(a) Basket cells were identified by their axon forming characteristic basket-like terminals onto the somata of nearby neurons. Light microscopic reconstruction of a fast-spiking interneuron in 50 μm-thick section (red, soma and dendrites; green, axon). Insets: Confocal fluorescence microscope images (Cy3, from another section of the same cell) show axon boutons arranged around unlabeled L2/3 cell somata (somata indicated by asterisks). Scale 10 μm. Axon boutons were confirmed immunopositive for parvalbumin in 28 of 39 successfully visualised basket cells (in 11 basket cells pv immunoreaction was unsuccessful or non-conclusive) (Supplementary file 1). (b) Basket cells show rapid action potential inward current kinetics compared to most other interneurons. (i) Action potential escape current trace (induced by a depolarizing step from Em in a voltage clamp) showing the inward current component in a parvalbumin immunopositive basket cell (pvBC). The inward current is slower in a nonFSIN interneuron immunonegative for parvalbumin (nonFSIN). Arrows indicate the action inward current width (black arrow, onset. Red and blue arrows, inward current recovers to the onset level). (ii) Histograms showing action inward current width in all basket cells (red, n = 64 including the supplementary data cells) and in nonFSINs (blue, n = 22) (bin 50 μs). Red and blue bars indicate anatomically identified basket cells and nonFSINs, respectively. Black bars indicate unsuccessfully visualized putative basket cells and a nonFSIN. (iii) Cumulative histogram of same data (D = 0.922, p=0.0001, two-sample Kolmogorov-Smirnov test). (c) Fast-spiking axo-axonic cells (AACs) were identified and excluded. Systematic visualization of recorded neurons allowed anatomical identification of occasional AACs and their exclusion from fast-spiking cell data. (i) AAC action potential inward current is fast (width from black to green arrow). (ii) Light microscopic reconstruction of one AAC in a 50 μm-thick section (green, soma and dendrites; black, axon). Note vertical axon terminal structures (some indicated by red arrows) characteristic of AAC. (iii) Confocal fluorescence images show biocytin-filled axon terminals (Cy3) of one recorded AAC forming bouton cartridges in close apposition to L2/3 pyramidal cell initial segments. Initial segments were visualized by anti-ankyrinG immunoreaction (Alexa 488) in 3 AACs (see Methods). In line with a previous animal study (Tamás et al., 1997), the human AACs also failed to show evidence (functional, n = 3; anatomical as close appositions, n = 5) for autapses (n = 5 AACs). It suggests that the seven unsuccessfully visualized fast-spiking interneurons with autaptic GABAergic transmission are likely to be basket cells (see Supplementary file 1).