Different physiological MRI noise between cortical layers.

Significantly higher temporal fluctuations of the blood oxygenation level-dependent (BOLD) signal in the living rat group compared to that in the dead rat group were observed in the cortex, suggesting the existence of physiological information in the signal fluctuations. A similar analysis shows significantly different fluctuations between visual cortical layers. The highest fluctuations were observed in layers 4 and 5 and the lowest in layer 1. Given the consistency with published electrophysiology studies anticipating high spontaneous activity in the deeper layers (particularly layer 4), and low activity in superficial layers, we hypothesize that the BOLD signal temporal fluctuations may reflect cortical neuronal activity. Temporal fluctuations in ultrahigh spatial resolution data of the rat brain were measured in two ways. In the first, analyses were performed according to known layer widths, and in the second equal lines of 117 micro along the cortex were selected. The second approach yielded temporal fluctuations along the cortex that resemble known neuronal density distributions including the intralayer structure, particularly within layer 5.