Magnetoencephalographic gamma-band responses to illusory triangles in humans

Kaiser, J., Bühler, M., and Lutzenberger, W.
MEG-Centre, Institute of Medical Psychology and Behavioural Neurobiology, University of Tübingen, Germany

Increased induced gamma-band activity to illusory triangles has been reported in EEG (Tallon-Baudry et al., J. Neurosci. 1996) where the activity was both spectrally and topographically widespread, i.e. it could not be attributed to specific cortical areas. While this is a typical feature of gamma-band responses in EEG, in MEG we have consistently found highly focal increases of activity in the higher gamma-band (50-90 Hz) during different types of auditory processing, suggesting that MEG may be more sensitive to local network synchronisation. Here we present a replication of the study by Tallon-Baudry et al. in MEG. N = 16 subjects had to respond to two consecutive presentations of a curved illusory triangle (targets) in sequences containing also straight illusory (Kanizsa) triangles, real triangles and no-triangle stimuli (with rotated inducer disks) at equal probabilities. Three blocks of 200 stimuli were presented (duration: 0.7 s, variable interstimulus interval between 2-3 s). Induced oscillatory responses were compared between Kanizsa and no-triangle stimuli and between Kanizsa and real triangles using a statistical probability mapping. Kanizsa triangles were distinguished from no-triangles by increased activity around 70 Hz over medial occipital cortex peaking at 240 ms and over bilateral lateral occipital areas at 430 ms after stimulus onset. Kanizsa stimuli differed from real triangles by increased spectral amplitudes at 90 Hz over parieto-occipital cortex between 100-450 ms after stimulus onset. These findings suggest that illusory triangles are encoded by networks both along the visual ventral and dorsal streams. Supported by DFG (SFB 550/C1).