Cerebral lateralisation
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are there differences between the two hemispheres or is there complete symmetry?
why always duplicate a brain region when you already have one doing the job?
could each half of the brain provide a different view of the world? perhaps have life of its own?
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hemispheric specialisations are best conceived as superimposed on the fundamental symmetry characteristic of brain structure and function
even if there are asymmetries, it does not mean that the task is strictly or always localised
distributed operations that might span both hemispheres are usually involved
each hemisphere has the machinery and is able to take over if necessary (plasticity)
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animal experiments, split-brain patients, unilateral lesion patients , kids below 5 (immature corpus callosum)
behavioural studies - patient W.J. (1961):
no changes in his personality after the operation but could name and describe visual stimuli presented only to his right visual field (disconnected
right hemisphere visual info from language centers on the left hemisphere)
nonverbal responses could be used with no problem (point, demonstrate function of etc)
could not arrange 4 red/white blocks in a simple pattern with his right hand (disconnected specialised systems in the right hemisphere from
motor apparatus in the left)
if both hands were allowed to try the task, the right hand sabotaged the good work done by the left (hemispheric competition?)
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left-hemisphere specialisation for language is true for 96% of the population (verified today by brain-imaging studies as well)
dominace of left hemisphere for language not strongly correlated with handedness: half the population of left-handers (8% of the population)
also have it
motor tasks with the right hand slow down more compared to the left hand while speaking
functional asymmetries might be reflected on structural asymmetries:
lateral fissure has a more prominent upward curl in the right hemisphere? visual cortex larger in right hemisphere?
planum temporale (encompassing Wernicke’s area) larger in the left hemisphere of 65% of right-handed people (11% the opposite and 24% no
asymmetry)
this asymmetry extends also to some subcortical structures (e.g. lateral posterior nucleus of the thalamus)
but 95% of right-handers show left-hemisphere language dominance!
newer, more accurate imaging techniques question the validity of these earlier findings on gross morphology differences
microanatomical differences (cell density, degree of myelination, columnar organisation, fine dendritic structure, connectivity, total dendritic
length, levels of various neurotransmitters) have also been reported but…
- callosal projections mainly and extensively
link together homotopic areas of the brain
- less frequently they also connect heterotopic
areas of the brain but with less extensive
connections
- mostly “association” cortices are connected primary cortices are not so much connected
by the callosum:
- there are a few connections between the
most eccentric regions of the visual field
between visual cortices
- very sparse connections in the primary
motor and somatosensory cortices
- callosal connections are usually thought as
synergistic - but could they also be
inhibitory, revealing a competition between
the two hemispheres for the control of
current processing?
- cats and monkeys loose approximately 70%
of their callosal axons during development
- autistic patients have a smaller callosum
(number and diameter of axons? proportion
myelinated? thickness of sheath? vessels?)
- gender, handedness, mental retardation,
schizophrenia correlates?
Hemisphere communication
specificity of callosal function
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callosum matures between 5-10
years of age (younger kids are bad in
combining intra-hemispheric info)
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callosum subdivisions are organised
into functional zones
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posterior regions (splenium) are more
concerned with visual information
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more anterior regions carry auditory
and tactile information
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anterior part seems to be involved in
high-order transfer of semantic
information
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results from two-stage procedure in
neurosurgery are enlightening:
transfer of visual, tactile and and
auditory info is severy disrupted at
first, by sectioning the posterior half
the transfer of semantic info about
the stimulus remains and is abolished
in the 2nd stage
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Wada test (50s): the ability to speak or to comprehend speech is disrupted for
up to several minutes
behavioural studies on laterilisation
the right hemisphere is dominant for
visuospatial tasks (inconsistent result?)
whereas the left for language and speech (but
split-brains can learn)
profound deficit in the interhemispheric
transfer of sensory (both vision and touch)
and motor information in split-brain patients
Navon (1977): local vs. local stimuli. RT times faster for identifying global elements + non-symmetrical interference
(global unaffected).
Robertson (1988): in patients with unilateral lesions: left-sided lesions slowed down local target identification whereas
patients with right-sided lesions were slower in global identification
Robertson (1993): split-brain patients are faster with local targets in the RVF and global targets in the LVF
both hemispheres can do it but differ in efficiency: left is local whereas right is global?
Auditory perception
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vision studies are easier
because of complete
laterilisation of input:
e.g. subjects are more adept to
recognise that a string of letters
forms a word when this string is
presented to the RVF
for auditory perception we can
use the dichotic listening task in
which we assume that the
ipsilateral projections from each
ear are suppressed:
a right-ear advantage is
revealed in linguistic memory
tasks whereas the opposite was
found for melody recognition
The spatial frequency hypothesis: right is low (global) and left is hight (local)
- Sergent (1982): the spatial frequency hypothesis as a computational basis for hemispheric
asymmetries in visual perception
- speech perception also depends largely on high sounds frequencies, whereas prosody (disrupted
more by lesions on the right hemisphere) depends on low frequencies
Sergent (1985): performance on a male-female task (low spatial-frequency) revealed a
LVF advantage, whereas performance on an identification task (high spatial-frequency) a
RVF advantage
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in vision: identification vs.
localisation (ventral vs. dorsal
pathways)
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studies of hemispheric
asymmetries have not been
limited to “what” but also to
“where” problems
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Kosslyn (1987): categorical vs.
coordinate representations of
spatial relationships
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left-hemisphere forms
categorical representations
(essential in recognising
objects, e.g. letters)
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right-hemisphere forms
coordinate representations
(essential for action)
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Kosslyn (1989): near-far task
seems to verify this hypothesis
BUT had problems with
replication, disappeared with
practice and also has
alternative explanations
the categorical vs. coordinate theory and an alternative SF explanation
Wolford (2000): split-brain patients hemipsheric differences in problem solving:
maximising (animals + right) vs. matching (humans + left)
Prototypes (left) vs. Exemplars (right)
- lesions in the right hemisphere make people deficient in both expressing
as well as recognising facial expressions
- in normal subjects, right hemisphere dominates when there is a conflict
Do the 2 hemispheres share a common attentional mechanism or not?
4 other theories on laterilisation:
Speech production requires the bilateral coordination of the muscles required fro articulation which might have created
pressure to localise control in a single focus in order to avoid slow interhemispheric communication and so a left-hemisphere
advantage for language followed, since this hemisphere had primary access to the speech production system (in general:
things happen faster when one ‘person’ decides!)
Davidson (1995): right vs. left prefrontal cortex in approach vs. withdrawal behaviors:
- left frontal lobe damage causes severe depression whereas right frontal lobe patients appear manic
- it also happens that language is the most social of all behaviours
- emotional stimuli differentially activate left and right depending on whether they are positive or negative in nature
- differential human EEG activation in reward vs. penalty trials in a financial task
- higher left-EEG in 3-year-olds which are quite content to leave their mother and play with toys (and vice versa)
Hemispheric specialisation
in animals
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birds do not have a corpus callosum and
are better in categorising (e.g. food/no
food) stimuli viewed in their right eye
left eye better in tasks that have to do
with learning specific colours, shapes,
sizes and spatial locations
this supports the prototype vs. exemplar
hypothesis?
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sectioning a canary’s hypoglossal nerve
in the left hemisphere severely disrupts
song production (right-hemisphere
lesions have little effect)
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nonhuman primates might prefer using
one hand over the other but there is no
consistent trend in motor preference
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Rhesus monkeys like humans have
better tactile discrimination of shape with
left hand
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split-brain monkeys have a righthemisphere advantage in face
recognition tasks and a left-hemisphere
advantage in in orientation discrimination
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left-hemisphere lesions in the Japanese
macaque produces a relative mild (and
transient) impairment in comprehending
the vocalisation of conspecifics