Milner And Goodale's Dimmary: Dorsal And Controtorical Interaction?

5. Dorsal and ventral interactions (?)

Although Milner and Goodale’s account has received several critiques which concern in particular the empirical data’s interpretation, broad aspects of their model are still acknowledged today. For example, several studies (e.g. James et al. 2003; Mahon et al….; McIntosh et al…) run with different brain scanning techniques, have demonstrated that observation of tools is likely to activate both ventral and dorsal areas, while pictures or non-graspable objects show a major ventral activity. Thus, to some extent, a dissociation between the two pathways’ function is conceivable.
At the same time, however, there is a growing body of evidence showing that the dissociation between the two streams may not
The superior parietal lobe controls bodily movements using a synthesis of pieces of information coming from the visual system and the motor system; thus, its neurons fire in the presence of visual stimuli but their activity is reinforced when the stimulus presentation is followed, for example, by an arm reaching movement directed towards the stimulus. Moreover, the superior parietal lobe has a prominent role in processing egocentrically-coded spatial information. Thus, this system calibrates fine-tuned movements and it is part of the so-called dorso-dorsal pathway. Generally, it has no fundamental role in object’s recognition since its connections with ventral areas are scarce and generally mediated by the inferior parietal lobe, which is for this reason called the ventro-dorsal pathway. It projects mainly to ventral premotor cortex’s areas (F4 and F5) and to the prefrontal lobe. The inferior parietal lobe is formed by different neural circuits, some of them regulating actions upon objects, while some other connect action with spatial representations, coded in an egocentric frame of reference. One of its main roles appears to be the transformation of objects’ sizes and shapes information into the appropriate motor schemas for acting upon them.
First, current interpretations of neuropsychological diseases or of behavioural experiments do not appear to favour Milner and Goodale’s model, at least with respect to the alleged dissociation between conscious ventral versus unconscious dorsal processing. Second, even if one accepts the hypothesis that fine-tuned action guidance might be unconscious, it should be acknowledged that this does not provide an argument concerning all possible dorsal stream’s contribution to visual awareness. As I pointed out referring to Rizzolatti and colleagues’ hypothesis of a three visual stream’s dissociation, part of the parietal cortex actually appears to be involved in producing some kind of conscious representations. In the following chapters, I further develop this point, arguing that a complete account proving that dorsal areas do not contribute to visual awareness is far from