- 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).
- 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.
- Excellent temporal resolution, good spatial resolution, has the capacity to determine causation, reversible so allows different conditions to be compared.
- Dissociations – may indicate that the two classes of letters are processed differently.