人类大脑无法判断对方性别

　　The identical facial features might be judged male when they are viewed in one part of our field of view, but thought of as female in another location, a recent study suggests.

　　Researchers from the Massachusetts Institute of Technology's McGovern Institute for Brain Research in Boston showed volunteers a series of faces, ranging from highly feminine to extremely masculine, and asked to say if they were male or female. Participants were told to fix their gaze on the centre of a computer screen as the faces flashed at different locations for 50 milliseconds.

　　The patterns were different for different people, the scientists reported. Some individuals judged an androgynous face as female every time it appeared in the upper right corner of the screen, while others saw it as male.

　　"Sampling bias" is the likely explanation, say the scientists. The visual cortex, which processes images, contains cells grouped according to which part of a scene they analyze. Within each group, there is thought to be a relatively small number of neurons devoted to interpreting the gender of faces. The smaller the image, the fewer cells are activated. As a result, cells in one group might be mostly those responding to male faces, while in a different part of the visual cortex female-responding cells dominate.

　　In addition, in the real world the effect is not noticed because there are so many other gender clues, such as hair and clothing. And experiments using computer-generated faces stripped of all other gender-identifying features showed how hard the brain finds it to tell sexes apart.

　　The findings challenge a longstanding belief of neuroscientists - that how the brain sees an object should not depend on where the object is located relative to the observer.