VISION

TABLE OF CONTENTS

Updated: Feb 21, 2008

LECTURE INFORMATION

KEY CONCEPTS IN THIS LECTURE

1. Light entering the eye activates light sensitive rods (sense shades of gray) and cones (sense colors) in the retina. The fovea of the retina is the area of greatest visual acuity and contains tightly packed cones. Rods and cones are unique receptors in that they are normally depolarized; light hyperpolarizes them. Convergence of light input occurs within the retina, ultimately causing ganglion cells to fire. Because of this convergence each ganglion has a receptive field. There are two functional types of ganglion cell: On Cells and Off Cells. These cells provide early discrimination of photic input.

2. Ganglion cells send this information to a relay point, the lateral geniculate nucleus in the thalamus, which codes it and passes it on to the primary visual cortex in the occipital lobe. The visual cortex consists of vertical columns of cells each of which contains color blobs (integrates color input), orientation cells (sensitive to specific position effects) and other specialized cells. Interpretation of these signals, however, occurs in the extrastriate cortices. The dorsal stream from the striate cortex determines where something is and ventral stream determines what it is. The extrastriate cortex has specialized areas responsible for storage of information on form (V3), color (V4) and motion (V5).

3. In other vertebrates, such as the predatory toad, the optic tectum in the midbrain maps visual objects, such as a potential prey item. Small, moving objects elicit predatory behavior (feeding behavior) while large objects trigger avoidance behavior (jumping away).

LECTURE OUTLINE

I. INTRODUCTION TO SENSORY SYSTEMS **

  A. Primary functions of the PNS

  B. Sensory and Motor Components
     1. Review of the sensory and motor systems
          a) Example: Withdrawal reflex

   C. Sensory receptors
     1. There are a number of sensory receptors	
          
  D. How do sensory receptors work?
     1. Graded response
     2. Receptor and generator potentials
     3. Dendritic action potentials occur
          a) Tonic verses phasic firing
               1) Compare and contrast
               2) Examples of tonic and phasic firing** 

II. VISION--THE EYE

  A. Structure/Function of the eye
     1.  Neuronal organization of the retina
          a. Structure/function of the rods and cones
               1) Rods sense levels of gray
               2) Three different cones sense color
               3) Light adaptation in rods and cones
          b. Fovea is area of greatest acuity--highest concentration
              of cones in primates--few rods
     2. Lens
          a. Responsible for light refraction and accomodation
     3. Iris regulates amount of light entering eye

  B. Tranduction and integration of light stimuli
     1. Light signal tranduction
          a. Rods hyperpolarize in response to light (Figs. 6.8, 6.9)
               1) Mechanism: Light acts on photoreceptor to close Na+
                   channels via GTP
               2) Photoreceptors and bipolar cells have no action 
                   potentials
               3) Light disinhibits biopolar cells causing ganglion cells to fire**
          b. Color vision
     2. Neuronal integration in the retina
          a. Rods and cones
          b. Bipolar cells
               1) Center-surround response 
          c. Horizontal and Amacrine cells
               1) Lateral inhibition and lateral movement
          d. Ganglion cells
               1) Ganglion cells have a receptive field
               2) On center-Off surround responses in ganglion cells
                    a) M and P ganglion cells code different light entities
                    b) Importance of ganglion cells in edge effects**
               3) Gratings response in ganglion cells
                    a) X and Y cells 
     3. Summary ( Animation on retinal processing)

III. INTEGRATION AND PROCESSING BEYOND THE RETINA

  A. What happens beyond the retina? **
     1. Retinal output resembles a Seurat painting, but that isn't 
         what we see
     2. An example of the complexities of visual perception

  B. Lateral geniculate nucleus (LGN) is the most important 
      afferent path from the eye
     1. LGN is a map and has 6 layers 
          a. Three layers receive input from the contralateral eye; 
              three from ipsilateral
          b. Magnocellular (grays; fast pathway), Parvocellular and 
              Koniocellular cells (slow pathway; color)
     2. Pathways other than the LGN (to the SCN and superior colliculus)

  C. Primary visual cortex (=Striate cortex, V1) 
     1. Cortical layers and vertical columns called modules
     2. A retinotopic map exists in the striate cortex
     3. Some of the cell types found in the modules
          a. Orientation cells (Single, Complex, and Hypercomplex)
               1)  How orientation cells work
          b. Gratings cells
          c. Retinal disparity cells determine if an object is off to 
              the side
          d. Color blobs detect color

  D. Secondary visual cortex (=visual association cortex, extrastriate cortex) 
     1. Functions of the extrastriate cortex include following movement, 
         form of the object, color, etc.
     2. Dorsal stream to the parietal lobe is involved in movement, 
         position, and 3 dimensional imaging
          a. V5 (=MT) receives input from V1 via V2 and V3a
          b. V5 is involved in discriminating movement
          c. V5 is associated with the somatosensory association
              cortex in parietal lobe
     3. Ventral stream to the temporal lobe is involved in form and color
          a. V4 receives input from the blobs/interblobs in V1 via V2 and V3
          b. V4 is involved in form, color, and face discrimination
               1) Face blindness (Prosopagnosia)
          c. V4 is associated with the TEO and TE regions of the temporal lobe
               1) TE has the largest receptive field; it is activated
                   by complex images (Fig. 6.37)
               2) These temporal areas could be the site of our visual memories 
                    and perception **

IV. SPECIAL TOPIC: NIGHT VISION IN THE CAT

  A. The cat has low acuity in light, but can see well under low light
     1. Large cornea and tapetum
     2. The iris is slit-shaped to control light entering the eye
     3. Lens is highly mobile so it focuses quickly and precisely
     4. 120 degree binocular vision

  B. The cat's retina has abundant rods (almost twice that 
      of humans) but few cones
     1. At night, cats see even slight movements from a great 
         distance ** 
     
V. FEATURE ANALYSIS IN THE TOAD

  A. The visual systems of amphibians and mammals are 
      quite different 
     1. Visual integration in the amphibian brain relies on the 
        optic tectum, not the cerebrum
     2. Ganglion cells in the retina can 
        discriminate a number of shapes

  B. Background on toad behavior and prey capture.
     1. Orient toward prey, Approach, Fixate on prey, and 
        Snap at Prey

  C. The amphibian CNS
     1. Role of optic tectum in visual integration
     2. Feature Analyzers in the optic tectum
          a. What distinguishes prey from predator?
     3. Optic tectum is a retinotopic map

  D. Network of thalamic and tectal nerve cells control a 
     number of motor pattern generators (MPGs) in the toad
     1. Behavioral sequences (Strings of MPG's) 
        often explain animal behavior

STUDY QUESTIONS

Using a flow chart format, outline the correct order of cellular events associated with light transduction in a rod.
Trace the path of light from the cornea to the retina. How is an image altered as it passes through the eye? What is accommodation and how is it achieved? What is the default state for the lens?
Define and give the physiological significance (how or why it is important) for the following terms:
- Retina
- Fovea
- Ganglion cell
- Lateral geniculate nucleus (LGN)
- Striate cortex (=Primary visual cortex, V₁)
- Modules in the striate cortex
- Extrastriate cortex (=Secondary visual cortex)
- Color blob
- On center ganglion cell
- Off center ganglion cell
- Simple orientation cell
- Complex orientation cell
- Hypercomplex orientation cell
- Texture cell
- Spatial disparity cell
- Lateral inhibition (in the retina)
- Adaptation (of a sensory receptor)
- V2, V3, V4, etc.
Transduction of a stimulus usually causes depolarization of a sensory receptor. What is the cellular basis for depolarization of a sensory receptor? One exception to this rule is the photoreceptor which is hyperpolarized by light. Explain how this hyperpolarization is eventually realized as an action potential leaving the retina. Which cells in the retina do not produce action potentials?
Contrast the receptive field properties of the simple, complex, and hypercomplex cells in the primary visual cortex (V₁) of mammals. How would you characterize these neurons as feature detectors? How similar are mammalian and amphibian sight?
How do on-center and off-center receptive fields permit the ganglion cells to see edge effects? How do the horizontal cells contribute to the surround feature of the ganglion cells receptive field? in

Is it likely that a visual image, such as the face of your mother, resides in a single cell in the extrastriate cortex? Explain your reasoning and defend your position.
Compare and contrast the organization of the amphibian and mammalian brain. How are the amphibian and mammlian brains different with respect to processing visual information?
What is the functional organization of the lateral geniculate nucleus (Table 6.2)?
What experimental approaches have been used to understand the visual perception in primates?
How do the dorsal stream and ventral stream function in visual perception of primates?
Except for primates, most mammals do not see color well and many, such as rodents, are color blind. What is a possible evolutionary reason for this absence? How might the functional organization of the striate and extrastriate cortices differ between primates and rodents?
Explain how the primary visual cortex is organized to identify the elements of an image.
What does the organization of the lateral geniculate nucleus tell you about its function?
The light information leaving the retina projects to a number of sites in the brain. Name three of these sites. How is light information used at each of these sites?
Review the general organization of the modules in the striate cortex (Fig. 6.30).
What are the properties of the magnocellular, parvocellular, and koniocellular divisions of the visual system?
How are the TEO and TE areas of the temporal lobe involved in vision? What are their functional characteristics?
How is the retinotopic map in the optic tectum of a toad important in the behavior of the animal? Explain. What is a motor pattern generator (MPG) and how they employed in animal behavior? What factors determine that a specific MPG is activated?
What are some special adaptations in the visual system of nocturnal mammals or of diving mammals that hunt in the dark of the ocean depths? What parts of the lateral geniculate nucleus and visual cortices might be particularly well-developed in these species? Explain. Compare the general characteristics (position of the eyes, etc.) of the visual system in a predator (e.g., cheetah) and its prey (e.g., gazelle).
Thought Question. This is a tough one. The ganglion cells in the retina and the cells in the lateral geniculate nucleus (LGN) both have center-surround receptive fields. However, the primary visual cortex (V1) has a receptive field that responds to the to the orientation of a line, but not to center-surround. Propose a neural circuit that would use center-surround receptive fields in the LGN to produce cells in the V1 that are sensitive to line orientation.
Although we know much about how the visual system functions, what important questions remain unanswered?

ADDITIONAL INFORMATION ON THE INTERNET

Seeing, Hearing, and Smelling the World. An interesting, animated web page from the Howard Hughes Medical Institute dealing with various aspects of our senses, including illusions.

Animations on integration in the visual cortex from the University of Western Ontario

Seeing what you don't see, an experiment on blindsight from Byrn Mawr.

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Introduction	Nerve Structure	Plasma Membrane	Nerve Physiology	CNS
Neurotransmitters	Neurosci. Methods	Vision	Audition & Smell	Autonomic & Motor
Endocrine Physiology	Reproduction	Territory & Aggression	Ingestive Behaviors	Biological Clocks & Sleep
	Navigation & Migration		Learning & Memory