Copyright ©Mark Nelson, 2002. All rights reserved.
Chapter 11: The Visual System
What you need to know
(exam questions will be a drawn from this subset of material)

What are two types of photoreceptors are found in the vertebrate retina? How do they differ?   (p. 255-256)
    - rods and cones
    - rods: B&W vision; more numerous; more sensitive (night vision); found in the periphery; highest density just outside the fovea
    - cones: color vision (different types have different absorption spectra); found mostly in the center (fovea) of the retina
    - an individual primate retina has about 120 million rods and 6.5 million cones

Where does transduction take place in the photoreceptor cell?  (p. 256)
     stacked membrane disks (like pennies in a roll of coins) found in the outer segment of rod and cone cells

What is the name of the photopigment in vertebrate rods?  (p. 256)
     rhodopsin

What is the dark current? How does it affect the resting membrane potential of a photoreceptor in the dark? (p. 257)
    - the dark current is an inward flow of ions into the outer segment of the photoreceptor cell in the dark
    - carried by Na+ ions
    - keeps the photoreceptor cell depolarized in the dark

How does the membrane potential of a vertebrate photoreceptor change when activated by light?  (p. 257)
     counter intuitively, activation by light causes a hyperpolarization of vertebrate photoreceptor cells

What are the key steps in the phototransduction process?  (p. 257)
     activation of rhodopsin by light => activation of transducin (a G protein) => activation of phosphodiesterase (PDE) =>
     breakdown of cGMP => closure of cGMP-dependent Na+ channels in the outer segment => hyperpolarization

What is the role of photoreceptor adaptation?  (p. 258-259)
     allows the visual system to operate under lighting conditions where the mean intensity can vary by many orders of magnitude
    (e.g. natural light levels vary by about 6-7 orders of magnitude between a cloudy, moonless night and a bright tropical beach)

What is one of the cellular mechanisms involved in photoreceptor adaptation?  (p. 259)
    - photoreceptor adaptation involves many mechanisms, one of which is related to free Ca++ in the outer segment;
     - small amount of Ca++ enters through open Na+ channels =>  
       buildup of free Ca++ in outer segment dampens synthesis of cGMP =>
       decrease in cGMP levels leads to a decrease in the number of open Na+ channels
       (negative feedback mechanism)
 
What are the five main cell types of the vertebrate retina?  (p. 259-261)
     photoreceptors, horizontal, bipolar, amacrine and ganglion cells

Which cell types can generate action potentials?  (p. 261)
     only amacrine and ganglion cells generate spikes, the others are non-spiking

What are the output cells of the retina, i.e., what cell type sends sensory axons to the CNS?  (p.260-261)
     ganglion cells

What's connected to what in the retina?  (p. 260-261)
     see Fig. 11-7 and Table 11-1

Which way do the photoreceptor cells "point" in the vertebrate retina?  (p.260)
     they point toward the back of the eyeball

How do the ganglion cell axons exit they eyeball?  (p. 260; Fig. 11-7)
     they exit in a bundle at a spot called the optic disk;
     this creates a blind spot because there are no photoreceptors at that point in the retina

What sort of receptive field organization do bipolar cells have?  (p. 261-264)
     center-surround; bipolar cells come in two types: on-center and off-center

What cell type is responsible for creating the "surround" of the bipolar cell's receptive field?  (p. 261-264)
     horizontal cells

Does an on-center bipolar cell depolarize or hyperpolarize when the center of it's receptive field is stimulated with light?  (p. 262)
     it depolarizes

What are the response properties of retinal ganglion cells to spots of light of various sizes and positions?  (p. 265)
     see Fig. 11-11

What are the differences between X and Y type ganglion cells in the cat visual system?  (p. 264-265)
     X cells have small receptive fields and show tonic responses to sustained stimuli
     Y cells have large receptive fields and have phasic responses; hence they are sensitive to moving stimuli
     Y cells are more numerous in the periphery (peripheral movement detectors)
What are the key difference between serial and parallel processing?  (p.267-268)
    - serial processing involves dividing a large task into several smaller tasks, and then working on each of those smaller task sequentially
    - parallel processing involves dividing a large task into several smaller tasks, and then working on each of those smaller tasks simultaneously
    - the brain uses both serial and parallel processing strategies in visual information processing

Where do visual signals go after they leave the retina?  (p. 268-269)
    - signals travel along axons in the optic nerve to the optic chiasm
    - the optic chiasm splits the signals from the two eyes so that the right visual field goes to the left side of the brain, and vice versa
    - the first stage of processing in the CNS after the optic chiasm is the lateral geniculate nucleus in the thalamus

What sort of receptive field properties are found in LGN neurons?  (p. 269)
     center-surround; similar to retinal ganglion cells

What is the functional role of the LGN?  (p. 269)
     the answer is only partially known; the LGN is described as a "relay station"
     the LGN is a potential target for attentional and arousal mechanisms to control information flow to the visual cortex
What is the next step in the visual processing pathway?  (p. 269-270)
     primary visual cortex

What is the difference between simple cell and complex cell response properties?  (p.270-271)
     simple cells respond best to oriented bars of light; the bar must have a proper position and orientation to maximally excite the cell
    complex cells also respond to oriented bars, but the position of the bar in the receptive field is unimportant

How is the visual cortex organized?  (p. 273-276)
     - in general, the visual cortex is organized topographically (retinotopic map), but it has lots of substructure
    - inputs from the two eyes are segregated into ocular dominance columns; cells within a column respond mainly to input from one eye
    - orientation columns are overlaid on the ocular dominance columns; cells in an orientation column respond best to stimuli at a particular angle
    - interspersed among the orientation columns are "blobs" that do not respond to orientation, but are involved in color processing

Which  invertebrates have single-lens eyes similar to those found in vertebrates? Do these animals have a "blind spot"? (p. 278-279)
     -squid and octopuses are remarkably similar in structure and function to vertebrate eyes
     -but they are thought to have evolved independently
    - the retina is flipped around in the squid and octopus so that the photoreceptors point inward
    - hence the ganglion cell axons leave from the "logical" side of the retina and there is no blind spot

What are the individual units that make up an insect compound eye?  (p. 280)
     - each compound eye is composed of thousands of ommatidia
    - each ommatidium has its own light focusing element (crystalline cone) and light-sensing element (retinula)