Vision

Vision

1. the normal stimulus for vision is electromagnetic energy (light waves) which are measured in nanometers (one billionths of a metre)

2. visual system is sensitive to wavelengths extending from about 700 nanometers (red) down to about 400 nanometers (blue-violet)

   • ROY G. BIV
     • order of the spectrum, from higher wavelengths to lower ones (red orange yellow green blue indigo violet)

3. the human eye

   • cornea
     • transparent protective structure at the front of the eye
   • pupil
     • an adjustable opening that can dilate or constrict to control the amount of light that enters the eye
     • the pupil's size is controlled by the muscles in the coloured iris that surrounds the pupil
     • low levels of illumination = dilation
       • lets more light into the eye to improve optical clarity hell yeah 
     • bright light = constriction of the pupil
   • lens
     • an elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects
     • focuses the visual image on the retina
       • light sensitive, a multi-layered tissue at the rear of the fluid filled eyeball
     • the lens reverses the image from right to left and top to bottom when it is projected on the retina
       • the brain reconstructs the visual input into the image that we perceive
   • myopia
     • good vision for nearby objects but difficulty seeing faraway objects
     • nearsightedness
     • the lens focuses the visual image in front of the retina (too near the lens)
       • blurred image for faraway objects
     • occurs when the eyeball is longer (front to back) than normal
   • hyperopia
     • farsightedness
     • occurs when the lens does not thicken enough
       • the image is focuses on a point behind the retina (it far from the lens)
   • retina and photoreceptors
     • rods
       • function best in dim light
       • primarily black and white brightness receptors
       • about 500 times more sensitive to light than cones
       • do not give rise to colour sensations
       • can be found throughout the retina except in the fovea
     • cones
       • colour receptors
       • function best in bright illumination
       • decrease in concentration as one moves away from the centre of the retina
         • periphery of the retina contains mainly rods
     • rods and cones send their messages to the brain via two additional layers of cells
       • bipolar cells
         • have synaptic connections with the rods and cones
         • bipolar cells synapse with a layer of about one million
           • ganglion cells
             • axons are collected into a bundle to form the optic nerve
       • visual acuity
         • our ability to see fine detail is greatest when the visual image projects directly onto the fovea
     • blind spot
       • the optic nerve formed by the axons of the ganglion cells exits through the back of the eye, not far from the fovea
         • no photoreceptors
         • we are unaware of the blind spot because our perceptual system "fills in" the missing part of the visual field

4. visual transduction

   • the process by whereby the characteristics of a stimulus are converted into nerve impulses
   • rods and cones translate light waves into nerve impulses through the action of photo molecules called photopigments
   • the absorption of light by these molecules produces a chemical reaction
     • changes the rate of neurotransmitter release at the receptor's synapse with the bipolar cells
       • the greater the change in the transmitter release, the stronger the signal passed on to the bipolar cell
         • the message is instantaneously on its way to the visual relay station in the thalamus and then onto the visual cortex of the brain

5. brightness vision and dark adaptation

   • the brightness sensitivity of both the rods and cones depends in part on the wavelength of the light
     • rods have greater brightness sensitivity than cones throughout the colour spectrum except at the red end where rods are insensitive
     • cones are most sensitive to low illumination in the greenish-yellow range of the spectrum
   • dark adaptation
     • progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination
       • after absorbing light, a photoreceptor is depleted of its pigment molecules
       • high light exposure = substantial amount of photopigment depleted
       • during the process, the photopigment molecules are regenerated and the receptors sensitivity increases greatly

6. colour vision

   • trichromatic theory
     • any colour on the visible spectrum can be produced by some combination of the wavelengths that correspond to the colours blue, green and red
       • additive colour mixture
     • young-helmholtz trichromatic theory
       • there are three types of colour receptors in the retina
       • individual cones are most sensitive to wavelengths that correspond to either blue green or red
         • each of these receptor classes sends messages to the brain, based on the extent to which they are activated by the light energy's wavelength
   • opponent process theory
     • formulated by ewald hearing in 1870
     • each of the three cone types responds to two different wavelengths
       • red or green
       • blue or yellow
       • black or white
   • colour transduction
     • dual process theory
       • combination of trichromatic and opponent-process theory to account for the colour transduction process
   • colour-deficient vision
     • trichromat
       • people with normal colour vision
     • dichromat
       • colour-blind in only one of the systems (red-green or yellow-blue)
     • monochromat
       • sensitive only to the black-white system and is totally colour blind

7. analysis and reconstruction of visual scenes

   • feature detectors
     • from retina, the optic nerve sends nerve impulses to a visual relay station in the thalamus
       • thalamus = the brain's sensory switchboard
       • input is routed to various parts of the cortex -- primary visual cortex
         • primary visual cortex is located in the occiptal lobe at the rear of the brain 
     • groups of neurons within the primary visual cortex are organized to receive and integrate sensory nerve impulses originating in specific regions of the retina
       • they fire selectively in response to stimuli that have specific regions of the retina
   • parallel processing
     • separate but overlapping modules within the brain simultaneously analyze an object's colors, shape, distance and movement by engaging in parallel processing
       • constructs a unified image of its properties
   • visual association cortex
     • information analyzed and recombined by the primary visual cortex is routed to other cortical regions (visual association cortex)

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