PHYSIOLOGICAL-PROPERTIES OF INHIBITORY INTERNEURONS IN CAT STRIATE CORTEX

Physiological and morphological properties of identified interneurons in the striate cortex of the cat were studied in vivo by intracellular recording and staining with biocytin. In conformity with in vitro stu dies, these non-pyramidal fast spiking cells have very brief action po tentials associated with a high rate of fall, and a large hyperpolariz ing afterpotential. These cells show high discharge rates, little or n o spike frequency adaptation in response to depolarizing current injec tion, as well as a diverse range of firing patterns. Three of the cell s were labeled and were found to be aspiny or sparsely spiny basket ce lls, with bitufted or radial dendritic arrangements, in layers II-IV. Their axonal arborizations were more dense near their somata and exten ded horizontally or vertically. Of 13 visually responsive cells tested , the receptive field properties of six cells and the orientation and direction preferences of eight cells were determined. Five of the succ essfully mapped cells had simple receptive fields while one had a comp lex receptive field type. The orientation and direction tuning propert ies of the overlapping set of eight cells showed a broad spectrum rang ing from unselective to tightly tuned. The majority exhibited a clear preference for orientation and none of the cells were clearly directio n selective. Quantitative analysis of the temporal properties of the s pike trains during visual stimulation and spontaneous activity reveale d that these cells do not exhibit any significant periodic activity, a nd fired at rates that were well below their maximum in response to de polarizing current pulses.