Week 1: Transcranial Magnetic Stimulation Study Guide With Questions Fully Solved.

What is TMS? - correct answer Non-invasive technique to create virtual, reversible cortical lesions that temporarily interrupt cognitive function in order to understand the function of specific brain regions Why do we use TMS, rather than patients? - correct answer Not enough patients with circumscribed lesions to study all cognitive functions Lesions in single, specialized areas are rare Recovery and brain elasticity can compensate for lesions overtime (other regions take over functions) How is TMS applied? - correct answer Externally with a coil (figure-eight coil) on scalp to produce a magnetic field that induces electrical currents in brain Currents depolarize neurons in a small area, causing them to fire randomly This increases neural noise, masking the neurons that are firing correctly Process can be modified to create fast sequences of pulses (rTMS, repetitive) What is the history of TMS? - correct answer Fritsch & Hitzig (1870)- electrically stimulated animals D'Arsonval (1896)- discovered magnetic stimulation of cortex Magnusson & Steven (1911)- developed head coil Barker & Freestone (1985)- developed TMS What are the use of coils in TMS, particularly the figure-eight one? - correct answer Diff coils generate different precisions of stimulation Figure-8: generates magnetic field in opposite direction, where the loops meet stimulation is strongest (allowing a more focal area than a round coil) What are the four uses of TMS for biological research? - correct answer Injection of neural noise using single pulse TMS The virtual lesion approach using repetitive TMS The probing excitability approach using single pulse TMS Probing information transfer using the paired pulse approach What is the injection of neural noise approach using single pulse TMS? - correct answer Injection of neural noise to interrupt cognitive processing (function is disrupted/interfered) at time window where region is required to find out its role in a process (causal involvement) Testing causality by interfering at time period where most required What is are two example studies that used the neural noise approach? - correct answer 3 alphabetical letters under difficult viewing conditions; TMS applied 2cm over visual cortex; measured effects on perception by varying time point of stimulation Found a critical period (40-120ms) as TMS affected the performance most at that time Backward masks presented after stimulus, to suppress perception of initial stimulus but TMS can disrupt processing of this mask (unmasking- cancels this mask)- detection rate at 90% with TMS, 60-140ms after mask (time course of processing) What else can be done with TMS, like the virtual lesion approach? - correct answer Approach: using repetitive TMS (10 mins) and extended neural noise to interrupt/enhance cognitive processing Measures whether (and for how long) a specific cognitive task is impaired (slowing, not total loss) What else can be done with TMS, like with the probing excitability approach for the motor cortex? - correct answer Helps measure how responsive the motor cortex is during a cognitive task (if required, already activated)- interest in how strong the motor cortex reacts to TMS Recording the motor evoked potentials (MEPs) using the electromyogram (EMG)- measuring how the motor cortex reacts to TMS 1. TMS over LH area on motor cortex 2. Neurons fire cause activity in RH 3. Activity of muscles recorded as MEPs What is a study involving this excitability approach with the motor cortex? - correct answer Stimulation of primary cortex (M1) during mental rotation elicited stronger MEPs compared to baseline like reading, suggesting M1 already activated during the process (strategy and diff stimuli doesn't play role) Study does not say if M1 was more excitable because adjacent regions were activated Where else is TMS useful, particularly using the paired-pulse approach to probe information? - correct answer Uses two pulses, in brief succession (one is sub-threshold, second is supra-threshold) Measures how strongly the first pulse influences the second What are two studies that involve the paired-pulse approach? - correct answer 1. In schizophrenia, apparent reduced cortical silence period (CSP; period of tonic motor activity following excitatory activity) Produce pulse to produce excitatory period, then measured excitatory through assessing effect of second impulse through MEPs Found that patients, compared to control, showed stronger responses to second pulse, suggesting deficits in motor inhibition 2. To see if primary visual cortex (V1) necessary for visual awareness V1 is first, V2 second found no effect V2 first, V1 second after 45ms, less phosphenes perceived; suggesting that back-projections from V2 to V1 required for awareness Where has TMS been clinically applied? - correct answer Treatment for depression (linked to imbalance in prefrontal activity between hemispheres) TMS stimulates one hemisphere