Dissociable neural responses to hands and non-hand body parts in human left extrastriate visual cortex

Abstract

Accumulating evidence points to a map of visual regions encoding specific categories of objects. For example, a region in the human extrastriate visual cortex, the extrastriate body area (EBA), has been implicated in the visual processing of bodies and body parts. Although in the monkey, neurons selective for hands have been reported, in humans it is unclear whether areas selective for individual body parts such as the hand exist. Here, we conducted two functional MRI experiments to test for hand-preferring responses in the human extrastriate visual cortex. We found evidence for a hand-preferring region in left lateral occipitotemporal cortex in all 14 participants. This region, located in the lateral occipital sulcus, partially overlapped with left EBA, but could be functionally and anatomically dissociated from it. In experiment 2, we further investigated the functional profile of hand- and body-preferring regions by measuring responses to hands, fingers, feet, assorted body parts (arms, legs, torsos), and non-biological handlike stimuli such as robotic hands. The hand-preferring region responded most strongly to hands, followed by robotic hands, fingers, and feet, whereas its response to assorted body parts did not significantly differ from baseline. By contrast, EBA responded most strongly to body parts, followed by hands and feet, and did not significantly respond to robotic hands or fingers. Together, these results provide evidence for a representation of the hand in extrastriate visual cortex that is distinct from the representation of other body parts.

Fig. 1.

Stimuli, averaged statistical maps and activation levels in experiment 1. A: exemplars of stimuli used in experiment 1: whole bodies, body parts, hands, tools, and chairs. B: group analysis results are shown in the right and left hemisphere of a single subject for the comparison of hands vs. chairs (H > C, shown in white) and for whole body and body parts vs. chairs (WB + BP > C, shown in orange). C–F: average peak activity (percentage bold signal change [%BSC]) for each stimulus category extracted from individual-subject brain areas functionally localized with the comparisons illustrated in B. Error bars represent SEs. G: group analysis results are shown in the right and left hemispheres of a single subject for the more selective comparison of hands vs. whole bodies, body parts, and chairs (H > WB + BP + C, shown in yellow) and for whole bodies and body parts vs. hands and chairs (WB + BP > H + C, shown in red). H–J: average peak activity (%BSC) for each stimulus category in the individual-subject brain areas localized with the comparisons illustrated in G. Error bars represent SEs.

Fig. 2.

Individual statistical maps for hand-responsive and body-responsive voxels in experiment 1. The position of hand-selective voxels localized by comparing hands vs. whole bodies, body parts, and chairs (H > WB + BP + C, highlighted in yellow) and of body-selective voxels (extrastriate body area [EBA]) localized by comparing whole bodies and body parts vs. hands and chairs (WB + BP > H + C highlighted in red) in experiment 1 is shown in the clearest transversal slice of each participant. Averaged statistical maps (bottom right image) for the hand-responsive area within left lateral occipital sulcus (LOS) and the body-responsive area in right and left EBA are overlaid onto the slice of a single participant for clarity. As clearly shown, although the hand-related response in left LOS is mostly lateralized to the left (left LOS = 14/14 participants; right LOS = 6/14 participants), EBA is bilaterally represented (left and right EBA = 14/14). In addition, in most of the participants (11/14) left LOS is localized anterior to left EBA. L, left; R, right.

Fig. 3.

Stimuli and activation levels for experiment 2. A: exemplars of stimuli used in experiment 2: fingers, body parts, hands, feet, and robotic hands. B and C: average time course (%BSC) for each stimulus category in experiment 2 in left LOS, functionally localized in each individual by comparing hands vs. whole bodies, body parts, and chairs (H > WB + BP + C) and left EBA functionally localized in each individual by comparing whole bodies and body parts vs. hands and chairs (WB + BP > H + C) for odd and even runs of experiment 1. Error bars represent SEs.