Impact of stimulus size and orientation on individual face decoding in monkey face-selective cortex

ABSTRACT Face-selective neurons in the monkey temporal cortex discharge at different rates in response to pictures of different faces. Here we tested whether the population response of neurons in the face-selective area ML (located in the middle Superior Temporal Sulcus) tolerates two affine transformations; one, picture-plane inversion, known to have a deleterious impact on the average response of face-selective neurons and the other, stimulus size, thought to have little or no impact on face-selective neurons. We recorded the response of 57 ML neurons in two monkeys. Face stimuli were presented at two sizes (10 and 5 degrees of visual angle) and two orientations (upright and inverted). The results indicate that different faces elicited distinct patterns of activity across ML neurons that were tolerant of changes in size. However, the results of the orientation manipulation were mixed; despite observing a reduced response to inverted faces, classifier performance was above chance for both upright and inverted faces and the classification score did not differ significantly for inverted and upright faces. We conclude that population responses in area ML to different faces are dependent on stimulus orientation but are more tolerant to changes in stimulus size.