Neurochemical responses to chromatic and achromatic stimuli in the human visual cortex.

In the present study, we aimed at determining the metabolic responses of the human visual cortex during the presentation of chromatic and achromatic stimuli, known to preferentially activate two separate clusters of neuronal populations (called "blobs" and "interblobs") with distinct sensitivity to color or luminance features. Since blobs and interblobs have different cytochrome-oxidase (COX) content and micro-vascularization level (i.e., different capacities for glucose oxidation), different functional metabolic responses during chromatic vs. achromatic stimuli may be expected. The stimuli were optimized to evoke a similar load of neuronal activation as measured by the bold oxygenation level dependent (BOLD) contrast. Metabolic responses were assessed using functional 1H MRS at 7 T in 12 subjects. During both chromatic and achromatic stimuli, we observed the typical increases in glutamate and lactate concentration, and decreases in aspartate and glucose concentration, that are indicative of increased glucose oxidation. However, within the detection sensitivity limits, we did not observe any difference between metabolic responses elicited by chromatic and achromatic stimuli. We conclude that the higher energy demands of activated blobs and interblobs are supported by similar increases in oxidative metabolism despite the different capacities of these neuronal populations.

(31)P-MRS using visual stimulation protocols with different durations in healthy young adult subjects.

Phosphorus magnetic resonance spectroscopy ((31)P-MRS) combined with visual stimulation in functional experiments allows the non-invasive dynamic study of brain energy metabolism. (31)P-MRS has been applied to several diseases and to healthy subjects, but works have shown variable findings and non-reproducible results, possibly caused by low numbers of subjects combined with different stimulation paradigms. In the present work, we used (31)P-MRS at 3 T with two different visual stimulation protocols with different block duration ("short" and "long") to evaluate metabolic changes under different workloads in 38 healthy subjects. We found a 15 % (short protocol-blocks of 1.5 min stimulation) and 3 % (long protocol-blocks of 5 min stimulation) increase in the inorganic phosphate (Pi) to α-adenosine triphosphate (α-ATP) ratio, and a 5 % (short protocol) and 2 % (long protocol) decrease in the nicotinamide adenine nucleotide (NADH + NAD(+)) to α-ATP ratio. The NADH + NAD(+) results are, to the best of our knowledge, the first functional magnetic resonance spectroscopy in vivo assessment of these compounds, but their interpretation is difficult since they cannot be separately quantified at 3 T. Our results show that longer stimulations produce smaller concentration changes in Pi/α-ATP and (NADH + NAD(+))/α-ATP ratios, which suggests a possible adaptation effect during longer stimulations that leads metabolic concentrations towards the initial equilibrium.