Differential Activation to Faces,Degraded Faces,and Their Inverted Counterparts

We investigated electrophysiological correlates of activation to faces, pointillised faces (degraded to a degree still allowing face perception), and their inverted counterparts. Evoked magnetic fields to the four stimulus categories were recorded by 122-channel magnetoencephalography (MEG) in six subjects. Simultaneously recorded 64-channel EEG provided visual evoked potentials in four subjects.

Analysis of the onset, peak latencies, and the signal strength of the face enhanced N170 response indicated the following. (1) In general, the MEG and EEG recordings supported each other. (2) Degrading the images delayed both the onset and peak latencies. Degrading the upright faces increased activity in some subjects, but activity to the degraded inverted faces was lower in all subjects. (3) Inversion delayed the onset latency of both the original and degraded faces by 4 – 10 ms on average. A similar shift was found for the peak latency of the inverted original face images. However, their pointillised counterparts peaked on average 26 ms later when inverted. The signal amplitude also reflected a differential effect of inverting original vs degraded faces; whereas original faces evoked larger responses in all six subjects, the signal to inverted degraded faces decreased in four subjects. The findings suggest that the analysis of both the onset (100 – 120 ms) and peak activity (150 – 170 ms) is important for understanding face processing. The effect of inversion on the evoked responses was found to be different for the original and pointillised faces.