We used magnetoencephalography (MEG) to investigate the timing and location of cortical activity related to perceived brightness. Participants passively observed 1 of 5 disks of different luminance (1, 3.2, 10, 32, and 100 cd/m²) during MEG recording, and rated the perceived brightness of the disk before and after the MEG recording. The perceived brightness showed an almost perfect log-linear dependence on luminance intensity. The MEG results showed that the stimulus presentation evoked neuromagnetic responses in the occipital region approximately 150 ms after stimulus onset. The average magnitude of the response was positively correlated with the subjective ratings of perceived brightness as well as the log-scaled stimulus luminance. These findings suggest that the neuromagnetic responses in the occipital cortex reflect subjective brightness perception and that the visual codex completes the brightness assignment as early as 150 ms after stimulus onset. The possible clinical application of these results is discussed.

Key words: brightness, luminance, MEG