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Chapter 3: Neurons and Perception

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  • At this intensity, the rod ganglion cell receives ten units of excitation and fires, but each cone ganglion cell receives only two units and therefore does not fire. Thus, the rods’ greater spatial summation enables them to cause ganglion cell firing at lower stimulus intensities than the cones’.
  • This slide provides the students with the stimulus so that they can experience it before it is explained to them on the next slide.
  • This slide provides the students with the stimulus so that they can experience it before it is explained to them on the next slide.
  • This slide provides the students with the stimulus so that they can experience it before it is explained to them on another slide.
  • This slide provides the students with the stimulus so that they can experience it before it is explained to them on another slide.
  • Important to emphasize that the receptive field is on the retina. Students tend to forget this as you work your way through the explanation of more specifically tuned neurons further into the system.
  • This figure marks the beginning of the section discussing pathways beyond the retina. A description of it can be used to set the discussion on the next slide. The more detailed slide of the anatomy appears in the slide after next.
  • This is a good summary of the previous material. It is a good emphasis point that the students can use to make sure they understand how the cells function and how the system works toward ever increasing complexity of stimuli for maximal firing rate.
  • This figure can be used to demonstrate the types of complex stimuli to which certain feature detectors respond. This provides an example of the end of the continuum from simple cells up through more “complex” cells in the striate cortex. Specialized neurons that respond to faces, etc. can also be mentioned. It is also good to think about getting the students to think about whether they should expect to see specific responses from neurons for other sensory modalities.
  • The results of this adaptation experiment should be compared with the following slide showing the response of a simple cortical cell.
  • This figure is used in the text as a demonstration for how adaptation to one set of gratings (the set on the left) affects another set (the one on the right). The next slide explains the results of the demonstration by showing the differing responses of specifically tuned neurons.
  • Neurons tuned to wide-bar gratings decreased firing to wide bars after adaptation to the wide-bar grating, which results in the top right bars as being perceived as narrower. Neurons turned to narrow bar gratings decreased firing after adaptation to the narrow bars, which results in the bottom right bars as being perceived as wider.

Transcript

  • 1. Chapter 3: Neurons and Perception
  • 2. Overview of Questions
    • How do electrical signals represent objects?
    • How does neural processing determine what we see?
    • What is the effect of the environment on developing visual systems?
    • What does it mean to say that perception is indirect?
  • 3. Convergence in the Retina
    • Rods and cones send signals vertically through
      • Bipolar cells
      • Ganglion cells
      • Ganglion axons
    • Signals are sent horizontally by
      • Horizontal cells
      • Amacrine cells
  • 4. Convergence in the Retina - continued
    • 126 million rods and cones converge to 1 million ganglion cells
    • Higher convergence of rods than cones
      • Average of 120 rods to one ganglion cell
      • Average of 6 cones to one ganglion cell
      • Cones in fovea have 1 to 1 relation to ganglion cells
  • 5. Convergence and Sensitivity
    • Rods are more sensitive to light than cones
      • Rods take less light to respond
      • Rods have greater convergence which results in summation of the inputs of many rods into ganglion cells increasing the likelihood of response
      • Trade-off is that rods cannot distinguish detail
  • 6.
    • Figure 3.2 The wiring of the rods (left) and the cones (right). The spot and arrow above each receptor represents light that stimulates the receptor. The numbers represent the number of response units generated by the rods and the cones in response to a spot of intensity of 2.0.
  • 7. Convergence and Detail
    • All-cone foveal vision results in high visual acuity
      • One-to-one wiring leads to ability to discriminate details
      • Trade-off is that cones need more light to respond than rods
  • 8.
    • Figure 3.3 Neural circuits for the rods (left) and the cones (right). The receptors are being stimulated by two spots of light.
  • 9. Lateral Inhibition of Neurons
    • Experiments with eye of Limulus
      • Ommatidia allow recordings from a single receptor
      • Light shown into a single receptor led to rapid firing rate of nerve fiber
      • Adding light into neighboring receptors led to reduced firing rate of initial nerve fiber
  • 10.
    • Figure 3.4 A Limulu s, or horseshoe crab. Its large eyes are made up of hundreds of ommatidia, each containing a single receptor.
  • 11.
    • Figure 3.5 A demonstration of lateral inhibition in the Limulus . The records on the right show the response recorded by the electrode in the nerve fiber of receptor A: (a) when only receptor A is stimulated; (b) when receptor A and the receptors at B are stimulated together; (c) when A and B are stimulated, with B at an increased intensity. ( From Mach Bands: Quantitative Studies on Neural Networks in the Retina, by F. Ratliff, 1965, figure 3.25, p. 107. Copyright © 1965 Holden-Day, Inc. Reprinted with permission. )
  • 12. Lateral Inhibition and Lightness Perception
    • Three lightness perception phenomena explained by lateral inhibition
      • The Hermann Grid: Seeing spots at an intersection
      • Mach Bands: Seeing borders more sharply
      • Simultaneous Contrast: Seeing areas of different brightness due to adjacent areas
  • 13.
    • Figure 3.6 The Hermann grid Notice the gray “ghost images” at the intersections of the white areas, which decrease or vanish when you look directly at the intersection.
  • 14. Hermann Grid
    • People see an illusion of gray images in intersections of white areas
    • Signals from bipolar cells cause effect
      • Receptors stimulated by dark areas inhibit the response of neighboring cells receiving input from white area
      • The lateral inhibition causes a reduced response which leads to the perception of gray
  • 15.
    • Figure 3.7 How lateral inhibition can explain the dark “ghosts” at the intersections. (a) View of four squares of the grid showing the position of receptor A, at the intersection, and the surrounding receptors in the “corridors.” (b) Perspective view of the squares and receptors in (a), showing that each receptor connects to a bipolar cell, and that the bipolar cells that surround A all send a large amount of lateral inhibition to A’s bipolar cell (indicated by the thick arrows). (c) and (d) Same as (a) above but focusing on receptor B, which is located in the corridor of the grid.
  • 16.
    • Figure 3.8 (a) Mach bands at a contour. Just to the left of the contour, near B, a faint light band can be perceived, and just to the right at C, a faint dark band can be perceived. (b) The physical intensity distribution of the light, as measured with a light meter. (c) A plot showing the perceptual effect described in (a). The bump in the curve at B indicates the light Mach band, and the dip in the curve at C indicates the dark Mach band. The bumps that represent our perception of the bands are not present in the physical intensity distribution.
  • 17. Mach Bands
    • People see an illusion of enhanced lightness and darkness at borders of light and dark areas
      • Actual physical intensities indicate that this is not in the stimulus itself
      • Receptors responding to low intensity (dark) area have smallest output
      • Receptors responding to high intensity (light) area have largest output
  • 18. Mach Bands - continued
      • All receptors are receiving lateral inhibition from neighbors
      • In low and high intensity areas amount of inhibition is equal for all receptors
      • Receptors on the border receive differential inhibition
  • 19. Mach Bands - continued
      • On the low intensity side, there is additional inhibition resulting in an enhanced dark band
      • On the high intensity side, there is less inhibition resulting in an enhanced light band
      • The resulting perception gives a boost for detecting contours of objects
  • 20.
    • Figure 3.10 Circuit to explain the Mach band effect based on lateral inhibition. The circuit works like the one for the Hermann grid in Figure 3.6, with each bipolar cell sending inhibition to its neighbors. If we know the initial output of each receptor and the amount of lateral inhibition, we can calculate the final output of the receptors. (See text for a description of the calculation.)
  • 21.
    • Figure 3.11 A plot showing the final receptor output calculated for the circuit of figure 3.10. The bump at B and the dip at C correspond to the light and dark Mach bands, respectively.
  • 22.
    • Figure 3.12 Simultaneous contrast. The two center squares reflect the same amount of light into your eyes but look different because of simultaneous contrast.
  • 23. Simultaneous Contrast
    • People see an illusion of changed brightness or color due to effect of adjacent area
      • An area that is of the same physical intensity appears:
        • Lighter when surrounded by a dark area
        • Darker when surrounded by a light area
  • 24. Simultaneous Contrast - continued
      • Receptors stimulated by bright surrounding area send a large amount of inhibition to cells in center
      • Resulting perception is of a darker area than when this stimulus is viewed alone
      • Receptors stimulated by dark surrounding area send a small amount of inhibition to cells in center
      • Resulting perception is of a lighter area than when this stimulus viewed alone
  • 25.
    • Figure 3.13 How lateral inhibition has been used to explain the simultaneous contrast effect.
  • 26.
    • Figure 3.14 The Benary cross.
  • 27.
    • Figure 3.15 White’s illusion. ( From Perception, 1981, 10, p. 215-230, fig 1a, p. 216. Reprinted with permission from Pion, Ltd., London. )
  • 28. Illusions not Explained by Lateral Inhibition
    • Benary Cross
      • People see differing brightness of triangles even though lateral inhibition is equal
    • White’s Illusion
      • People see light and dark rectangle even though lateral inhibition would result in the opposite effect
  • 29. Explanation of Benary Cross and White’s Illusion
    • Belongingness
      • An area’s appearance is affected by where we perceive it belongs
      • Effect probably occurs in cortex rather than retina
      • Exact physiological mechanism is unknown
  • 30. Neural Circuits
    • Groups of neurons connected by excitatory and inhibitory synapses
    • A linear circuit has no convergence and only excitatory inputs
      • Input into each receptor has no effect on the output of neighboring circuits
      • Each circuit can only indicate single spot of stimulation
  • 31.
    • Figure 3.16 Left: A linear circuit with no convergence. Right: Response of neuron B as we increase the number of receptors stimulated.
  • 32. Neural Circuits - continued
    • Convergent circuit with only excitatory connections
      • Input from each receptor summates into the next neuron in the circuit
      • Output from convergent system varies based on input
      • Output of circuit can indicate single input & increases output as length of stimulus increases
  • 33.
    • Figure 3.17 Circuit with convergence added. Neuron B now receives inputs form all of the receptors, so increasing the size of the stimulus increases the size of neuron B’s response.
  • 34. Neural Circuits - continued
    • Convergent circuit with excitatory and inhibitory connections
      • Inputs from receptors summate to determine output of circuit
      • Summation of inputs result in:
        • Weak response for single inputs & long stimuli
        • Maximum firing rate for medium length stimulus
  • 35.
    • Figure 3.18 Circuit with convergence and inhibition. Because stimulation of the receptors on the side (1, 2, 6, and 7) sends inhibition to neuron B, neuron B responds best when just the center (3 - 5) are stimulated.
  • 36. Receptive Fields
    • Area of retina that affects firing rate of a given neuron in the circuit
    • Receptive fields are determined by monitoring single cell responses
    • Stimulus is presented to retina and response of cell is measured by an electrode
  • 37.
    • Figure 3.19 Recording electrical signals from a fiber in the optic nerve of an anesthetized cat. Each point on the screen corresponds to a point on the cat’s retina.
  • 38. Center-Surround Receptive Fields
    • Excitatory and inhibitory effects are found in receptive fields
    • Center and surround areas of receptive fields result in:
      • Excitatory-center-inhibitory surround
      • Inhibitory-center-excitatory surround
  • 39.
    • Figure 3.20 (a) Response of a ganglion cell in the cat’s retina to stimulation: outside the cell’s receptive field (area A on the screen); inside the excitatory area of the cell’s receptive field (area B); and inside the inhibitory area of the cell’s receptive field (area C). (b) The neuron’s receptive field is shown without the screen.
  • 40. Center-Surround Antagonism
    • Output of center-surround receptive fields changes depending on area stimulated:
      • Highest response when only the excitatory area is stimulated
      • Lowest response when only the inhibitory area is stimulated
      • Intermediate responses when both areas are stimulated
  • 41.
    • Figure 3.21 Response of an excitatory-center-inhibitory-surround receptive field as stimulus size is increased. Color indicates the area stimulated with light. The response to the stimulus is indicated below each receptive field. The largest response occurs when the entire excitatory area is illuminated, as in (b). Increasing stimulus size further causes a decrease in firing due to center-surround antagonism. ( From “Integrative Action in the Cat’s Lateral Geniculate Body,” by D. H. Hubel and T. N. Wiesel, 1961, Journal of Physiology, 155, 385-398. figure 1. Copyright © 1961 by The Physiological Society, Cambridge University Press. Reprinted with permission. )
  • 42. Pathway Beyond the Retina
    • Pathway to visual processing area
      • Optic nerve
      • Lateral geniculate nucleus (LGN)
      • Superior colliculus
      • Striate cortex
      • Extrastriate cortex
  • 43.
    • Figure 3.22 (a) Side view of the visual system, showing the three major sites along the primary visual pathway where processing takes place: the retina, the lateral geniculate nucleus, and the visual receiving area of the cortex. (b) Visual system seen from underneath the brain showing how some of the nerve fibers from the retina cross over to the opposite side of the brain at the optic chiasm.
  • 44. Windows Mac OS 8-9 Mac OS X Right Brain/Left Brain
  • 45. Striate Cortex
    • Simple cortical cells
      • Side-by-side receptive fields
      • Respond to spots of light
      • Respond best to bar of light oriented along the length of the receptive field
    • Orientation tuning curves
      • Shows response of simple cortical cell for orientations of stimuli
  • 46. Striate Cortex - continued
    • Complex cells
      • Like simple cells
        • Respond to spots of light
        • Respond to bars of light
      • Unlike simple cells
        • Respond to movement of bars of light in specific direction
  • 47. Striate Cortex - continued
    • End-stopped cells
      • Respond to:
        • Moving lines of specific length
        • Moving corners or angles
      • No response to:
        • Stimuli that are too long
  • 48.
    • Figure 3.23 (a) The receptive field of a simple cortical cell . This cell responds best to a vertical bar of light that covers the excitatory area of the receptive field. The response decreases as the bar is tilted so that it also covers the inhibitory area. (b) Orientation tuning curve of a simple cortical cell for a neuron that responds best to a vertical bar (orientation = 0). ( From “Receptive Fields of Single Neurons in the Cat’s Striate Cortex, “ by D. H. Hubel and T. N. Wiesel, 1959, Journal of Physiology, 148, 579-591, figure 2. Copyright © 1959 by The Physiology Society, Cambridge University Press. Reprinted with permission. )
  • 49.
    • Table 3.1 Properties of neurons in optic nerve, LGN, and cortex.
  • 50. Feature Detectors
    • Neurons that fire to specific features of a stimulus
    • Pathway away from retina shows neurons that fire to more complex stimuli
    • Cells that are feature detectors:
      • Simple cortical cell
      • Complex cortical cell
      • End-stopped cortical cell
  • 51.
    • Figure 3.25 How a neuron in a monkey’s temporal lobe responds to a few stimuli. This neuron responds best to a circular disc with a thin bar. (From “Coding Visual Images of Objects in Interotemporal Cortex of the Macaque Monkey,” by K. Tanaka, H. A. Siato, Y Fukada, and M Moriya, 1991, Journal of Neurophysiology, 66, 170-189. Copyright © 1981 by the American Physiological Society. Reprinted by permission.)
  • 52. Selective Adaptation
    • Neurons tuned to specific stimuli fatigue when exposure is long
    • Fatigue causes adaptation to stimulus
      • Neuron’s firing rate decreases
      • Neuron will fire less when stimulus presented again
    • Selective means that only those neurons that respond to the specific stimulus adapt
  • 53. Method for Selective Adaptation
    • Measure sensitivity to range of one stimulus characteristic
    • Adapt to that characteristic by extended exposure
    • Re-measure the sensitivity to range of the stimulus characteristic
  • 54. Stimulus Characteristics for Selective Adaptation
    • Gratings are used as stimuli
      • Made of alternating light and dark bars
      • Angle relative to vertical can be changed to test for sensitivity to orientation
      • Difference in intensity can be changed to test for sensitivity to contrast
  • 55.
    • Figure 3.26 (a) Gratings that vary in orientation. (b) Gratings that vary in contrast.
  • 56. Method for Contrast Sensitivity
    • Measure contrast threshold by decreasing intensity of grating until person can just see it
    • Calculate the contrast sensitivity by taking 1/threshold
    • If threshold is low, person has high contrast sensitivity
  • 57. Method for Orientation Sensitivity
    • Use a high contrast grating
    • Measure sensitivity to different orientations
    • Adapt person to one orientation
    • Re-measure sensitivity to all orientations
    • Psychophysical curve should show selective adaptation for specific orientation if neurons are tuned to this characteristic
  • 58.
    • Figure 3.27 (a) Results of a psychophysical selective adaptation experiment. This graph shows that the participant’s adaptation to the vertical grating causes a large decrease in her ability to detect the vertical grating when it is presented again, but has less effect on gratings that are tilted to either side of the vertical. (b) Orientation tuning curve of the simple cortical neuron from Figure 3.23b.
  • 59. Size Adaptation
    • Gratings have bars of different sizes
    • Similar method as for contrast and orientation
    • Size adaptation changes the perception of similarly sized bars
  • 60.
    • Figure 3.28 Stimuli for selective adaptation to size. See text for instructions .
  • 61.
    • Figure 3.30 How neurons that respond best to narrow (N), medium (M), and wide (W) bars respond to the medium-bar grating on the right of Figure 3.28. (a-b): Response before adaptation. (c) Response after adaptation to the wide-bar grating at the top left. (d) Response after adaptation to the narrow-bar grating on the bottom left.
  • 62. Selective Rearing
    • Animals reared in specific environment
      • Limits type of stimuli present
      • Neural plasticity would result in lack of ability to see characteristics unavailable in environment
      • Shows that neurons need environment to develop fully
  • 63.
    • Figure 3.31 (a) Striped tube used in Blakemore and Cooper’s (1970) selective rearing experiments. (b) Distribution of optimal orientations for 52 cells from a cat reared in an environment of horizontal stripes, on the left, and for 72 cells from a cat reared in an environment of vertical stripes, on the right. (Blakemore & Cooper, 1970).
  • 64. Perception is Indirect
    • Stimuli in environment impinge on receptors
    • Transduction takes place causing electro-chemical impulse in neurons
    • Provides accurate information about world
    • Feels direct but that is an illusion
    • This is true for all senses