Perception and Pain

Sensory Receptors

• Three basic categories of skin receptors:

  • Mechanoreceptors:
    • Respond to sensitive indentations or changes in pressure on the skin.
  • Thermoreceptors:
    • Respond to changes in temperature.
  • Nociceptors:
    • Respond primarily to pain.

• Do all three receptors respond to most stimulation on the skin?

  • Yes, they all respond, but to varying degrees.

• How does the brain determine the sensation on the skin?

  • By using the ratio principle.
    • The ratio of firing among the three skin receptors.

• Example:

  • A strong touch will cause mechanoreceptors to fire strongly.
  • Warm fibers will respond to pressure.
  • If there's pain or damage, nociceptors will fire.
  • The brain determines the sensation based on the pattern of activation among the receptor types.

Acuity

• Different parts of the body vary in sensitivity to:

  • Distance.
  • Depth.
  • Noticing two distinct points.

• Psychophysical methods:

  • Two-point threshold:
    • The minimum distance between two points on the skin that can be detected as two distinct points.

• Acuity:

  • The capacity to process detail.

• Acuity in the eyes:

  • Mediated by the fovea in the retina.
    • Populated by cones, which are responsible for detailed vision.

• Acuity in the cochlea:

  • Inner hair cells are responsible for the perception of frequency (pitch), which is more acuity-related.
  • Outer hair cells are responsible for the perception of loudness (amplitude).

• Acuity differences across the skin:

  • The smaller the two-point threshold, the more acute that part of the body is.
  • Lips have a smaller two-point threshold than the back.

Somatosensory Cortex and Motor Cortex

• Brain lobes:

  • Frontal lobe.
  • Parietal lobe.
  • Temporal lobe.
  • Occipital lobe.

• Somatosensory cortex:

  • Receives afferent (sensory) nerve responses related to touch from the thalamus.

• Motor cortex:

  • Sends out efferent (motor) nerve signals to initiate touch.

• Location:

  • The somatosensory cortex and motor cortex are located next to each other in the parietal lobe.

• Contralateral representation:

  • The right side of the brain controls the left side of the body, and vice versa.

• Mapping:

  • The body is represented upside down.
  • The amount of brain tissue devoted to each body part varies.
  • Areas with high acuity (face, lips, hands, tongue) have larger areas of the brain devoted to them.

• Relationship:

  • The smaller the two-point threshold, the more sensitive to detail (more acute).
  • The smaller the two-point threshold, the larger the part of the brain devoted to processing that area.

• Brain as a computer:

  • The brain has subroutines that are differentially devoted to processing different parts of the body.

• Brain Plasticity:

  • If one part of the brain is damaged, adjacent areas can take over its functions.

Homunculus

• Homunculus:

  • A whimsical representation of the amount of brain tissue devoted to different parts of our body.
  • High acuity areas have a large somatosensory cortex representation.
  • Low acuity areas have a smaller representation.

• Inner emptiness:

  • The idea that there's a little person (homunculus) inside our head is incorrect.
  • There is no infinite regress of homunculi.
  • It's all