人類大腦無法判斷對方性別
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。英國《每日郵報》25日報道稱，最新研究顯示，同樣一張面孔，從某個角度看上去是男性，而從另一個角度看上去就會變成女性。
來自波士頓麻省理工學院麥戈文腦研究所的研究人員做了這樣一個實驗：他們給受試者展示了一組從高度女性化到極端男性化的人臉照片，並要求受試者據此回答所見對象是男性還是女性。受試者還被告知，這些臉孔照片會在電腦屏幕的不同位置閃現出來並持續50毫秒，此時他們只能將視線落在電腦屏幕中央。
科學家們彙報研究結果時稱，不同的人得出的結論是不一樣的。每當一個較爲中性的面孔從電腦右上角出現時，某些受試者會稱該臉孔屬於一個女人，而另外一些人則稱這是一張男人的臉。
科學家們解釋道，產生這種判斷差異的可能原因是“抽樣偏差”。原來，視覺皮質負責處理圖片信息，其內含有分析不同視角的細胞羣。每一個細胞羣內，只有一小部分神經細胞負責辨別人類面孔的性別信息。而人類所看到的圖片越小，被激活的神經細胞就越少。結果，細胞羣內被激活的大部分是那些同辨別男性面孔相關的神經細胞，而視覺皮質的另一部分被激活的大部分是可辨別女性面孔的神經細胞。
此外，現實中這種視覺差異並不爲人注意的原因在於有太多的性別線索供人類判斷對方性別，比如髮型和衣着。而科學家用電腦將人類面孔所有可判斷性別的外在特徵處理掉後，發現實際上人腦很難判斷對方的性別到底是什麼。
長期以來神經科學家們一直認爲大腦看待物體的方式同觀察視角無關，而該研究結果證明原先的理論並不成立。
