Extended Data Fig. 4: Activating local populations: opsin expression and light responses.

a. Widefield image of the visual cortex in Mouse I (see Fig. 3), showing the pattern of blood vessels (gray), the expression of ChRmine-mScarlet (red) and the location of the 3 probe insertions (black symbols). The asterisk marks the probe insertion shown in the subsequent panels. b. Example coronal section obtained with serial section two-photon tomography after perfusion. This image shows the green channel, revealing the track of the Neuropixels Opto probe (arrows), which had been immersed in DiO before insertion. The background expression in the hippocampus is due to autofluorescence. c. The same coronal section, viewed in the red channel, reveals expression of ChRmine-mScarlet in a patch of cerebral cortex but not in the underlying hippocampus. The DiO marking the probe track results in a faint additional red signal, particularly visible in the hippocampus (arrows). d. Superposition of the previous two images, showing the probe track in green and opsin expression in red. e. Example spike waveform from a unit that passed quality control, showing the spike shape recorded at 10 × 2 sites at baseline (1 s before emitter onset, left), during 400 ms emitter stimulation (middle), and during 400 ms external surface illumination (right). Each trace is the average of 100 spike waveforms. Shaded area (hardly visible) shows ± 1 s.e. f-j. Five more units stimulated by the same emitter (#11). These units appear in Fig. 3b, c.