Dorsal And Ventral Visual Systems Research Paper

2. Evidence From Private Neuro-Behavioral Studies

In their seminal paper, Ungerleider and Mishkin (1982) argued that the two streams of visual processing play complementary roles in the perception of incoming visual information. According to their account, the ventral stream governs the recognition of objects, while the dorsal stream mediates the localization of those same objects. Apparent support for this idea came from work with monkeys. Lesions of ITC produced deficits in the animal’s ability to discriminate visually between objects, but did not affect its performance on a task requiring it to judge the location of a ‘landmark.’ Conversely, lesions of the PPC produced deficits in performance on the landmark task, but did not affect object discrimination learning. Although the evidence available at the time was consistent with this proposal, recent research fits better with a distinction not between sub-domains of perception, but between perception on the one hand and the guidance of action on the other (Jeannerod 1997, Milner and Goodale 1995).

3. Evidence From Visual Neurons In The Ventral And Dorsal Streams

Neurons in areas such as ITC are tuned to the features of objects, and many of them show remarkable categorical specificity; some of these cells maintain their selectivity irrespective of viewpoint, retinal image size, and even color. These neurons are affected little by the monkey’s concurrent behavior, but many are affected by how often the visual stimulus has been presented, and by its association with reward. Evidently the ventral stream is more concerned with the enduring characteristics and significance of objects than with moment-to-moment changes in the visual array.

Neurons in the dorsal stream show quite different properties from those in the ventral stream. Neurons in the PPC are activated by visual stimuli according to the response the monkey makes to those stimuli. For example, some sells respond when the stimulus is the target of an arm reach; others when it is the object of a grasp response; others when it is the target of a saccadic eye movement; others when the stimulus is moving and elicits a slow pursuit eye movement; and still others when the stimulus is stationary and becomes the object of fixed gaze (Jeannerod 1997, Milner and Goodale 1995).

These findings can readily explain why posterior parietal lesions impair landmark task performance: the monkey simply fails to orient toward the landmark. They also explain one of the most striking effects of PPC lesions, namely an inability to accurately reach for a moving or stationary food morsel, and why they fail to shape and orient their hands and fingers appropriately to pick it up. Evidently visual networks in the dorsal stream compute more than just spatial location.

4. Visuomotor Deficits Following Brain Damage In Humans

The first systematic description of a patient with bilateral PPC damage was published by Balint in 1909 (See Harvey 1995). Balint’s patient was unable to accurately reach toward objects with his right hand. In many respects, this disorder, which Balint called optic ataxia, resembles the problems of the PPC-lesioned monkey. Patients with optic ataxia not only fail to reach in the right direction but also have difficulty orientating their hand and forming their grasp appropriately to pick up target objects (Perenin and Vighetto 1988). Often they are unable to use visual information to form their grip as they reach toward an object, and have to grope for it. For example, they make errors in hand rotation as they try to reach toward and through a large oriented slot. Also, they often fail to show the normal proportional opening of the hand when reaching out to grasp objects of different sizes. Yet despite their visuomotor failures, these patients have comparatively little difficulty in giving perceptual reports of the orientation and location of the very objects they fail to grasp.

5. Perceptual Deficits Following Brain Damage In Humans

On the other side of the equation, an impairment of ventral stream function occurs in humans who suffer from the condition known as visual form agnosia. These patients not only fail to recognize faces or objects, they fail even to identify simple geometric shapes. One such patient, DF, has been extensively studied during the 1990s. The value of these studies depends on DF’s uniquely fortuitous pattern of brain damage, which gives rise to severe visual form agnosia as an isolated symptom, in the absence of any motor impairment or mental deterioration. All other similar patients in the literature either have a less severe form agnosia, or an impaired ability to cooperate or to make precise limb and hand movements.

What DF can do is as important as what she can’t. Her attempts to make a perceptual report of the orientation of an oriented slot show little relationship to its actual orientation, whether her reports are made verbally or by manual means. However, when she is asked to insert her hand or a hand-held card into the slot, she moves her hand or the card toward the slot in the correct orientation and inserts it accurately. Indeed her hand begins to rotate in the appropriate direction as soon as it leaves the start position. In short, although she cannot report the orientation of the slot, she can act upon it with considerable skill.

Similar dissociations between perceptual report and visuomotor control were also observed when DF was asked to deal with properties of objects such as their size and shape (Goodale et al. 1991). Thus, she showed excellent visual control of anticipatory hand posture when reaching out to pick up blocks of different sizes that she could not distinguish perceptually. Just like healthy subjects, DF adjusted her finger-thumb separation well in advance of contacting the object, in a perfectly normal and linear fashion in relation to the target width. Yet when she was asked to use her finger and thumb to give a perceptual indication of the object’s width, her responses were unrelated to the actual stimulus dimensions, and showed high variation from trial to trial. Functional MRI data have shown that the ventral stream is severely disconnected from visual inputs in DF, so we infer that the calibration of these various residual visuomotor skills must depend on intact mechanisms within the dorsal stream.

Various studies of DF show that she is able to govern many of her actions using visual information of which she has no awareness. But it is clear that this is only true of actions that are directly targeted at the visual stimulus. She cannot successfully use the same visual information to guide an identical response but displaced to another location. Presumably the difference is that the displaced response is an arbitrary or symbolic one. DF seems to depend on a visual processing system dedicated to motor control, which will normally only come into play when she carries out natural goal-directed actions. She has no explicit awareness of the shapes and sizes that she is able to grasp using this system.

There are temporal as well as spatial limits on DF’s ability to drive her motor behavior visually. In one study, DF was shown a rectangular block, which was then removed while her eyes were closed, and then she reached out and pretended to grasp it. The preparatory grip size of normal subjects still correlates well with object width even after a 30 second delay. In DF, however, all evidence of grip scaling during her reaches had evaporated even after a delay of 2 seconds. The problem seems to be that a delayed reach is not a natural movement, requiring subjects to imagine the object and then ‘pantomime’ an appropriate grasp. This strategy would not have been open to DF: she could not generate a visual image of something that she didn’t perceive in the first place. Apparently her preserved visuomotor brain can only respond to present or imminent states of the visual world, and disregards past states that may no longer be relevant.

Recently, a complementary ‘improvement’ in both reaching and grasping accuracy has been recorded in two bilateral optic ataxic patients after a 5-second delay (Milner and Dijkerman 2001). Presumably these patients were able to throw off the dominance of the damaged dorsal stream under the delay condition, allowing them to make use of their better-preserved ventral system.

6. Operation Of The Two Streams In Normal Vision

It may be that like DF, healthy observers also control their actions using visual information that is not present in their awareness. In providing such visual guidance the dorsal stream may act pretty much in isolation, and independently of any acquired ‘knowledge base.’ Our implicit assumptions about the perceptual stability and constancy of the world are based on such stored knowledge. These assumptions sometimes cause us to misjudge what we see, as when a shift in a large frame makes a stationary spot inside it seem to shift in the opposite direction. In contrast, the dorsal system, by and large, is not deceived by such spatial illusions. Instead, it directs our saccadic eye movements and hand movements to where a target really is, instead of where our perceptual system tells us it is (e.g., Bridgeman et al. 1997). Similarly, under appropriate circumstances geometric illusions can be seen to affect visually-guided reaching and grasping (Haffenden and Goodale 1998) far less than they affect perceptual judgments. In other words, you may perceive an object as bigger than it really is, but you open your finger-thumb grip correctly when reaching for it.

It seems that the processing accomplished by the ventral stream both generates, and is in turn informed by, stored abstract visual knowledge about objects and their spatial relationships. It may be that these ‘top-down’ influences are what render our perceptual representations accessible to awareness (Lamme and Roelfsema 2000). This would fit with the idea that coded descriptions of enduring object properties, rather than transitory egocentric views, are needed for mental manipulations like the planning of action sequences and the mental rehearsal of alternative courses of action.

7. Integration Of Visual Function

Although the dorsal and ventral streams have separate functions, there are multiple connections between them, and indeed adaptive goal-directed behavior in humans and other primates must depend on a successful integration of their complementary contributions. Thus the execution of a goal-directed action might depend on dedicated control systems in the dorsal stream, but the selection of appropriate goal objects and the action to be performed depends on the perceptual machinery of the ventral stream. At the level of visual processing, however, the visuomotor modules in the primate PPC function quite independently from the ITC mechanisms generating perception-based knowledge of the world. Only this latter, perceptual, system can provide suitable raw materials for thought processes to act upon. In contrast, the other is designed to guide actions purely in the ‘here and now,’ and its products are consequently useless for later reference.

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