Effects of visual attention on BOLD signal variance

The responses of sensory neurons are noisy, and laboratory studies typically deal with this variability by averaging responses to many stimulus presentations. Recently, it has been observed that the noise signals carry important information about the brain activity, especially by observing the trial-to-trial noise correlation of spiking activity across populations of neurons (Ecker et al. 2010). The trial-to-trial fluctuations in the responses of pairs of neuron are affected by attention, and this has influence on behavior (Cohen et al. 2009, Mitchell et al. 2009). In particular, it was found that attention decreased the noise correlation of neural responses in V4, indicating a more efficient encoding or an increase of information content. Yet the results of these electrophysiology studies left it unclear whether such effects would also occur elsewhere in the cortex, and whether similar effects can be observed in the BOLD signal. In the present study we asked human participants to perform a difficult, attention-demanding task on a complex visual motion display during a prolonged period of time, alternated by equally long periods of visual stimulation without any task. Brain activity was recorded using fMRI. We then analyzed changes in the mean BOLD signal during both conditions, as well as the signal variance within the time-series of each condition. During attention, the BOLD signal variance decreased in several regions, including V5/MT, the temporal parietal junction, and in additional medial-frontal regions. Mean BOLD signal increased in early visual cortex, V5/MT, and in the parieto-frontal attention network. The results demonstrate firstly that the variance of BOLD activity can be altered by visual attention. Secondly they show that there is only a partial overlap between regions whose BOLD signal increases and those whose BOLD signal variance changes. This suggests that changes in variance and in net amplitude may reflect distinct brain processes related to attention