The Dorsal Stream

- the 'where' and the 'how'

The first clues for the existence of the dorsal stream of visual information were furnished by lesion studies in monkeys. A lesion is an umbrella term denoting some kind of damage in an area of the brain. When selected areas in the posterior parietal cortex of the brain were cut by scientists in the late 20th century, tests indicated that the spatial perception of monkeys was greatly affected. Their ability to recognize objects, however, remained intact.

The function that these parietal lesions affected is known as a visuomotor type of function; the types of movement that vision prompts. Upon seeing an object, humans with intact dorsal streams can automatically reach and grasp for it. To do so, they have to have a good idea of where in space the object is located. Hence, this is the 'where' pathway. They must also know, on a subconscious level, what shape and size to put their hand into. With respect to movement, this adds a 'how' component. You do both of these things without thinking, as when reaching for an adapter stuck into a socket, or your phone lying on the table. Have you ever wondered how your hands form these grasping shapes? Many body parts must coordinate for you to be able to take hold of something. Most of the time, this coordination happens without a conscious command.

A cool idea to think about is that the dorsal stream - this grasping, visuomotor system allowing you to manipulate your environment - may have evolved before your ability to declaratively identify the objects you spent your time handling. This secondary ability is the domain of the ventral stream, and though it's a key component for a deeper rational understanding of the world, it may not have been necessary for a long time, and maybe still isn't. This hints at a rather unsurprising idea: that humans may have evolved to be reactionary animals first, and thinkers only after.

The dorsal stream was long thought* to be a purely magnocellular pathway, tracing back to the parasol, or 'M' retinal ganglion cells. These cells unfurl large dendritic 'trees' and gather input from a wide expanse of rods and cones, but are achromatic (giving no color information to the brain). Parasol cell axons connect to layers 1 and 2 of the lateral geniculate nucleus, from where they dominantly synapse in layer 4C-alpha of of the primary visual cortex. Important in terms of the dorsal stream is the ability of M cells to detect motion. Another way of putting this is that their receptive fields are sensitive to the changing position of a stimulus in space at small differences in time - what's known as a 'high temporal frequency'.

The dorsal stream originates in area V1 of the occipital cortex and moves to V2. From there it proceeds anteriorly and dorsally (forward, along the upper portion of the brain). It goes to area V5, which is also known as the 'middle temporal' area, MT. V5 is located along the posterior and bottom side of the superior temporal gyrus. It's a hub for the detection of direction. It has neurons organized retinotopically and in columns, with each column showing a preference for a specific direction.

When a column of V5 cells preferring a certain direction is artificially stimulated with a micro-electrode, the eyes of a monkey will move slightly toward that direction. It appears that the signals arising from V5 cortical columns stimulated by an object moving through the visual field are vectorially averaged by the brain. A vectorial average is simply a weighted average of many small inputs, each of which contains information about both direction and magnitude.

The dorsal stream terminates in the back section of the parietal lobe. Faults and lesions in this area caused by brain tumors or strokes lead to a host of symptoms, all of which involve a reduced sense of spatial awareness in humans. I've discussed these at this section's end.

Given this overview of dorsal stream function, let's go briefly down into the cellular level. We're already familiar with area V1, where both the dorsal and ventral streams originate, which is described in previous sections as home to simple and complex cells. V1 cells, as a whole, are primarily sensitive to contrasts, edges, directions of movement and spatial frequency of stimuli.

Area V2, the dorsal stream's first stop and a surprisingly understudied part of the brain, is more of an enigma. Lesions made in monkey V2 areas severely affect their performance in spatial tasks, but don't affect contrast sensitivity, which is needed for identifying objects clearly. V2 receives the most feedforward connections from V1 of any brain area, and the stimuli its cells respond to have been hypothesized to arise from combining V1 cell receptive fields. As a result, individual V2 cells respond to similar visual stimuli as V1 cells: chiefly to the orientation of bars of light, and also to their color and spatial frequency. The receptive field of a V2 cell is in general far more complex, however. A V2 receptive field, for example, can be composed of regions responding to different orientations.

V2 sends out many connective axon streams. One stream gives extensive feedback to V1. Three others travel to areas V3, V4 and V5.

The next area of interest in the dorsal stream is area V5 (alternatively area MT). It is easy to distinguish in histological sections because of its heavy myelination, which is a characteristic of a cortical area through which a great deal of fast traffic flows. As I mentioned above, V5 is a columnar hotbed of direction- and speed-selective neurons. Nearly 90% of its constituent neurons show strong response to a specific direction.

Apart from V2, it gets its inputs directly from V1, and also from V3. Subcortical regions of the brain, too, are involved - showing you how quickly things get complicated within the cortical regions - with the koniocellular layers of the LGN and axons from the pulvinar nucleus of the thalamus both projecting to V5.

Since it receives so many inputs, originating in so many different structures, trying to explain how each source contributes to the characteristics of MT neuron receptive fields is a daunting task. Merely describing these fields is more accessible, and was first done with our familiar micro-electrode technique. When direction and speed selectivity emerged as defining characteristics there was a bit of confusion, because that's what V1 is sensitive to, too. Its exact role still remains unclear, but the most widely accepted provisional idea is that V5 is the area key in integrating many local motion signals into the global motion of a complex object moving through the visual field.

It doesn't do this work by itself. There are at least three cortical areas of undefined expanse that surround the V5 area like a belt. These are all higher-order visual areas, involved in aspects of our visual worlds such as optic flow, which is the name for what happens when you move through the world at some speed but it looks instead like the landscape is moving.

The dorsal stream leaves V5 to terminate in the back part of the parietal lobe, the posterior parietal cortex (PPC). This is a domain of 'higher-order functions' and is called 'associative' because it combines inputs from different brain areas, mixing motor, auditory and visual information. It's bounded by the post-central sulcus in front, and the parieto-occipital sulcus behind. It is itself split into two lobules by the intra-parietal sulcus. The upper half is the superior parietal lobule; the lower the inferior parietal lobule.

Electrical stimulation of the PPC shows that it is involved in many different functions. In macaque monkeys this cortical area has been subdivided into as many as 15-20 specialized areas. For example, electric shocks applied to the anterior part of the sulcus that divides the PPC indicate a role in grasping objects by hand. Going backward along it, eye movements and defensive arm movements to protect the head are evoked.

Broadly described, the PPC links the eyes and the limbs, especially the hand and arm - it is a visuomotor area. It can do this because it projects to the motor areas located in front of the central sulcus, though these connective streams are largely ill-defined.

For our discussion, one key role it plays is in regulating proprioception, the 3-dimensional awareness of where our limbs are located in space. People that lose this body image can lose their awareness of some parts of their bodies completely. This can lead to bizarre situations, like sufferers waking up and noticing an arm or leg in bed that's not their own, and casting themselves bodily onto the floor as they try to get rid of it.

The second key role is the one that birthed the concept of the dorsal stream: visuomotor function. People with defects in this area are afflicted with optic ataxia, discussed in the paragraphs below.

But the parietal cortex is much more interesting than just these two features. As I mentioned above, it is an 'association' area, a term which in neuroanatomy denotes the confluence site of multiple sensory inputs. It seems that it's these senses, put together, that give us our sense of embodiment. This sense of embodiment has given rise to some of the most striking experiments in neuroscience, during which the parietal lobes of epileptic patients undergoing surgery were electrically stimulated and 'out of body experiences' (OBEs) were produced as a result.

Damage in the Dorsal Stream

Optic ataxia: this neurological condition is seen most commonly with damage to the posterior parietal region of the cortex. People that have it are able to recognize objects in their environment with ease, but are unable to successfully reach and grasp them. They're unable to reach in the right direction, as well as form the correct shape in the correct orientation with their hands. Sometimes, there's also an inability to adapt the size of the grasp to the size of the object to be grabbed.

*This clear distinction has since been refuted. It's now clear that both the dorsal and ventral visual streams receive input from both the magno- and parvo-cellular layers of the LGN.