Linearity of the fMRI response in category-selective regions of human visual cortex

Abstract

The goal of this study was to determine the linearity of the blood oxygen level-dependent (BOLD) response, as measured by functional magnetic resonance imaging (fMRI), in category-selective regions of human visual cortex. We defined regions of the temporal lobe that were selective to faces (fusiform face area, FFA) and places (parahippocampal place area, PPA). We then determined the linearity of the BOLD response in these regions to their preferred and nonpreferred stimuli. First, we tested the principle of scaling. As we increased the visibility of the stimulus, there was a corresponding linear increase in the fMRI signal in the FFA and PPA to their preferred stimulus (face and place, respectively). In contrast, responses in the FFA and PPA to the nonpreferred stimulus did not conform to the principle of scaling. Next, we asked whether the fMRI response in these regions of visual cortex conformed to the principle of additivity. To assess this, we determined whether the response to a long stimulus block could be predicted by adding the response to multiple shorter duration blocks. Although the fMRI response in the FFA and PPA was generally linear to the preferred stimulus, a more nonlinear response was apparent to the nonpreferred stimulus. In conclusion, the linearity of the BOLD response in the human ventral visual pathway varied across cortical region and stimulus category. This suggests that measures of linearity may provide a useful indication of neural selectivity in the brain.

Figure 1

Examples of face and place images at different levels of phase coherence (%).

Figure 2

The location of the fusiform face area (FFA) and parahippocampal place area (PPA) in one subject. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 3

Mean peak amplitudes for face and place images in the FFA and PPA at 20, 40, 60, and 80% phase coherence. Error bars represent 95% confidence intervals.

Figure 4

Detection and within‐category discrimination for faces and places at different levels of phase coherence. Error bars represent ±1 standard error.

Figure 5

Mean predicted and measured responses to face images in the FFA. The responses to 2, 4, and 8 s blocks were used to predict the measured response at 4, 8, and 16 s.

Figure 6

Mean predicted and measured responses to place images in the PPA. The responses to 2, 4, and 8 s blocks were used to predict the measured response at 4, 8, and 16 s.

Figure 7

Mean predicted and measured responses to place images in the FFA. The responses to 2, 4, and 8 s blocks were used to predict the measured response at 4, 8, and 16 s.

Figure 8

Mean predicted and measured responses to face images in the PPA. The responses to 2, 4, and 8 s blocks were used to predict the measured response at 4, 8, and 16 s.