APICAL DENDRITES DRIVE SURROUND RESPONSES IN THE VISUAL CORTEX

Pyramidal neurons possess a prominent apical tuft, which is thought to modulate somatic output based on contextual surround signals in the visual cortex. It remains unclear, however, what sensory inputs the tuft receives and how they shape visual selectivity and size tuning, the latter being a proxy of contextual modulation. To causally determine how the apical dendrite modulates somatic output, we recorded responses of layer-5 pyramidal neurons to drifting gratings of increasing sizes in awake mice’s primary visual cortex. Neurons were semi-sparsely labelled with GCaMP7s, and responses were measured before and after pruning the apical tuft using two-photon dendrotomy. For both pruned neurons (32 neurons in 5 mice) and within-field-of-view controls (42 neurons in 5 mice), we quantified receptive fields, orientation and direction selectivity; we also classified neurons as preferring small or large-sized gratings. Across neurons, pruning only mildly reduced responses to gratings, without significantly affecting orientation or direction selectivity. Surprisingly, while responses of small-preferring neurons remained stable, apical pruning significantly reduced responses of neurons preferring large stimuli (p < 0.05). In a separate cohort, imaging of apical synapses with iGluSnFRs revealed that large-preferring neurons integrated excitatory inputs covering a larger retinotopic field than small-preferring neurons. Individual synaptic RF centres were also more dispersed, producing a larger centroid offset relative to the soma. Together, these results indicate that the apical tuft provides a net excitatory contribution that supports integration over a large visual space, rather than mediating surround suppression or direction or orientation selectivity.