BISC 421 1st Edition Lecture 24 Outline of Current LectureI.Critcal Period Current LectureCritical PeriodTHE CRITICAL PERIODAltricial Creatures•We are altricial creatures meaning we need our parents to take care of us when we are little-‐ WE ARE BORN TOO YOUNG •Our brains require a critcal period in which our brain is making connectons.Right Hemisphere Co-Opted for Sign Language Learned During Childhood but not AdulthoodSigned languages such as American SignLanguage (ASL) are natural languages that are formally similar to spoken languages, and thus present an opportunity to examine the effects of language structure and modality on the neural organizaton for language. Natve learners of spoken languages show predominantly left-lateralized patterns of neural actvaton for language processing, whereas natve learners of ASL show extensive right hemisphere (RH) and LH actvaton. We demonstrate that the RH angular gyrus is actveduring ASL processing only in natvesigners (hearing, ASL-English bilinguals) but not in those who acquired ASL after puberty (hearing, natve English speakers).Differences in actvaton between natve- and late-learning ASL signers, forEnglish sentences (top) and for ASL sentences (bottom).•Don't need to understand too much about this slide•Brain development and brain plastcity•Brain of natve English speaker who also speaks American sign leanguage•Special region in the brain lights up for sign language•Brain rewires based on what it needs to do-‐ brain is very plastcOcular Dominance as a Model to Understand Mechanisms of DevelopmentalPlastcity•In eyeballs you only get input from one eye•Not untl you get to the cerebral cortex that you get input from both eyes•Some cells only respond to input from right eye and some from left eye-‐ calledocular dominance•Some cells are equally happy to get input from both eyes – called binocular vision important for depth perceptonThe Critcal (or Sensitve) PeriodEyelid suture is a model for conditons such as cataract, which can produce amblyopia (lazy eye or reduced vision through an eye –even when the optcs of that eye are ultmately repaired)•Curve 1-‐7 where 1 is only input from one eye and 7 input from the other eye and others in between•Closed the kittens eyelid and let it grow up and then uncover the eye-‐ ganglion cell isfine but the brain only sees the input from the other eye-‐ even through the eye is fine the brain does not wire to it•If they deprive an adult they find that the cat can stll see • Model for conditons that are common in the populaton-‐ cataracs and lazy eyeAutoradiograhpy: Transneuronal Labeling With Radioactve Amino Acids•An experiment that took radioactve amino acid and stck it in eye and can label the cells that get input from the injected eye •Can see that only half of cortex is labeled – the white and black are equal in weightNormal Animal with Normal Ocular Dominance ColumnsAutoradiograph* of the visual cortex of a mature monkey that received a monocular injecton of 3H tritated proline.Equally white and black which corresponds to the ocular dominance distributonWhen One Eye is Sutured at Birth and Opened During Adulthood, the CorrespondingColumns are Shrunken.Lid suture at one week, at 11 months the deprived eye was injected with 3H. A darkfield micrograph was taken, so that light patches correspond with labeled areas.•When the eye is sutured and then uncovered we can see that the good eye takes over the cortexWhen One Eyelid Is Sutured at 4 Weeks and Opened During Adulthood, the Corresponding Columns Are Slim. Lid suture at four weeks, injected with 3H proline in the normal (undeprived) eye; The dark (deprived eye) columns are slim.•Even after 4 weeks it is not so devastatng as a one week because the brain is not as plastcDeprivaton at seven weeks has a modest effectLid suture at seven weeks, label injected in the normal eye. Slight shrinkage of the unlabeled (dark) columns.•The brain is even less plastc at 7 weeks (think of a kid at a year old)At 8 weeks, the profile is the same as the adult. Thus the critcal periodis over by three months for a monkey (in human, the critcal period lasts for years).•At 8 weeks the profile is the same as adults•For humans this critcal period lasts for yearsAnatomical Explanaton of Ocular Dominance PlasticityTerminal Arborizatons of Lateral GeniculateNucleus Axons CanChange Rapidly in Response toMonocular Deprivaton•If you suture one eyelid and look at cells in the thalamus-‐ we see that there are lots of axons in cortex coming from the non deprived cell and opposite for the deprived cells•These structural changes are cemented in adulthood Our brains are so plastc during development that they actuallyreflect the unique patterns of our retna. Shadows formed by the optc disks carve monocular domains in the primary visual cortex. Brain wiring can be changed by patterns of sensory input.Strabismus –- deficits caused by the inability of the two eyes to focus on the same target.Esotropia – one eye deviates inwardscongenital esotropia accommodatve esotropia,the eye tlts inwards while focusing on a target.Exotropia– one eye deviates outwards •Some only use one eye to navigate world•Strabismus-‐ both eyes see perfectly well but the person cannot make coordinatng or conjugate eye movements. Don't have depth percepton-‐ don't have binocular vision •Opposite is exotropia-‐ one eye fixates outwardsOcular Dominance and StrabismusOcular dominance histograms obtained by electrophysiological recordings in normal and(experimentally induced) strabismic cats.•Cut eye muscle on one side of cat and found that there were no binocular cellsHebb’s Postulate"When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased."D. O. Hebb, 1949 Representaton of Hebb's postulate as it might operate during development of the visual system-- often phrased as “neurons that fire together wire together”.Conclusion: Appropriately synchronized patterns of actvity are required to form the appropriate binocular connectons•The reason why we get binocular vision •Neurons that fire together wire