Sensation and Perception

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Sensation and Perception
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Sensation and Perception

• Sensation The process of detecting a stimulus
  and converting it to a neural event.
• Transduction Change energy into an electrical
  signal the nervous system can use.
• Eye Retina, Ear Cochlea, Pain Nerve endings
• Percpeption Your awareness/experience of the
  sensation and your reaction to it.

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Perception

• Interpreting this information after the signal is
  transmitted to the CNS (cortex).
• Light (510 nm) certain receptors certain
  pathway to CNS specific cells in cortex
  intercortical connections green

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Five Primary Senses

• Vision
• Hearing (audition)
• Haptic touch, proprioception, and pain
• Chemical Senses
• Smell (olfaction)
• Taste

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Why Study the Senses?

• Scientific Helps us understand the NS and its
  organization.
• Experiments with the senses help us understand
  the nature of the NS.

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• Aesthetics We use the senses to obtain pleasure.
• Eg. Listen to music, look at art, watch TV, taste
  good food.
• Clinical Reasons Prevent and treat sensory
  deficits or diseases.
• Many people have vision or hearing problems that
  can be prevented or treated.
• Eg. Glasses, hearing aids, lens implants, retinal
  surgery, ear implants, development of the senses.

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The Visual System

• Light The stimulus. Electromagnetic radiation.
  
• Electrically charges particles, photons, that
  radiate in space.
• Visible light is a small portion of the
  electromagnetic spectrum (Figure)

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The Eye

• The receptor for light. Has several components.
• Structural Orbital bones, sclera, fluid
  (aqueous, and vitreous humor).
• Movement Two sets of extraocular muscles
  (rectus, oblique). Tracking and Saccades.
• Interocular muscles (ciliary muscles) allow the
  lens to change shape.
• Optical
• Neural

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Early Stages of Vision

• Optics The refraction of light to focus on the
  retina.
• Cornea Transparent outer portion of the eye.
  Where refraction begins.
• Problems astigmatism, keratoconus.
• Pupil Regulates amount of light.
• Iris Two sets of muscles (circular, radial)
  which control the size of the pupil.
• Problems coloboma - a cleft in the iris. Leads
  to blurry vision.

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• Lens Small onion-shaped structure that changes
  shape (accommodates) to focus light onto the
  retina (focal point).
• Near object gets fatter to increase refractive
  power.
• Far object flattens itself to reduce refractive
  power. Figure.
• In tune with the length of the eyeball (16-17
  mm).
• Problems cataracts, myopia, hyperopia,
  presbyopia

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• If the eye is too short, image is focused behind
  the retina (farsightedness, hyperopia). Treated
  with convex lens.

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• If the eye is too long, image focused is in front
  of the retina (nearsightedness, myopia). Treated
  with concave lens.

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• Receptors/CNS
• Retina Innermost layer where transduction takes
  place. Consists of several layers.
• Photoreceptors Rods and Cones.
• Rods 120 million, mainly periphery, elongated,
  dim light.
• Cones 8 million, mainly central, short and
  stubby, daylight and color vision (Figures)
• Packing of cones determines the limits of visual
  acuity, ie, the smallest object/pattern that can
  be resolved.
• Rods and cones contain photopigments. Rods
  Rhodopsin

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• Three cone photopigments are sensitive to
  different parts of the spectrum.
• Short Wavelength Sensitive (SWS) Most sensitive
  to 430 nm light.
• Mid Wavelength Sensitive (MWS) Most sensitive to
  530 nm light.
• Long Wavelength Sensitive (LWS) Most sensitive
  to 560 nm light (Figure).
• A single cone doesnt allow perception of color.
• Problems If one or more of these cones is
  missing, or deficient, leads to color
  deficiencies.

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• Dichromacy Missing 1/3 pigments.
• Protanopia Lack LWS cone pigment. Red-green
  defect. Confuse light from 550-700 nm.
• Deuteronopia Missing MWS pigment. Red-green
  defect. Confuse light from 550 570.
• These are hereditary and sex-linked (X).
• Tritanopia Missing SWS pigment. Blue-yellow
  defect. Confuse light from 445 480.

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• Anomalous Trichromacy Possess all three cones
  pigments, but one is irregular.
• Protanomaly Abnormal LWS pigment.
• Deuteronomaly Anbormal MWS pigment.
• Show abnormal color matching.
• Also hereditary and sex-linked.
• Monochromacy Possess one cone pigment. Very
  rare. See shades of grey.

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• Collector Cells Horizontal, amacrine, bipolar
  cells. Pass signals on to ganglion cells.
• Retinal Ganglion Cells Form outer layer of the
  retina. Carry neural signals out of the eye.
• Fovea A pit in the central retina where vision
  is sharpest.
• Outer layers are peeled away (Figure)
• Consists of mainly cones.
• Possesses more exclusive connections with the
  ganglion cells.
• Many rods connect to a single retinal ganglion
  cells (convergence).
• Problem Macular degeneration.

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Physiology of the Visual System

• Comes from single-cell recordings in the ganglion
  cells.
• Small spot of light was shone on the retina.
  Eventually get an on response.
• Move and get a off response.
• Single cell responds to increase or decrease in
  light.
• Specific to one area.

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• There is center/surround organization.
• May be on center/off surround.
• May be off center/on surround.
• This antagonistic arrangement is known as a
  ganglion cells receptive field.

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• Vital for seeing details and accentuates
  differences (gratings).
• Not an anatomical structure, its a physiological
  structure.
• Smaller in the central retina (fovea .01 mm
  30? .5 mm).
• Contributes to sharper vision in the fovea.

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Two Types of Ganglion Cells

• Magnocellular (M cells) 20, larger, possess
  thicker axons, larger receptive fields.
• Fast transmission.
• Respond to small differences in light.
• No response to color.
• Poor acuity.
• Parvocellular (P cells) 80, smaller, with thin
  axons, smaller receptive fields.
• Respond slowly to quick changes in light.

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• Respond to different colors in different parts of
  the receptive fields.
• High acuity.
• These two types of cells remain separate and are
  part of two separate systems.
• Magnocellular Respond to fast moving, dim
  objects.
• Parvocellular Respond to stationary objects in
  great detail. Perceive color.
• They operate in parallel.

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Lateral Inhibition

• When you look at adjacent patches of light with
  different levels of brightness, you see a sharp
  edge.
• Due to sideways influence of adjacent cells at
  single cell levels and macrolevels.
• When a cell is stimulated, it tries to inhibit
  adjacent cells.
• See Figure of Mach bands.

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a bc d

• Cells A and B receive the same amount of
  stimulation.
• Cells C and D receive the same amount of
  stimulation.
• Inhibition is proportionate to stimulation.
• A and B, and C and D inhibit equally.

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• However C inhibits B more.
• Thus, Cs response is greater.

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Central Visual Pathways

• All visual info. leaves the eye via the optic
  nerve. This is a blind spot (no photoreceptors
  Fig).
• Consists of myelinated axons of RGCs.
• Problem Multiple Sclerosis.
• Optic nerves meet, cross over at the optic chiasm
  to go over to the opposite side of the brain.
• Contralateral fibers cross over.
• Ipsilateral fibers do not.
• Mixed fibers are called optic tracts (Figure).

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1. Primary (Geniculostriate) Pathway

• Handles higher visual functions of color vision,
  spatial vision, and depth perception.
• Spatial vision The ability to detect objects or
  patterns (visual acuity).
• 80 of fibers connect to the lateral geniculate
  nucleus (LGN) Figure.
• Nucleus in the thalamus with 6 cell layers 2
  M-cell layers 4 P-cell layers.
• Segregated according to input (contralateral -
  1,4,6 ipsilateral - 2,3,5).

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• All have strong center/surround arrangement.
• There are differences between P and M cells in
  the LGN.
• Spatial resolution P-cells have higher acuities.
  Important for spatial vision.
• Temporal resolution M-cells respond faster to
  flickering lights.
• Color vision Only P-cells are sensitive to
  color. There are color-opponent cells.
• Red center/green surround (vice-versa)
• Blue center/yellow surround (vice-versa)

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Color-opponency begins at the retina (RGCs)
Cones MWS
LWS
R
G-
Cones SWS MWS
LWS
B
Y-
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• Continues on to the visual cortex (Area V1) of
  the occipital lobe which consists of 6 layers.
• Connects with layer 4.
• P- and M-cells remain segregated.
• Cortical magnification Larger proportion of the
  cortex devoted to fovea/central retina. Critical
  for spatial vision.
• Cells in the cortex may be simple or complex and
  are extremely specialized.
• Simple clear on/off zones (oval)

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-

- -
-
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• Complex No clear on/off zones.
• 1) Orientation detectors Both simple and
  complex.
• 2) Direction detectors complex cells respond to
  targets that move in a specific direction.
• 3) Size detectors Simple and complex cells have
  preferred stimulus sizes.
• 4) Shape detectors Simple and complex cells
  respond maximally to a certain shape.

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• Contains binocular cells, ie, cells driven by
  both eyes. Most have a dominant eye.
• Cells feeding into a binocular cell are matched
  on preference and visual field.
• These cells depend heavily on experience and are
  critical for stereopsis (depth perception.
• Stereopsis True 3-D vision.
• Problems Cataracts, strabismus
• After the primary cortex, connections go to area
  V2 and beyond.

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Secondary Pathway

• 20 of optic tract fibers go to the superior
  colliculus of the midbrain (Figure).
• An important pathway in lower species.
• Involved in eye reflexes (eg, optokinetic
  nystagmus, OKN).
• Also initiates, maintains, and guides tracking
  movements.
• Not spatially organized. Tend to respond to
  stimuli of any shape.

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• Multisensory respond to audition and vision.
• Respond to peripheral stimulation.
• An older system to detect peripheral targets, and
  initiate movements to follow it.