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So I've been reading up on the Two-streams Hypothesis*, and it bothers me that the explanations both sides give only extend to tasks involving one type of visual information. For example, an experiment might require subjects to identify the category of an object (ventral stream) or correctly grasp an object (dorsal stream).

But surely there are tasks in the real world that require both sorts of information simultaneously...

For example, suppose I'm out picking apples: I want to put the ripe apples in my basket and knock the rotten apples to the ground. The ripeness categorization would presumably be a ventral task, whereas the action (grasping/knocking down) relies on dorsal information.

Has there been any work on what mechanisms are behind our ability to perform such tasks?

The closest I've come to an answer is this paper about we use rules to select actions, but the actions discussed don't involve the spatial properties of the rule defining object.

*Brief summary of my understanding of the Two-streams Hypothesis: it posits two separate pathways in the brain for processing visual objects: a dorsal pathway for spatial and affordance information, and a ventral pathway for property and category information.

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Good to be back :) –  zergylord Mar 27 '13 at 4:08
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up vote 6 down vote accepted

From personal observation, the consensus seems to be that the two streams hypothesis is an oversimplification of the truth, albeit a useful one.

The primary reason that this hypothesis is seen as an oversimplification is because there is a lot of cross-talk between the two streams. For example, Zanon et al. (2010) provides evidence for functional connectivity using EEG and TMS. Himmelbach & Karnath (2005) provide evidence from neuropsychological patients with optic ataxia.

More direct evidence comes from Borra et al. (2008) and Felleman & Van Essen (1991), who provide evidence for cross-talk between streams in monkeys by tracing cortical connections.

Mahon et al. (2007) adresses your specific question, stating:

In the course of manipulating and using objects, it is necessary to integrate the output of object recognition processes (ventral stream) with information about object motion (left middle temporal gyrus) and the motor commands necessary to realize the function of the objects (left inferior parietal lobule). The efficacy of such an information processing network would be increased if the organization of object recognition processes already anticipated the processing requirements of computations implemented ‘downstream.’ (see article for more details)

There have also been a few updated reviews of the two streams hypothesis, such as Milner & Goodale (2008), that might clear up some misconceptions.

Another oversimplification is that the functions of each stream are not entirely unique, as is often stated. For instance, areas in both the dorsal and the ventral stream contain neurons which respond to shape encoding (Lehky & Sereno, 2006), despite that this is generally considered a 'ventral' task.

Unfortunately the misconception that these two streams function completely independent of each other, and perform completely independent functions, is still being perpetuated in text books and journal articles.

References

Zanon, M., Busan, P., Monti, F., Pizzolato, G., & Battaglini, P. P. (2010). Cortical connections between dorsal and ventral visual streams in humans: evidence by TMS/EEG co-registration. Brain topography, 22(4), 307-317.

Himmelbach, M., & Karnath, H. O. (2005). Dorsal and ventral stream interaction: contributions from optic ataxia. Journal of Cognitive Neuroscience, 17(4), 632-640. PDF

Borra, E., Belmalih, A., Calzavara, R., Gerbella, M., Murata, A., Rozzi, S., & Luppino, G. (2008). Cortical connections of the macaque anterior intraparietal (AIP) area. Cerebral Cortex, 18(5), 1094-1111.

Felleman, D. J., & Van Essen, D. C. (1991). Distributed hierarchical processing in the primate cerebral cortex. Cerebral cortex, 1(1), 1-47.

Mahon, B. Z., Milleville, S. C., Negri, G. A., Rumiati, R. I., Caramazza, A., & Martin, A. (2007). Action-related properties shape object representations in the ventral stream. Neuron, 55(3), 507-520. PDF

Milner, A. D., & Goodale, M. A. (2008). Two visual systems re-viewed. Neuropsychologia, 46(3), 774-785. PDF

Lehky, S. R., & Sereno, A. B. (2007). Comparison of shape encoding in primate dorsal and ventral visual pathways. Journal of neurophysiology, 97(1), 307-319. PDF

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full disclosure: i haven't read all of these articles, so please correct where appropriate –  Jeff Mar 27 '13 at 5:25
    
Wow, thanks for the great response! That Mahon quote was just the kind of thing I was looking for. Looks like I've got my readings for the next week :) –  zergylord Mar 27 '13 at 6:42
    
thanks for the good question! i have heard it a few times before but not never bothered to look up many of the primary sources until now. –  Jeff Mar 27 '13 at 15:20
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