Psychophysical properties of the trunk midline

This study was carry out to obtain direct evidence that the body midline actually is perceived and to assess some psychophysical properties of this line. Twelve normal, right-handed male subjects were asked to make accurate pointing movements toward the midline of the anterior trunk on the basis of their mental representation of this line. Each hand was used to point while the head was either aligned with the trunk or tilted 30 degrees to the right or left. Analysis of end-positions of pointing on trunk images acquired by an image analysis system indicated that the trunk midline indeed is perceived as a straight line. Three putative trunk midlines were taken into consideration on the basis of anatomic landmarks, and it was found that the mental representation of the trunk midline came nearest to the line orthogonal to the intermammary line crossing its midpoint. The performing hand and the position of the head relative to the trunk both had an effect on the mental representation of the trunk midline. These findings suggest that somatosensory signals from the trunk, as well as proprioceptive input from the neck, contribute to the elaboration of the subject's mental representation of the trunk midline.