Sensory information processing in neocortex is associated with rhythmic synchronized gamma frequency firing of sensory cortical units and similar frequency oscillations of the field potentials. Different aspects of the gamma activity (20-80 Hz) have been suggested as correlates of attention, arousal and sensory binding. It is clear that attention has a modality selective influence, while arousal has a more general effect on the sensory systems. We used an experimental conditioning paradigm to separate these differential effects of attention and arousal on spontaneous neocortical gamma activity. We recorded field potentials with epidural electrodes placed above the auditory cortical areas of cats. The animals performed a simple instrumental alimentary conditioning task with different modality (visual and auditory) conditioned stimuli. When they attended to the auditory conditioned stimulus, both frequency and power increase of spontaneous gamma activity were detected. However when they attended visual, we found no power increase of gamma activity recorded above auditory areas, while the frequency increase was the same as in the "attend auditory" condition. We conclude that the power modulation of gamma activity is modality specific and thus can be attributed to selective attention, whereas the frequency modulation of gamma activity shows no modality specificity, it is influenced by the arousal level.