Noradrenergic modulation of cat area 17 neuronal responses to moving visual stimuli.

Orientation and velocity tuning curves were obtained from neurons in cat primary visual cortex before, during and after the iontophoretic application of norepinephrine (NE). While no consistent effects on orientation tuning were observed, the most consistently observed effect of NE on velocity tuning was enhancement of the selectivity for stimulus speed and direction. This was evidenced by a narrowing of velocity tuning curves in 16/28 of the neurons we encountered as well as by an increase in the measured direction index for 13/28 cells. In contrast to NE, acetylcholine and gamma-amino-n-butyric acid (GABA) simply increased and decreased, respectively, the visually-evoked discharges, and in no case altered the shape of the velocity tuning curve. These data suggest that the spatiotemporal tuning of visual cortical neurons may be dependent not only on local interactions between visual afferents and target neurons, but in addition, depend on the activation state of the noradrenergic coeruleo-cortical projection system.