and hence offers a much more comprehensive picture of changes related to effects of a given mental stimulus. the spectral density estimates would be least affected by artefacts that lack periodicity, and filtering would reduce the effect of noise. the changes at the c-peak may show cortical long-term potential (cltp) or cortical long-term depression (cltd), which has been proposed to be suggest equivalents of cortical activity during learning and cognitive processes. the flow velocity tracings are monitored during paradigm 1 comprising a checkerboard square as object perception are compared to whole face (paradigm 2) and facial element sorting task (paradigm 3). fast fourier transform calculations are used to obtain the spectral density and cross amplitude plots in the left and right middle cerebral arteries. the c-peak also called memory (m-peak) cortical peak could be seen arising during paradigm 3, a facial element sorting task requiring iterative memory recall as a subject constantly spatially fits the puzzle by matching each facial element in paradigm 3 to that stored in memory (paradigm 2) before proceeding to form the picture of the whole face. accuracy although tcd is not always accurate due to the relative velocity of blood flow, it is still useful for diagnosis of arterial occlusions in patients with acute ischemic stroke, especially when using the middle cerebral artery. a research study has been performed to compare power motion doppler of tcd (pmd-tcd) with ct angiography (cta), both are valid, but pmd-tcd accuracy is not higher than 85 percent. the advantages of pmd-tcd is portability (so it can be used in the bed side or in the emergency room), subjects patients to less radiation compared to cta (so can be repeated, if necessary for monitoring) and is less expensive than cta or magnetic resonance angiography.