Structural representations of fingers rely on both anatomical and spatial reference frames

Finger agnosia refers to a neurological condition in which patients with left posterior parietal lesions fail to identify their fingers, despite having relatively preserved abilities in sensation and skilled action. This dissociation suggests that the structural body representations (BSRs) may be distinct from sensorimotor representations. However, recent research has reported that postural changes modulate representation of hand structure, revealing dynamic interactions between structural and sensorimotor body representations. However, it is unknown how and to what extent anatomical and spatial proximity contribute to shape the hand structural representation. We investigate this question using the “in-between” test in which participants estimate how many unstimulated fingers are in-between two touched fingers of the left hand placed palm down. The first phalange of the participants’ fingers was touched on the left or right side. Judged finger numerosity was greater when fingers were stimulated on far sides (i.e., opposite sides of the two fingers) compared to when they were stimulated on close (i.e., sides facing each other’s) or mid-distance (i.e., sides facing in the same direction) sides. Therefore, fingers identification was modulated both by anatomical and spatial proximity in external space between touches. This demonstrates that BSRs rely on both anatomical and external reference frames.