Baseline conditions and subtractive logic in neuroimaging

Abstract

Discrepancies in the patterns of cortical activation across studies may be attributable, in part, to differences in baseline tasks, and hence, reflect the limits of the subtractive logic underlying much of neuroimaging. To assess the extent of these effects, three of the most commonly used baseline conditions (rest, tone monitoring, and passive listening) were compared using phoneme discrimination as the experimental task. Eight participants were studied in a fMRI study with a 4.1 T system. The three baseline conditions systematically affected the amount of activation observed in the identical phoneme task with major affects in Broca's area, the left posterior superior temporal gyrus, and the left and right inferior parietal regions. Two central findings were: 1) a differential effect of baseline within each region, with the rest baseline condition producing the greatest amount of activation and the passive listening condition producing the least, and 2) systematic baseline task activation in the inferior parietal regions. These results emphasize the relativity of activation patterns observed in functional neuroimaging, and the necessity to specify the baseline processes in context to the experimental task processes.

Figure 1

A typical slice prescription and the ROIs, which include the inferior frontal gyrus (IFG), the posterior superior temporal gyrus (pSTG), and the inferior parietal region (IP).

Figure 2

Each graph depicts the amount of activation in a given ROI as a function of baseline condition. The graphs are all on the same scale. Error bars represent the square root of MS _e/n, where MS _e is the pooled error term for both of the independent variables [Loftus and Mason, 1994]. The left IFG, left temporal and left and right IP regions all show a monotonic decrease in the amount of activation as a function of baseline condition. In addition, the right homologue patterns are similar to those for the corresponding left region, with the exception of the right IFG.

Figure 3

The graphs depict the amount of baseline task activation in a given ROI. The amount of activation is defined as the mean number of deactivated voxels. Note that the graphs are on the same scale. Error bars represent the square root of MS _e/n, where MS _e is the pooled error term for both of the independent variables [Loftus and Masson, 1994].