PSYC20006 Lecture Notes - Lecture 2: Mental Rotation, Cortisol, Evoked Potential
10 May 2018
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Lecture 2
- TMS: non-invasive, virtual lesion, induce neural noise, TMS coils generate two very
strong magnetic fields that rotate in opposite directions → causes current flow that enters
the brain (not very deeply, just top of cortex), know precisely where is stimulated (usually
using figure eight coils)
- Current flow that enters cortex quickly → depolarises neurons (makes neurons fire)
- Firing is random → called noise (not predictable or related to any systematic effects)
- Systematic firing → brain making you do stuff and process things
- Injection of neural noiseapproach findings: time window (40 – 120 ms) is when the
primary visual cortex handles the information and needs to process it in order for one to
be aware → to pass it on to higher brain regions that one can report what is happening;
stimulate more to left → right visual field drops out (that letter cant be reported, stimulate
more to right → left visual field drops out; stimulate above center → bottom letter was
gone, cant do reverse because of a bone and cant get through with TMS
- Another study of injection of neural noiseapproach findings: mask letters (present the
letters → after very brief moment present something else for longer, also called
backwards masking, dont give visual cortex enough time to process first letters
- Researchers investigated whether a visual mask can itself be masked using single-
pulse stimulation, thereby unmasking the stimulus → Usually, backwards masks are
presented after the stimulus, used to suppress perception of the briefly presented visual
stimulus → As TMS can be used to disrupt processing of stimuli, it could potentially also
disrupt processing of the mask, thereby preventing that the stimulus is suppressed
- Without TMS, at 100 ms SOA detection rate was 0.37. With TMS following the mask,
detection rate increased to 0.9 → Unmasking was found between 60 and 140 ms
stimulation after the mask → This technique can inform us about time-course of
processing
- The virtual lesion approach: constantly induce neural noise → virtual lesion that stays
for a while → slows down cognitive process that region is involved in; Using repetitive
TMS to interrupt or enhance cognitive processing; It is also possible to inhibit cognitive
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functions for a longer period of time by applying repetitive TMS (rTMS); It can then be
measured whether (and for how long) a specific cognitive task is impaired (usually
slowing instead of total loss of function)
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- The “probing excitability” approach using single-pulse TMS: For the motor system in
particular, one option is to test how responsive (or “excitable”) the motor cortex is during
a cognitive task; The idea is that if the motor cortex is required for a cognitive task, then
it should already be activated when single-pulse TMS is delivered; Instead of aiming at
disrupting cognitive functions (and measuring the effect of TMS on performance), the
measure of interest is how strongly the motor cortex “reacts” to the pulse itself’
- The excitability of the primary motor cortex can be measured by recording “motor
evoked potentials” (MEPs) using the electromyogram (EMG), which is electrical activity
of muscles
- Left motor cortex controls right side, right motor cortex controls left side
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- One can then measure MEPs for each stimulation and compare average MEPs
between experimental conditions
- Is the primary cortex (M1) involved in the mental rotation of objects?; Some
neuroimaging studies found activation of M1 during mental rotation – which is odd as
nothing is ‘really’ rotated!; Maybe M1 one is simply involved in inner speech?
- Rotating things in head (imagination) takes as long as in reality
- Stimulation of M1 during mental rotation elicited stronger MEPs as compared to
baseline, reading aloud and reading silently; Evidence that M1 is more excitable during
mental rotation might be already activated, and hence, involved in the cognitive process
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