Do stimuli or tasks determine lateralized auditory cortex responses? An MEG study

We studied hemispheric differences of the evoked magnetic fields induced by frequency modulated tones (FMs) and pure sinusoidal tones, by means of magnetoencephalography (MEG). Seven subjects were exposed to three conditions: 1) passive listening to sinusoidal 1 kHz tones (sine-exposure condition), 2) passive listening to linear FMs (FM-exposure condition), and 3) directional categorization (up versus down) of FMs (FMtask condition). The underlying question was whether different stimuli determine the hemispheric activation of auditory cortex, or rather the task performed by the subjects. With binaural presentation we observed an N100m-latency difference between hemispheres for FMs and a smaller difference for sinusoidal tones. The righthemispheric response to FMs was faster than the left one, both during task end exposure. We also observed different frequency content of the raw (non-averaged) data recorded for the three different conditions. Specifically, the FM-task condition produced a distinct spectral content compared to the two exposure conditions with tones and FMs, which induced a similar spectral pattern. Keywords—Directional categorization task, evoked magnetic fields, frequency modulated tones, magnetoencephalography