Recordings were considered acceptable when the blood flow velocities could be detected bilaterally, with a clear envelope of the velocity spectrum during the entire cardiac cycle. The visual-evoked Pifithrin-�� price paradigm consisted of 10

cycles, each with a resting phase of 20 s with closed eyes and a stimulating phase of 40 s of silent reading text columns. The text that the study subjects read was the same for all participants and free of strong emotional content. Changes between phases were signalled acoustically using a tone. The complete test cycle had a total duration of 10 min, and was repeated in each position – supine, sitting and 70° head-up tilt (HUT). The reading test and its reliability have been already validated against a checkerboard stimulation paradigm [19]. All signals were visually inspected to identify artifacts or noise, and narrow spikes were removed by linear interpolation. The heart–MCA distance was used to obtain estimates of ABP in the MCA (ABP-MCA). Cerebrovascular resistance index (CVRi) was estimated by the ratio of mean ABP to mean BFV for each heartbeat. For both PCA and MCA, the instantaneous relationship between

ABP and BFV was also used to estimate the critical closing pressure (CrCP) of the cerebral circulation, by extrapolation of the linear regression BFV = a × ABP + b, as previously described [20], [21] and [22]. The inverse of the linear regression slope was also obtained for each cardiac cycle, and it is referred to as “resistance area product” (RAP = 1/a) to differentiate it from CVRi C59 molecular weight [22] and [23]. The CrCP can be obtained from the value of ABP where BFV = 0, that is, see more CrCP = −b/a. All beat-to-beat estimates were interpolated with a third-order polynomial and resampled at 0.2-s intervals to generate a time series with a uniform time base. To become independent from the insonation angle, all parameters were

normalized by the averaged value of the 5 s period prior to activation [8], [15], [17] and [19]. Ten cycles of 20 s rest (closed eyes) and 40 s stimulation (silent reading) were averaged for each volunteer at each position. For each parameter, two different variables were calculated from the evoked CBF in response to visual task: (1) the maximal velocity variation during the 40 s of stimulation and (2) the averaged last 20 s corresponding to a stable phase of flow evoked response. The first one corresponds to the classical overshoot phase used in other fTCD investigations [24], [25], [26], [27], [28] and [29]; the second was included since it seems to be the most stable phase during activation, after the initial overshooting, and allows some comparison with the gain parameter of a second order analysis [14], [15] and [19]. Data were expressed as mean ± standard deviation (SD). Normal distribution of all variables was confirmed by Shapiro–Wilk test and homogeneity of the variances was assured by Levene’s test.