Evaluation of activity-dependent functional pH and T_1ρ response in the visual cortex

Recent experiments suggest that T₁ relaxation in the rotating frame (T_1ρ) detects localized metabolic changes in the human visual cortex induced by a flashing checkerboard task. Possible sources of the T_1ρ signal include pH, glucose, and glutamate concentrations as well as changes in cerebral blood volume. In this study we explored the relationship of the T_1ρ signal changes related to cerebral blood volume changes by employing inferior saturation pulses. Our hypothesis was that there would be a contribution of cerebral blood volume to the functional T_1ρ signal, but a majority of the signal would correspond to metabolic changes. In addition, the relationship between T_1ρ and pH was explored by manipulating the frequency of the flashing checkerboard and imaging with T_1ρ, BOLD, and ³¹P spectroscopy. We hypothesized that T_1ρ and pH changes would be sensitive to the stimulation frequency. To test this hypothesis, we used a full-field visual flashing checkerboard and varied the frequency between 1, 4, and 7Hz. Supporting our hypotheses, we found that approximately 73% of the measured signal change corresponds to metabolism in vivo and that increasing stimulation frequency increased responses measured by all three imaging modalities. The activation area detected by T_1ρ overlapped to a large degree with that detected by BOLD, although the T_1ρ response area was significantly smaller. ³¹P spectroscopy detected a greater acidosis with the higher stimulation frequencies. These observations suggest that, similar to the BOLD response, the magnitude of the T_1ρ and pH response depends on stimulation frequency and is thus likely to be activity-dependent.