S & P Intro to Vision

Visible Light

• electromagnetic spectrum

• light as wave or particle

• nanometer = 10^-9 of a meter

The Eye - transduces light to neutral firing

• Pupil - where light enters the eye

• Cornea - first site of focusing on surface of eye (80%)

• Lens - Flexible focuser in the eye (20%)

• Retina - contains rods and cones, the transducers of light

• Optic nerve - carry the electrical signal from the eye to the brain

• Sclera - outer white part of your eye

Lens Accommodation

• The accommodates for close vision by tightening the ciliary muscles, allowing the pliable crystalline lens to become more rounded

• Ciliary muscles relaxed, fibers taut, lens at minimum strength for distant vision

• Ciliary muscles contracted, fibers slack, lens rounds to greater strength for close vision

• Light rays from distant objects are nearly parallel and don’t need as much refraction to bring them to a focus

• Light rays from close objects diverge and require more refraction for focusing

Focusing Problems

• Emmetropia - normal vision

• Hyperopia - farsighted

• Myopia - nearsighted

• Astigmatism - distortion

• Presbyopia - old vision

Corrective Lenses

• Lenses focus light at the back of the eye

• Concave - nearsighted

• Convex - farsighted

Transduction of Light

• Light isomerizes (changes from one shape to another) Rhodopsin

• Leads to action potential

• A rod will respond to 1 photon

• 7 photons needed to see

Eye Structure and Retinal Array

• the retina lines the back of the eye and consists of rod and cone photoreceptors as four types of neurons:

  • second-order bipolar

  • horizontal cells

  • third-order retinal ganglion cells (RGCs)

  • amacrine cells

• muller glial cells fill the spaces between the neurons

• the pigment epithelium, critical for photoreceptor function, underlies the retina

• photoreceptors and RGCs are most susceptible to blinding retinal disease

• progress in combating photoreceptor degeneration has been made, but there are few strategies to address RGC loss

Distribution of Rods and Cones

• Fovea - only cones (-50,000)

• Periphery - rods and cones (6 million cones, 120 million rods)

• Macular degeneration - destroys fovea

• Retinitis pigmentosa - destroys periphery

Blind Spot

• site where optic nerve exits the eye

• no receptors there

• brain fills in so you don’t see it

Dark Adaptation Curve

• Scotopic - night vision

• Photopic - day vision

More on Dark Adaptation

• Measure cone adaptation - use light placed on the fovea - no rods

• Measure rod adaptation - need a rod monochromat - no functioning cones

  • can measure the isolated response of the rods

  • can’t use normal vision observer as there are cones in the periphery

• Process of adaptation

  • once light is out, both rods and cones increase sensitivity

  • 3-5 minutes cones finish adapting

  • around 25-30 minutes, rods finish adapting

  • rod-cone break is where rods start to determine the curve of adaptation

Differences in Adaptation - Rods and Cones

• Why do rods take longer to adapt than cones?

• Visual pigment regeneration

  • when light hits the retina, rhodopsin isomerizes

  • this causes the visual pigment to bleach

  • as some pigments are bleaching, some are regenerating

  • this process is faster in cones than rods

• Detached retina - separates retina from pigment epithelium (contains regenerating enzymes)

  • caused by blow to the head

  • prevents process of pigment regeneration

  • person becomes blind in area of detachment

Spectral Sensitivity of Rods and Cones

• Threshold is the inverse of sensitivity

  • high sensitivity means low threshold

  • low sensitivity means high threshold

  • higher sensitivity means you need less light to see it

  • lower sensitivity means you need more light to see it

Purkinje Shift - at dusk

• shift from red to blue

• shift from cones to rods as light decreases

  • rods are more sensitive to blue wavelengths

• hues change as the spectral sensitivities of the two receptors are different

Absorption Spectrum

• how much light is absorbed

• peak is most absorbed wavelength

• spectral sensitivity is related to absorption spectrum

Neural Convergence

• Order of processing:

  • linear processing is Receptors → bipolar cells → ganglion cells → “brain”

  • lateral processing by horizontal cells and amacrine cells

  • Horizontal cells - processing between receptors and bipolar cells

  • Amacrine cells - processing between ganglion cells

  • Neural convergence - one neuron receives signals from many cells

Rods vs Cones

• Rods - more convergence, better in low light because of convergence and greater sensitivity

• Cones - little to no convergence, better resolution due to little convergence, not good in low light

• Get lateral inhibition with rods

Lateral Inhibition - related to convergence

• Limulus (horseshoe crab) - huge eyes or ommatidia

• easy to study lateral inhibition

• light excites Cell A

• add light to B, cell A fires even less

• add more light at B, cell fires even less

• cell A is being inhibited by cell B activity

Simultaneous Contrast

• examples of lateral inhibition at work

Direct or Indirect Perception?

• We think that we perceive directly

• We perceive through our senses and it’s all in our head - more indirect

• Seems very real to us

• But our senses distort the world (lateral inhibition is just one example)