Distinct cortical networks for the detection and identification of human body
Introduction
Identification and recognition of individuals rely to a large extent on visual information about features of faces and bodies and so do judgments on the emotional state and intentions of the respective others. Accordingly, it is argued that there are areas of the cerebral cortex specialized for the processing of body and face related information. These are the EBA and FBA (extrastriate and fusiform body area) and OFA and FFA (occipital and fusiform face area), respectively (Puce et al., 1996, Kanwisher et al., 1997, Downing et al., 2001, Peelen and Downing, 2005, Schwarzlose et al., 2005, Downing et al., 2006, Peelen et al., 2006). These are the EBA and FBA (extrastriate and fusiform body area) and OFA and FFA (occipital and fusiform face area), respectively (Puce et al., 1996, Kanwisher et al., 1997, Downing et al., 2001, Peelen and Downing, 2005, Schwarzlose et al., 2005, Downing et al., 2006, Peelen et al., 2006).
The first localizations of areas in the human visual cortex responding selectively to images of bodies and body parts have been performed with functional Magnetic Resonance Imaging (fMRI) in humans (Downing et al., 2001) and in monkeys (Pinsk et al., 2005, Tsao et al., 2003), and confirmed with additional methods such as Evoked Potentials (EP) (Kovacs et al., 2005, Thierry et al., 2006), Intracranial Recordings (Pourtois et al., 2007) and Transcranial Magnetic Stimulation (TMS) (Urgesi et al., 2004, Urgesi et al., 2007a, Urgesi et al., 2007b). Single-cell recordings in non-human primates also point to a remarkable functional selectivity of neurons for shapes of bodies and body parts in the inferior temporal cortex (Gross et al., 1972, DeSimone et al., 1984, Wachsmuth et al., 1994, Tsao et al., 2006, Kiani et al., 2007).
However, the notion of distinct, category specific cortical regions is still a matter of debate. There is also evidence that the representations of faces and objects in ventral temporal cortex are widely distributed and overlapping (Haxby et al., 2001, Ishai et al., 2000, O'Toole et al., 2005) and that the specialization of the face selective area is dependent on expertise (Gauthier et al., 1999). Thus, local specialization and distributedness coexist and further investigations are needed to unveil the nature of cortical representations.
Faces and bodies constitute a unique object category because they can be further classified according to identity and ownership. Together with gait, gesture and prosody faces and bodies are crucial for the assessment of a person's identity. Visual information extractable from faces and bodies allows for judgments on gender, age, and ethnicity and ultimately the identity of a person. This includes as special case the identification of a body or a face as belonging to oneself or someone else.
The distinction between self and other and the representation of the self rely on numerous sources of information. Proprioceptive and visual signals contribute to the representation of the dynamic body scheme and the awareness of ownership of the body (Jackson and Decety, 2004, Uher et al., 2005, Pellijeff et al., 2006, Shimada et al., 2005, Vogeley et al., 2001). Furthermore, investigations of the sense of agency revealed differential cortical activation patterns for the anticipation, visualization, and observation of one's own movement versus that of others (Chaminade and Decety, 2002, Cunnington et al., 2006, Grezes et al., 2004, Ramnani and Miall, 2004). Because of the prominent role of facial cues for the discrimination of an person's identity the studies on self-representation and self vs. others distinction are based on the distinction between one's own face and that of others (Platek et al., 2006, Uddin et al., 2005, Uddin et al., 2006). In the present study we investigated networks involved in self-other distinction that rely on non-facial cues. To this end we presented images of headless bodies and examined whether self-body-identification is reflected in the differential activation of the cortical areas involved in the encoding of bodies or body parts (EBA and FBA) or whether it involves in addition specific networks devoted to the distinction between self and other in a more general sense.
We examined subjects with event related fMRI while they performed two explicit two-alternative, forced-choice identification tasks: 1. distinction of one's own body from other unfamiliar bodies and 2. distinction of a familiar body from other unfamiliar bodies. This paradigm was used in order to dissociate ‘familiarity’ from the distinction between ‘self’ and ‘other’. In a previous study, based on a block design and passive viewing conditions, we found that cortical areas responding to bodies and self-other distinctions were not restricted to the extrastriate visual cortex (Hodzic et al., 2008). In the current study, we applied a block design to identify body related areas and an event related design for the segregation of areas involved in the self-other distinction, manipulating the identity, familiarity and the viewpoint of the presented body images.
Section snippets
Methods
Ten human volunteers (mean age 27 years, 5 females) with normal or corrected-to-normal vision took part in this study. All participants gave their informed written consent to the procedure in accordance with institutional guidelines and the Helsinki declaration (www.wma.net/e/ethicsunit/helsinki.htm).
For the compilation of the stimulus material subjects wearing dark blue bikinis or swimming suits were photographed in neutral, frontal, upright posture against a non-reflecting gray background.
Behavioural measures
The reaction times (RT) and accuracy of the responses during the identification experiments were as follows: condition ‘self’; RT = 1027 ms, 98% correct, condition ‘familiar other’; RT = 1029 ms, 95% correct, condition ‘other 1’ (other in ‘self’ run); RT = 1032 ms, 98% correct, and condition ‘other 2’ (other in ‘familiar other’ run); RT = 1030 ms, 99% correct (Fig. 1).
Repeated measures ANOVA revealed that the accuracy for the condition ‘familiar other’ was significantly lower than for the condition
Discussion
The goal of this study was to differentiate between cortical areas devoted to the analysis of body related information: 1.) the processing of visual, body specific features (body-detection), 2.) the distinction between familiar and unfamiliar bodies (body-identification) and 3.) the distinction of one's own body (self-identification).
Acknowledgment
We are grateful to S. Weigelt for contributions to experimental design and data analysis.
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