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Understanding Hemispheric Differences in Cognitive Neuroscience

Explore hemispheric asymmetries, corpus callosum functions, dichotic listening, handedness impact, and language components.

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Understanding Hemispheric Differences in Cognitive Neuroscience

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  1. Last Lecture • Hemispheric differences in the Bisected Brain • Functions of the corpus callosum • Hemispheric Asymmetries in the Normal Brain • Methods and theories

  2. This Lecture • Dichotic Listening • The corpus callosum & resource allocation • Handedness • Functions of the corpus callosum • The aphasias

  3. Announcements • Quiz next week • Study guide on the web after class today • Cognitive Neuroscience chapter 8 only sections on Language and the Brain (303-312)

  4. With dichotic input the ipsilateral ear’s input is supressed. Both ears project bilaterally Contralateral projection is stronger right left right left right left ga ga ga ba ba ba - - ga ba ba ga - Dichotic Listening

  5. The Corpus Callosum and Processing Resources • Resource: basic raw materials needed to get a task done. • Each hemisphere has some competancy with most tasks. • As task difficulty increases, both hemispheres are recruited to meet task demands.

  6. Two Comparison Conditions Within Field Comparison Across Field Comparison A B A B + + A A A B B A A A Unilateral Processing Bihemispheric Processing (fromBanich & Belger, 1989)

  7. A B A B A B C D C D + + + A A a Three Levels of Task Difficulty 3-Item Physical 5-Item Physical 5-Item Name Physical Identity Match: A-A Name Identity Match: A-a

  8. 750 WITHIN ACROSS 650 550 450 350 250 3-PI 5-PI 5-NI Sometimes, Two Hemispheres are Better than One... • For easier tasks within field matches are faster. • For harder tasks across field matches are faster. Reaction Time

  9. Functions of the Corpus Callosum • Unify the sensory world • e.g. copies of input • Information transmission • to access specialized processors (more on this with case V.J) • to share products of cognitive processes • Allocation of resources (Banich task)

  10. Handedness • 10% of the population is Left Handed • Left handers have different brain organization What is left-handedness?

  11. Case V.J.- Clues to handedness(Baynes et al) • a left hander w/ normal speech & language who underwent callosotomy. After surgery: • left hand single-letter writing - perfect • right hand entirely agraphic left hand right hand

  12. Left hand Right hand

  13. Isolation of grapho-motor module • In VJ, writing dissociates from other language abilities. • Writing in RH • All others LH • In JW & VP all language abilities are co-localized in LH.

  14. Implications of V.J. • Writing module can be dissociated from other language modules. • Handedness may depend on laterality of writing module. • In all Left handers access to this RH module may require the c. callosum. • Laterality of other language modules may also vary in Left handers.

  15. What are the other components of language?

  16. Components of Sound-based Language • Phonemes- units of sounds that make up words • Morphemes- smallest unit of meaning • Syntax - grammar-- rules for combining words • Lexicon - stored representations of words (auditory/visual) • Semantics - meanings that correspond to lexical items • Prosody- intonation of speech (Right Hemisphere province) • Articulation- movements of the mouth and vocal tract for producing speech • Discourse - Linking of sentences into coherent narrative. Production and comprehension of these components are required for language.

  17. Broca's aphasia • speech: labored, slow & nonfluent (awkward articulation, search for phonemes). • paraphasic errors (phonemic: pelsil for pencil) • responses make sense but are ungrammatical. • greatest difficulty: verbs, articles, pronouns (generating & repeating) --> telegraphic speech • Comprehension is relatively spared

  18. 44 39 22

  19. Cookie Theft Picture (BDAE)

  20. Lesion locus of Broca’s aphasia

  21. Is Broca’s aphasia simply a problem with vocal-motor coordination? After all...Broca’s area is near motor face area... NO!! • No dysfluency after hemifacial paralysis from RH damage • Dysarthria - a separate disturbance- lack of coordination of vocal track.

  22. Broca’s aphasia has cognitive/linguistic properties • Better fluency for memorized phrases. • Singing may be more fluent than speech • --> Melodic intonation therapy • Why should coordination problems affect verbs/function words more than nouns??? • Written output shows same errors as speech! These features suggest more than a problem with vocal coordination.

  23. Wernicke-Geschwind model • Broca's a.: forms detailed coordinated plans for language production (spoken, written, covert/rehearsal) • Explains dysfluency and poor articulation Butcomprehension is not perfect... • Schwartz, Saffran and Marin, 1980 • Poor syntax comprehension • Broca's aphasics poor at judging grammaticality Active: The horse kicked the cow. Passive: The cow was kicked by the horse.

  24. Agrammatism... • difficulty using & understanding grammar Modification of the W-G model Broca's area: • Plan for coordinating language production • understanding and using syntax. Active: The horse kicked the cow. Passive: The cow was kicked by the horse.

  25. 44 39 22

  26. Wernicke's aphasia • Speech: phonetically & grammatically normal but meaningless. • generally fluent, unlabored, well articulated. • normal intonation (prosody). • words used inappropriately, nonsense words (neologisms) --> "word salad" • meaning expressed in roundabout way (circumlocution). Cookie Theft description... • "Mother is away here working her work to get better, but when she's looking into the two boys looking in the other part. She's working another time..." • Comprehension: severely impaired.

  27. Interpretation of Wernicke's aphasia According to Wernicke-Geschwind model Wernicke's Area • stores memories of sound sequences that constitute words. • translates auditory input into phonological forms that can then access semantics. • meaning is stored in other cortical areas • During spontaneous speech, "cognitive" areas send input to Wernicke's area: • Cognition -> Wernicke's A. (22) -> arcuate fasciculus -> Broca's A. (44)-> Face area

  28. READING

  29. Reading According to theWernicke-Geschwind model... Wernicke’s A. essential for reading... • Visual processing --> Angular gyrus --> Wernicke's A.--> Semantics • Angular G. (39) translates visual code to a form that can access auditory word code in Wernicke’s.A. • Semantics must be accessed via the auditory (phonological) form of the word. • Implication: Reading requires phonological recoding via Wernicke’s area… or does it??

  30. PET evidence challenges this accountPosner, Peterson, Raichle and colleagues (1988) • If auditory recoding is necessary for reading then reading and listening should activate the same brain areas • They do not... • Implication- visual representation can access meaning directly LISTENING READING

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