Cortical neuron response selectivity derives from strength in numbers of synapses

Abstract

Single neocortical neurons are driven by populations of excitatory inputs, forming the basis of neural selectivity to features of sensory input. Excitatory connections are thought to mature during development through activity-dependent Hebbian plasticity 1 , whereby similarity between presynaptic and postsynaptic activity selectively strengthens some synapses and weakens others 2 . Evidence in support of this process ranges from measurements of synaptic ultrastructure to slice and in vivo physiology and imaging studies 3,4,5,6,7,8 . These corroborating lines of evidence lead to the prediction that a small number of strong synaptic inputs drive neural selectivity, while weak synaptic inputs are less correlated with the functional properties of somatic output and act to modulate activity overall 6,7 . Supporting evidence from cortical circuits, however, has been limited to measurements of neighboring, connected cell pairs, raising the question of whether this prediction holds for the full profile of synapses converging onto cortical neurons. Here we measure the strengths of functionally characterized excitatory inputs contacting single pyramidal neurons in ferret primary visual cortex (V1) by combining in vivo two-photon synaptic imaging and post hoc electron microscopy (EM). Using EM reconstruction of individual synapses as a metric of strength, we find no evidence that strong synapses play a predominant role in the selectivity of cortical neuron responses to visual stimuli. Instead, selectivity appears to arise from the total number of synapses activated by different stimuli. Moreover, spatial clustering of co-active inputs, thought to amplify synaptic drive, appears reserved for weaker synapses, further enhancing the contribution of the large number of weak synapses to somatic response. Our results challenge the role of Hebbian mechanisms in shaping neuronal selectivity in cortical circuits, and suggest that selectivity reflects the co-activation of large populations of presynaptic neurons with similar properties and a mixture of strengths.

Synaptic diversity naturally arises from neural decoding of heterogeneous populations

“…inhibitory weights) were not explicitly studied in this work. We focused on excitatory weights as there are now a number of studies reporting the functional diversity of dendritic spines on cortical neurons (Chen et al, 2013, 2011; Iacaruso et al, 2017; Jia et al, 2010, 2011; Kerlin et al, 2019; Scholl et al, 2017, 2019a; Wertz et al, 2015; Wilson et al, 2016, 2018), which are the recipients of excitatory afferents. Few techniques currently exist to access inhibitory populations in vivo (Scholl and Fitzpatrick, 2020).…”

Targeting Functionally Characterized Synaptic Architecture Using Inherent Fiducials and 3D Correlative Microscopy

“…8f, 8g) and correlated to their functional properties. Our workflow allowed for reliable data collection and the number of function-paired spines analyzed was about ten times higher than previous reports [see Scholl et al (2020) for numerical results].…”

“…Viral injections, anesthesia, cranial window preparation, and presentation of visual stimuli for behavioral experimentation in ferrets are described in depth in our previous works (Scholl et al, 2017(Scholl et al, , 2020. Briefly, layer 2/3 neurons of the primary visual cortex of female ferrets (n = 3) were driven to express the calcium indicator GCaMP6s via a Cre-dependent viral expression system.…”

“…Functional imaging of dendritic spines was performed with in vivo 2-photon imaging (Fig. 1a) to characterize visually driven calcium activity of single cortical pyramidal neurons [see Scholl et al (2020) for more details]. After fixation, we sliced the brain parallel to the indention of the cranial window to ensure subsequent imaging was performed in the same plane as the 2-photon microscopy images.…”

Anatomy and activity patterns in a multifunctional motor neuron and its surrounding circuits

“…Nearly a decade ago, back-to-back papers in Nature described two-photon calcium imaging in the mouse visual cortex followed by serial transmission electron microscopy of a small volume of that same cortex (Bock et al, 2011) and two-photon calcium imaging in the mouse retina followed by serial block face electron microscopy of that same retina (Briggman et al, 2011). This approach, "Correlated Light and Electron Microscopy" or CLEM (de Boer et al, 2015), has since allowed the quantification of synaptic connections between functionally identified neurons that control eye movement in the larval zebrafish (Vishwanathan et al, 2017), a reconstuction of the connections in the olfactory bulb of another larval zebrafish in which neural responses to eight different odors had been recorded prior to sectioning (Wanner and Friedrich, 2020), and a critical re-assessment of the mechanisms of selectivity to particular sensory features in neurons of the mammalian visual cortex (Scholl et al, 2019).…”