5 - vision, touch and pain

• Vision


  The process of perceiving the world through the sense of sight
• You see an object when it emits or reflects light that stimulates receptors that transmit information to your brain
• Coding


  How the brain encodes and interprets sensory information
• Law of specific nerve energies


  Whatever excites a particular nerve establishes a special kind of energy unique to that nerve
• If someone electrically stimulated the auditory receptors in your ear, you would perceive sound
• If your entire brain was flipped upside down, your perceptions would not change
• Eye


  • Light enters through the pupil, is focused by the lens and cornea, and is projected onto the retina
  • Light from the left side of the world strikes the right half of the retina, and vice versa
  • Light from above strikes the bottom half of the retina, and light from below strikes the top half
• Route of visual information within the retina


  Receptors -> Bipolar cells -> Ganglion cells -> Optic nerve
• The blind spot is the point where the optic nerve exits the eye, which has no receptors
• Fovea


  Tiny area of the retina specialized for acute, detailed vision
• Fovea


  • Each receptor connects to a single bipolar cell, which connects to a single ganglion cell
  • Provides 70% of the input to the brain, dominating our vision
• Many bird species have two foveas per eye, one pointing ahead and one pointing to the side
• Predatory birds have more receptors on the top half of their retinas, while prey species have more on the bottom half
• Receptors send their messages to bipolar and horizontal cells, which in turn send messages to amacrine and ganglion cells. The axons of the ganglion cells form the optic nerve, which exits the eye at the blind spot and continues to the brain.
• Birds of prey have many receptors on the upper half of the retina, enabling them to see down in great detail during flight. But they see objects above themselves poorly, unless they turn their heads.
• Convergence of input onto bipolar cells


  In the fovea, each bipolar cell receives excitation from just one cone (and inhibition from a few surrounding cones), and relays its information to a single midget ganglion cell. In the periphery, input from many rods converges onto each bipolar cell, resulting in higher sensitivity to faint light and low sensitivity to spatial location.
• Rods and cones

  • Rods, abundant in the periphery of the human retina, respond to faint light but are not useful in daylight because bright light bleaches them. Cones, abundant in and near the fovea, are less active in dim light, more useful in bright light, and essential for color vision.
• Differences between foveal and peripheral vision


  • Foveal vision: Cones, good detail vision, good color vision.
  • -》Peripheral vision: Proportion of rods increases, responds well to dim light, poor detail vision, poor color vision.
• Although rods outnumber cones by about 20 to 1 in the human retina, cones provide about 90 percent of the brain's input.
• People vary substantially in the number of axons in their optic nerve and the size of the visual cortex, largely for genetic reasons.
• Photopigments


  Chemicals in rods and cones that release energy when struck by light. Consist of 11-cis-retinal (a derivative of vitamin A) bound to proteins called opsins.
• Light converts 11-cis-retinal to all-trans-retinal, thus releasing energy that activates second messengers within the cell.
• Trichromatic (Young-Helmholtz) theory


  People perceive color through the relative rates of response by three kinds of cones, each one maximally sensitive to a different set of wavelengths.
• Long- and medium-wavelength cones are far more abundant than short-wavelength (blue) cones. Consequently, it is easier to see tiny red, yellow, or green dots than blue dots.
• The short-wavelength (blue) cones are about evenly distributed across the retina, but the other two kinds are distributed haphazardly, with big differences among individuals.
• Cones in human retina


  • Short-wavelength cones are evenly distributed
  • Medium- and long-wavelength cones are distributed haphazardly with big differences between individuals
  • Some people have more than 10 times as many of one kind as the other
• Variations in cone distribution produce only small differences in people's color perceptions
• In the retina's periphery, cones are so scarce that you have no useful color vision
• Opponent-process theory of color vision


  1. Brain has mechanisms that perceive color on a continuum from red to green, yellow to blue, and white to black

  2. After prolonged exposure to one color, the fatigued response causes you to perceive the opposite color
• The opponent-process theory cannot fully explain all color afterimage phenomena
• Retinex theory of color vision


  • Cortex compares information from various parts of the retina to determine brightness and color for each area
  • Visual perception requires reasoning and inference, not just retinal stimulation
• Color vision deficiency


  Better term than "color blindness" as complete colorblindness is rare
• Most people with red-green color deficiency are men
• Women with one normal gene and one color-deficient gene are slightly less sensitive to red and green than average
• Genetically engineered mice with an additional cone type showed behavioral evidence of color vision
• Adult monkeys with red-green color deficiency learned to discriminate red from green after gene therapy added a third cone type
• Vision processing


  1. Light strikes retina

  2. Retina sends message to brain

  3. Brain interprets activity of sensory neurons

  4. Brain processes coded sensory information
• You see because light strikes your retina, causing it to send a message to your brain. You send no sight rays out to the object.
• Law of specific nerve energies


  The brain interprets any activity of a given sensory neuron as representing a particular type of sensory information
• Sensory information is coded so that the brain can process it. The coded information bears no physical similarity to the stimuli it describes.