eye - begins w/ capture of light energy by photoreceptors 

• eyespot - predecessor to the eye
  • cluster of photoreceptors
  • sensitive to light, but cannot form images
• sclera - white of the eye, made of connective tissue
• light enters eye through transparent cornea
• iris - colored portion of eye
  • can decrease size of pupil (opening)
• lens focuses image onto the retina on the back of the eye
• light has to pass layer of bipolar cells and ganglion to reach rods/cones
• rods - photoreceptor for black/white vision
  • rhodopsin - photopigment in rods
• cones - photoreceptor for color vision
  • found mostly in fovea (central region of retina)
  • photopsin - photopigment in cones
  • 3 different cones >> 3 color sensitivities (blue, green, red)
• dark >> photoreceptors release neurotransmitter that inhibits bipolar neurons >> less action potential goes to brain
• occipital lobe of brain interprets messages from the eye
  • blind spot - where nerves come out of the eye, leading to the brain
• myopic (near-sighted) - image focused in front of fovea
• hyperopic (far-sighted) - image focused behind fovea
• color blindness - sex-linked trait
  • due to lack of certain type of cones
  • trichromats - people w/ normal color vision
• binocular vision - ability to see 3D images and sense depth
  • due to 2 eyes viewing object from different angles
  • less binocular vision >> larger overall field of view

other sensory experiences - other parts of electromagnetic spectrum used to sense environment 

• heat - wavelengths longer than visible light
  • poor environmental stimulus in water
  • pit viper - only vertebrate known to sense infrared radiation
• electricity - good environmental stimulus in water
  • all aquatic animals general electrical currents from muscle mov’t
• magnetism - eels, sharks, bees, birds navigate according to earth’s magnetic lines
  • used in migration