruited specific groups of neurons, generating an artificial “photoensemble” (i.e., a group of optically activated neurons). To quantify this, we used multidimensional population vectors to analyze population activity and found that photoensembles activate different populations of neurons than visual stimuli do, with only 20.17 ± 9.4% neurons in common. Although the number of ensembles was similar in both experimental conditions, photoensembles activated more neurons than visually evoked ones. Neurons belonging to photoensembles or visual ensembles had a widespread spatial distribution and were spatially intermingled. Visual ensembles remained stable after population photostimulation, which indicates that repetitive photostimulation did not disrupt preexisting cortical ensembles. We noted that some photostimulated cells became coactive spontaneously (see below), as if the artificial photoensemble had been imprinted into the cortex. Moreover, the activation of a single cell was able to recall these imprinted ensembles demonstrating pattern completion. Pattern completion has been described in the hippocampus and is a property of attractor neural networks. Single-cell activation did before population phtostimulation not produce considerable alterations of overall network activity. However, after population photostimulation, photoactivation of selected members (8 ± 2.5%) of the imprinted ensemble consistently recalled an associated group of cells. These recalled ensembles, evoked by single-cell stimulation, did not disrupt the overall network activity and were interspersed in time with ongoing cortical ensembles. Though the number of ensembles during single-cell photostimulation before and after population training remained stable, single-cell photostimulation after population training reliably enabled the recall of a specific group of neurons that was not coactive before; this occurred 64.5 ± 12.63% of the time. The spatial location of neurons in recalled ensembles had a broader distribution than the occasional neurons that were indirectly activated before population training. Also, after population training, the number of calcium transients in nonphotostimulated neurons remained constant whereas it increased in photostimulated neurons, ruling out the possibility that population photostimulation changed the basal level of activity in the whole network. This modification of the functional connectivity between photostimulated neurons required a minimal number of trials, indicating that the observed changes were driven by an alteration in the circuit triggered by repeated photostimulation of a specific population of neurons. To investigate whether imprinted ensembles were persistently integrated in ongoing cortical activity, we imaged the same area on consecutive days. Single-cell photostimulation still enabled the recall of previously imprinted ensembles on consecutive days (fig. S6). Our analysis of ongoing activity from nonphotostimulated and photostimulated neurons showed that imprinted ensembles recurred spontaneously, even on consecutive days and after additional population photostimulation. Although cross-correlations between nonresponsive neurons were not altered, they were increased between photostimulated neurons and remained stable the next day. Thus, optogenetic activation of identified neurons enhanced their local functional connections for at least 1 day. Recalled ensembles shared similar characteristics—such as number of neurons and spatial distribution—with ongoing ensembles, but the mean distance between active neurons was shorter, which indicates that the effect of the photostimulation is local. Recalled ensembles often had neurons that did not belong to ongoing ensembles, demonstrating that recalled ensembles are indeed novel and not just dormant preexisting ensembles. However, given that cortical connections are likely not in a tabula rasa state, we expect that imprinted ensembles may recruit segments of physiologically relevant circuit motifs. Previously, electrical or optogenetic stimulation has been used to show that coactivation of neuronal groups can produce physiologically relevant behaviors. Here, we show the possibility of training individual neurons to build artificial neuronal ensembles, which then become spontaneously active. Our results are consistent with the finding that neurons responding to similar visual stimuli have a higher interconnectivity, as well as with the similarity between visually evoked and spontaneous ensembles. In both cases, recurrent coactivation of a neuronal group would enhance functional connectivity, imprinting ensembles into the circuit. More than 60 years ago, Hebb proposed that repeated coactivation of a group of neurons might create a memory trace through enhancement of synaptic connections. Because of technical limitations, this hypothesis has been difficult to test with single-cell resolution in awake animals. By combining novel imaging and photostimulation techniques and analytical tools, our work can be interpreted as a confirmation of the Hebbian postulate and as a demonstration that To investigate whether imprinted ensembles were persistently integrated in ongoing cortical activity, we imaged the same area on consecutive days. Single-cell photostimulation still enabled the recall of previously imprinted ensembles on consecutive days. Our analysis of ongoing activity from nonphotostimulated and photostimulated neurons showed that imprinted ensembles recurred spontaneously, even on consecutive days and after additional population photostimulation. Although cross-correlations between nonresponsive neurons were not altered, they were increased between photostimulated neurons and remained stable the next day. Thus, optogenetic activation of identified neurons enhanced their local functional connections for at least 1 day. Recalled ensembles shared similar characteristics—such as number of neurons and spatial distribution—with ongoing ensembles, but the mean distance between active neurons was shorter, which indicates that the effect of the photostimulation is local. Recalled ensembles often had neurons that did not belong to ongoing ensembles, demonstrating that recalled ensembles are indeed novel and not just dormant preexisting ensembles. However, given that cortical connections are likely not in a tabula rasa state, we expect that imprinted ensembles may recruit segments of physiologically relevant circuit motifs. Previously, electrical or optogenetic stimulation has been used to show that coactivation of neuronal groups can produce physiologically relevant behaviors. Here, we show the possibility of training individual neurons to build artificial neuronal ensembles, which then become spontaneously active. Our results are consistent with the finding that neurons responding to similar visual stimuli have a higher interconnectivity, as well as with the similarity between visually evoked and spontaneous ensembles. In both cases, recurrent coactivation of a neuronal group would enhance functional connectivity, imprinting ensembles into the circuit. More than 60 years ago, Hebb proposed that repeated coactivation of a group of neurons might create a memory trace through enhancement of synaptic connections. Because of technical limitations, this hypothesis has been difficult to test with single-cell resolution in awake animals. By combining novel imaging and photostimulation techniques and analytical tools, our work can be interpreted as a confirmation of the Hebbian postulate and as a demonstration that cortical microcircuits can perform pattern completion. [end of paraphrase]
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