Spatial homogeneity and task-synchrony of the trial-related hemodynamic signal.

There is growing evidence that functional brain images in alert task-engaged subjects contain task-related but stimulus-independent signals in addition to stimulus-evoked responses. It is important to separate these different components when analyzing the neuroimaging signal. Using intrinsic-signal optical imaging combined with electrophysiology we had earlier reported a particular 'trial-related signal' in the primary visual cortex (V1) of alert monkeys performing periodic fixation tasks. This signal periodically modulated V1 tissue blood volume, in time with anticipated trial onsets. Unlike visually evoked blood volume changes, however, this signal was present even in total darkness. Further, it could not be predicted by concurrently recorded spiking or local field potentials. Here we use our earlier recording techniques to analyze the spatial distribution of this trial-related signal over our imaged area (10mm square, subdivided into a 16×16 grid, i.e. at 625 μm resolution). We show that the signal is spatially coherent and essentially homogeneous over the imaged region and fails to be predicted by concurrent electrode recordings even at the resolution of a single grid square at the electrode tip. As a corollary we show that the signal is critically linked to the animals' engagement in a task. Not only does the trial-related signal entrain accurately and precisely to any task timing at which the animal was willing to perform; the signal also loses the entrained trial-locked pattern dramatically, within a single trial, when the animal stops performing correctly. Thus the signal is very unlikely to be an ongoing task-independent vascular oscillation. These findings will help categorize the likely distinct varieties of non-stimulus-related signals evoked during behavioral tasks, and lead to a further understanding of the elements comprising the net neuroimaging response.