Evidence for a Third Visual Pathway Specialized for Social Perception

Abstract

Existing models propose that primate visual cortex is divided into two functionally distinct pathways. The ventral pathway computes the identity of an object; the dorsal pathway computes the location of an object, and the actions related to that object. Despite remaining influential, the two visual pathways model requires revision. Both human and non-human primate studies reveal the existence of a third visual pathway on the lateral brain surface. This third pathway projects from early visual cortex, via motion-selective areas, into the superior temporal sulcus (STS). Studies demonstrating that the STS computes the actions of moving faces and bodies (e.g., expressions, eye-gaze, audio-visual integration, intention, and mood) show that the third visual pathway is specialized for the dynamic aspects of social perception.

Keywords: V5/MT; body perception; face perception; neuroanatomy; social perception; superior temporal sulcus (STS).

Figure 1,. Key Figure. Cortical connectivity of the third pathway (in red).

The third pathway begins in primary visual cortex (V1) and projects into the posterior banks of the superior temporal sulcus (STS) via the motion-selective area V5/MT. The cortico-cortical connections of the third pathway are independent of the ventral pathway (shown in green) and the dorsal pathway (shown in blue).

Figure 2.. Cortico-cortical connections of the macaque occipitotemporal cortex.

A direct cortical pathway from primary visual cortex (V1) to the middle temporal (MT) motion processing area is highlighted in red. The third pathway in macaques projects from V1 to MT. MT then has direct anatomical connections with the medial superior temporal (MST) and fundus of the superior temporal (FST) cortices. FST then feeds into the more anterior (rostral) regions of the dorsal bank (d) and fundus (f) of the STS [4].

Figure 3.. The third pathway exhibits no visual field bias.

A. Face-selective areas in the human occipitotemporal cortex. The posterior superior temporal sulcus is in the third pathway. The occipital face area and fusiform face area are both in the ventral pathway. B. There is a contralateral visual field bias in face-selective areas in the ventral pathway (10,11,32,33), while the pSTS had no visual field bias [10]. This same lack of a visual field bias is also seen in the dorsal bank and fundus of the macaque STS [5,30]. C. Group social interactions necessitate directing and redirecting attention to different individuals across the visual field. The lack of visual field bias in the pSTS is consistent with the functional role of the lateral pathway in social cognition.

Figure 4.. Biological motion processing in the macaque STS

A. Macaque face patches are organised in a manner consistent with two functionally distinct pathways. Dorsal Patches (purple) AF and MD respond selectively to faces in natural motion. Ventral patches (red) PL, ML and AL respond selectively to static face images. Face patch MF exhibits a split response to moving and static faces consistent with the dorsal / ventral distinction [18]. B. Results from an fMRI study of macaques viewing natural videos that contained animals (left) or no animals (right) [19]. Surface maps show the percent variance explained by biological motion. The brain on the bottom left shows how biological motion drives the neural response from early visual cortex and into the STS.