Conscious and preconscious adaptation to rhythmic auditory stimuli: a magnetoencephalographic study of human brain responses

This study was triggered by the experimental evidence that subjects require d to tap in synchrony with a heard rhythm spontaneously time their tapping to variations in rhythm frequency even when these variations are so small t hat they are not consciously detectable. We performed a series of magnetoen cephalographic (MEG) measurements, aimed at investigating whether the respo nse of the auditory cortex discriminates randomly administered series of br ief tones differing from each other only by their interstimulus intervals ( ISI). Moreover, by combining psychophysical measurements, conscious and pre conscious adjustments of tapping to rhythm variations were compared with br ain cortical responses. The ISIs were varied by 2% or 20% from a "central" value of 500 ms. Subjects always consciously detected the 20% ISI changes a nd easily adjusted their tapping accordingly, whereas they never consciousl y detected the 2% ISI changes, even though they always correctly adjusted t heir tapping to them. Analysis of the auditory evoked Fields (AEFs) showed that the intensity of the M100 component decreased with decreasing ISI both for 20% and 2% variations in a statistically significant manner, despite t he fact that the 2% variation was not consciously perceived. The M100 behav ior indicated that connections between auditory and motor cortexes may exis t that are able to use the information on rhythm variations in the stimuli even when these are not consciously identified by the subject. The ability of the auditory cortex to discriminate different time characteristics of th e incoming rhythmic stimuli is discussed in this paper in relation to the t heories regarding the physiology of time perception and discrimination.