The coarse mental map of the breast is anchored on the nipple

The sense of touch plays a key role in our experience of our body and our interactions with the world, from the objects we manipulate to the people we touch. While tactile sensibility and its neural basis have been extensively characterized for the glabrous skin of the hand, much less is known about touch on other parts of the body. Despite the important role of the breast in affective touch, mediating the feel of a hug, and in sexual function, as one of the principal erogenous zones, little is known about its sensory properties. To fill this gap, we investigated the spatial acuity of the breast and compared it to that of the hand and back, body regions that span the range of tactile spatial acuity. First, we had subjects judge the relative locations of two consecutive touches on the breast, hand, or back. We found that the tactile acuity of the breast was far lower than that of the hand, and even lower than that of the back, heretofore the paragon of poor acuity. Furthermore, breast acuity decreased with breast size, consistent with the hypothesis that innervation capacity does not scale with body size. Second, we showed that touches to different regions of the nipple were largely indistinguishable, suggesting that the nipple is a sensory unit. Finally, we examined the ability of subjects to report the location of touches on a three-dimensional model of their breast and found localization errors to be systematically biased toward the nipple.