Prof. Tang’s group mapped the precise spatiotemporal functional organization of excitatory synaptic inputs onto macaque V1 neurons
Cortical neurons sample from their dendritic synaptic inputs as the basic unit of computation. Whereas in vitro patch-clamp and intracellular recordings have advanced our understanding of how dendritic synaptic integration occurs in single neurons, and in vivo studies have extended those methods to highlight how dendritic activity contributes to cortical functions such as orientation and direction selectivity, sample size and spatial resolution have limited the utility of these methods. High-resolution two-photon calcium imaging of dendrites, in contrast, can achieve long-term functional mapping of individual dendritic inputs in the intact brain. Because the temporal sequence of synaptic inputs has been verified to be of vital importance to dendritic nonlinear integration, high-speed dendritic imaging can provide important new insights into dendritic computational mechanisms. A recently developed genetically encoded glutamate-sensing fluorescent reporter, iGluSnFR, which has a high signal-to-noise ratio (SNR) and fast kinetics, promises to map the spatiotemporal functional organization of dendritic excitatory inputs.
In this study, we performed two-photon dendritic imaging with iGluSnFR in awake macaque monkeys, and obtained fine functional spatiotemporal maps of dendritic excitatory inputs in individual V1 neurons (Fig. 1a). We found a functional integration and trade-off between orientation-selective and color-selective inputs in basal dendrites of individual V1 neurons (Fig. 1b), which presented a direct evidence of dendritic level computations for orientation and color integration mechanisms.
Fig. 1 Excitatory synaptic inputs onto dendrites of macaque V1 neurons. a, The spatial organization of dendritic excitatory inputs on individual V1 neurons. b, Functional integration and trade-off between orientation/color inputs in individual V1 neurons’ basal dendrites.
Synaptic inputs had a propensity to spatially cluster on dendrites with respect to individual visual features, while tending to be tuned across an array of feature dimensions. One local pair of synaptic inputs might share similar orientation preferences while possessing quite dissimilar RFs (Fig. 2a). In contrast, another pair of synaptic inputs might have both different orientations and dissimilar RFs (Fig. 2b), or different orientations and similar RFs. This wide scattering served to maximize the pool of potential matches between dissimilar features within local dendritic branches. As such, it provides a potential computational substrate for multidimensional feature integration at the dendritic level in V1 superficial layer neurons.
Fig. 2 Synaptic inputs are functionally scattered in multidimensional feature space. a, Two ROIs on one dendritic draft share similar orientation preferences while having distinct RFs. b, Another associated ROIs have both different orientation preferences and different RFs.
Furthermore, we found apical dendrite inputs had larger receptive fields and longer response latencies than basal dendrite inputs (Fig. 3), suggesting a dominant role for apical dendrites in integrating feedback in visual information processing. It remains a challenge to directly dissociate feedback vs feedforward inputs, and our results provide indirect evidence for the provenance of the signals. Future studies will aim to obtain direct evidence by combining new techniques, such as optogenetics or electrophysiology, and specific behavioral tasks, such as a top-down modulated selective attention task.
Fig. 3 Comparison of excitatory dendritic inputs on apical dendrites versus those on basal dendrites. a, Apical and basal dendritic shafts of one example neuron. b, Orientation preference and RF of dendritic inputs on AD and BD. c, Summed orientation preference of orientation-selective ROIs on AD and BD of each neuron. d, AD RF size versus BD’s of each neuron. e, Time courses of response on AD versus BD.
Link of the paper：https://www.nature.com/articles/s41467-020-14501-y