Inagaki, M., Fujita, I.
Dependence of spatial frequency tuning on stimulus size in face-responsive neurons of the temporal visual cortex and the amygdala.
Psychophysical performance of face discrimination in human subjects varies as a function of spatial frequencies (SFs) in the face images. Although retina-based SFs (cycles/deg) vary with image size, face discrimination performance is similar regardless. Face discrimination depends more on relative SFs normalized by image size or image-based SFs (cycles/image) than on absolute retina-based SFs. We reasoned therefore that if neurons in a brain area contribute to face discrimination, they should be tuned to image-based SFs rather than retina-based ones. Here, we tested this for face-responsive neurons in the temporal visual cortex and the amygdala of two Japanese monkeys (Macaca fuscata) by assessing the effects of image size on SF tuning. We made band-pass filtered face images of 7 center image-based SFs (2, 2.8, 4, 5.7, 8, 11.3, 16 cycles/image) each with 5 image sizes (3.8, 5.4, 7.7, 11, 15.3 degrees) for a total of 35 face images. We recorded 115 and 105 face-responsive neurons from the temporal visual cortex (A18 - A24) and the amygdala (A22 - A25), respectively. Among them, 47 and 41 neurons were selective for image-based SFs in at least 2 image sizes and their responses were well-fitted by a set of Gaussian functions in the temporal visual cortex and the amygdala, respectively. We assessed effects of image size on SF tuning by evaluating the shift in preferred image-based SFs. In the temporal visual cortex, most neurons only minimally changed the preferred image-based SFs across different image sizes, indicating that these neurons were tuned to image-based SFs independent of image size. In the amygdala, on the other hand, many neurons showed a shift in the preferred image-based SFs depending on image size, indicating that these neurons were not ideally tuned to image-based SFs. The size effects on the preferred image-based SFs in the temporal visual cortex were smaller than those of the amygdala (p = 0.005, Kruskal-Wallis test). Overall, neuronal response properties of the temporal visual cortex are more consistent with face discrimination abilities than are those of the amygdala. Our results demonstrated a difference in face representations between the temporal visual cortex and the amygdala, suggesting that different types of visual processing make face representations in the temporal visual cortex and the amygdala, respectively.