Asymmetry in cerebral blood flow velocity with processing of facial images during head-down rest

INTRODUCTION: Ability to interpret facial expression is crucial for non-verbal communication among humans, and could be affected by changes in cerebral circulation during exposure to microgravity or its simulation. METHODS: There were 16 subjects (8 men and 8 women) who were exposed to 24 h of -6 degrees head-down rest (HDR). Transcranial Doppler ultrasonography was used to monitor mean blood flow velocity (MBFV) in the middle cerebral arteries bilaterally during processing of facial images before, at 6 and 24 h of HDR, and after HDR (Pre-, 6H-, 24H-, and Post-HDR, respectively). The laterality index was assessed as side-to-side differences in MBFV relative to Pre-HDR for each condition. RESULTS: For Pre-HDR, both objects and faces were right lateralized in men (p < 0.001) and showed a left lateralization tendency in women (p > 0.05). At 6H-HDR, both object and faces were left lateralized in men (p < 0.05), but right lateralized in women (p < 0.001). At 24H-HDR, both men and women were left lateralized (p < 0.05). For Post-HDR, both remained left lateralized for all tasks (p < 0.05). DISCUSSION: HDR alters cerebral lateralization for object and facial stimuli, with opposing tendencies in men and women. The gender differences may reflect peculiarities in processing strategy for object and faces between men and women. Men use a right hemisphere processing strategy for faces and women a left hemisphere strategy. The superiority of processing of faces by women compared with men has been attributed to left hemisphere based strategy. HDR alters lateralization patterns and may thus alter processing strategies for faces.