The activity of complex neurones of area 17 was recorded in anaesthetized cats in response to sinusoidal drifting gratings of various orientations, spatial frequencies and contrasts. The responses of complex cells present two different firing patterns: spikes organized in clusters and spikes which do not show this organization ("isolated spikes"). The clustered component is the only one in the complex cell discharge which is tuned for spatial frequency and orientation, while the isolated spike component is correlated with the contrast of the stimulus.