LEC3 PS20106

Slide1 : Lecture 3 Cerebral Lateralisation Biological Psychology PS20106 Dr Gemma Calvert


Plan of lecture : Plan of lecture Historical overview Popular theories of hemispheric functions Anatomical asymmetries Methods for testing functional asymmetries Wada Neuropsychological patients Split brain patients Normal volunteers Brain imaging Theories of hemispheric specialisation


Historical Overview : Historical Overview Cerebral hemispheres: equal size,weight & surface area and appear symmetrical But for centuries the effects of unilateral brain damage have shown major functional differences Most prominent has been effect of LH damage on language function (Broca 1861) In 1864, Hughlings Jackson suggested that the human brain cannot be duplicating function in both hemispheres, the LH is dominant (in right-handers)


Right Brain – the neglected hemisphere : Right Brain – the neglected hemisphere Cerebral dominance concept LH controls speech & higher functions, RH subordinate/no special function) Arose as no clear evidence of specific functional impairments after RH damage Later, Jackson argued RH dominates in visual perception (patient with RH damage had problems recognizing objects, persons or places) These ideas, ahead of their time, went largely ignored until 1930’s, when more data showing specialised roles for RH were collected using newly introduced psychological tests in large patient groups Historical Overview


Right Brain: early structure/function studies : Right Brain: early structure/function studies Patients with RH damage consistently do more poorly on non-verbal tests involving Manipulation of geometrical figures Puzzle assembly Completion of missing parts of patterns/figures Tasks with form, distance & space relationships NB – these are not everyday tasks so often unnoticed Patients self-reports also included Disturbances in orientation and awareness (neglect) Unable to recognise familiar faces Singing/music Historical Overview


Slide6 : Free Viewing Perceptual Asymmetries When asked to indicate which image appears darker overall, people typically pick the stimulus that is darker on the left side, irrespective of whether it is on the top or bottom. Normal participants show a robust preference of approximately 65% for the leftward stimulus. This is an example of a naturally occurring leftward bias of attention


Slide7 : Asymmetries in facial communication The Mona Lisa, like most painted portraits, turns her left cheek toward the artist so that the left side of her face is featured more than the right. This leftward bias is stronger for portraits of women than men. Research suggests that the key factor underlying the bias is related to the desire to express emotion. The left side of the face, because it is controlled by the emotional right hemisphere, is more expressive of emotion.


Slide8 : Portraits showing the right side are judged as happier whereas portraits featuring the left side of the face are judged to express sadness more strongly. When asked to say which image is happier, people tend to pick image A (right side of the face). Asymmetries in communication of emotional valence


Slide9 : Popular dichotomies of left and right brain function


Wada Testing : Wada Testing Wada test unambiguously reveals the language hemisphere. This technique estimates 96% have LH language


Anatomical Asymmetries : Anatomical Asymmetries Handedness is not strongly correlated with language dominance (50% of left handers have LH language) Therefore most anatomical studies have looked for structural asymmetries in brain regions associated with language function Heschl’s gyrus (larger on right) Sylvian fissure (slopes higher on right) RH more pronounced in front right, LH in left back Planum temporale: larger on left (65%) 11% > right and 24% no asymmetry BUT, reports of correlation with dichotic listening, DVF tasks Dyslexics reduced left PT


Slide12 : Anatomical Asymmetries


Testing hemispheric function : Testing hemispheric function Wada testing √ Neuropsych testing of stroke victims √ Split brain patients Psychological tests in healthy volunteers


Inter-hemispheric communication : Inter-hemispheric communication The two cerebral hemispheres are interconnected by the largest fiber system in the brain, the corpus callosum In humans, this bundle of white matter includes > 200 million axons Most callosal projections link together homotypic areas (directly opposite), a few project to heterotopic (non-opposite) areas


Slide15 : Inter-hemispheric communication


10-07 : 10-07 Communication across the callosum enables information from both visual fields to contribute to the activity of receptive field cells It has been suggested that callosal connections could play a role in synchronizing oscillatory activity in cortical neurons as an object passes through these receptive fields In this view, callosal connections facilitate processing by pooling together diverse inputs Other researchers however suggest that callosal fibers may be inhibitory, providing a means for each hemisphere to compete for control of current processing


Commissurotomy : Commissurotomy Because the corpus callosum is the primary means of communication between the 2 hemispheres - severing these fibers can therefore inform about the functional specialisation of each hemisphere In 1960’s, Roger Sperry and Michael Gazzaniga carried out numerous studies in patients who had undergone commissurotomy (severing of the callosal fibers) to contain the spread of epileptic seizures The procedure allows subsequent assessment of how the two hemispheres operate in relative isolation (visual information presented to one hemisphere does not transfer)


Split brain patients : Split brain patients Patient report “As I reach to retrieve something to wear in the closet with my right hand, my left hand comes up and takes something different” Reports like this are common, but the effects go away soon after surgery Suggests that the cerebral commissures transmit information that is inhibitory in nature – ie one hem leads to callosal transmissions that serve to moderate, decrease or stop certain activities in the other Problems also associating names with faces (have to associate a feature instead – Dick has glasses) – disconnection of speaking hemisphere from RH perceptual regions?


Slide19 : Each visual field projects to one hemisphere By presenting information briefly (to avoid rapid eye movements to the opposite visual field) into each isolated visual field, and consequently, hemisphere, researchers are able to test the functions or preferences of each hemisphere The split-brain patient reports through the speaking hemisphere only the items flashed to the right half of the screen and denies seeing left-field stimuli or recognizing objects presented to the left hand The left hand correctly retrieves objects presented in the left visual field of which the subject verbally denies having any knowledge


Slide20 : Partial resection of the CC allows researchers to investigate specific transfer functions of the CC by region. E.g. when the splenium is spared, visual information is transferred normally, but not tactile information. Anterior transfers semantic info, posterior transfers sensory info.


Slide21 : The right hemisphere is capable of understanding language, but not syntax. It also has trouble making inferences: can’t say how individual words are causally related, and usually is unable to produce words. However, some patients can learn to use their right hemisphere to produce words – but they can’t share this information with the left hemisphere (i.e. not able to say if object A (shown to left hem) and object B (shown to right hem) are the same. Split brain: language skills of the RH


Slide22 : Report on the ability of both hemispheres to reproduce a picture. A patient with severed connections between the cerebral hemispheres was asked to draw the cross and cube seen in the middle of the picture. Despite the fact that he was right-handed, he was almost entirely incapable of reproducing the pictures with his right hand (controlled by the left hemisphere), whereas he was able to do it relatively well with his left hand (which is controlled by his right hemisphere) Split brain: visuo-spatial skills in LH vs RH


Split brain: perception and attention : Split brain: perception and attention Although object identification processes work in isolation, crude information re. spatial locations can be shared. A cue presented in one hemisphere can be acted upon by the other hemisphere that saw nothing.


Slide24 : Non-linguistic functions involving complex visual and spatial processes Perception of part-whole relationships (arcs to circles task) Block design Copying drawings Split brain: implications for RH function


Split brain: implications for language : Split brain: implications for language Confirms that in most people, control of speech is localised in LH BUT, systematic testing of different components of language suggests RH has some linguistic abilities RH can match printed words and pictures, but cannot usually match words that rhyme indicating that the RH lacks ability to do phonological decoding RH grammatical abilities match that of 5 yr old child


What is the basis of observed RH superiority in visual-spatial tasks? : What is the basis of observed RH superiority in visual-spatial tasks? Although both hemispheres may be equally skilled in perceiving spatial relationships, RH could be dominant for the expression of visual understanding, just as LH is dominant for expression of language understanding This distinction emphasizes an asymmetry in the ability to perform the complex motor acts required by these tasks (sensory to motor mapping is disconnected) Alternatively, there may be true diffs in perceptual abilities in LH and RH: L hand of (R handed) split brain subs much better at matching unseen tactile object with corresponding visual image, esp as shapes became less geometrical and more free form LH difficulty became more marked as objects became less describable verbally


Problems with split brain research : Problems with split brain research Epilepsy poor model of normal brain Potential for re-organisation of language in early development Information could still transfer via subcortical structures


Testing hemispheric asymmetries in normal subjects : Testing hemispheric asymmetries in normal subjects Divided visual field studies (although information transferred via CC, RTs faster when information presented to hemisphere specialised for the task Dichotic listening studies


Slide29 : Dichotic listening tasks (a) Subjects report more words presented to right ear (LH) (b) Left ear (RH) better at recognising melodies


Slide30 : Chimeric faces task. Normal subjects tend to report the expression presented to the right hemisphere (falling in left visual field). Does not pit hemispheres on RTs. Free Viewing Perceptual Asymmetries


Problems with laterality research : Problems with laterality research Effects are small and inconsistent Information can be transferred by CC Biased reporting of positive findings


Neuroimaging : Neuroimaging


Slide33 : Neuroimaging While most people show a left-biased but still bilateral organisation of language, others have strongly lateralised pattern


Inferences from normal studies : Inferences from normal studies LH specialised for speech perception and production, detection of rapid cues, memory encoding RH specialised for emotion, face processing, music, visuo-spatial processing, spatial attention, memory retrieval


Inferences from normal studies : Specialisation is relative not absolute RH in split-brain can comprehend words RH can evolve (unmask?) language abilities following LH stroke (plasticity) Imaging suggests most tasks involve both hemispheres, each performing complementary functions Researchers attempt to establish some common denominator to explain different hemispheric biases (what do RH tasks have in common – is there something fundamental about the RH processing capabilities?) Inferences from normal studies


Theories of hemispheric specialisation : Theories of hemispheric specialisation Instead of breakdown by type of task (verbal v spatial), may be better categorised as different ways of dealing with the information Different operations/computations are more or less efficiently carried out in one/other hemisphere (i.e. just happens these tasks tap into these different underlying operational/low level biases. LH language dominance consequent on LH’s superior analytic skills, of which language is but one manifestation Similarly, RH superior visuospatial performance derived from its synthetic, holistic manner of dealing with information New ways of conceptualising LH/RH differences


New ways of conceptualising hemispheric differences : New ways of conceptualising hemispheric differences Left Hemisphere Matches on basis of verbal descriptions of properties of blocks and 2D patterns Matches by function Right Hemisphere Matches blocks on basis of overall appearance Matches by appearance


Slide38 : Information can be represented at multiple scales At its most global scale, this drawing is of a house We can also recognise and focus on the component parts of the house (local features, doors, windows, roof etc). This description is hierarchical in that the finer levels of description are embedded in the higher levels Theories of hemispheric specialisation


Slide39 : David Navon (1977) created stimuli that could be identified on one of two levels – the global level (“H”), or local level (“F’s”. Initial findings showed that the perceptual system first extracts the global shape with interference occurring when two levels are discordant. Suggests that 2 distinct systems might represent local and global information. Theories of hemispheric specialisation


Slide40 : Although both hemispheres can abstract either level of representation, they differ in the efficiency with which they represent local and global information Note that the drawings made here by LH and RH damaged patients are consistent for both linguistic and nonlinguistic stimuli, suggesting a task-independent representational deficit.


Slide41 : RTs to hierarchical letter stimuli presented in LVF or RVF in normal subjects. Subjects judged whether the stimulus contained (at least) one of two letters H or L. Implication: LH more adept at representing local information and the right hemisphere was better with global information. This is consistent with the pattern observed in neurological patients. (Sergent, 1982)


Slide42 : Neuroimaging Components of tasks seem to be distributed across the hemispheres. Global-local distinction RH vs LH


So what is lateralised? : So what is lateralised? Two hemispheres may work in concert to perform a task, even though their contributions may vary (no requirement for a master director who decides which hem does what). E.g. language not exclusive LH, RH may also contribute although the types of representations it derives not as efficient for certain types of tasks. LH does not defer to RH on visuospatial tasks but processing the information in a different way. In this view, much of the clinical hemispheric data tell us more about our tasks, than the computations of LH and RH.


What is lateralised theoretical arguments : What is lateralised theoretical arguments Focal regions vs diffuse organisation (Semmes, 1968) LH collection of focalised regions RH diffusely organised Two modes of cognitive processing (Levy, Sperry, Bogen) LH Logical, analytical, computer-like, analyses information sequentially, abstracts out relevant details, attaches verbal labels RH Synthesizer, concerned with overall stimulus configuration, organises information in terms of gestalts or wholes Distant inference from data, no direct support of this model LH specialised for certain motor functions (Kimura) LH specialised for certain kinds of motor function, both verbal and non-verbal Neurological control of speech evolved out of a system of motor control of gesture


The Frequency Hypothesis : The Frequency Hypothesis Frequency hypothesis (Sergent, 1982) LH neurons specialised for high frequency AV info, RH neurons specialised for low frequency AV info


Categorical vs coordinate hypothesis : Categorical vs coordinate hypothesis Categorical vs coordinate representations in memory (Kosslyn, 1987) LH categorical (high frequency), RH coordinate (low frequency


Summary : Summary Shift from strict dichotomy to hemispheric biases Shift from description of different tasks each hemisphere is better at, to emphasis of different processing/computational styles Recognition of parallel processing efficiency Understanding that some systems are lateralised (sensory systems), others can still use subcortical routes (attention)