Sensitivity vs. acuity
- Sensitivity refers to detectability of pressure or deformation of skin.
- Sensitivity is highest on lips and fingers; lowest on back and stomach.
- Females generally show higher sensitivity.
- Vibrotactile sensitivity is greatest at 200 Hz. Palm shows greatest vibrotactile sensitivity. (Lip reading in deaf persons can be facilitated by transforming sound to vibrotactile stimulation.)
- Acuity is different from sensitivity, and refers to the discriminability of structures indenting the skin. Figure of merit is the two-point threshold.
- Two-point threshold is smallest in the finger (2 mm). Two-point threshold on the forearms is 30 mm; on the back it is 70 mm.
- Fingers are analogous to the fovea in the retina (but different because they work best when scanned over an object).
- High acuity oftentimes goes hand in hand with high sensitivity (e.g., lips, fingers), but not always.
- Spacing of dots is designed to match the acuity (i.e., two-point threshold) available on the finger. Embossed Roman characters would appear blurred by the point spread function of skin.
- Limited surface area means that characters cannot simply be enlarged. Thus, a new symbol set is required.
- Experienced Braille readers can read at about 100 words per minute (as compared with 250-300 for visual readers).
- RA fibers = “rapidly adapting”; SA fibers = “slowly adapting”
- Punctate fibers have small RF’s, 4-10 ridges in skin; Diffuse fibers have large RF’s (whole finger or greater part of palm).
- SA punctate fibers have highest resolution.
- RA fibers are more sensitive - they are the fibers that become active when a stimulus is barely perceptible; SA fibers, by contrast, will not be firing at this point.
- RA and SA fibers enter the dorsal horn and proceed via lemniscal tract to the brain.
- Fibers cross over to contralateral thalamus and cortex; RA and SA fibers segregate in thalamus (like magno/parvo segregation in LGN).
- Meissner corpuscles (upper layer of skin, small) and Pacinian corpuscles (lower layer of skin, large) are innervated by RA fibers.
- Merkel disks and Ruffini endings (middle layers of skin) are innervated by SA fibers.
- Meissner corpuscles reside in the ridges of the skin, 40-50/mm
.
- Ruffini - diffuse, sense stretch.
- Pacinian - largest, least numerous, each innervated by a single RA fiber; extremely sensitive, especially to high-frequency vibration.
- The body surface is “mapped” onto the cortex, but it is only locally topographic. Fractures occur on larger scale.
- Regions of high acuity on the skin have a proportionally larger amount of suface area of cortex devoted to representing them.
- 50% of S-I in mammals is devoted to the head.
- Some neurons exhibit selectivity to the direction of movement across the skin.
- “Barrel cortex” in rodents is a specialized region for representing the whiskers.
- Plasticity: somatosensory representation changes with experience. Attention influences plasticity, increases discrimination.
- “Phantom limbs” arise from remapping of body parts after damage or amputation.
- Haptics: cortex must merge proprioceptive (kinesthesis) and somatosensory information.