Seeing takes time

Weizmann Institute scientists and doctors led by Rafael Malach found that recognizing a visual object coincided with a burst of neural activity. That recognition was not instantaneous, however, but required a minimum amount of time.

The experiments were performed on epileptic volunteers with pre-surgical electrodes implanted in their brains. These subjects were shown a screen shot of a recognizable image, called the ‘target’, consisting of a face, house or other man-made object. At different time intervals after the target image, a meaningless picture, called the ‘mask’ was put on the screen. The researchers could compare the electrical signals from the brain with the subjects’ claims of recognition of the target image.

Malach states:

We found that there was a rapid burst of neural activity occurring in the high-order visual centers of the brain – centers that are sensitive to entire images of objects, such as faces – whenever patients had correctly recognized the target image.

The volunteers were able to recognize the target and have the corresponding burst of neural activity if the target was left on the screen for at least 150 milliseconds.