Biology subjects

Epp, S. M.

Publications and source records attributed to Epp, S. M..

3 recordsLinked to original sources

bioRxiv2024

Simultaneous quantification of oxygen and glucose consumption during visual stimulation in the human cortex

The brain is fueled by oxidative glucose metabolism, yet activation may uncouple oxygen and glucose consumption. Because the two have never been measured simultaneously in humans across conditions, we combined multiparametric quantitative BOLD (mqBOLD) imaging with functional PET (fPET) during rest and visual stimulation. Participants (n = 12 with complete data) were scanned on an integrated PET/MR system during alternating six-minute blocks of checkerboard stimulation and fixation, with continuous arterial sampling. Cerebral metabolic rates of oxygen (CMRO2) and glucose (CMRglc), and cerebral blood flow (CBF) were quantified per condition and combined into the oxygen-to-glucose index (OGI = CMRO2/CMRglc), where six indicates fully oxidative glucose use. At rest, OGI was 4.6-5.0 in task-responsive visual regions and 4.9 elsewhere. Stimulation increased CMRglc (12-26%) and CBF (10-22%) more than CMRO2; (3-10%, not significant), lowering OGI by 12% in metabolically defined visual regions. Beyond the visual cortex, OGI fell 8%, significantly in 47% of cortical regions. The mechanisms differed, however: glycolytic upregulation in visual cortex (CBF, CMRglc up) versus oxidative downregulation elsewhere (CBF, CMRO2; down; CMRglc unchanged). Simultaneous imaging thus shows that focal activation shifts metabolism toward glycolysis across the cortex, with consequences for interpreting metabolic contrasts outside engaged regions.

neuroscience↗

bioRxiv2023

Two distinct modes of hemodynamic responses in the human brain

Functional MRI measures brain activity by monitoring changes in blood oxygen levels, known as the blood-oxygen-level-dependent (BOLD) signal, rather than measuring neuronal activity directly. This approach crucially relies on neurovascular coupling, the mechanism that links neuronal activity to changes in cerebral blood flow (CBF). However, it remains unclear whether this relationship is consistent for both positive and negative BOLD responses across the human cortex. Here we found that about 40% of voxels with significant BOLD signal changes during various tasks showed reversed oxygen metabolism, particularly in the default mode network. These "discordant" voxels differed in baseline oxygen extraction fraction (OEF) and regulated oxygen demand via OEF changes, while "concordant" voxels depended mainly on CBF changes. Our findings challenge the canonical interpretation of the BOLD signal, indicating that quantitative fMRI provides a more reliable assessment of both absolute and relative changes in neuronal activity.

neuroscience↗

bioRxiv2018

Higher performing older adults upregulate brain signal variability in response to feature-rich sensory input

The extent to which brain responses differ across varying cognitive demands is referred to as \"neural differentiation,\" and greater neural differentiation has been associated with better cognitive performance in older adults. An emerging approach has examined within-person neural differentiation using moment-to-moment brain signal variability. A number of studies have found that brain signal variability differs by cognitive state; however, the factors that cause signal variability to rise or fall on a given task remain understudied. We hypothesized that top performers would modulate signal variability according to the complexity of sensory input, upregulating variability when processing more feature-rich stimuli. In the current study, 46 older adults passively viewed face stimuli and house stimuli during fMRI. Low-level analyses of our stimuli showed that house images were more feature-rich than faces, and subsequent computational modelling of ventral visual stream responses (HMAX) revealed that houses were more feature-rich especially in V1/V2-like model layers. Notably, we then found that participants exhibiting greater face-to-house upregulation of brain signal variability in V1/V2 (higher for house relative to face stimuli) also exhibited more accurate, faster, and more consistent behavioral performance on a battery of offline visuo-cognitive tasks. Further, control models revealed that face-house modulation of mean brain signal was relatively insensitive to offline cognition, providing further evidence for the importance of brain signal variability for understanding human behavior. We conclude that the ability to align brain signal variability to the complexity of perceptual input may mark heightened trait-level behavioral performance in older adults.

neuroscience↗