Vision I - Receptors and Neural Processing

INTRODUCTION
- Effective distance: somatosensation, gustation, olfaction, audition, vision
- Evolution: light sensitivity (fish thru skin), motion (frog), color (esp. good in primate)
- 70% of sensory receptors are in the eye
Nature of light - particle (photon) and wave properties
Wavelengths of visible light: 360nm-760nm (blue to red)

PARTS OF THE EYE
- Eye is part of the brain (CNS)
- Pupil and Iris (greek for 'rainbow') - iris controls amount of light entering eye through the pupil
Vitreous humor: jelly like substance fills eyeball
Cornea and Lens are the focusing elements / Ciliary muscles
Accomodation: a tightening of the ciliary muscles to increase the curvature of the lens (gets thicker), bringing near objects into focus
Presbyopia "old eye"- with age lens hardens and ciliary muscles weaken
holding the newspaper at arm's length
Retina - a sheet lining the back of the eye containing 5 layers of cells
Rods, Cones, Horizontal, Bipolar, Amacrine, and Ganglion cells
Horizontal and Amacrine cells run across retina; collect info from a # of receptors and/or bipolars

<<< FIGURE: Cells of the Retina >>>

Light must pass through 4 layers of cells to get to receptors
Pigment epithelium: nutrients and enzymes, clean up, and rapid regeneration of pigment
Contains melanin (dark pigment) to absorb stray light
Ganglion cell axons (1 million) bundle up to form the optic nerve
Blind spot: area with no receptors, where optic nerve leaves the eye
Decussation of fibers at the optic chiasm

THE RETINA AND TRANSDUCTION
The receptors job is to transduce light (photons) into electrical energy (neural impulses)
Receptor inner and outer segments

Visual Pigment Molecule - made up of Opsin protein and light-sensitive Retinal molecule (from Vitamin A) contained in discs (folded membranes) of outer segment
Molecule is transmembrane
Retinal absorbs light and changes shape; becomes catalyst and triggers a chemical reaction that generates electrical signal
Bleaching: Retinal seperates from opsin after conformation change

Rod (120million) and Cone (6million) Distribution

Fovea - contains only cones, area of best focus
2 types of Ganglion cells: small ones, P-cells (parvo) & big ones, M-cells (magno)

DUPLICITY THEORY OF VISION (von Kries, 1896)
Retina has two types of receptors that are morphologically & functionally diff. and operate under diff. conditions
Dark adaptation: increase in sensitivity when illumination changes from light to dark (going into a movie theater)
2 Stages of dark adaptation

1. rapid phase / 3-4 min / Cones
2. gradual, slow phase / 10-30 min / Rods

Why? - pigment regeneration increases sensitivity of rods & cones; rod's pigment takes longer to regenerate
"2 visual systems"- cones for 'daytime' vision, rods for low light levels
Spectral sensitivity curve: plot of sensitivity -vs- wavelength
Created by the absorption spectra of their pigments:
1 type of rod pigment - 500nm
3 type of cone pigments - 419nm (short), 531nm (medium), 558nm (long)
Purkinje shift: when rods take over from cones during dark adaptation, we become more sensitive to short wavelengths of light (blue end of spectrum)

NEURAL CIRCUITS
The influence of how the brain is "wired up"
Network of nerve fibers; connections between receptor and neuron; interneural connections, too
126 million receptors converge on 1 million ganglion cells

Linear: receptor-neuron direct link; not affected by stim other receptors
Convergent: when 2 or more receptors synapse on a single neuron
Inhibition: by release of some NTs, neuron less likely to fire

INTRO to RECEPTIVE FIELDS
Electrophysiology - single or multi-(group) cell recordings of activity
Recording from a ganglion cell, we can shine a point of light on various places on the screen and tell: what are of retina (receptors) the cell is "hooked-up" to, the kind of convergence and inhibition in the network

Receptive field: region of the retina that, when stimulated, influences the firing rate of the neuron

<<< FIGURE: The Receptive Field >>>

Influence can be: neuron fires @ higher rate=excitatory (+) ON response
" " " lower rate=inhibitory (-) OFF response
Ganglion cells have center-surround organization of RFs
Center-surround antagonism
can be On Center-Off Surround as well as Off Center-On Surround)

NEURAL WIRING & SENSITIVITY
? Which receptor has better sensitivity ?

Rods have higher sensitivity than cones
Rod's higher sensitivity comes from 3 sources

1. high convergence on ganglion cells - spatial summation
2. rods larger, absorb more light
3. generates larger electrical response than cones

NEURAL WIRING & ACUITY
? Which receptor has better acuity ?

Fovea has best acuity because of cone density and low convergence

SUMMARY of ROD and CONE DIFFERENCES