Residual fMRI sensitivity for identity changes in acquired prosopagnosia

Abstract

While a network of cortical regions contribute to face processing, the lesions in acquired prosopagnosia are highly variable, and likely result in different combinations of spared and affected regions of this network. To assess the residual functional sensitivities of spared regions in prosopagnosia, we designed a rapid event-related functional magnetic resonance imaging (fMRI) experiment that included pairs of faces with same or different identities and same or different expressions. By measuring the release from adaptation to these facial changes we determined the residual sensitivity of face-selective regions-of-interest. We tested three patients with acquired prosopagnosia, and all three of these patients demonstrated residual sensitivity for facial identity changes in surviving fusiform and occipital face areas of either the right or left hemisphere, but not in the right posterior superior temporal sulcus. The patients also showed some residual capabilities for facial discrimination with normal performance on the Benton Facial Recognition Test, but impaired performance on more complex tasks of facial discrimination. We conclude that fMRI can demonstrate residual processing of facial identity in acquired prosopagnosia, that this adaptation can occur in the same structures that show similar processing in healthy subjects, and further, that this adaptation may be related to behavioral indices of face perception.

Keywords: adaptation; expression; fMRI; face perception; identity; prosopagnosia; sensitivity.

Figure 1

Coronal T1-weighted MRI brain images of the three patients, standardized to Talairach space. Slices were taken every 12 mm, from y = + 48 mm to y = −84 mm. B-AT1 has large bilateral lesions of the anterior temporal lobes following herpes encephalitis (+12 to −36 mm). R-AT1 has a small surgical lesion in the right anterior temporal lobe, additionally affecting the right hippocampus and amygdala (0–12 mm). R-IOT1 has a single right inferior occipitotemporal lesion from his prior hemorrhage (−48 to −84 m).

Figure 2

Schematic representation of an experimental trial. In all three experimental conditions the first image was the same. The second image in the pair was either a new picture with the same identity and same expression as the first image, a picture of a different person with the same expression or a picture of the same person with a different expression. An image pair was presented within every TR (2 s) and fixation trials were randomly intermixed with experimental trials.

Figure 3

A representative example of the hemodynamic response (HDR) as calculated by the deconvolution analysis. In this case the time point at 6 s (encircled) would be considered the HDR-peak and the value at this time point would be used for analysis.

Figure 4

Core system regions-of-interest identified with the functional localizers (all brains standardized to Talairach space). All six regions of the core system were identified in B-AT1 and R-AT1. Due to the location of the lesion, R-IOT1 does not display a right OFA or right FFA. However, a right posterior STS (pSTS) was identified along with all three core regions in the left hemisphere.

Figure 5

Release from adaptation in response to identity changes as seen in the fusiform face areas of the three patients. Significant release from adaptation was observed in the right FFA of both B-AT1 and R-AT1. In contrast, R-IOT1 who did not have a right FFA due to damage, exhibited significant release from adaptation in the left FFA. ^*p < 0.05.

Figure 6

Release from adaptation in response to identity changes as seen in the occipital face areas of the three patients. Significant release from adaptation was observed in the left FFA of all three patients and only in the right OFA of B-AT1 (R-IOT1 did not have a right OFA due to damage). ^*p < 0.05.