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Scientific Understanding of Consciousness |
Perception, Face Recognition
Science 5 November 2010: Vol. 330. no. 6005, pp. 845 – 851 Functional Compartmentalization and Viewpoint Generalization Within the Macaque Face-Processing System Winrich A. Freiwald1 and Doris Y. Tsao2 1 The Rockefeller University, 1230 York Avenue, New York, NY (paraphrase) Primates can recognize faces across a range of viewing conditions. Representations of individual identity should thus exist that are invariant to accidental image transformations like view direction. We targeted the recently discovered face-processing network of the macaque monkey that consists of six interconnected face-selective regions and recorded from the two middle patches (ML, middle lateral, and MF, middle fundus) and two anterior patches (AL, anterior lateral, and AM, anterior medial). We found that the anatomical position of a face patch was associated with a unique functional identity: Face patches differed qualitatively in how they represented identity across head orientations. Neurons in ML and MF were view-specific; neurons in AL were tuned to identity mirror-symetrically across views, thus achieving partial view invariance; and neurons in AM, the most anterior face patch, achieved almost full view invariance. Primates can recognize faces accurately despite a plethora of transformations in size, position, makeup, illumination, and, perhaps the most drastic in terms of low-level feature characteristics, head orientation. A biological substrate for primate face recognition is likely provided by face-selective cells and by face-selective brain regions, which can be identified by functional magnetic resonance imaging (fMRI) experiments. In macaques, fMRI reveals six discrete face-selective regions, consisting of one posterior face patch [posterior lateral (PL)], two middle face patches [middle lateral (ML) and middle fundus (MF)], and three anterior face patches [anterior fundus (AF), anterior lateral (AL), and anterior medial (AM)], spanning the entire extent of the temporal lobe. Why are there multiple face patches? Answering this question requires understanding the representation of faces in each patch. The six patches form strong, specific connections to each other. This suggests that the representations in each distinct patch are not independent but constitute transformations of each other. In particular, electrical microstimulation in the middle face patches activates both AL and AM. Determining how ML, MF, AL, and AM represent faces was the goal of the current study. The greatest obstacle to object recognition is the huge amount of variation that can occur in the retinal images cast by a 3D object. Our finding of individual-selective responses with a high degree of invariance across head orientations in AM was obtained with an image set containing faces never encountered in real life. Thus, whatever learning has occurred before the experiments, it has resulted in a face representation that allows generalization of selectivity for new faces. The face system may already incorporate all a priori knowable invariances of a bilaterally symmetric 3D object, a face, into a canonical face space, such that for an a posteriorly encountered novel face, a largely invariant response can be generated without necessity for further learning. Although experience with an actual individual is not a necessary condition for representations in AM, such experience may yield an even more invariant representation. Four results provide new insights into the functional organization of the macaque face-patch system. First, all face regions recorded from contained a high degree of face-selective neurons (lower estimates using only the FOB stimulus set: 97% in ML/MF, 86% in AL, and 89% in AM). Extrapolating from these findings, it seems likely that the entire network of temporal lobe face patches constitutes a dedicated brain system for the processing of one high-level object-category, faces. Second, structure and function are highly correlated. A face patch at a particular anatomical location harbors a specific face representation that is qualitatively different from the face representation in a face patch at a different location, and further differences are likely to exist between face representations in the different face patches. Thus, attaching a name to a given face patch is now shown to be meaningful because it signifies functional identity across individuals. Together with earlier results, this shows that the face-processing system is a network composed of multiple, functionally specialized nodes. Third, the system contains one region (AM) that provides for population coding of identity across view conditions, using a hybrid representation of both coarse and sparse elements. Fourth, the finding of an entire face patch, yet only one, containing neurons with mirror-symmetric tuning is worth particular emphasis because it raises the possibility that such a representation constitutes a critical computational step for object recognition. Even though inferotemporal cells with mirror-symmetric tuning have been reported before, this is the first time that such cells have been shown to be agglomerated within a single intermediate node within a form-processing network. Why would this step be useful? One possibility, following earlier work on view-tuned face-selective neurons in the STS, is that this stage serves to extract socially important information from head orientation, because face (and gaze) aversion carries similar social meaning whether for leftward or rightward orientation. Alternatively, such a stage may provide computational advantages for efficient coding. Why are the three view stages of the face-processing system located in separated regions and not next to each other? One possibility is that the face-processing system interdigitates with representations for other objects implementing the same three view stages. The pattern of viewpoint generalization revealed here for faces may thus turn out to be a general organization principle of the entire inferotemporal cortex. (end of paraphrase)
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