Biology - Lecture 3 (Dissociations & TMS)

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  • Lecture 3 - Dissociations & TMS
    • Dissociations – may indicate that the two classes of letters are processed differently. 
      • Single dissociations: not sufficient to draw conclusion that there is a qualitative difference between vowel and consonant letters.
      • Double dissociation: evidence that lesions in one region affect verbs and legions in other region affects nouns.
    • Single case vs. group studies:   
      • Group level analysis can be suboptimal, reduces the contribution of irrelevant factors.
      • Single case studies have their risks, patients may be affected by nuisance factors that are difficult to identify. 
    • Transcranial Magnetic Stimulation (TMS)
      • A large current is briefly discharged into a coil of wire held on subject’s head. Current generates a rapidly changing magnetic field around coil of wire, and this field passes into the brain (through neurons).    
      • Effects:
        • TMS can increase or reduce excitability.
        • Results in a disorganisation of neural activity, typically resulting in impaired performance.
        • Effect is similar to that of neurological lesion.
      • Spatial resolution
        • Typically 10-20mm; 5-10mm at best. Influenced by distance from the scalp, connectivity between target and adjacent region.    
      • What can one infer from TMS?
        • Functional-anatomical inference: is area x essential for performing task y?
        • Chronometric (temporal) inference: at what time t does stimulation affect task y?
        • Process dissociation inference: can stimulation selectively disrupt process x without disrupting processes y and z?
        • Process interaction inference: if disrupting process x increases effectiveness of process y, it shows the two processes normally compete.
      • Pros
        • Excellent temporal resolution, good spatial resolution, has the capacity to determine causation, reversible so allows different conditions to be compared. 


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