What Is a Visual Stream?

Abstract

The dual stream model of the human and non-human primate visual systems remains Leslie Ungerleider's (1946-2020) most indelible contribution to visual neuroscience. In this model, a dorsal "where" stream specialized for visuospatial representation extends through occipitoparietal cortex, whereas a ventral "what" stream specialized for representing object qualities extends through occipitotemporal cortex. Over time, this model underwent a number of revisions and expansions. In one of her last scientific contributions, Leslie proposed a third visual stream specialized for representing dynamic signals related to social perception. This alteration invites the question: What is a visual stream, and how are different visual streams individuated? In this article, we first consider and reject a simple answer to this question based on a common idealizing visualization of the model, which conflicts with the complexities of the visual system that the model was intended to capture. Next, we propose a taxonomic answer that takes inspiration from the philosophy of science and Leslie's body of work, which distinguishes between neural mechanisms, pathways, and streams. In this taxonomy, visual streams are superordinate to pathways and mechanisms and provide individuation conditions for determining whether collections of cortical connections delineate different visual streams. Given this characterization, we suggest that the proposed third visual stream does not yet meet these conditions, although the tripartite model still suggests important revisions to how we think about the organization of the human and non-human primate visual systems.

Figure 1.

Versions of the dual visual stream model in the rhesus monkey brain. Shaded regions represent vision-related regions in the occipital, temporal, and parietal cortices. Color coding of regions as blue (dorsal) and red (ventral) follows that of Ungerleider (1995), which is an extension of the model depicted in (C). The same scheme was used to code the equivalent areas in (A) and (B). (A) The initial model of the dual visual steam (Mishkin, Ungerleider, & Macko, 1983). Arrows depict the ventral and dorsal streams, each beginning in area OC and splitting off in prestriate cortex (areas OB and OA) to reach into ventral inferior temporal cortex, or OTC (areas TEO and TE) and dorsally to the posterior parietal cortex, or OPC (PG). (B) An updated account of the dual visual stream model, incorporating forward and backward connections between the two streams (Ungerleider & Desimone, 1986). Solid arrowheads depict “forward” connections while line arrowheads depict “backward” projections. Solid arrow lines depict projections from peripheral and central representations alike, while dashed arrow lines depict projections only from peripheral field representations. “d” is meant to indicate that the projection from V1 to V3 only applies to area V3d. “m” indicates that V2 only projects to the medial portion of VIP. This model also includes projections to additional extrastriate areas such as areas VIP and MSTp in area PG and area FST (VIP: ventral intraparietal area, MSTp: medial superior temporal visual area, peripheral representation, FST: fundus of the superior temporal visual area). All definitions used in (A) apply to (B). (C) Another updated version of the model, including a proposed third visual branch in the superior temporal sulcus (STS) as well as many more projections (Distler et al., 1993). This model also emphasizes area TEO as responsible for a large degree of “cross-stream” projection between the ventral and dorsal streams. “d” is meant to indicate that the projections from V2 to V3 and FST to V3 only apply to area V3d. All definitions used in (A) and (B) apply (C).

Figure 2.

How streams are situated in the proposed taxonomic hierarchy of neural organization. The width of the scheme indicates (in the abstract) the total extent of the visual system, which is subdivided (both structurally and functionally) into separate streams, pathways, and mechanisms. Red and blue indicate division of the system into two separate streams, with the opacity decreasing at each subordinate level. The number of divisions depicted at each subordinate level is the minimum number necessary for a neural system to subsume multiple streams, pathways, and mechanisms. This visualization does not capture how pathways or mechanisms might project from one stream to another, or how multiple pathways might serve different visual functions.