Multi-timescale measurements of brain responses in visual cortex during functional stimulation using time-resolved spectroscopy

Abstract

Studies of neurovascular coupling (hemodynamic changes and neuronal activation) in the visual cortex using a timedomain single photon counting system have been undertaken. The system operates in near infrared (NIR) range of spectrum and allows functional brain monitoring to be done non-invasively. The detection system employs a photomultiplier and multi-channel scaler to detect and record emerging photons with sub-microsecond resolution (the effective collection time per curve point is ~ 200 ns). Localisation of the visual evoked potentials in the brain was done using knowledge obtained from electroencephalographic (EEG) studies and previous frequency-domain optical NIR spectroscopic systems. The well-known approach of visual stimulation of the human brain, which consists of an alternating black and white checkerboard pattern used previously for the EEG study of neural responses, is applied here. The checkerboard pattern is synchronized with the multi-channel scaler system and allows the analysis of time variation in back-scattered light, at different stimulation frequencies. Slow hemodynamic changes in the human brain due to HbHbO 2 changes in the blood flow were observed, which is evidence of the system's capability to monitor these changes. Monocular visual tests were undertaken and compared with those done with an EEG system. In some subjects a fast optical response on a time scale commensurate with the neural activity associated with the visual cortex was detected. Future work will concentrate on improved experimental protocols and apparatus to confirm the existence of this important physiological signal.

Fast optical imaging of human brain function

“…Although initially this approach produced images with low spatial resolution, current methodology (based on the use of a large density of recording channels and of particular measurement procedures described in the section Materials and Methods ) has pushed the spatial resolution to a sub-cm level (Gratton and Fabiani, 2003 ). With this methodology, our group (Gratton et al, 1995 ; for a review, see Gratton and Fabiani, 2009 ), as well as others (e.g., Steinbrink et al, 2000 ; Wolf et al, 2002 ; Franceschini and Boas, 2004 ; Lebid et al, 2005 ; Tse et al, 2006 ; Kubota et al, 2008 ; Medvedev et al, 2008 ) have shown that fast optical signals can be recorded consistently, with a combination of spatial and temporal resolution that is adequate for the type of neuroimaging research considered above. However, other groups have questioned this possibility (Steinbrink et al, 2005 ; Radakrishnan et al, 2009 ).…”

Optical imaging of temporal integration in human auditory cortex

“…Wolf et al ., 2000, 2003; Tse et al ., 2006; Tse & Penney, in press) and others using different methodologies (e.g. Steinbrink et al ., 2000; Franceschini & Boas, 2004; Lebid et al ., 2005; see Gratton et al ., 2006 for a comparison of different recording methods). By contrast, only one laboratory has reported inability to obtain a fast optical signal using photon time‐of‐flight measures (Syré et al ., 2003).…”