Functional Transcranial Doppler Ultrasound for Measurement of Hemispheric Lateralization During Visual Memory and Visual Search Cognitive Tasks

Functional transcranial Doppler ultrasound (fTCD) is a noninvasive sensing modality that measures cerebral blood flow velocity (CBFV) with high temporal resolution. CBFV change is correlated to changes in cerebral oxygen uptake, enabling fTCD to measure brain activity and lateralization with high accuracy. However, few studies have examined the relationship of CBFV change during visual search and visual memory tasks. Here a protocol to compare lateralization between these two similar cognitive tasks using fTCD is demonstrated. Ten healthy volunteers (age 21±2 years) were shown visual scenes on a computer and performed visual search and visual memory tasks while CBFV in the bilateral middle cerebral arteries was monitored with fTCD. Each subject completed 40 trials, consisting of baseline (25 s), calibration (variable), instruction (2.5 s), and task (20 s) epochs. Lateralization was computed for each task by calculating the bilateral CBFV envelope percent change from baseline and subtracting the right side from the left side. The results showed significant lateralization (p < 0.001) of the visual memory and visual search tasks, with memory reaching lateralization of 1.6% and search reaching lateralization of 0.5%, suggesting that search is more right lateralized (and therefore may be related to "holistic" or global perception) and memory is more left lateralized (and therefore may be related to local perception). This method could be used to compare cerebral activity for any related cognitive tasks as long as the same stimulus is used in all tasks. The protocol is straightforward and the equipment is inexpensive, introducing a low-cost high temporal resolution technique to further study lateralization of the brain.