Difference between human and chimpanzee experiments | Expected result if this difference is great enough to significantly affect the pattern of brain activation | Evaluation of data |
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Humans received intravenous, 10 mCi doses of fluorodeoxyglucose, whereas chimpanzees received oral, 15 mCi doses. | This might cause differences in the overall level of activity across the brain (i.e., human scans might have greater absolute brightness). | Each scan was normalized to its own mean intensity value, so the average intensity of each image is exactly equal to 1. Identical thresholding methods were applied to both species. Above-threshold voxels in each ROI were normalized as a percentage of the total above-threshold voxels in the entire brain. These steps control for possible differences in the overall level of brain activity. |
Human stimuli were 2D videos, whereas chimpanzee stimuli were 3D live demonstrations. | PMv is involved in processing 3D shape from visual disparity (stereopsis) (Georgieva et al., 2009; Joly et al., 2009). This might cause greater PMv activation in chimpanzees than humans. In addition, a lower percentage of macaque mirror neurons are activated by videos than by live demonstrations (Caggiano et al., 2011). This might cause greater chimpanzee activation in regions homologous to those that contain mirror neurons in macaques. | Humans had greater activation than chimpanzees in both ventral premotor cortex and inferior parietal cortex. This suggests that species differences in the pattern of brain activation are not due to differences in the dimensionality of the stimuli. |
Human stimuli consisted only of a hand, the lower portion of the arm, the tabletop, and graspable object, whereas chimpanzee stimuli included an entire human demonstrator as well as the testing box in which the grasping actions were performed. | In monkeys, observation of only a hand grasping an object activates more rostral regions of VLPFC and PMv (areas 46, 45A, 45B, and rostral F5), whereas observation of an entire demonstrator grasping an object also activates caudal F5 (Nelissen et al., 2005). This might cause frontal activations to extend more caudally than in chimpanzees than humans. | Frontal activations extended more caudally in humans than chimpanzees; PMv contained a significantly greater proportion of total brain activation in humans than chimpanzees (Figure 11). This suggests that species differences in the pattern of brain activation are not due to differences in scene complexity. |
Chimpanzee stimuli included a more prominent reaching action than human stimuli. | Observed reaching activates more dorsal regions of premotor cortex than observed grasping (Filimon et al., 2007). This might cause greater PMd activation in humans than chimpanzees. | PMd activation was not significantly different between humans and chimpanzees (Figure 11). This suggests that species differences in the pattern of brain activation are not due to differences in the prominence of the observed reaching action. |
Possible methodology-related differences in regional brain activation were either controlled for using normalization procedures or are opposite to what we observed; therefore, these differences are not likely to have contributed to differences in observed activation.
PMv, ventral premotor cortex; PMd, dorsal premotor cortex; VLPFC, ventrolateral prefrontal cortex; IPL, inferior parietal lobule.