In Search of Baseline: Absolute and Relative Measures of Blood Flow and Oxidative Metabolism in Visual Cortex Stimulated at Three Levels of Complexity

Abstract

Objective: Functional imaging of regional cerebral blood flow (rCBF) or the blood-oxygenation-level dependent (BOLD) signal is based on the premise that changes of flow and energy metabolism are linked to the same incremental excitation of neurons. The baseline conjecture holds that a single default mode of brain function gives rise to coupled changes of blood flow and oxidative metabolism that raise oxygen’s extraction fraction, when activity falls, and lower the extraction fraction when activity rises. To locate the hypothetical baseline in visual cortex in relation to functional demands, we compared increments of (rCBF) and oxidative metabolism (rCMRO2) during stimulation of visual cortex at three different levels of complexity. Materials and Methods: Healthy volunteers underwent positron emission tomography (PET) with [15O] water and [15O] oxygen while they viewed three stimuli in random order; a cross-hair fixation stimulus without color or spatial frequency (FX), an achromatic white disc stimulus with an 8 Hz on-off rate (WD), and a circular yellow-blue chromatic checkerboard with an 8 Hz color-reversing frequency (YB). For the baseline conjecture to hold, the sum of any pair of contrasts must equal the third at any site of significant change, such that the sum of the contrasts [YB-WD] and [WD-FX] must equal the contrast [YB-FX]. Results: The rCMRO2 and rCBF changes averaged 21% and 52% of the whole-brain average, implying excitation by increasing stimulus complexity. The color reversal contrast ([YB-WD]) revealed bilateral sites of significant change of rCMRO2 and a single left hemisphere site of significant rCBF change in the general region of Brodmann’s areas (BA) 17 and 18 of the lingual gyrus. The spatial frequency contrasts ([YB-FX] and [WD-FX]) revealed single sites of significant change of rCMRO2 and rCBF also in the lingual gyrus but close to the midline. For the midline sites, the [YB-FX] and [WD-FX] contrasts were identical and revealed a baseline between the FX and WD/YB stimulus conditions. For the lateral sites, the [YB-WD] stimulus contrast revealed a baseline between the YB and WD stimulus conditions. Conclusion: The intermediate positions of the baselines indicated the existence in BA 17/18 of at least two populations of cells, a lateral group responding to the absence and presence of color reversal in the presence of a spatial frequency, and a midline group responding only to the absence and presence of spatial frequency.