Are VEP correlated fast optical signals detectable in the human adult by non-invasive near-infrared spectroscopy (NIRS)?

The potential of near infrared spectroscopy (NIRS) to detect vascular changes in cerebral cortical tissue elicited by functional stimulation has been established (1). The vascular response is considered the result of neuronal activity in the investigated area and forms the basis of imaging techniques such as BOLD-contrast fMRI and PET. In the animal optical methods have been shown to detect optical changes in the illuminated tissue which exactly follow the time course of electrical potential changes, thus optical techniques can potentially assess both the 'fast' neuronal and the 'slow' vascular response. Lately a group of investigators has reported data showing that the 'fast optical signal' is detectable in the adult human in response to a visual stimulus ("optical VEP" termed EROS, i.e. event related optical signal). We failed to reproduce these results, with an almost identical instrumentation and experimental protocol. The negative result is reported in this paper. To check the expected magnitude of such changes we performed a simulation based on data reported in the animal and a human head model of optical properties. The result indicates that changes in optical properties to be expected in a non-invasive approach in the human adult are about three orders of magnitude smaller than those reported previously by the group of Gratton and co-workers (2). Also they are so small that they are below the noise level of the presently available NIRS monitors.