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Explain the role of individual, psychological, and contextual factors on…
Explain the role of individual, psychological, and contextual factors on the neurophysiological effects of TMS
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TMS is non-invasive neurostimulation and involves electromagnetic current generated via primary coil.
Barker et al (1985) introduced first TMS stimulator and coil that generated visible motor responses (to naked eye).
Repetitive transcranial magnetic stimulation (r TMS) is use for major depressive disorder (Libor Ustohal 2018).
The neurophysiological effects of TMS include the fact it produces magnetic field that interferes with neuron electrical activity by inducing electric currents in neural tissue of brain areas. (The Oxford Handbook of TMS 2008)
Surface of brain contains many neuronal layers that are made up of excitatory and inhibitory neurons, TMS stimulates different neuron populations to generate responses.
To stimulate pathways for mood, the motor threshold must be known, and this is the minimum stimulus intensity to promote motor-evoked potential (Libor Ustohal 2018).
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Sleep deprivation is another individual factors that may influence TMS outcomes. 8 healthy subjects underwent threshold-tracking TMS once, then again after a minimum 24-hour period of sleep deprivation and it was found that sleep deprivation had reduced inhibition and facilitation. There were changes in inhibition-facilitation balance in the primary motor cortex of normal subjects.(Civardi et al 2001).
One study using 42 healthy control subjects highlighted the effects of age on TMS outcomes. The subjects underwent complex testing regarding resting motor threshold (RMT), which is minimum stimulus intensity to cause a liminal motor-evoked response at rest, and Active Motor Threshold which is similar to the RMT except it looks at the intensity to promote motor-evoked response in relation to isometric contraction of a tested muscle at 20% maximum.
It was found there was slightly higher AMT and RMT in older indiividuals compared to younger but the difference was not significant, however, mean cortical silent period was shorter in older controls, meaning they had impaired efficiency of intracortical circuits. This study suggests individual factors such as age may influence TMS outcomes (Oliveiro et al 2006).
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In one study that used 50 unique head models and investigating cerebrospinal fluid volume and brain-scalp distance, there was a correlation with these factors and increased spread, lower intensity for stimulation.
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Individual factors are like individual differences and are factors within people that may affect TMS outcomes
Many individual factors like age, sleep deprivation and cerebrospinal fluid volume can affect TMS outcomes.
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A narrative review investigated effect of expectations before or during non-invasive brain stimulation that may influence brain and behaviour. It was found that expectations may influence motor control, perception and working memory.
Placebo effect is the reduction of pain perception and sensation due to positive verbal suggestions whilst nocebo effect is worsening of symptoms due to negative verbal suggestions. It was suggested expectations may hijack sensation and perception to reduce surprises.
Effectiveness of non-invasive brain stimulation depends on baseline activation of state region and expectations can influence the brain state that is associated with TMS.
The dorsolateral prefrontal cortex is involved in expectations and due to the role of this area in working memory and attention, expectations have potential to influence these factors.
The dorsolateral prefrontal cortex may change activity due to expectations, which may affect the way this area interacts with Non invasive brain stimulation (Braga et al 2021).
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Contextual factors are factors such as verbal and non-verbal interactions between patient and clinician and study location which may cause the patient to travel to location.
Contextual factors may influence psychological factors such as expectations (Braga et al 2021) which would lead to neurophysiological changes previously mentioned.
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Contextual healing (Miller et al 2008) is the idea clinical encounters and the clinician-patient interaction can encourage healing within patients, therefore researchers and clinicians should aim to perfect verbal and non-verbal interactions to enhance contextual healing
To conclude, neurophysiological effects of TMS can be influenced by many factors, so more research should be done into improving various factors and optimising TMS to ensure the best treatment outcomes.