Recognizing faces is an innate ability in primates; even the youngest infants respond to Mom's face. So, a fascinating and central question in neurobiology is where in the hierarchy of visual processing face recognition takes place.
Through a series of precise experimental manipulations of perception in human subjects, Farshad Moradi and his colleagues have gained new insight into the process. They have found that identifying a face depends on actually seeing it, as opposed to merely having the image of the face fall on the retina.
In one set of experiments, the researchers took advantage of a phenomenon called "binocular rivalry" to present face images to subjects in circumstances under which the retinal input would remain perceptually invisible.
In such binocular rivalry experiments, a different image is presented simultaneously to each eye. Since the visual system can only pay attention to one image at a time, the other remains "invisible"--suppressed from visual awareness. The researchers found that in such experiments the recognition of the face depended on actually perceiving it. In contrast, they found, such lower-level "aftereffects" as recognizing the orientation of the face were not affected by lack of visual awareness.
"Thus, the competition between incompatible or interfering visual inputs to reach awareness is resolved before those aspects of information that are exploited in face identification are processed," wrote the researchers.
In further experiments, they also explored whether face recognition was affected by suppressing awareness of the face image with distracting tasks such as memorizing images or sounds--a phenomenon called "inattentional blindness." The researchers found that the requirement to pay attention to visual distractors, but not auditory distractors, eliminated the face recognition. Thus, they found, since auditory distractors didn't reduce face recognition, such recognition is not affected by competing processing of other sensory stimuli.
Also, the researchers found that asking the subjects to preoccupy themselves by practicing imagining the faces did not interfere with recognition--indicating that face recognition is a function of visual processing, rather than "top-down" cognitive processing.
"In summary, our findings establish a close relationship between configural face adaptation and visual awareness," wrote the scientists. "If you don't see a face, you will not adapt to its identity, even though you may adapt to other aspects of the face, such as orientation or color.
"It appears paradoxical that some aftereffects such as negative afterimage or orientation-dependent aftereffect do not require seeing the inducing stimulus. Contrariwise, the result reported here for the identify aftereffect is more in line with common expectation. Together, these findings provide insight into brain organization and the neural correlates of conscious perception," they wrote.
Farshad Moradi, Christof Koch, and Shinsuke Shimojo: "Face Adaptation Depends on Seeing the Face"
Source: Eurekalert & others
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