Transcript Mind, Brain & Behavior

Mind, Brain & Behavior
Friday
February 21, 2003
Types of Cones
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Three types of cones respond preferentially to
different wavelengths of light:
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Short wavelengths (419 nm) – blue
Middle wavelengths (531 nm) – green
Long wavelengths (559 nm) – red
All other colors can be produced by
combining different proportions of blue, green
and red.
Anomalous Vision
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Monochromats – people with only rods or
with only one type of cone.
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Dichromats – caused by genetic mutation on
the X chromosome so occurs in men (1% red
blind, 2% green blind).
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Unable to see color – not the same as
achromatopsia due to damage to cortex.
Hybrid gene causes red/green blindness.
Blue blindness rarer and not sex-linked.
Divariant Vision in the Fovea
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Short wavelength cones (blue) are missing in
the fovea.
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The lens focuses short wavelength light in front
of the retina (chromatic aberration).
Because color vision is divariant in the area of
greatest visual acuity, color vision is not used
for fine spatial discrimination.
Color Opponency
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Certain colors are never seen in combination:
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Reddish green, bluish yellow.
Red and green mix to form yellow; yellow and
blue mix to form white.
Hering’s opponent process theory –
perceptual cancellation occurs because colors
are processed as opponent pairs.
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Three color-opponent channels.
Color Processing
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The brain compares responses of three types
of cone cells.
Inputs from the three types of cones are
combined in different ways.
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The brain computes responses of specific cones
but also all cones in the retina (background) to
compensate for ambient light (constancy).
Area V4 responsible for color constancy –
damage results in loss of color experience.
Formation of Visual
Circuits
Chapter 25
Importance of Sensory Experience
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Wiring of the visual circuits in the brain
depends on stimulation of the visual
pathways.
Connections to LGN are stimulated by
spontaneous synchronized firing occurring
independently in each eye before birth.
Connections to cortex are stimulated by visual
experience, synchronized for each eye but
different across the two eyes, after birth.
Hebbian Wiring
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Neurons that fire together form circuits during
critical periods when there is plasticity.
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Synchronized firing of many neurons together is
needed to form ocular dominance columns.
The postsynaptic target cell releases a neural
growth factor that is taken up by the active
neuron.
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Presynaptic terminals take up the growth factor
and add axon terminals, strengthening contact.
Effects of Sensory Deprivation
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Congenital cataracts removed at age 10 –
inability to see form (shape, patterns) but no
impairment of color vision.
Loss of one eye – effect depends on when the
eye closure occurred during development.
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The two eyes compete for space. If one eye is
closed early on, no columns form.
Later on, the spared eye forms larger columns by
spreading into the closed eye’s area.
Developmental Timelines
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Different areas of the brain and different
layers in the same region have different
critical periods.
Integration of input from the two eyes in layer
3 of striate cortex occurs after formation of
the ocular dominance columns.
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Closing one eye at that time impairs depth
perception.
Other Critical Periods
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Language, music, mathematics need to be
acquired before puberty.
Social competence depends on social
stimulation and experience in early childhood.
Studies of deprivation:
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Hospitalism (anaclitic depression)
Genie – child kept locked in a basement until
middle childhood – never acquired language.