ID No. 16
Are some parts of the human body more sensitive to touch than others? If someone’s foot were to be scratched, would they have the same sensory reaction had it been their elbow?
Being able to perceive pressure, pain, temperature, and other physical sensations is an essential characteristic for survival. Sensory neurons—which are the reason feeling is possible—constantly relay messages through the nervous system to the brain. They dictate somatosensation—every input from skin, muscles, tendons, and organs is transmitted by sensory neurons to the central nervous system, which in turn uses these inputs in controlling behavior (Bjorklund ; Gray, 2014).
It has been established that the somatosensory cortex, working in conjunction with the network of sensory neurons throughout the body, is responsible for the perception of touch (Bjorklund ; Gray, 2014). Research has uncovered that different areas of the somatosensory cortex are dedicated to different areas of the body, in proportion not to how large the area is but rather to its sensitivity and degree of fine motor control (Bjorklund ; Gray, 2014). More sensitive areas and areas with higher fine motor control therefore have larger areas of the somatosensory cortex devoted to processing their input.
Seeking to quantify the difference in touch sensitivity, Nolan (1985) stimulated different areas of the body and noted his subjects’ reactions. In his experiment, he tested the difference in cutaneous sensation between different areas of the face and the trunk. To evaluate the difference in sensitivity, he established eleven different areas, all which corresponded to a specific section of peripheral nerve. He then touched each area with either one or two points of an EKG caliper, varying the distances until the subject could discriminate two points correctly eight out of ten times.
We performed an experiment similar to Nolan’s, looking to calculate the two-point discrimination values for both the cheek and the back. Due to our previous understanding of the proportional mapping of the somatosensory cortex, we hypothesized that discrimination values would be lower for the cheek than for the back.
Partners took turns being both the participant and the experimenter. First, the handedness and sex of the participant were documented. Then the participant closed their eyes. Using a caliper, the experimenter first tested discrimination values on the side of the face corresponding to the handedness of the participant. In a series of 32 trials, the experimenter pressed the caliper into the cheek of the participant at varying distances in random order, ensuring that no pattern was followed. The experimenter was careful to apply the pressure at an area of the cheek other than the cheekbone. There were 10 trials performed at 0 cm; that is, the cheek was poked by a singular point 10 times throughout the testing. The other 22 trials consisted of 11 distances, ranging from 0.2 cm to 5 cm, adjusted on the caliper by the experimenter before each trial. After each trial, the participant responded whether they felt 1 or 2 points of contact, and the experimenter recorded their response. The same method was used for the upper back—the experimenter varied the distance of the calipers in the 32 trials, ensuring a random sequence of 1 or 2 pokes, on the side of the upper back corresponding to the handedness of the participant. Once again, the experimenter ensured they were applying contact to an area without any bone. The experimenter then recorded the sensory threshold: the distance at which the participant correctly responded that there had been 2 points applied, and after which their responses were perfect. For example, if the participant had felt 2 points of contact at 2 cm, and successfully responded 2 points for every distance until 5 cm, their sensory threshold would be 2 cm. If the sensory threshold was not established at 5 cm, it was set at 5.5 cm. After determining the sensory threshold for both the cheek and the upper back, partners reversed roles.
Results and Discussion
The mean two-point discrimination for the cheek and the upper back are 1.78 cm (SD = 0.82 cm) and 4.25 cm (SD = 1.19 cm) respectively.
t(148) = 22.33, p ; .001.
These results support our hypothesis that the cheek is more sensitive than the upper back. The mean two-point discrimination of the cheek is lower than that of the upper back, and the difference is statistically significant. Additionally, the standard deviation for two-point discrimination for the cheek is lower than that of the upper back. These findings align with those of Nolan (1985). This discrepancy suggests that there are less sensory neurons in the upper back area tested, and therefore that the somatosensory cortex area devoted to the cheek is proportionally larger than that devoted to the upper back.
Up until this point in the study, it has been assumed that the topographical mapping of the somatosensory cortex has been fixed—that unique areas in the cortex are statically devoted to unique areas in the body. But Ramachandran and Rogers-Ramachandran (2000) suggest that neural plasticity result in the somatosensory cortex being malleable.
In their research, it was shown that when an arm was amputated, sensory input was re-mapped onto the face (Ramachandran ; Rogers-Ramachandran 2000). In the study, subjects would claim they “felt” sensory information such as hot and cold in their phantom limb when touched on a specific area of their face. This finding seems to provide clear evidence for their “remapping hypothesis,” which states that perceived stimuli to phantom limbs is a result of the topographical rearrangement of dedicated areas in the somatosensory cortex (Ramachandran ; Rogers-Ramachandran 2000). Previously dedicated areas must have been remapped, which explains the phantom sensations.
The Ramachandran and Rogers-Ramachandran (2000) study provides critical insight into the neuronal plasticity of the somatosensory cortex. Since the cortex is proven to have the ability to remap itself, then there may be ways other than amputation for this effect to appear. Humans originally walked the earth barefoot. Due to the advent of footwear, did the bottoms of our feet become less sensitive to surround stimuli? Is it possible that topographical remapping can occur not only within someone’s lifespan, as is the case with a phantom limb or severe nerve damage, but also through generations of evolution?
In our experiment, there were several limitations to the method that could have affected the data. First, there is no guarantee that all participants were touched in the same exact area of the cheek and upper back. Second, the experimenter may have varied the pressure from trial to trial for both the cheek and upper back. Third, some participants elected not to remove their clothing for the upper back testing. Fourth, the testing environment may not have been the same temperature when all the trials were performed. It would be useful in a future experiment to standardize all these variables. In that case, the means recorded may provide a more accurate representation of discrimination, and the standard deviations may be lower.
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