Chapter 6

• Pupil


  Donut-shaped bands of contractile tissue that regulate the retina
• Iris


  Gives eyes their characteristic color, light enters the eye through the pupil
• Pupil size adjustment


  Represents a trade-off between sensitivity (ability to detect dim objects) and acuity (ability to see object details)
• When pupils are constricted, the image on the retina is sharper and there is greater depth of focus
• Ciliary muscles


  Cause the lens to assume its natural cylindrical shape
• Accommodation


  The process of adjusting the lens configuration to bring images into focus on the retina
• Eyes are coordinated so each point in the visual world is projected to corresponding points on the two retinas
• Binocular disparity


  The difference in the position of the same image on the two retinas, greater for close objects than distant objects
• Types of retinal neurons

  • Receptors
  • Horizontal cells
  • Bipolar cells
  • Amacrine cells
  • Retinal ganglion cells
• Amacrine cells and horizontal cells are specialized for lateral communication
• There is a gap in the receptor layer for the retinal ganglion cell axons to leave the eye, called the blind spot
• Fovea


  An indentation at the center of the retina, about 0.33 cm in diameter
• Completion


  The visual system uses information from around the blind spot to fill in gaps in retinal images
• Surface interpolation


  The process by which the visual system extracts edge information and infers the appearance of large surfaces
• Types of visual receptors

  • Cones
  • Rods
• Duplexity theory


  Cones and rods mediate different kinds of vision
• Photopic vision


  Predominates in good lighting, provides high-acuity colored perceptions
• Scotopic vision


  Occurs in low light, lacks detail and color of photopic vision
• Differences between photopic and scotopic vision result from differences in convergence - many rods converge on a single ganglion cell, while few cones converge
• Spectral sensitivity curve


  A graph of the relative brightness of lights of the same intensity at different wavelengths
• Types of spectral sensitivity curves

  • Photopic
  • Scotopic
• Purkinje effect


  A visual effect observed during the transition from photopic to scotopic vision
• Types of involuntary fixational eye movements

  • Tremor
  • Drifts
  • Saccades
• Transduction


  The conversion of light to neural signals by visual receptors
• Rhodopsin


  A red pigment extracted from rods, its light-absorbing capacity is regained when returned to the dark
• The absorption spectrum of rhodopsin is related to the human scotopic spectral sensitivity curve
• Retina-geniculate-striate pathways


  Conducts signals from each retina to the primary visual cortex via the lateral geniculate nuclei
• Signals from the left visual field reach the right primary visual cortex, and vice versa
• Retinotopic organization


  The retina-geniculate-striate system is organized like a map of the retina, with adjacent stimuli exciting adjacent neurons
• Types of retinal ganglion cell layers

  • Parvocellular (small cell bodies)
  • Magnocellular (large cell bodies)
• Mach bands


  Nonexistent stripes of brightness and darkness that enhance contrast at edges
• Receptive field


  The area of the visual field within which a stimulus can influence a neuron's firing
• Receptive fields are smaller in the foveal area than the periphery, are circular, monocular, and have excitatory and inhibitory regions
• On-center cells


  Respond with "on" firing to light in the center, inhibition to light in the periphery
• Off-center cells


  Respond with inhibition and "off" firing to light in the center, "on" firing to light in the periphery
• Types of primary visual cortex neurons

  • Simple cells
  • Complex cells
• Binocular complex cells have receptive fields in each eye that are in the same position and have the same orientation preference
• Retinal ganglion cells have 20-40 distinct types, with receptive fields selective for various features like orientation, motion, and direction
• Lateral geniculate cells also have receptive fields selective for orientation, motion, and direction
• Contextual influences


  Influences on a visual neuron's activity caused by stimuli outside its receptive field