Lesson 4 - The Visual System

• The Central Nervous System consists of the brain and the spinal cord
• • Light: Electromagnetic radiation; travels as wave
  • Amplitude: height of waves; perception of brightness
  • Wavelength: distance between peaks; perception of color
• • Purity: how varied the mix of wavelengths is
  • Saturation: relative amount of whiteness in a color, or richness of colors; intensity of color
• Two Purposes of the Eyes:

  • Channeling light to neural tissue that receives it.
  • Housing the tissue.
• • Cornea: where light enters the eye
  • Lens: focuses the light rays on the retina
• Accommodation: the curvature of the lens adjusts to alter visual focus
• Nearsightedness: close objects seen clearly; distant objects blurry
• Farsightedness: distant objects seen clearly; close objects appear blurry
• Iris: colored ring of muscle, constricts or dilates via amount of light
• Pupil: regulates amount of light coming into the eye
• Retina: absorbs light, processes images, sends visual information to the brain
• Optic Nerve: bundle of neurons that carries visual information from the retina to the brain
• Optic Disk/Blind Spot: point where the optic nerve exists the eye and where they are no photoreceptors
• Rods: Black and white/low light vision; more sensitive in dim light–peripheral vision; greatest density just outside the fovea. Rods outnumber cones by a huge margin
• Cones: Color and daylight vision; do not respond well in dim light. Cones provide better visual acuity (sharpness/precise detail)
• Photoreceptors: Light-sensitive cells in the retinathat convert light energy to neural impulses
• Rods: Sensitive to dim light but not colors
• Cones: Sensitive to colors but not dim light
• Fovea: area of sharpest vision in the retina; densely packed cones
• Retina: Light-sensitive layer at the back of the eyeball
• Dark Adaptation: Eyes become more sensitive to light in low illumination (enter dark theater on bright day). Complete in 30 minutes; major progress in first 10 minutes
• Light Adaptation: Eyes become less sensitive to light in high illumination (leaving school to go to your car/bus)
• Bipolar Cells: combine information from photoreceptors; send results to ganglion cells
• Ganglion Cells: integrate information into single firing rate to optic nerve
• Horizontal Cells: connect receptors
• Amacrine Cells: connect bipolar to bipolar; ganglion to ganglion
• Receptive Field: area on the retina that, when stimulated, affects the firing of that cell. Come in a variety of shapes and sizes
• Lateral Antagonism: neural activity in a cell opposes activity in surrounding cells
• Discovered feature detectors: neurons that respond selectively to lines, edges, etc.
• Simple Cells: respond most strongly to bars of light in their “favorite” orientation
• Complex Cells: respond most strongly to moving bars of light in their “favorite” orientation
• Hypercomplex Cells: respond most strongly to moving bars of light of a particular length or angle
• BASICS OF COLOR VISION:

  • Wavelength – Intensity (amplitude)
  • Color – Brightness
  • Wavelength – determines color
  • Longer = red / Shorter = violet
  • Amplitude – determines brightness
  • Purity – determines saturation
  • Hue: the qualitative experience of color of the light stimulus
  • Saturation: purity/vividness of color sensations
  • Brightness: intensity of light
  • Color: psychological sensation derived from the wavelength of visible light–color, itself, is not a property of the external world
• Subtractive Color Mixing: remove wavelengths of light leaving less there
• Additive Color Mixing: superimposing lights, putting more light in the mixture than exists with one light by itself
• Afterimages: visual sensations that linger after the stimulus is removed; color of the image is the complement of the color you originally stared at
• color blindness - inability to distinguish colors
• Reversible figures: drawings that have two interpretations that can shift back and forth
• Perceptual sets: motivational forces can foster perceptual sets
• Inattentional blindness: failure to see objects because attention is focused elsewhere