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