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Scientific Understanding of Consciousness |
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Thalamocortical Signals Selectively Amplified via Recurrent Inputs
Nature 532, 370–374 (21 April 2016) Anatomy and function of an excitatory network in the visual cortex Wei-Chung Allen Lee,et.al. Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA Neuro-Electronics Research Flanders, a research initiative by imec, Vlaams Instituut voor Biotechnologie (VIB) and Katholieke Universiteit (KU) Leuven, 3001 Leuven, Belgium Biomedical Applications Group, Pittsburgh Supercomputing Center, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA Allen Institute for Brain Science, Seattle, Washington 98103, USA [paraphrase] Circuits in the cerebral cortex consist of thousands of neurons connected by millions of synapses. A precise understanding of these local networks requires relating circuit activity with the underlying network structure. For pyramidal cells in superficial mouse visual cortex (V1), a consensus is emerging that neurons with similar visual response properties excite each other, but the anatomical basis of this recurrent synaptic network is unknown. Here we combined physiological imaging and large-scale electron microscopy to study an excitatory network in V1. We found that layer 2/3 neurons organized into subnetworks defined by anatomical connectivity, with more connections within than between groups. More specifically, we found that pyramidal neurons with similar orientation selectivity preferentially formed synapses with each other, despite the fact that axons and dendrites of all orientation selectivities pass near (<5 μm) each other with roughly equal probability. Therefore, we predict that mechanisms of functionally specific connectivity take place at the length scale of spines. Neurons with similar orientation tuning formed larger synapses, potentially enhancing the net effect of synaptic specificity. With the ability to study thousands of connections in a single circuit, functional connectomics is proving a powerful method to uncover the organizational logic of cortical networks. Pyramidal cells in the rodent primary visual cortex (V1) respond to highly specific visual features, resulting in diverse receptive field preferences, in contrast with the less selective responses of most inhibitory neurons. A model is emerging in which V1 responses arise from the selective amplification of thalamocortical signals through recurrent inputs from other pyramidal neurons. Evidence for functionally specific cortical amplification has been seen in physiological and optogenetic studies; however, firm anatomical evidence at the synaptic level has been lacking. A recent study also showed that neurons with similar orientation preference share stronger connections; however, it is unknown if this effect is due to stronger synapses, more synapses, or perhaps spatially clustered synapses. To test these hypotheses, we measured the receptive-field properties and reconstructed the detailed anatomy of the same visual cortical neurons, identifying actual synapses versus axonal-dendritic appositions. We combined in vivo cellular resolution optical imaging with ex vivo electron microscopy (EM) reconstructions. We measured cellular calcium responses, which reflect the firing of action potentials, using the genetically encoded indicator GCaMP3 to characterize the sensory responses of ~300 μm × 300 μm × 200 μm volume of an awake, behaving mouse. Visual stimuli consisted of drifting sinusoidal gratings of different spatial and temporal frequencies, orientations, and directions. In addition to cell bodies in layers 2/3 (L2/3), we measured signals from large calibre apical dendrites that continued beyond the depth of our imaging volume and had branching morphologies consistent with deep layer pyramidal cells. From their responses, we estimated the peak preferred orientation for each cell, with neurons’ visual preferences typically maintained across 12 days of chronic imaging. After locating the functionally imaged region using vascular landmarks, we cut a series of ~3,700 serial EM sections, which were imaged with a transmission electron microscope camera array (TEMCA) at ~4 nm × 4 nm × 40 nm per voxel. The EM-imaged region spanned 450 μm × 450 μm × 150 μm, consisting of ~10 million camera images and ~100 TB of raw data. We traced the processes of excitatory pyramidal neurons located within the middle third of the EM volume (450 μm × 450 μm × 50 μm, using software (CATMAID) allowing distributed annotation of large image data sets. Teams of trained annotators traced and validated wire-frame models of the dendritic and axonal arbors of neurons selected for reconstruction (chosen because they exhibited visual responses), and located all outgoing synapses along the annotated axons. For each synapse, we traced the postsynaptic dendrite centripetally until they reached either the cell body or the boundary of the aligned EM volume. We did not retrogradely trace axons providing input to selected neurons because of the lower probability that they originated from cells in the volume. Henceforth, we limit our discussion to the excitatory network from selected pyramidal cells onto spines of excitatory targets. Functional connectomics promises to build bridges between the in vivo activity of neurons, network connectivity, and neuronal structure. Here, we used this approach to demonstrate five aspects of the excitatory cortical circuit. Independent of physiology, we demonstrate (1) a modular network organization that previously had only been inferred indirectly from smaller cortical subnetworks, and (2) that multiple synapses between pairs of neurons occur far above chance levels, often closely spaced on the dendrite (<20 μm), although there seems to be no specific mechanism favouring local clustering over widespread spacing. Further, we demonstrate (3) an anatomical substrate of functionally specific connections between neurons, and (4) that this specificity does not result from the spatial arrangement of the neuropil, but instead must operate at the scale of dendritic spines (<1–5 μm). Finally, we show (5) that synapse size correlates with physiology, with larger synapses found between neurons with similar peak orientations. Such specific connectivity is consistent with intracortical amplification of afferent signals, overcoming the strong inhibitory tone in the awake cortex. As methods for automated reconstruction improve, each of these findings will come into greater focus, leading to a richer understanding of network structure and function. [end of paraphrase]
Return to — Neural Network |
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Thalamocortical Signals Selectively Amplified via Recurrent Inputs
Nature 532, 370–374 (21 April 2016) Anatomy and function of an excitatory network in the visual cortex Wei-Chung Allen Lee,et.al. Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA Neuro-Electronics Research Flanders, a research initiative by imec, Vlaams Instituut voor Biotechnologie (VIB) and Katholieke Universiteit (KU) Leuven, 3001 Leuven, Belgium Biomedical Applications Group, Pittsburgh Supercomputing Center, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA Allen Institute for Brain Science, Seattle, Washington 98103, USA [paraphrase] Circuits in the cerebral cortex consist of thousands of neurons connected by millions of synapses. A precise understanding of these local networks requires relating circuit activity with the underlying network structure. For pyramidal cells in superficial mouse visual cortex (V1), a consensus is emerging that neurons with similar visual response properties excite each other, but the anatomical basis of this recurrent synaptic network is unknown. Here we combined physiological imaging and large-scale electron microscopy to study an excitatory network in V1. We found that layer 2/3 neurons organized into subnetworks defined by anatomical connectivity, with more connections within than between groups. More specifically, we found that pyramidal neurons with similar orientation selectivity preferentially formed synapses with each other, despite the fact that axons and dendrites of all orientation selectivities pass near (<5 μm) each other with roughly equal probability. Therefore, we predict that mechanisms of functionally specific connectivity take place at the length scale of spines. Neurons with similar orientation tuning formed larger synapses, potentially enhancing the net effect of synaptic specificity. With the ability to study thousands of connections in a single circuit, functional connectomics is proving a powerful method to uncover the organizational logic of cortical networks. Pyramidal cells in the rodent primary visual cortex (V1) respond to highly specific visual features, resulting in diverse receptive field preferences, in contrast with the less selective responses of most inhibitory neurons. A model is emerging in which V1 responses arise from the selective amplification of thalamocortical signals through recurrent inputs from other pyramidal neurons. Evidence for functionally specific cortical amplification has been seen in physiological and optogenetic studies; however, firm anatomical evidence at the synaptic level has been lacking. A recent study also showed that neurons with similar orientation preference share stronger connections; however, it is unknown if this effect is due to stronger synapses, more synapses, or perhaps spatially clustered synapses. To test these hypotheses, we measured the receptive-field properties and reconstructed the detailed anatomy of the same visual cortical neurons, identifying actual synapses versus axonal-dendritic appositions. We combined in vivo cellular resolution optical imaging with ex vivo electron microscopy (EM) reconstructions. We measured cellular calcium responses, which reflect the firing of action potentials, using the genetically encoded indicator GCaMP3 to characterize the sensory responses of ~300 μm × 300 μm × 200 μm volume of an awake, behaving mouse. Visual stimuli consisted of drifting sinusoidal gratings of different spatial and temporal frequencies, orientations, and directions. In addition to cell bodies in layers 2/3 (L2/3), we measured signals from large calibre apical dendrites that continued beyond the depth of our imaging volume and had branching morphologies consistent with deep layer pyramidal cells. From their responses, we estimated the peak preferred orientation for each cell, with neurons’ visual preferences typically maintained across 12 days of chronic imaging. After locating the functionally imaged region using vascular landmarks, we cut a series of ~3,700 serial EM sections, which were imaged with a transmission electron microscope camera array (TEMCA) at ~4 nm × 4 nm × 40 nm per voxel. The EM-imaged region spanned 450 μm × 450 μm × 150 μm, consisting of ~10 million camera images and ~100 TB of raw data. We traced the processes of excitatory pyramidal neurons located within the middle third of the EM volume (450 μm × 450 μm × 50 μm), using software (CATMAID) allowing distributed annotation of large image data sets. Teams of trained annotators traced and validated wire-frame models of the dendritic and axonal arbors of neurons selected for reconstruction (chosen because they exhibited visual responses), and located all outgoing synapses along the annotated axons. For each synapse, we traced the postsynaptic dendrite centripetally until they reached either the cell body or the boundary of the aligned EM volume. We did not retrogradely trace axons providing input to selected neurons because of the lower probability that they originated from cells in the volume. Henceforth, we limit our discussion to the excitatory network from selected pyramidal cells onto spines of excitatory targets. Functional connectomics promises to build bridges between the in vivo activity of neurons, network connectivity, and neuronal structure. Here, we used this approach to demonstrate five aspects of the excitatory cortical circuit. Independent of physiology, we demonstrate (1) a modular network organization that previously had only been inferred indirectly from smaller cortical subnetworks, and (2) that multiple synapses between pairs of neurons occur far above chance levels, often closely spaced on the dendrite (<20 μm), although there seems to be no specific mechanism favouring local clustering over widespread spacing. Further, we demonstrate (3) an anatomical substrate of functionally specific connections between neurons, and (4) that this specificity does not result from the spatial arrangement of the neuropil, but instead must operate at the scale of dendritic spines (<1–5 μm). Finally, we show (5) that synapse size correlates with physiology, with larger synapses found between neurons with similar peak orientations. Such specific connectivity is consistent with intracortical amplification of afferent signals, overcoming the strong inhibitory tone in the awake cortex. As methods for automated reconstruction improve, each of these findings will come into greater focus, leading to a richer understanding of network structure and function. [end of paraphrase]
Return to — Neural Network Return to — Thalamocortical system Return to — Reentry and Recursion Return to — Plasticity of Neural Connections Return to — Vision |