Vision

• Route within the retina

  Bipolar cells send messages to ganglion cells, which then join together to form the optic nerve. Amacrine cells process visual information and send it to other cells
• Negative afterimages result from fatiguing a response by opponent-process cells, leading to a color replacement effect
• Receptors


  Specialized to absorb one kind of energy and transduce it into an electrochemical pattern in the brain
• Retinex theory is proposed to account for color constancy
• Photopigments in rods and cones release energy when struck by light and consist of 11-cis-retinal bound to proteins called opsins
• Midget ganglion cells in the fovea of humans and other primates are small and each receives input from a single cone
• The fovea is a tiny area specialized for acute, detailed vision found in the central portion of the macula
• Perception


  Animals need to perceive the things around them. Objects emit energy that stimulates the receptors that transmit information to an animal’s brain. The brain codes the information, but it does not resemble what is actually seen until it is interpreted
• Rods respond to faint light and are abundant in the periphery of the human retina. Cones are essential for color vision and are abundant in and near the fovea
• Eye


  • Pupil, Iris, Lens, Cornea, Retina
• Coding in the brain


  Impulses in certain neurons indicate light, while impulses in other neurons indicate sound. Frequency of response controls for the intensity of a feeling, like pain
• The blind spot is the point where the optic nerve exits the eye and has no receptors
• Visual receptors

  • Rods
  • Cones
• Opponent-process theory


  Negative color afterimage occurs when staring at a colored object under bright light and then looking at a plain white surface. It involves perceiving color in terms of paired opposites like white-black, red-green, and yellow-blue
• Retinex Theory
• Color Constancy is the ability to recognize the color of objects despite changes in lighting, not explained by trichromatic theory or opponent-process theory
• Further Processing


  Receptive field is a portion of the visual field that excites or inhibits a specific cell. Parvocellular neurons have small receptive fields and respond to details and color. Magnocellular neurons have larger receptive fields and respond to moving stimuli. Koniocellular neurons have various functions and connect to LGN, thalamus, and superior colliculus
• Color Constancy
• Generation of negative afterimages


  Fatiguing a response by opponent-process cells, resulting in a red afterimage when the stimulating color is removed
• Ventral and Dorsal Paths
• Processing in the Retina


  Retina contains many receptors, cells respond to specific visual patterns to extract meaningful data. Lateral inhibition sharpens boundaries of visual objects
• Retinex Theory


  Proposed to account for color constancy by combining information from various parts of the retina in the cortex to determine brightness and color perception
• Overview of the Mammalian Visual System

  Rods and cones make synaptic connections with horizontal cells and bipolar cells. Horizontal cells inhibit bipolar cells, which synapse with amacrine and ganglion cells. Ganglion cell axons form optic nerves, which meet at the optic chiasm. Axons from each eye cross to the opposite side of the brain. Most ganglion cell axons go to the lateral geniculate nucleus (LGN) of the thalamus, then to the visual areas of the cerebral cortex
• Simple and Complex Receptive Fields


  Simple cells have fixed excitatory and inhibitory zones. Complex cells respond to patterns of light in a particular orientation. End-stopped cells have a strong inhibitory area at one end of the receptive field
• Damage in one stream

  Results in different deficits
• Ventral Stream


  • Through the temporal cortex, the perception pathway or the "what" pathway, important for identifying and recognizing objects
• Visual information pathway

  Primary visual cortex (V1) sends information to secondary visual cortex (area V2), which further processes and transmits it to additional areas
• Damage to the dorsal stream (parietal cortex) affects the ability to reach out and grab objects accurately
• Cell receptive field


  • Responds to a bar-shaped pattern of light within its broad receptive field, as long as the bar does not extend beyond a certain point
• Damage to the ventral stream affects the ability to make sense of a television program due to trouble identifying what things are
• Dorsal Stream


  • Through the parietal cortex, the action pathway or the "how" pathway, important for visually guided movements