Notes: Vision (Part 1)

(Part II of vision will come from Brad's lecture)

History:
10/26/12 - formatting change (no content change)
10/17/12 - Minor word editing
10/12/12 - Original posted version

Where we are going:
    How is a distorted and upside-down 2-D retinal image transformed into the 3-D world we perceive?

Light

No species can see in the dark, but some are capable of seeing when there is little light
Light can be thought of as
 - Particles of energy (photons)
 - Waves of electromagnetic radiation (has a wavelength)
Humans see light between 380-760 nanometers in wavelength

Properties of light:

________________ – perception of color

________________ – perception of brightness


Source: http://www.perret-optic.ch/optometrie/Vision_des_couleurs/vis-couleur_gb.htm

The Eye

Focusing an image

______________ - contractions of ciliary muscles to deform the lens and change the focus




Optional video: Children that can voluntarily control their pupils
http://www.youtube.com/watch?v=YIKm3Pq9U8M

Distance cues from eyes

_________________________ - eyes turn slightly inward for closer objects

_________________________ - the eyes have a slightly different perspective
    the closer an object, the more obvious the difference in perspective

Saccades

Saccade - quick shifts in eye direction to observe a scene
    not the same as the slow tracking when following an object (a car crossing in front of you)

Source: http://en.wikipedia.org/wiki/Eye_tracking

Source: http://www.androidblues.com/visualperception.html

Retina

Transduction

transduction - conversion of one form of energy into another

visual transduction - turning _____________ into a _____________ _____________

how does this happen: pigment absorb photons and react

Rods & Cones

________________________ - cells specialized for visual transduction

rods - specialized for seeing ________________________
    more sensitive to photons than cones
    signals from many rods are pooled into one retinal ganglion cell

cones - specialized for seeing________________________ (more later)
    in most humans, there are 3 different cones sensitive to 3 different wavelengths of light



Rhodopsin - a ______________ that changes shape when it absorbs ______________
    (you won't be tested on cyclic GMP, just rhodopsin in general:)
    cyclic GMP keeps sodium channels open
    when rhodopsin absorbs light, it breaks up cyclic GMP

    when light hits rhodopsin, this:
        increases/decreases the amount of Na+ entering the cell
        depolarizes/hyperpolarizes the cell
        increases/decreases glutamate release

Rhodopsin: Source: wikipedia.org/wiki/Rhodopsin

_________________ - the ability to see when light is dim, requires _________________ photoreceptors

_________________ - the ability to see details (resolution), requires ________________ photoreceptors

Other retinal cells

Bipolar cells

    typically only connect to a few rods or cones (never both)

                                    "BB" Bipolar mnemonic:
    OFF bipolar cells - when it's Bright, off Bipolar cells are off
    ON bipolar cells -  when it's Bright, on Bipolar cells are on
                (reminder:  when it's Bright, rods/cones are off)

    ON bipolar cells reverse the signal from rods/cones

        rods/cones _________________ glutamate release when they absorb light

        ON bipolar cells _____________________ to glutamate (WTF?!?)

        bright -> rods/cones hyperpolarize -> less glutamate -> less hyperpolarization -> depolarize

Amacrine / Horizontal cells - involved in lateral inhibition (more later)
    typically connect to many rods/cones

Retinal ganglion cells - carry the signal from retina out of the eye
    may receive input from only a few cones or many rods
    their axons form the optic nerve

Distribution of rods and cones in the retina:


_________________ - the center of the retina
    where the lens focuses the image
    a high/low concentration of cones
    a high/low concentration of rods

_________________ - the area of the retina where the axons from the retinal ganglion cells leave the eye

Trick for seeing in the dark - don't look directly at what you want to see.
    Why does this work?


Spectral sensitivity
    - pigments will absorb more light from some wavelengths than for others
    - spectral sensitivity = a profile of absorption/reaction across different wavelengths

Visual pathway

Source: http://www.dgward.com/physo101/sm06_pages/labs/Peripheral%20Vision%20and%20Visual%20Pathways.htm

Lateral geniculate nucleus (LGN)
    - part of the thalamus, which is a relay station between most sense systems and the cortex
    - exact role is unclear
    - maybe involved in: making visual information more efficient, focusing attention, saccades

Visual cortex (more later)
    - performs the processing on visual information to allow us to perceive visual scenes/stimuli

Information from LEFT visual field goes to RIGHT visual cortex (and vice versa)
    NOT left EYE to right visual cortex

Retinotopic mapping
    - If two retinal ganglion cells that are close together in the retina, their axons end close together in the visual cortex
    - The retina is "mapped on" to the cortex

M & P channels
    - two parallel channels of axons running through LGN
    - magnocellular layers (magno=big, M layers) = movement, "big picture"
    - parvocellular layers (P layers) = color, detail, stationary/slow tracking
    - demonstrates principle of parallel processing / functional segregation (more later)

Low-level Visual Processing

"low-level" refers to early in the visual pathway & dealing with simple visual stimuli like brightness, edges & color.
"high-level" refers be areas that receive the pre-processed information from low-level parts of the visual system and that process more advanced stimuli like motion, faces, object-recognition & visual space

Receptive Fields

Definition: The area of visual space that stimulates or inhibits a neuron (or neural tissue)

The stimulus might be simple or complex. Examples:
  - Some neurons might be stimulated by any light in a precise spot in the top right corner of the visual field
  - Some neurons might be stimulated by a vertical edge anywhere on the left
  - Some neurons might be stimulated by faces anywhere in the visual field

Receptive fields become larger
    farther away from the fovea - don't need to know exact location, just want to notice something
    at higher levels of the visual system - just want to react to a face (for example), doesn't matter where it is

Hubel & Weisel

Videos: Intro & long version

Center-surround

A simple but common kind of receptive field: Center-surround

Behavior:

    is _______________ by light in the middle of receptive field

    is _______________ by light surrounding the middle of receptive field
 
Result:
    is most active if it is the only field receiving input
    is not very active if it is receiving input along with all surrounding areas
    increases contrast
    uses lateral inhibition

Edges

Why do edges appear "accented"?


Source: http://www.vdic.com/casedatabase/casedatabase_visual_perception.asp

Lateral Inhibition

lateral inhibition increases contrast between strong and weak signals
When a neuron fires, it inhibits its neighbors

Source: http://www.d.umn.edu/~jfitzake/Lectures/DMED/SensoryPhysiology/GeneralPrinciples/LateralInhibition.html

Color

In most humans, there are three kinds of cones
    - each with a different photo-sensitive pigment called iodopsins
    - each of the three iodopsins is sensitive to different wavelengths of light

Number of cones varies
    - some animals & people (with color blindness) have only 2 kinds of cones
    - some animals (birds in particular) have 4 kinds of cones




(Do not need to know: component/trichromatic theory or color constancy)

High-level Visual Processing

(content for this section is provided in Brad's presentation)

Sensation vs Perception

Visual cortex

Dorsal vs Ventral stream

Face recognition

Damage & Pathologies

• At the level of retina
• In primary visual cortex
• In secondary visual cortex / association cortices
  

Principles of Visual Processing

• Parallel processing
• Hierarchical
• Functionally segregated

These principles apply to many neural systems besides vision