Occipital cortex activation studied with simultaneous recordings of functional transcranial Doppler ultrasound (fTCD) and visual evoked potential (VEP) in cognitively normal human subjects: effect of healthy aging.

We evaluated effect of aging, gender and eye (sighting) dominance on relationship between visual evoked flow response (VEFR) and visual evoked potential (VEP), which refers to neurovascular coupling. The VEFR was defined as a percentage increase of the ratio of mean blood flow velocity in the contralateral (according to the side of dominant eye processing) posterior cerebral artery P2 segment to those in ipsilateral middle cerebral artery from the baseline during half-field stimulation. Vasoneural coupling index (CI) was defined as "100 x VEFR/VEP P100 amplitude". Compared to the healthy elderly subjects (n: 19; female/male: 6/13, mean age: 69.7 +/- 7), younger participants (n: 28; female/male: 16/12; mean age: 31.1 +/- 4.7) had significantly higher VEFR for both sides: 18.9 +/- 6.7% versus 11.2 +/- 6.7%, p < 0.001 and 17.3 +/- 7.7% versus 11.8 +/- 5.5%, p: 0.007, for the hemisphere contralateral to dominant and nondominant eye (D and ND side), respectively. Albeit absence of any correlation between their latencies, VEP and VEFR amplitudes were well correlated. However, this was significant only for younger subjects and more evident in D side. The CI was higher in young subjects compared to those in old ones (6.49 +/- 2.79 versus 4.75 +/- 2.35, respectively, p = 0.007). But, this age-related trend remained as borderline when sides were analyzed individually: In the young subjects CI was 5.99 +/- 2.21 and 6.96 +/- 3.22 for D and ND sides, while those were 4.27 +/- 2.60 and 5.19 +/- 2.07 in old ones. This study confirmed diminished visual evoked flow in relation with advancing age, and suggested that "weakened" neurovascular coupling (as evidenced by a decreased VEP and VEFR correlation along with decreased CI) as one of the underlying mechanisms.