Abstracts

Human brain mapping has undergone a revolution in recent years as a result of advances in imaging techniques such as fMRI and optical recording of intrinsic signals (ORIS). Both techniques are based on the coupling and uncoupling between electrophysiological activity, cerebral blood flow and hemoglobin oxygenation. Controversy exists regarding events which occur in the first few hundred milliseconds following neuronal activation. While some investigators demonstrate an initial dip in oxygenated hemoglobin (HbO2), others report no such event. We investigated the change in deoxygenated hemoglobin (Hbr; 605/630nm) as well as changes in cerebral blood volume (CBV; 546 nm) with high resolution, high speed ORIS in the human brain following bipolar cortical stimulation. Bipolar cortical stimulation was delivered to 8 patients undergoing craniotomy for resection of medically intractable epilepsy as biphasic trains of 1 msec pulses at 60 Hz for 2-3 seconds (1-4 mA). A sterile glass footplate was placed on the surface of the brain to reduce cortical pulsation and a CCD camera was draped sterilely and suspended over the brain. Changes in light reflectance at each incident wavelength were calculated by dividing each image (10 Hz framerate) by an image prior to the onset of cortical stimulation. Experiments consisted of 6-12 trials at each incident wavelength. Bipolar stimulation induced a highly reproducible change in light reflectance which could be seen after only a single trial. Intertrial 2-dimensional correlation coefficients were highly significant (r=0.75;p[lt]0.001). At 605 and 630 nm, a clear dip in oxygenation was seen within 200 ms after stimulation that peaked at [sim] 2 s. This signal was highly focal, compared with the inverted signal that appeared later in the draining veins consisted with a less well-localized BOLD signal. The signal recorded at 546 nm (CBV) was monophasic and was also apparent as early as 200 ms after stimulation. Although highly localized in the first 2 s, the CBV signal then spread to adjacent gyri and peaked at [sim]5-8 s. The average amplitude of the peak of the monophasic signal recorded at 546 nm (CBV: 9.69%) was larger than the initial dip (3.46%) and BOLD signal (-8.48%). Stimulation at increasing amplitudes revealed a nonlinear increase in the ORIS. We find a clear decrease in oxygenation as early as 200 ms after bipolar stimulation of the human cortex. This initial dip is highly localized compared with a later increase in oxygenated hemoglobin (BOLD signal). Changes in CBV occur more rapidly after cortical stimulation than expected and are also highly localized in the first 2 s. However, this rapid increase in CBV may influence the magnitude of the optical signal recorded at 605/630nm since an overall increase in CBV may raise Hbr as well as HbO2. (Supported by NIH (NINDS), Dana Foundation, CURE Foundation.)