Steady-state visually evoked potential correlates of object recognition.

In present high density electroencephalogram (EEG) study, we examined steady-state visual evoked potential (SSVEP) correlates of object recognition. In SSVEP tasks a visual stimulus is presented repetitively at a specific flickering rate and typically elicits a continuous oscillatory brain response. This response is characterized by the same fundamental frequency as the initiating stimulus. The stimulus material consisted of a series of pictures depicting familiar and unfamiliar objects which have been successfully applied in previous EEG studies on object recognition. In particular, we presented familiar and unfamiliar objects at rates of 7.5, 12 and 15Hz. At all three driving frequencies, we found specific SSVEPs that furthermore showed significant amplitude differences between familiar and unfamiliar objects. The familiar/unfamiliar effects were localized to early occipital, lateral occipital and temporal areas by means of VARETA (Variable Resolution Electromagnetic Tomography). Interestingly, the morphology of the familiar/unfamiliar effect differed between flicker rates. The 12 and 15Hz conditions revealed higher SSVEP amplitudes for familiar as opposed to unfamiliar objects, whereas in the 7.5Hz condition the effect was reversed. We concluded that SSVEPs are sensitive to stimuli's semantic content. Thus, SSVEP paradigms open new venues to study object recognition. Nonetheless, selecting appropriate driving frequencies is non-trivial, because flicker rate might have an influence on the observed effects.