Single cell responses in cat visual cortex to visual stimulation during iontophoresis of noradrenaline

We studied how iontophoresis of noradrenaline (NA) changes responsiveness of individual cells in the feline visual cortex when their visual receptive fields are stimulated with the appropriate visual stimulus. We found three populations of cortical cells which either increased, decreased or did not change their visual responsiveness during NA iontophoresis. About equal numbers of cells belonged to each of these three groups. In the majority of such cells that changed visual responsiveness during NA iontophoresis and that had measureable amounts of spontaneous activity, the ratio of visually evoked to spontaneous activity (signal-to-noise ratio) improved during NA iontophoresis. This improvement was independent of the direction of changes in the response magnitude to visual stimulation. There was a differential effect of NA on simple and complex visual cortical cells: Although most simple cells (86%) clearly changed their responsiveness during NA iontophoresis, the effects were seen in only one-third of complex cells. Furthermore, the effects on complex cells were usually weak compared to those typically seen in simple cells. In some cases the effects of NA were more complicated than an overall enhancement of suppression of the cortical cell's responses to visual stimulation. The possible dual role of NA in the visual cortex is briefly discussed.