Neural mechanisms of visual backward masking revealed by high temporal resolution imaging of human brain

Backward masking is one of the potent ways to reveal the neural mechanism of visual awareness in humans. Although previous neuroimaging studies have reported that the visual masking involves the attenuation of hemodynamic signals to the masked stimulus in visual ventral regions such as the fusiform and inferior temporal gyrus, the temporal profiles of this attenuation as a whole neural population is mostly unclear. Here we used magnetoencephalography and investigated the neural response changes in higher visual region induced by backward masking. The combination of our previous random dot blinking method with the sensor-based analysis isolated the neural responses in the higher visual cortex relating to shape perception. The results revealed that, as the visibility of the target stimulus was reduced by the mask following it, the neural response to the target in the ventral regions showed gradual decreases both in its peak amplitude and peak latency. Furthermore, this decrease in the peak amplitudes was significantly correlated with the behavioral accuracy of the target identification, while the peak latency was not. These results indicate that backward masking simultaneously produces two types of neural changes in higher visual regions: attenuation of the populational neural activity itself and temporal interruption of this activity by the subsequent mask response. Especially, our data suggest that the response attenuation in higher visual response is a main cause of the perceptual impairment observed in the backward masking paradigm.