Biphasic changes in tissue partial pressure of oxygen closely related to localized neural activity in guinea pig auditory cortex

An understanding of the local changes in cerebral oxygen content accompanying functional brain activation is critical for making a valid signal interpretation of hemodynamic-based functional brain imaging. However, spatiotemporal relations between changes in tissue partial pressure of oxygen (PO₂) and induced neural activity remain incompletely understood. To characterize the local PO₂ response to the given neural activity, the authors simultaneously measured tissue PO₂ and neural activity in the identical region of guinea pig auditory cortex with an oxygen microelectrode (tip < 10 μm) and optical recording with voltage-sensitive dye (RH 795). In addition, a laser displacement gauge and a laser-Doppler flowmeter were used to monitor the spatial displacement and regional cerebral blood flow, respectively, in the PO₂ measurement region. In the activated region, tissue PO₂ initially decreased during the ∼3-seconds after the onset of acoustic stimuli, and then increased during the next ∼5 seconds. Such biphasic changes are consistently found in cortical layers I to IV. In addition, amplitude of the biphasic change was closely related to detected peak height of the optical signal changes. The results suggest that the initial decrease in tissue PO₂ is coupled to the induced neural activity and depends on response time of local increase in cerebral blood flow.