Dorsal and Ventral Visual Streams

Identification of the temporal- and parietal-lobe visual pathways led researchers bn a search for the possible functions of each. One way to examine these functions is to ask why evolution would produce two different destinations for the pathways in the brain. The answer is that each route must create visual knowledge for a different purpose.

David Milner and Mel Goodale (1995) proposed that these two purposes are to iden tify what a stimulus is (the "what" function) and to use visual information to control movement (the "how" function). Many authors have emphasized the role of the latter

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pathway as a "where" function. The problem is that "where" is a property of"what" a stimulus is as well as a cue for "how" to control movement to a place. We therefore will use the "what-how" distinction suggested by Milner and Goodale.

This "what" versus "how" distinction came from an analysis of where visual information goes when it leaves the David Milner Mel Goodale striate cortex. Figure 8-14 shows the two distinct visual pathways that originate in the striate cortex, one progressing to the temporal lobe and the other to the parietal lobe. The pathway to the temporal lobe has become known as the ventral stream, whereas the pathway to the parietal lobe has become known as the dorsal stream.

To understand how these two streams function, we need to return to the details of how the visual input from the eyes contributes to them. Both the geniculostriate and the tectopulvinar systems contribute to the dorsal and ventral streams.

GENICULOSTRIATE PATHWAY

The retinal ganglion-cell fibers from the two eyes distribute their connections to the two lateral geniculate nuclei (left and right) of the thalamus in what at first glance appears to be an unusual arrangement. As seen in Figure 8-11, the fibers from the left half of each retina go to the left LGN, whereas those from the right half of each retina go to the right LGN. But the fibers from each eye do not go to exactly the same place in the LGN.

Each LGN has six layers, and the projections from the two eyes go to different layers, as illustrated in anatomical context in Figure 8-10 and alone in Figure 8-15. Layers 2,3, and 5 receive fibers from the ipsilateral eye (i.e., the eye on the same side), whereas layers 1,4, and 6 receive fibers from the contralateral eye (i.e., the eye on the opposite side). This arrangement provides for combining the information from the two eyes and for segregating the information from the P and M ganglion cells.

Axons from the P cells go only to layers 3 through 6 (referred to as the parvocellular layers), whereas axons from the M cells go only to layers 1 and 2 (referred to as the magnocel-lular layers). Because the P ganglion cells are responsive to color and fine detail, layers 3 through 6 of the LGN must be processing information about color and form. In contrast, the M cells mostly process information about movement, and so layers 1 and 2 must deal with movement.

Before we continue, you should be aware that just as there are six layers of the LGN (numbered 1 through 6), there are also six layers of the striate cortex (numbered I through VI). That there happen to be six layers in each of these locations is an accident of evolution found in all primate brains. Let us now see where these LGN cells send their connections in the visual cortex.

You learned in Chapter 2 that layer IV is the main afferent (incoming) layer of the cortex. Layer IV of the visual cortex has several sublayers, two of which are known as IVCa and IVCp. Layers 1 through 4 of the LGN go to IVCb, and LGN layers 5 and 6 go to IVCa. As a result, a distinction between the P and M functions continues in the cortex

As illustrated in Figure 8-16, input from the two eyes also remains separated in the cortex but through a different mechanism. The input from the ipsilaterally

Dorsal Ventral Visual Stream

Figure 8-14

Visual Streaming Visual information travels from the occipital visual areas to the parietal and temporal lobes, forming the dorsal and ventral streams, respectively.

Figure 8-14

Visual Streaming Visual information travels from the occipital visual areas to the parietal and temporal lobes, forming the dorsal and ventral streams, respectively.

Figure 8-15

I Geniculostriate Pathway

Information travels from the right side of each retina to the right LGN.

Left eye (contralateral)

Information travels from the right side of each retina to the right LGN.

Left eye (contralateral)

Dorsal And Ventral Visual Streams

Figure 8-16

Maintaining Separate Visual Input

(Left) Information from the two eyes is segregated by layers in the lateral geniculate nucleus, and the lateral geniculate nucleus maintains this segregation in its projections to the visual cortex. Information from each eye travels to adjacent columns in cortical layer IV. (Right) A horizontal plane through V1 shows a zebralike effect of alternating ocular-dominance columns in the cortex. Photograph from "Functional Architecture of Macaque Monkey Visual Cortex," by D. H. Hubel and T. N. Weisel, 1977, Proceedings of the Royal Society of London B, 198, Figure 23.

0 Link to the area on the optic chiasm in the module on the Visual System on your CD to investigate the visual pathways to the LGN.

0 On your CD, find the primary visual cortex area in the Visual System module to see the visual connections to the occipital cortex. Notice in particular how the cortex is layered in this region and how this layering parallels that seen in the LGN.

Cortical column. Cortical organization that represents a functional unit six cortical layers deep and approximately 0.5 mm square and that is perpendicular to the cortical surface.

Primary visual cortex (Vi). Striate cortex that receives input from the lateral geniculate nucleus.

Extrastriate (secondary) cortex.

Visual cortical areas outside the striate cortex.

Blob. Region in the visual cortex that contains color-sensitive neurons, as revealed by staining for cytochrome oxidase.

Horizontal section of striate cortex

Blob Area Striate Cortex

Cortical visual area

Horizontal section of striate cortex

Striate Cortex Color

Ocular dominance columns

Cortical visual area

Lateral geniculate nucleus

Ocular dominance columns connected LGN cells (that is, layers 2, 3, and 5) and the input from the contralaterally connected LGN cells (layers 1, 4, and 6) go to adjacent strips of cortex. These strips, which are about 0.5 millimeter across, are known as cortical columns. We return to the concept of cortical columns shortly.

In summary, the P and M ganglion cells of the retina send separate pathways to the thalamus, and this segregation remains in the striate cortex. The left and right eyes also send separate pathways to the thalamus, and these pathways, too, remain segregated in the striate cortex.

TECTOPULVINAR PATHWAY

As already noted, the tectopulviar pathway is formed by the axons of the remaining M ganglion cells. These cells send their axons to the superior colliculus in the midbrain's tectum, which functions to detect the location of stimuli and to shift the eyes toward stimuli. The superior colliculus sends connections to the region of the thalamus known as the pulvinar.

The pulvinar has two main divisions: medial and lateral. The medial pulvinar sends connections to the parietal lobe, whereas the lateral pulvinar sends connections to the temporal lobe. One type of information that these connections are conveying is related to "where," which, as noted earlier, is important in both "what" and "how" functions.

The "where" function of the tectopulvinar system is useful in understanding blind-sight in D. B. His geniculostriate system was disrupted but his tectopulvinar system was not, thus allowing him to identify the location of stimuli that he could not identify. Let us now look at how visual information proceeds from the striate cortex through the rest of the occipital lobe to the dorsal and ventral streams.

OCCIPITAL CORTEX

As shown in Figure 8-17, the occipital lobe is composed of at least six different visual regions, known as V1, V2, V3, V3A, V4, and V5. Region V1 is the striate cortex, which, as already mentioned, is sometimes also referred to as the primary visual cortex. The remaining visual areas of the occipital lobe are called the extrastriate cortex or secondary visual cortex. Because each of these occipital regions has a unique cellular structure (cytoarchitecture) and has unique inputs and outputs, we can infer that each must be doing something different from the others.

You already know that a remarkable feature of region V1 is its distinct layers, which extend throughout V1. These seemingly homogeneous layers are deceiving, however. When Margaret Wong-Riley and her colleagues (1993) stained the cortex for the enzyme cytochrome oxidase, which has a role in cell metabolism, they were surprised to

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(A) Medial view of functional areas

(A) Medial view of functional areas

Charting Dorsal Ventral Stream

(B) Lateral view of functional areas y3A y3 y2

y1 y2 y4

y3 y3A

(B) Lateral view of functional areas

Visual Cortex V1v2v3

V1 = Primary visual cortex V2-V5 = Extrastriate cortex y3A y3 y2 y1

V1 = Primary visual cortex V2-V5 = Extrastriate cortex find an unexpected heterogeneity in region V1. So they sectioned the V1 layers in such a way that each cortical layer was in one plane of section, much like peeling off the layers of an onion and laying them flat on a table. The surface of each flattened layer can then be viewed from above.

As Figure 8-18 illustrates, the heterogeneous cytochrome gaining now appeared as random blobs in the layers of VI. In fact, these darkened regions have become known as blobs, and the less-dark regions separating them have become known as interblobs. Blobs and interblobs serve differ-! ent functions. Neurons in the blobs take part in color perception, whereas neurons in the interblobs participate in form and motion perception. So, within region V1, input that arrives in the parvo- and magnocellular pathways of the geniculostriate system is| segregated into three separate types of information: color, form, and movement.

This information is then sent to region V2, which lies next to region V1. Here thd color, form, and movement inputs remain segregated. This segregation can again be seen through the pattern of cytochrome oxidase staining, but the staining pattern is different from that in region V1. Figure 8-19 shows that region V2 has a pattern of thick and thinj stripes that are intermixed with pale zones. The thick stripes receive input from the

Figure 8-17

Visual Regions of the Occipital Lobe

Interblob Regions
Figure 8-18

Heterogeneous Layering The blobs in region V1 and the stripes in region V2 are illustrated in this drawing of a flattened section through the visual cortex. The blobs and stripes can be visualized by using a special stain for cytochrome oxidase, which is a marker for mitochondria.

Lateral geniculate nucleus

Blobs (color)

Striate cortex (VI)

Interblob regions

Form

Movement

Extrastriate cortex

Dorsal stream

Parietal lobe (PG)

Extrastriate cortex (y2)

V3 (dynamic form) V4 (color form)

Extrastriate cortex

Ventral stream

Temporal lobe (TE)

Dorsal And Ventral Stream

Parietal lobe Dorsal stream

Ventral stream Temporal lobe

Parietal lobe Dorsal stream

Ventral stream Temporal lobe

Figure 8-19

Charting the Dorsal and Ventral Streams The dorsal stream, which controls visual action, begins in region V1 and flows through region V2 to the other occipital areas and finally to the parietal cortex, ending in an area of the parietal lobe referred to as PG. The ventral stream, which controls object recognition, begins in region V1 and flows through region V2 to the other occipital areas and finally to the temporal cortex, ending in an area of the temporal lobe referred to as TE. The flow of information from the subregions of V1 (blobs and interblobs) is to the thick, thin, and pale zones of V2. Information in the thin and pale zones goes to regions V3 and V4 to form the ventral stream. That in the thick and pale zones goes to regions V3A and V5 to form the dorsal stream.

ls le

Visual field. Region of the visual world that is seen by the eyes.

Receptive field. Region of the visual world that stimulates a receptor cell or neuron.

movement-sensitive neurons in region V1; the thin stripes receive input from V1 s color-sensitive neurons; and the pale zones receive input from V1's form-sensitive neurons.

As diagrammed in Figure 8-19, the visual pathways proceed from region V2 to the other occipital regions and then to the parietal and temporal lobes, forming the dorsal and ventral streams. Although many parietal and temporal regions take part, the major regions are region G in the parietal lobe (thus called region PG) and region E in the temporal lobe (thus called region TE).

Within the dorsal and ventral streams, the function of the visual pathways becomes far more complex than a simple record of color, form, and movement. Rather, the color, form, and movement information is put together to produce a rich, unified visual world made up of complex objects, such as faces and paintings, and complex visuo motor skills, such as catching a ball. The functions of the dorsal and ventral streams are therefore complex, but they can be thought of as consisting of "how" functions and "what" functions. "How" is action to be visually guided toward objects, whereas "what identifies what an object is.

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Responses

  • freddie
    Does the tectopulvinar system contribute to the dorsal stream?
    5 years ago
  • almaz
    What lobe do each of the dorsal & ventral visual streams go to?
    4 years ago
  • Clinton
    Is there a pathway that links the dorsal and ventral pathway?
    4 years ago
  • may chubb-baggins
    Where is the temporal, dorsal and ventral part of the retina?
    3 years ago
  • spencer
    Did dorsal or ventral nervous system evolved first?
    3 years ago
  • arnor
    What does the ventral stream do?
    3 years ago
  • Louie
    What does the ventral stream process?
    3 years ago
  • josefiina
    What are the anatomical differences between the geniculostriate and tectopulvinar visual pathways?
    3 years ago
  • dylan
    What visual function does the dorsal stream regions perform?
    2 years ago
  • lena
    Does the dorsal stream go through the visual cortex?
    2 years ago
  • eric
    Where does the ventral stream projects?
    5 months ago
  • eoghan
    How to remember locations and functions of dorsal and ventral visual processing streams?
    24 days ago

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