Cerebral Asymmetry Flashcards
anatomical asymmetries
Right hemisphere:
-larger and heavier (fewer neurons, but greater myelination)
-extends further anteriorly
Left Hemisphere:
-extends further posteriorly
-sylvian fissure is a bit longer, with a gentler slope
Temporal Lobes:
-planum temporale (wernicke’s area) is larger by volume in the LH (consistent with language dominance)
-heschl’s gyrus (A1) has 2x volume in the RH (makes sense, since prosody of sound (intonation) is dominant in right)
commissurotomy
- extreme surgical procedure to sever the fibres of the corpus callosum, preventing interhemispheric communication
- results in a split-brain, in which sensory info is sent only to one hemisphere
chimeric face
- study using split brain patients, but instead of flashing pics/words in each field, you flash a face made from a combo of two, such that each half will end up in a separate hemi
- ask them to match the face they saw and respond as quickly as possible (allow freedom to point at an image, say a number, etc)
- more often than not, they pick the face that is presented in the LVF, which ends up in the RH
- suggests that facial recognition/processing is RH dominant (interesting, since fusiform face area is usually bigger in RH)
wada technique
- since blood supply starts out unilateral, you can briefly anesthetize one hemi of the brain by injecting sodium amobarbital directly into one carotid artery (either left or right) via catheter to femeral artery in groin
- the patient lays on a table with both hands up and is asked to count backwards from 100
- if the LH is anesthetized, the right arm will fall and the patient will stop speaking; they may be unable to answer qs asked by the clinician, and may even have impairment memory of the testing after the fact
- if right hemi anesthetized, LH will fall, but counting should be aight
- used prior to surgery as screening tool to decide where key functions are impaired (provides info for cost-benefit analysis)
auditory laterality
-info from each ear ends up in both hemis, but there is a contralateral dominance (once the ipsilateral signals merge at the superior olives, there are more fibres/connections dedicated to contralateral processing, and therefore more rapid conducting
dichotic presentation
- present two different sets of simultaneous info, one to each ear, then ask participants to respond
- result is ipsilateral suppression, and most often the subject will say wat they heard in the right ear (bc its contralateral pathway to the LH is emphasized, and the generally dominant language structures there can process faster that the RH can process info brought in from the left ear)
- R ear (and therefore LH) seem to have a language advantage, while L ear (and RH) may have the advantage in matching melodies (processing of intonation is stronger in RH)
theories of perceptual asymetries
Direct Access Theory:
-the hemisphere that gets the info first, but if that hemisphere isn’t dominant for that type of information then it might be slower
Callosal Relay Model:
-once information reaches the contralateral hemisphere, it may need to be transferred to the side that is best/dominant for that type of info, which may degrade performance (time-wise)
Activating-Orienting Model:
-attentional bias leads to information saliency (bc the hemi is dominant for specific stimuli, it might be biased to pay that more attention; ex eye tracking shows that most eye movements focus on the left side of a face, putting it in LVF and into the RH (dominant for facial processing))
spatial frequency hypothesis
suggests the RH is dominant for low frequency processing (in terms of sound, but also tasks like using low frequency, global processing to determine that two dissimilar images aren’t the same person), whole LH is dominant for high frequency (sounds, but also inspection of fine tuned info and comparison to determine that two similar pictures show the same man)
-trials show that hemispheric differences in auditory processing of pitch are relative to the pitch range (as opposed to having an absolute cutoff)
hemispheric processing and chimeric stimuli
- study that showed split brain patients a chimeric image such that each constituent half ends up the the contralateral hemi
- when asked to freely choose what the image looked like, patients would pick the image that resembled what they saw in the LFV (suggesting a RH dominance for the processing of appearance)
- when asked to match by function or use, patients would select whatever complemented what was seen in the RVF (suggesting left hemi dominance for function)
global vs local processing
- patients with general left of right hemi damage were tested using target stimuli of small objects making up a larger one (such as an M made up of little zs, or a triangle made of smaller rectangles)
- in either test (which ruled out a language dominance confound effect for the lettered stim), patients with RH damage lost the global, overall processing, being only able to remember the component details (ex. just a bunch of small zs), while patients with LH damage got the big picture, but missed out on the detailed, local processing (ex. just the big M)
- suggests RH is involved in global processing, while LH is involved in local processing
inter-hemispheric interaction
-how does division of info into hemispheres?
ask if a digit presented on the bottom is the same as either of the top two: people were faster and more accurate when the matching numbers were presented to the same hemisphere (via the same VF) as opposed to opposite hemis (in which case info must be exchanged between the two hemis)
in a more difficult ordinal task, ask whether the sum of the bottom number and either top number is 10: see opposite results (answers are quicker and more accurate when the relevant info is spread between both hemis, suggesting that some process like math may engage both hemis to get the job done, and splitting the info ensures they both get activated at the same time)
EEG
(electroencephalography)
- functional imaging technique with high temporal resolution but relatively poor spatial resolution
- electrical potential is measure via an electrode array on the surface of the head
- event-related potentials are time-locked and will be visible on the EEG; not single action potentials, but rather a sum of general signaling
- deflections in the ongoing EEG can be caused by exogenous components (loud noise, bright light, touch (env things)) or endogenous ones (recalling an emotional event, shifting attention to pain or pleasure)
EEG electrode placement
- A capital letter indicates the general region over the scalp (C for vertex, F for frontal, O for occipital, etc)
- this is followed by a subscript: z for midline placement, odd for left, or even for right; the magnitude often represents the distance from the midline (the greater the number, the further away)
- used to be placed by hand, but we developed tin electrode caps then high density sensor arrays
