Body posture affects tactile discrimination and identification of fingers and hands

It is an unresolved question whether the posture of single fingers relative to each other is represented in the brain within an external frame of reference. In two experiments, we investigated postural influences on the processing of tactile stimuli at fingers and hands. Healthy subjects received two simultaneous tactile stimuli at the fingertips while the fingers of both hands were either interleaved or not. In speeded response tasks, they were asked to discriminate (experiment 1) or to identify (experiment 2) the touched body parts, either regarding hand laterality or finger type. The results demonstrate that both finger discrimination and finger identification are influenced by body posture. We conclude that the assumption of a solely somatotopic representation of fingers is not tenable and that an external reference system must be available for the detection of single fingers. The results are discussed in terms of a mental segmentation of external space, based on body posture and task requirements.