Transcript Chapter 2: Introduction to Physiology of Perception

Chapter 2: Introduction to the
Physiology of Perception
Chapter 2: Physiology of Perception
Overview of Questions
• How are physiological processes involved in
perception?
• How is light transformed into electricity in the eye?
• How is what we see determined by the properties
of the receptors in our retinas?
History of the Mind
Basic Brain Structure
• The brain has modular organization
– The sensory modalities have primary
receiving areas
• Vision - occipital lobe
• Audition - temporal lobe
• Tactile senses - parietal lobe
Neurons: Communication & Processing
• Key components of neurons:
– Cell body
– Dendrites
– Axon or nerve fiber
• Receptors - specialized neurons that
respond to specific kinds of energy
– Note: “Receptors” will be used again to
describe the structures that capture
neurochemical signals
Basics of Neural Signals
• Neurons are surrounded by a solution of ions
– Ions carry an electrical charge
– Sodium ions (Na+) - positive charge
– Chlorine ions (Cl-) - negative charge
– Potassium ions (K+) - positive charge
– Electrical signals are generated when such ions cross
the membranes of neurons
Neuroscience Overview
• The Brain: https://www.youtube.com/watch?v=LQ4DlE1Xyd4
•
The Neuron: https://www.youtube.com/watch?v=6qS83wD29PY
•
Membrane Potential: https://www.youtube.com/watch?v=tIzF2tWy6KI
•
Action Potential: https://www.youtube.com/watch?v=W2hHt_PXe5o
•
Synaptic Transmission: https://www.youtube.com/watch?v=WhowH0kb7n0
Recording Neural Signals
• Microelectrodes are used to record from single
neurons
– Recording electrode is inside the nerve fiber
– Null electrode is outside the fiber
– Difference in charge between them is -70 mV
– This negative charge of the neuron relative to its
surroundings is the resting potential
Recording Neural Signals - continued
• Electrical signals or action potentials occur
when:
– Permeability of the membrane changes
– Na+ flows into the fiber making the neuron
more positive
– Then K+ flows out of the fiber making the
neuron more negative
– This process travels down the axon in a
propagated response
Neuron and Neural Impulse
Properties of Action Potentials
• Action potentials remain the same size
• Increase in stimulus intensity can increase
the firing rate of neurons
• Refractory period is 1 ms - upper firing rate is
500 to 800 impulses per second
• Spontaneous activity of action potentials
occurs without stimulation
Changes in firing rate due to changes in intensity
Synaptic Transmission of Neural Impulses
• Neurotransmitters are:
– Released by the presynaptic neuron from
vesicles
– Received by the postsynaptic neuron on
receptor sites
– Matched like a key to a lock into specific
receptor sites
– Used as triggers for voltage change in the
postsynaptic neuron
Synaptic Transmission
Types of Neurotransmitters
• Excitatory transmitters - cause depolarization
– Neuron becomes more positive
– Increases the likelihood of an action
potential
• Inhibitory transmitters - cause
hyperpolarization
– Neuron becomes more negative
– Decreases the likelihood of an action
potential
Light and Vision
•
http://www.nytimes.com/2008/12/23/health/23blin.html?scp=2&sq=blindsight&st=cse
•
Reading 3
Light is the Stimulus for Vision
• Electromagnetic spectrum
– Energy is described by wavelength
– Spectrum ranges from short wavelength
gamma rays to long wavelength radio
waves
– Visible spectrum for humans ranges from
400 to 700 nanometers
– Most perceived light is reflected light
The electromagnetic spectrum
Focusing and Transduction
Light and the Eye
Windows
Focusing Images on the Retina
• The cornea, which is fixed, accounts for about
80% of focusing
• The lens, which adjusts shape for object
distance, accounts for the other 20%
– Accommodation results when ciliary
muscles are tightened which causes the
lens to thicken
• Light rays pass through the lens more
sharply and focus near objects on retina
• Accommodation
Focusing Images on Retina - continued
• The near point occurs when the lens can no
longer adjust for close objects
• Presbyopia - “old eye”
– Distance of near point increases
– Due to hardening of lens and weakening of
ciliary muscles
– Corrective lenses are needed for close
activities, such as reading
Retinal Processing - Rods and Cones
• Differences between rods and cones
– Shape
• Rods - large and cylindrical
• Cones - small and tapered
– Distribution on retina
• Fovea consists solely of cones
• Peripheral retina has both rods and cones
• More rods than cones in periphery
Figure 2.15. The distribution of rods and
cones in the retina.
Figure 2.16 The mosaic of rods and cones in
the peripheral retina of a monkey.
Retinal Processing - Rods and Cones continued
– Number
• 120 million rods
• 5 million cones
• Blind spot - place where optic nerve leaves the
eye
– We don’t see it because:
• One eye covers the blind spot of the other
• It is located at edge of the visual field
• The brain “fills in” the spot- Remember, we
don’t see with our eyes!
Diseases that Affect the Retina
• Macular degeneration
– Fovea and small surrounding area are destroyed
– Creates a “blind spot” on retina
– Most common in older individuals
• Retinitis pigmentosa
– Genetic disease
– Rods are destroyed first
– Foveal cones can also be attacked
– Severe cases result in complete blindness
Figure 2.18 (a)
Simulated macular
degeneration and (b)
retinitis pigmentosa
Transduction of Light into Nerve Impulses
• Receptors have outer segments, which contain:
– Visual pigment molecules, which have two
components:
• Opsin - a large protein
• Retinal - a light sensitive molecule
• Visual transduction occurs when the retinal absorbs
one photon
– Retinal changes it shape, called isomerization
Figure 2.20 Model of a visual pigment molecule.
Psychophysical Study of Isomerization
• Experiment by Hecht et al. (1942)
– Determine the absolute threshold for detecting
a light
– Determine how many visual pigment molecules
the threshold level light would affect
– Results showed that only one photon was
needed to excite one visual pigment molecule
Figure 2.21 The observer in Hecht et al.’s (1942)
experiment could see a spot of light containing 100
photons. Fifty photons reached the retina, and 7 photons
were absorbed by visual pigment molecules. Each of
these visual pigment molecules were most likely
contained in different rods. Hecht concluded that (1) it
takes only 1 photon to activate a rod receptor and (2) we
see the light when 7 rod receptors are activated
simultaneously. Rods are sensitive!!
Physiological Reaction after Isomerization
• Isomerization triggers an enzyme cascade
– Enzymes facilitate chemical reactions
– A cascade means that a single reaction
leads to increasing numbers of chemical
reactions
Enzyme Cascade: Isomerization of one visual pigment
molecule activates about a million other molecules.
Measuring Dark Adaptation
• Three separate experiments are used
• Method used in all three experiments:
– Observer is light adapted
– Light is turned off
– Once the observer is dark adapted, she
adjusts the intensity of a test light until she
can just see it
Measuring Dark Adaptation - continued
• Experiment for rods and cones
– Observer looks at fixation point but pays
attention to a test light to the side
– Results show a dark adaptation curve:
• Sensitivity increases in two stages
• Stage one takes place for 3 to 4 minutes
• Then sensitivity levels off for 7 to 10
minutes - the rod-cone break
• Stage two shows increased sensitivity
for another 20 to 30 minutes
Figure 2.23 Viewing conditions for a dark adaptation experiment. The image of the fixation point falls on the
fovea, and the image of the test light falls in the peripheral retina.
Measuring Dark Adaptation - continued
• Experiment for cone adaptation
– Test light only stimulates cones
– Results show that sensitivity increases for
3 to 4 minutes and then levels off
• Experiment for rod adaptation
– Must use a rod monochromat
– Results show that sensitivity increases for
about 25 minutes and then levels off
Visual Pigment Regeneration
• Process needed for transduction:
– Retinal molecule changes shape
– Opsin molecule separates
– The retina shows pigment bleaching
– Retinal and opsin must recombine to respond to
light
– Cone pigment regenerates in 6 minutes
– Rod pigment takes over 30 minutes to regenerate
– What does this mean?
Spectral Sensitivity of Rods and Cones
• Sensitivity of rods and cones to different parts
of the visual spectrum
– Use monochromatic light to determine
threshold at different wavelengths
– Threshold for light is lowest in the middle of
the spectrum
– 1/threshold = sensitivity, which produces
the spectral sensitivity curve
Figure 2.26 (a) The
threshold for seeing a light
versus wavelength. (b)
Relative sensitivity versus
wavelength -- the spectral
sensitivity curve.
Figure 2.27 Spectral sensitivity curves for rod vision (left) and cone
vision (right). The maximum sensitivities of these two curves have
been set equal to 1.0. However, the relative sensitivities of the rods
and the cones depend on the conditions of adaptation: the cones are
more sensitive in the light, and the rods are more sensitive in the dark.
Spectral Sensitivity of Rods and Cones continued
• Rod spectral sensitivity shows:
– More sensitive to short-wavelength light
– Most sensitivity at 500 nm
• Cone spectral sensitivity shows:
– Most sensitivity at 560 nm
• Purkinje shift - enhanced sensitivity to short
wavelengths during dark adaptation when the
shift from cone to rod vision occurs
Spectral Sensitivity of Rods and Cones continued
• Difference in spectral sensitivity is due to
absorption spectra of visual pigments
• Rod pigment absorbs best at 500 nm
• Cone pigments absorb best at 419nm,
532nm, & 558nm
– Average of all 3 equals 560nm
• These match the spectral sensitivity curves
• Psychophysics (thresholds) can tell us about
the physical properties of rods and cones.
Figure 2.29 Absorption spectra of the rod pigment (R), and the short- (S), medium- (M), and long
wavelength (L) cone pigments. (From J. K. Bowmaker and H. J. A. Dartnall, “Visual Pigments of Rods and
Cones in a Human Retina,” Journal of Physiology, 298, 1980, 501-511. Copyright © 1980. Reprinted with
permission of the author.)