2017-10-17 08:24:47

Prelude

I can see clearly now.

Today's topics

  • Common principles in sensation & perception
  • Case study: Vision

Common Principles

  • Topographic maps

Tonotopic (frequency) maps in auditory cortex

Retinotopic maps in visual cortex

Common principles

  • Non-uniform sensitivity

Two-point touch thresholds

Acuity variations across visual field

Hearing threshold varies across frequency

Hierarchical processing

Parallel processing

Parallel processing

Case study: Vision

Electromagnetic (EM) radiation

Features of EM radiation

  • Wavelength/frequency
  • Intensity
  • Location/position of source
  • Reflects off some materials
  • Refracted (bent) moving through other materials

EM radiation provides information across space (and time)

Reflectance spectra differ by surface

Optic array specifies geometry of environment

Color == categories of wavelength

  • Eyes categorize wavelength into relative intensities within wavelength bands
  • RGB ~ Red, Green, Blue
    • Long, medium, short wavelengths
  • Color is a neural/psychological construct

RGB monitors

How a camera works

The biological camera

The biological camera

Parts of the eye

  • Cornea - refraction (2/3 of total)
  • Pupil - light intensity; diameter regulated by Iris.
  • Lens - refraction (remaining 1/3; focus)

Parts of the eye

  • Retina - light detection
    • ~ skin or organ of Corti
  • Pigment epithelium - regenerate photopigment
  • Muscles - move eye, reshape lens, change pupil diameter

Eye forms image on retina

  • Image inverted (up/down)
  • Image reversed (left/right)
  • Point-to-point map (retinotopic)
  • Binocular and monocular zones

Retinal image

Eyes views overlap

The fovea

The fovea

  • Central 1-2 deg of visual field
  • Aligned with visual axis
  • Retinal ganglion cells pushed aside
  • Highest acuity vision == best for details

Acuity varies across fovea

Acuity varies across fovea

What part of the skin is like the fovea?

What part of the skin is like the fovea?

Photoreceptors detect light

Photoreceptors detect light

  • Rods
    • ~120 M/eye
    • Mostly in periphery
    • Active in low light conditions
    • One wavelength range

Photorceptors detect light

  • Cones
    • ~5 M/eye
    • Mostly in center
    • 3 wavelength ranges

Photoreceptors "specialize" in particular wavelengths

How photoreceptors work

  • Outer segment
    • Membrane disks
    • Photopigments
      • Sense light, trigger chemical cascade
  • Inner segment
    • Synaptic terminal
  • Light hyperpolarizes photoreceptor!
    • The dark current

Retina

  • Physiologically backwards
    • How?
  • Anatomically inside-out
    • How?

Retina

  • Physiologically backwards
    • Dark current
  • Anatomically inside-out
    • Photoreceptors at back of eye

Retinal layers

Retinal layers

  • From photoreceptors…
  • To Bipolar cells
    • <-> and Horizontal cells
  • To Retinal ganglion cells
    • <-> and Amacrine cells

Center-surround receptive fields

Center-surround receptive fields

  • Center region
    • Excites (or inhibits)
  • Surround region
    • Does the opposite
  • Bipolar cells & Retinal Ganglion cells ->
  • Most activated by "donuts" of light/dark
    • Local contrast (light/dark differences)

What's a reddish-green look like?

What's a reddish-green look like?

Opponent processing

Opponent processing

  • Black vs. white (achromatic)
  • Long (red) vs. Medium (green) wavelength cones
  • (Long + Medium) vs. Short cones
  • Can't really see reddish-green or bluish-yellow
    • "Oppose" one another at cellular/circuit level

From eye to brain

From eye to brain

  • Retinal ganglion cells
  • 2nd/II cranial (optic) nerve
    • Optic chiasm (\(\chi\) - asm): Partial crossing of fibers
    • Nasal hemiretina (lateral/peripheral visual field) cross
    • Left visual field (from L & R retinae) -> right hemisphere & vice versa
  • Lateral Geniculate Nucleus (LGN) of thalamus (receives 90% of retinal projections)

From eye to brain

  • Hypothalamus
    • Suprachiasmatic nucleus (superior to the optic chiasm): Synchronizes day/night cycle with circadian rhythms
  • Superior colliculus & brainstem

Next time…

  • Wrap up on vision
  • Principles of action