Mind, Brain & Behavior Friday February 21, 2003

Types of Cones  Three types of cones respond preferentially to different wavelengths of light: 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  Monochromats – people with only rods or with only one type of cone. Unable to see color – not the same as achromatopsia due to damage to cortex.  Dichromats – caused by genetic mutation on the X chromosome so occurs in men (1% red blind, 2% green blind). Hybrid gene causes red/green blindness.  Blue blindness rarer and not sex-linked.

Divariant Vision in the Fovea  Short wavelength cones (blue) are missing in the fovea. 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  Certain colors are never seen in combination: 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. Three color-opponent channels.

Color Processing  The brain compares responses of three types of cone cells.  Inputs from the three types of cones are combined in different ways. 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  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  Neurons that fire together form circuits during critical periods when there is plasticity. 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. Presynaptic terminals take up the growth factor and add axon terminals, strengthening contact.

Effects of Sensory Deprivation  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. 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  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. Closing one eye at that time impairs depth perception.

Other Critical Periods  Language, music, mathematics need to be acquired before puberty.  Social competence depends on social stimulation and experience in early childhood.  Studies of deprivation: Hospitalism (anaclitic depression) Genie – child kept locked in a basement until middle childhood – never acquired language.