Table 3.
Substitutive outcomes used in transcranial direct current stimulation research.
| Outcome | Definition | Pros | Cons |
|---|---|---|---|
| Neuropsychological tests | Paper or computerized tests that explore cognitive performance. | Non-expensive, relatively easy to apply, specific tests can be used according to the brain area under study. | Low signal-to-noise ratio, as performance depends on the rater, the subject's characteristics (age, educational level); learning effects, thus requiring a control group. Relatively non-specific. |
| Neurophysiological measurements | Techniques that record electrical brain activity (EEG, qEEG,ERP) as to examine changes in brain activity. | Strong temporal relationship (i.e., very sensible to change), able to index subclinical changes. | Relatively non-specific (measurement of several brain networks) and medium to low spatial relationship. Devices should be adapted to minimize electrical noise of tDCS. |
| Transcranial Magnetic Stimulation (TMS) | TMS used as a tool to index cortical excitability | Relatively affordable and easy to apply, provide several measures of cortical excitability. | Measures obtained with TMS have considerable intra and inter-subject variability; usually applied over the motor cortex only. |
| Neuroimaging methods - Radiotracers | Methods such as PET and SPECT that use radioactivity to assess brain metabolism. | Good temporal relationship, able to index subclinical changes, can be used during "online" tDCS. | Poor spatial resolution, invasive, PET is expensive and not always available (requires a cyclotron). |
| MRI-based neuroimaging methods | MRI-based analyses (e.g. functional, structural, spectroscopy, DTI) that provide static and dynamic neuroimages. | Not-invasive, excellent spatial and temporal resolution (according to the method), specific. | Analyses are difficult and can yield false-positive results, expensive, not always available, tDCS devices and MRI cannot be used simultaneously. |
| Blood chemistry | Blood measurement of substances expressed by the CNS that cross BBB. | Minimally invasive, easy to perform, samples can be frozen and analyzed later, gives a quantitative measurement, sensible to change. | Minimal spatial resolution (brain metabolites are usually not specific of a particular area), low temporal resolution (metabolites must cross the BBB), lack of important biological blood markers in neuropsychiatry. |
tDCS= transcranial direct current stimulation; EEG= electroencephalography; qEEG= quantitative EEG; ERP = evoked-related potentials; PET = positron-emission tomography; SPECT = single photon emission computed tomography; MRI = magnetic resonance imaging; DTI = diffusion tensor imaging; CNS = central nervous system; BBB= blood-brain barrier.