Mikio Inagaki, Ichiro Fujita
Dependence of Spatial Frequency Tuning on Stimulus Size in Face-Responsive Neurons: Comparison between the Temporal Visual Cortex and the Amygdala.
Psychophysical performance of face discrimination in human subjects varies as a function of the spatial frequencies (SFs) in the image. Changes in image size, which accompany changes in retina-based SFs (cycles/deg), only minimally influence face discrimination performance. 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 given brain area contribute to face discrimination, they should be tuned to image-based SFs rather than retina-based ones. We tested this for face-responsive neurons in the temporal visual cortex and the amygdala of two Japanese monkeys (Macaca fuscata). We recorded extracellular neuronal activities from awake, behaving monkeys and examined the responses of single neurons to band-pass filtered face images (center image-based SFs: 2, 2.8, 4, 5.7, 8, 11.3, 16 cycles/image) of various stimulus sizes (3.8, 5.4, 7.7, 11, 15.3 degrees). For 47 temporal cortex neurons and 44 amygdala neurons, we analyzed the effects of stimulus size on SF tuning by assessing 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 systematic shift in the preferred image-based SFs depending on image size, indicating that their responses were influenced by retina-based SFs. The size effects on the preferred image-based SFs in the temporal visual cortex were smaller than those of the amygdala (p = 0.018, Mann-Whitney test). Thus, the reference frame for SF tuning of the temporal cortex neurons is more consistent with face discrimination abilities than that of the amygdala neurons. 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.