neural development brain is an intricate machine how is it wired up? hard-wired in genes? learned? philosophical question: nature versus nurture two forms of memory: genes and synapses ---------------------------------------------------------------------- three stages of development genesis of neurons differentiation migration outgrowth of axons and dendrites growth cone cell adhesion molecules neurotrophic factors refinement of connections critical period for development of certain behaviors deprivation during critical period produces permanent effects deprivation after critical period produces little or no effect development of social behaviors imprinting (instinct to acquire a behavior) feral children Harlow: newborn monkeys isolated for 6 mos to 1 yr. crouched in a corner rocked back and forth like autistic children what is the neural basis of critical periods? development of visual behavior dark-reared animals children born with cataracts amblyopia partial or complete loss of eyesight not caused by abnormalities in the eye background: ocular dominance retina, LGN, layer 4c cells are monocular most cells above and below 4c are binocular lid suture expts (Hubel and Wiesel) 10 day-old kitten eyes just about to open sewed shut one eyelid lid opened after ten weeks eyes looked normal (pupil contraction reflex) abnormal cortical responses ocular dominance histogram only 15 % of cells prefer closed eye sluggish responses weak orientation tuning LGN responses are normal, though anatomy shows atrophy light or form deprivation? diffuser: contact lens or nictitating membrane critical period for cat: 4th wk to 4th month before 4th wk, cats don't use vision much during first few wks of critical period, even a few days of closure has dramatic effect on ocular dominance histogram 1 yr lid suture in adult cat: no effect critical period for monkey: birth to 1 yr 4 yr lid suture in adult monkey: no effect recovery opening: no change in ODH after week or more of deprivation reverse suture during critical period, ODH reverses after critical period, no ODH change sight recovers, surprisingly ocular dominance columns inject one eye with radioactively labeled amino acid label is transported to cortex ocular dominance columns can be visualized stripes of sutured eye narrow ocular dominance columns are not fully-formed at birth LGN afferents overlap extensively at birth postnatal retraction into columns competition is it atrophy due to disuse? double suture expt. half cells respond normally 1/4 respond abnormally 1/4 don't respond cooperation activity-dependence strabismus commonest cause of amblyopia surgical treatment age-dependent effects adults: double vision children alternating: good vision in both eyes suppression: bad vision in suppressed eye inject tetrodotoxin into each eye, OD columns do not form stimulate optic nerves synchronously, no OD columns asynchronously, OD columns