The impact of stimulus size and orientation on individual face coding in monkey face-selective cortex

Jessica Taubert,Bruno Rossion,Rufin Vogels,Goedele Van Belle

doi:10.1038/s41598-018-28144-z

Jessica Taubert, Bruno Rossion + Show 2 more

Open Access

https://doi.org/10.1038/s41598-018-28144-z

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Abstract

Face-selective neurons in the monkey temporal cortex discharge at different rates in response to pictures of different individual faces. Here we tested whether this pattern of response across single neurons in the face-selective area ML (located in the middle Superior Temporal Sulcus) tolerates two affine transformations; picture-plane inversion, known to decrease the average response of face-selective neurons and the other, stimulus size. We recorded the response of 57 ML neurons in two awake and fixating monkeys. Face stimuli were presented at two sizes (10 and 5 degrees of visual angle) and two orientations (upright and inverted). Different faces elicited distinct patterns of activity across ML neurons that were reliable (i.e., predictable with a classifier) within a specific size and orientation condition. Despite observing a reduction in the average response magnitude of face-selective neurons to inverted faces, compared to upright faces, classifier performance was above chance for both upright and inverted faces. While decoding was largely preserved across changes in stimulus size, a classifier trained with one orientation condition and tested on the other did not lead to performance above chance level. We conclude that different individual faces can be decoded from patterns of responses in the monkey area ML regardless of orientation or size, but with qualitatively different patterns of responses for upright and inverted faces.

Highlights

Single neurons that respond selectively to face compared to non-face visual stimuli were identified in the inferior temporal (IT) cortex of non-human primates over forty years ago[1]
The aim of the present study is to test whether the identity of 12 randomly selected faces can be decoded from single neuron output in area middle patch (ML), and, most importantly, the extent to which this performance depends on stimulus size and picture-plane inversion
Neurons in ML responded stronger to upright than inverted faces (main effect of Orientation, F(1,56) = 13.19, p < 0.001), which extends previous findings in this region[3,5] and shows that the preference for upright faces remains even when averaging across multiple identities and stimulus sizes (Fig. 2A)

Summary

Introduction

Single neurons that respond selectively to face compared to non-face visual stimuli were identified in the inferior temporal (IT) cortex of non-human primates over forty years ago[1]. Studies have shown that activity in area ML, combined with MF (a face-selective patch in the fundus of the middle STS region), is identity-selective[3,10,11,15,18,21] Based on these observations, the aim of the present study is to test whether the identity of 12 randomly selected faces can be decoded from single neuron output in area ML, and, most importantly, the extent to which this performance depends on stimulus size and picture-plane inversion. Average firing rate of neurons whether each neuron’s preferences have changed These previous studies did not test whether the unique patterns of responses across neurons elicited by different face identities were altered when the faces were turned upside down. On a single neuron basis, this influence would manifest as a change in the preferred facial identities dependent on either size or inversion or both

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