early visual processing

Issac Newton (1704)

Light acts like a particle

James Clerk Maxwell (1873)

Light wavelike has properties (produces diffraction patterns)

Visible light

• light is electromagnetic radiation (like gamma rays, radio, radar, etc.)

• visible from ~380 to ~760 nm (billionths of a metre)

• the eye transduces light energy → neural impulses

• Issac Newton

• James Clerk Maxwell

• Ḥasan Ibn al-Haytham

Ḥasan Ibn al-Haytham (b.c.965-d.c.1040)

• called the “father of optics” and a “pioneer of modern optics

• wrote Book of Optics (1011-1021):

  • vision produced by light reflecting from surfaces into the eye

  • visual perception occurs in the brain

  • perception is subjective and affected by individual experience

• laid the foundation for the scientific method

The eye

• Light first strikes cornea

• Passes through aqueous humour

• Passes through pupil (hole in the centre of the iris)

• Passes through crystalline lens

• Passes through vitreous humour to retina

Cornea

• where light hits first

• Concentrates light rays

Pupil (the hole in centre of the iris)

• dilates (gets larger) in the dark to let in more light

• contracts in bright light to protect the eye

• sunglasses should have UV protection to guard against retinal and corneal damage

  • e.g., iggak (caribou antler goggles) worn by the Inuit protect against snow blindness (sunburned corneas)

Accommodation

ciliary muscles change shape of the lens, altering its focal length, which keeps image focused on retina

Crystalline lens

• accommodation

• elasticity reduces with age, making near point (minimum distance at which you can focus) move farther away: presbyopia (elder eye)

Retina

• receptors (rods and cones) point to the back of the eye

• Synapse with bipolar cells (have two long extensions)

• Which connect to ganglion cells

• Horizontal cells

• Amacrime cells

Ganglion cells (2 types)

P-cells and M-cells

Horizontal cells

make lateral connections among receptors and bipolar cells

Amacrine cells

laterally connect among bipolar and ganglion cells

Rods and Cones

• duplex retina theory (Schultze, 1866)

• Duplicity theory (von Kries, 1896)

• Fovea centralis

Duplex retina theory (Schultze, 1866)

• observed that retinas of nocturnal animals (e.g., owls) only contained rods

• diurnal animals (e.g., pigeons) only contained cones

• animals active during day and night had both rods and cones

Duplicity theory (von Kries, 1896)

related rods and cones to scotopic (dark) and photopic (light) vision

Fovea centralis

used for directed looking

• densest concentration of receptors in the eye

• only has cones (peripheral retina contains rods & cones)

Functional differences between rods and cones:

• ambient illumination changed from light to dark

• after a while, eyes adapt: sensitivity increases

Expt. 1: dark adaptation curve

• procedure: room lights go out; test light shined in observer’s periphery (rods & cones)

• test light adjusted to absolute threshold repeatedly as time passes

• result: threshold decreases (sensitivity increases) with time

Expt. 2: cone adaptation

• repeat expt.1, but shine test light on fovea

• result: explains part of the curve

Expt. 3: rod adaptation

• problem: how do you measure rods alone?

• solution: rod monochromats, due to a genetic defect, have only rods on their retinas (in the periphery)

What causes functional differences between rods and cones?

• different pigments in rods and cones

• Boll (1876) found photosensitive pigment in rods: bleached in the light and regenerated in the dark

• Rhodopsin comprised of retinal and opsin

• when hit by light, retinal changes shape (isomerization), causing a chain of events that culminates in a neural signal

• absorption spectrum

Absorption spectrum

amount of each wavelength absorbed by each type of pigment

What components is rhodopsin made of

retinal and opsin