We first measured each participant’s ability to judge the relative position of two touches applied in succession at two nearby locations on the skin of the hand, back, and breast using a punctate probe. The first of the two touches (the reference) was at the same position on each trial, and the second touch (the comparison) was either above or below the first at a pre-specified distance. For the hand, distances ranged from 1 to 10 mm; for the other body regions, distances ranged from 2.5 to 40 mm, anticipating lower precision based on preliminary testing (Figure 1). The participant’s task was to report whether the test stimulus was located above or below the reference stimulus. We then assessed the participant’s performance as a function of the distance between the test and reference and characterized the distance from the reference required to reliably locate the test stimulus – the just noticeable difference (JND) – for each participant and region.

As expected, the hand demonstrated significantly greater spatial precision (median [25th, 75th] percentiles: 2.16 [1.73, 2.93] mm) than either the back (5.72 [4.36, 8.16] mm) or either breast region (6.63 [5.41, 7.73] mm and 8.00 [5.72, 10.80] mm, for the lateral and medial breast, respectively) as indicated by the lower JNDs (Figure 1A, B and 1-way ANOVA: F_[3,126]=22.7, p<0.01; Tukey’s HSD post-hoc t-test: p<0.01). Interestingly, the medial breast yielded even shallower functions and lower spatial precision than did the back (Figure 1A, B and one-way ANOVA with Tukey’s HSD post-hoc t-test: p=0.0284), although no statistical differences were observed between the lateral breast and either the back or medial breast (p=0.3974 and 0.7377, respectively). In other words, touches needed to be between three and four times as far apart on the breast than on the hand to yield equivalent location discrimination performance.

The spatial precision of the hand has been shown to depend on the size of the hand, with smaller hands yielding better precision (Peters et al., 2009). With this observation in mind, we investigated whether the inter-participant differences in breast precision might be driven in part by differences in breast size. To determine the relative expansion (increase in surface area) of each participant’s breast, we computed the difference between their bust (circumference of the torso at the nipple line) and underbust (circumference of the torso at the inframammary fold). Comparing this value with their spatial precision, we found that the precision of the lateral breast decreased (JNDs increased) as breast size increased (Figure 1C, Pearson’s correlation with Bonferroni PHC (n=4): r=0.734, p<0.01), a phenomenon that did not extend to the medial breast (Figure 1C), hand, or back (Figure 1—figure supplement 1, p>0.05 for all), implying that any expansion is preferentially limited to the skin of the lateral breast and that the observed relationship between breast size and precision was not spurious.

While the result that the medial breast has the lowest spatial resolution is not necessarily surprising given the lack of mechanoreceptors near the surface of the skin (Gutiérrez-Villanueva et al., 2020), the magnitude of the correlation was unexpected. Indeed, we observed that the JNDs for each participant were, on average, only marginally smaller than the measured diameter of the nipple (80% ± 52%). This would imply that two points on opposite halves of the nipple (80% diameter apart) might be confused as the same point. Given that the NAC is inherently a heterogeneous structure as it is composed of three anatomically distinct regions of the breast (the nipple, the areola, and the nearby outer breast), we reasoned that this approach may be biased and thus not reflect the functional ability to locate sensations on the breast.

To assess if participants could reliably perceive tactile events at the level of either the areola or nipple, we delivered punctate touches to each quadrant on the nipple or areola and the participant reported the quadrant in which the touch had been delivered (Figure 2A). Specifically, we marked the edge of the nipple or the 50% point between the center of the nipple and the edge of the areola in each of the cardinal directions such that the proportional distance was constant across participants. When touching different quadrants of the areola, participants were able to reliably locate stimuli (Figure 2B, mean ± standard deviation: 55% ± 21% where chance is 25%, one-sided Monte-Carlo test, Z=15, p<0.01), with 8 of the 10 participants performing significantly above chance (p<0.05 after Bonferroni PHC where n=10). Of the remaining participants, one performed marginally above chance (40%, p=0.25 after Bonferroni PHC), whereas the other systematically misperceived the location and performed well below chance (7.5%, p=1 after Bonferroni PHC). On the nipple, however, participants were consistently worse at locating stimuli than the areola (Wilcoxon signed-rank test, p=0.0137) where only three of the ten participants outperformed chance, although the group as a whole outperformed chance (Figure 2B, 36% ± 13%; Z=5.5, p<0.01). This finding suggests that the ability to locate tactile events is significantly worse at the nipple than at the areola, consistent with equivalent innervation and the smaller size of the nipple, and reaffirms our earlier finding that spatial precision at the medial breast is poor. Finally, to determine if this task was also influenced by spatial expansion, we sought to compare the breast size with performance on the quadrant task. Unfortunately, only four of the ten participants returned for measurement and while negative trends were observed, the sample size is insufficient for formal analysis.

In the experiments described above, we investigated the participants’ ability to distinguish the relative locations of two touches to the breast. Next, we examined their ability to identify the absolute location of a tactile stimulus on their torso. To this end, we presented a single touch to the breast or the back with a punctate probe at one of 25 locations after which each participant marked the location of the touch on a three-dimensional digital image of the participant’s own breast or back (Figure 3A and B).

To estimate the degree to which participants could accurately localize touch events, we computed the error for each body part across participants and found that, contrary to the precision task, participants had lower errors on their breasts in comparison to their back (Figure 3—figure supplement 1A; paired t-test, t[9]=7.4, p<0.001). Next, we computed this error as a function of the distance from the center landmark for each region (nipple or the inferior angle of the scapula, i.e. the bottom corner, Figure 3C and D) and found that in the case of the breast, the errors increased systematically as the distance from the nipple increased consistent with the quadrant localization task (Linear Mixed Model [LMM]: distance: n=248, B=0.234, t=9.194, p<0.001, Table 1). This association was not found for the back (distance: n=248, B=0.093, t=1.714, p=0.087, Table 2). Then, using an LMM to compare the groups, we found both a significant interaction between the effect of distance and the location (location x distance: n=496, B=–0.139, t=–2.723, p=0.006, Table 3). Given that any errors are likely a result of two sources of noise: bias and response variability (imprecision), we next sought to quantify the relative contribution of the two sources. We estimated the bias for each point by computing the centroid of all reports for a given stimulus across trials and measured the error between the centroid and the stimulus location. To measure the imprecision, we computed the mean pairwise distance between each of the reported locations for a given stimulus location and the mean of those reported locations (Figure 3C and D). We found that, in both the breast and back, the error that originated from systematic bias significantly outweighed that of the imprecision (location x error type: n=995, B=–23.05, t=12.728, p<0.001).

As bias was prevalent in the localization task, we next sought to determine if the bias was systematic across stimulus locations. Consequently, we assessed the tendency for the error vector to be uniformly distributed in two dimensions (Figure 3—figure supplement 1B, C). Importantly, because the back is a relatively flat surface in comparison to the breast, error vectors were only computed in the horizontal plane and depth was excluded when computing the angular error. When computing the absolute angle between stimulus location and the average reported location (blue vectors in Figure 3A and B) and computed the distribution across participants (Figure 3E and F). Across participants, the distribution of biases did not significantly deviate from uniformity (permutation test for uniformity: R=0.1314, p=0.9331 and R=0.1856, p=0.7171, respectively). Next, when we assessed if the bias with respect to the center point (nipple or scapula) was consistent, we found that both the breast and back tended to exhibit biases (R=0.3140, p=0.0100 and R=0.6164, p<0.0001, respectively), with the effect roughly doubled for the breast.

To further quantify this observation, we computed the vector strength – a measure of circular uniformity – of the biases for individual participants. For the breast we did not detect significant non-uniform distributions across participants, although some participants demonstrated biases (e.g. towards the top of the shoulder, Figure 3—figure supplement 1D–F; vector strength Monte-Carlo test: p<0.05 for 4/10 participants). Examining the back, significant non-uniform distributions were observed for all but one participant; however, the direction was inconsistent across participants, implying that reports were not biased towards any single landmark. Next, we computed the distribution of relative angles between the bias vector and the center point of our task and found that seven of ten participants were biased towards the nipple (vector strength Monte-Carlo: p<0.05), and two were biased towards the scapula, suggesting that while individual biases are present, they vary substantially across the population.

Given these findings, we conclude that the breast has lower tactile precision than the hand and is instead comparable to the back. Moreover, localization of tactile events to both the back and breast are inaccurate, but localizations to the breast are consistently biased towards the nipple.

The poor spatial precision of the breast – about four times lower than that of the hand and slightly worse than the back – replicates previous findings (Weinstein, 1968) achieved using less reliable methods (two-point threshold and a ‘same-different’ paradigm). Tactile spatial precision is determined by density of innervation: more neural tissue is devoted to more highly innervated body regions, and this increased central representation is a key contributor to the increased precision. The low spatial precision of the nipple and areola is consistent with a histological study revealing these regions to be sparsely innervated (Gutiérrez-Villanueva et al., 2020).

We found a significant relationship between breast size and spatial precision: women with larger breasts tended to exhibit lower spatial precision on their breasts, consistent with previous findings that tactile precision scales with body size. Indeed, the spatial precision of the hand has been shown to depend on the size of the hand, with smaller hands exhibiting better precision (Peters et al., 2009; Wong et al., 2013). These results are consistent with the hypothesis that the number of tactile nerve fibers does not scale with body size, so fibers are more sparsely distributed on bigger bodies, leading to lower precision. Consequently, the precision of the breast is likely determined initially by torso precision and then as a result of subsequent expansion.

The mental representation of the body is not uniform and veridical (Longo, 2022). Localization tends to be more precise when stimuli are applied near anatomical points of reference that form perceptual anchor points (Weber, 1834). For example, the navel and spine act as anchor points along the abdomen (Cholewiak et al., 2004; Van Erp, 2005): touches to the navel or spine are never mistaken for touches anywhere else on the abdomen. Furthermore, touches to locations near these two anchor points are mislocalized following a bias toward these areas: touches near the navel are pulled toward the navel, and touches near the spine are pulled toward the spine. Similar biases are observed on the back of the hand, where touches are mislocalized to be closer to the fingers than they actually are (Mancini et al., 2011), and on the forearm, where they are pulled toward the wrist (Fuchs et al., 2020). Analogously, we found that precision is highest near the nipple, and perceived locations are pulled toward the nipple, suggesting that the nipple plays a pivotal role in the mental representation of the breast. It must be noted, however, that these same biases may be inadvertently influenced by our study design. As participants were annotating 3D meshes of their breasts, the cognitive importance of the nipple may have also caused them to report sensations as closer to the nipple regardless of the actual percepts. This observation motivates standard reporting methodologies for morphologically diverse body parts such as the (Nielsen et al., 2026).

Understanding the spatial precision of the breast is particularly important given a recent proliferation of efforts – including using autologous tissue, synthetic grafts, and neuroprosthetic or bionic approaches – to restore sensation to the breast following mastectomy (Lindau and Bensmaia, 2020; Courtiss and Goldwyn, 1976). In particular, this work shows that the spatial resolution of an implantable sensor sheet should depend on the distance from the nipple. Individual sensors should be placed in each quadrant of the NAC, while subsequent sensors should be placed radially with increasing separation (0.5–2 cm inter-sensor spacing) up to a 5 cm radius, at which point resolution would remain constant at approximately 2 cm. These values, however, depend on the spatial resolution of the stimulation technology, which can vary significantly for both peripheral nerve stimulation (Charkhkar et al., 2018) and intracortical microstimulation (Greenspon et al., 2025b). Moreover, little is understood about the tactile coding of the primary afferents of the breast, and further research will be needed to both inform stimulus patterning and inform safe stimulation parameters (Graczyk et al., 2016; Hobbs et al., 2025; Greenspon et al., 2025a).

The breast has unique sensory properties because (McGlone et al., 2014) it mediates lactation and nursing, (Corniani and Saal, 2020) it comprises distinct regions – the nipple, areola, and outer breast – which differ in the type of skin and patterns of sensory innervation, (Craig and Lyle, 2001) it has erectile function and gives rise to erogenous sensations, and (Johansson and Vallbo, 1979) it undergoes variable expansion across individuals during puberty. First, we find that spatial precision on the breast is lower than that of the back, previously regarded as the body region with lowest tactile precision. Second, spatial precision is lower in larger breasts, presumably due to sensory innervation being fixed prior to expansion during puberty. Third, the nipple itself has poor precision and yet plays a major role in how participants perceive tactile events on their breast.

A total of 48 healthy adult women (24.2±3.1, 19–32 years) participated in this study: Thirty-four in the location discrimination task (mean ± SD, age range; 22.6±2.7, 19–28 years), ten in the medial breast quadrant localization task (25.3±3.8, 19–32 years), and ten in the absolute localization task (24.7±2.9, 20–28 years). Some participants performed multiple tasks. Experimental procedures were performed in accordance with the relevant guidelines and regulations and were approved by the Institutional Review Board of the University of Chicago (IRB 18–0135). The informed consent process included documentation of written consent from each participant, and the participants were compensated for their participation. We excluded any women who, by self-report, were currently pregnant or breastfeeding, had a history of breast surgery, or any diagnosis of neurological illness.

In addition to self-reported bra size, we collected standardized, objective measurements from each participant. Specifically, we measured the bust – that is the circumference of the chest at the level of the nipple – and subtracted from it the under-bust, the circumference at the level of the inframammary fold (mean ± SD, range; 12.4±4.6, 7.6–25.4 cm). We also measured the diameter of the areola and nipple (areola: 37.4±11.9, 21–80 mm; nipple: 12.7±2.7, 7–20 mm).

Touches were delivered manually with a 1 mm tipped XP-Pen (XP-Pen USA, CA, USA), which allowed the experimenter to lightly press the stimulus against the skin and ensure that the skin was indented uniformly across touches. The XP-Pen was selected instead of monofilaments because, in pilot experiments, several participants reported discomfort with the sharp edges of the monofilaments on their breasts while also allowing for the application of a constant force. The participants were asked to report any discomfort with the application of the stimulus or the inability to feel the application of the stimulus reliably. Note that spatial precision is consistent across stimulus amplitudes as long as the touch is sufficiently above threshold (Gibson and Craig, 2002), which was the case here.