We mapped the visual response along 104 interneuron dendrites (total imaged length 6.3 mm) with regions of interest placed every micrometer along the dendrite (Fig. 6d). Visually-evoked GCaMP6s responses were detected in 5.7 mm (90 %) of dendritic length, with 1.79 mm (28%) showing significant orientation-tuning (p < 0.01, ANOVA across 8 conditions). The majority of imaged dendrites (33/54; longer than 50 μm) had multiple domains preferring different orientations. The OSI (0.44 ± 0.15; mean ± s.d.) of tuned dendritic segments was higher than for interneuron somata (p < 0.005, Supplementary Fig. 12), but lower than for typical pyramidal neurons (p < 0.001, Supplementary Fig. 9). Back-propagation44 of broadly tuned somatic action potentials probably lowers the OSI of the dendritic calcium signals. Consistent with this, both the dendritic OSI and the percentage of orientation selective sites increased with distance from the soma (Supplementary Fig. 13a).
