Illusory contours and specific regions of human extrastriate cortex: evidence from rTMS

Functional magnetic resonance imaging studies showed that perception of illusory contours is associated with extrastriate cortex activation prevailing on the right side. 1 Hz repetitive transcranial magnetic stimulation (rTMS) is able to induce lasting inhibition of cortical activity. The objective of the study was to investigate the role of extrastriate cortex in illusory contour perception inducing 1 Hz rTMS interference in healthy subjects. Eight healthy subjects underwent 1 Hz rTMS (600 pulses) through a figure-of-eight coil over right and left occipital cortex (O1 and O2 of 10/20 EEG system); sham magnetic stimulation on the same sites and right motor cortex rTMS (in three subjects) were given as control. Subjects performed a computerized task requiring perception of illusory and real contours of Kanizsa squares in baseline and after rTMS. After stimulus presentation the subject made a forced-choice decision about the regularity or irregularity of stimulus contour, by hitting as fast as possible one of two keys on the computer keyboard. Reaction times (RT) were measured. Right occipital stimulation significantly increased RT for illusory contour perception (vs. baseline, P < 0.05). No significant RT changes were observed in the other experimental conditions with respect to the baseline condition. It is concluded that 1Hz rTMS of right extrastriate cortex can disrupt perception of illusory contours and the effect appears to be side-specific, being evident only after right occipital stimulation. This study supports the critical role of right extrastriate cortex in illusory contour perception.