Oscillatory gamma band and slow brain activity evoked by real and imagery musical stimuli

There is growing evidence that oscillatory brain activity is involved in Gestalt recognition processes. However, it is still a question at issue whether this activity is correlated to high-level cognitive top-down processes or to perceptual bottom-up processes [1]. In contrast imaginary events are supposed to be pure top-down processes, which makes them a possibility to differenciate between bottom-up and top-down processes. There are several studies investigating imaginary events in a variety of modalities. In the visual modality Rogers et al. [2] found a late positive component following omissions during a train of stimuli. In an action viewing experiment Hari et al. [3] showed a rebound of oscillatory 20 Hz activity similar to a suppression during action. In visual as well as auditory modality Simson et al. [4, 5] observed late positive components that were topographically similar to P3 components and independent of modality. In an experiment with omissions in a train of tones Raij et al. [6] showed bilateral activiation of auditory cortex and frontal areas. Zatorre et al. [7] and Halpern et al. [8] showed in PET studies the activation of similar areas during the perception and imagination of melodies. These experiments demonstrated the activation of brain areas and processes during real as well as imaginary sensory input. In our experiment we examined the slow evoked and oscillatory induced (non-phase locked) brain activity in gamma ( 20 Hz) frequency range for real and imagined musical and non-musical stimuli. It was hypothesised that the activity during the imagined stimuli should be generated by a top-down process and it was expected to give evidence of the functional role of oscillatory activity in sound processing and representation of auditory objects.