Two-streams hypothesis
[1] The hypothesis, given its initial characterisation in a paper by David Milner and Melvyn A. Goodale in 1992, argues that humans possess two distinct visual systems.
The dorsal stream (or, "where pathway") leads to the parietal lobe, which is involved with processing the object's spatial location relative to the viewer and with speech repetition.
[6] In 1982, Ungerleider and Mishkin distinguished the dorsal and ventral streams, as processing spatial and visual features respectively, from their lesion studies of monkeys – proposing the original where vs what distinction.
Visual action planning and coordination, on the other hand, uses absolute metrics determined via egocentric frames of reference, computing the actual properties of objects relative to the observer.
Damage to the posterior parietal cortex causes a number of spatial disorders including: The ventral stream is associated with object recognition and form representation.
The ventral stream gets its main input from the parvocellular (as opposed to magnocellular) layer of the lateral geniculate nucleus of the thalamus.
From there the information moves to the beginning of the dorsal pathway, which is located at the boundary of the temporal and parietal lobes near the Sylvian fissure.
This is evident because its ability to acquire new vocabulary, be disrupted by lesions and auditory feedback on speech production, articulatory decline in late-onset deafness and the non-phonological residue of Wernicke's aphasia; deficient self-monitoring.
The articulatory network 1, which processes motor syllable programs, is located in the left posterior inferior temporal gyrus and Brodmann's area 44 (pIFG-BA44).
"Functionally, conduction aphasia has been characterized as a deficit in the ability to encode phonological information for production," namely because of a disruption in the motor-auditory interface.
Contemporary perspectives however, informed by empirical work over the past two decades, offer a more complex account than a simple separation of function into two-streams.
[29] Recent experimental work for instance has challenged these findings, and has suggested that the apparent dissociation between the effects of illusions on perception and action is due to differences in attention, task demands, and other confounds.
[30][31] There are other empirical findings, however, that cannot be so easily dismissed which provide strong support for the idea that skilled actions such as grasping are not affected by pictorial illusions.
The dissociation between visual agnosia and optic ataxia has been challenged by several researchers as not as strong as originally portrayed; Hesse and colleagues demonstrated dorsal stream impairments in patient DF;[36] Himmelbach and colleagues reassessed DF's abilities and applied more rigorous statistical analysis demonstrating that the dissociation was not as strong as first thought.
[37] Goodale & Milner themselves have proposed the analogy of tele-assistance, one of the most efficient schemes devised for the remote control of robots working in hostile environments.
Thus the emerging perspective within neuropsychology and neurophysiology is that, whilst a two-systems framework was a necessary advance to stimulate study of the highly complex and differentiated functions of the two neural pathways; the reality is more likely to involve considerable interaction between vision-for-action and vision-for-perception.
Robert McIntosh and Thomas Schenk summarize this position as follows: We should view the model not as a formal hypothesis, but as a set of heuristics to guide experiment and theory.
The differing informational requirements of visual recognition and action guidance still offer a compelling explanation for the broad relative specializations of dorsal and ventral streams.
