Two action systems in the human brain.

Area of Science:

• Neuroscience
• Cognitive Science
• Visual Processing

Background:

• The Ungerleider and Mishkin (1982) and Milner and Goodale (1995) models proposed distinct dorsal and ventral visual streams.
• Recent research has expanded on these models, detailing neural circuits and behavioral observations.
• The classic "two visual systems" hypothesis struggles to explain certain action disorders like limb apraxia and optic ataxia.

Purpose of the Study:

• To elaborate on the distinction between dorsal and ventral visual processing streams.
• To propose a revised model accounting for new neuroanatomical and behavioral evidence.
• To differentiate multiple dorsal pathways from the ventral stream.

Main Methods:

• Review of neuroanatomical studies detailing neural circuits.
• Analysis of behavioral observations, including disorders of action.
• Comparison of functional characteristics across proposed visual pathways.

Main Results:

• Evidence suggests at least two distinct dorsal visual routes in the human brain: the "Grasp" and "Use" systems.
• These dorsal systems can be differentiated from the ventral stream based on neuroanatomy, representational specificity, and processing time.
• The "Grasp" and "Use" systems offer a better explanation for observed dissociations in action disorders.

Conclusions:

• The classic two-stream model requires revision to incorporate multiple dorsal pathways.
• The proposed "Grasp" and "Use" systems provide a more comprehensive framework for understanding dorsal stream functions.
• This refined model better accounts for the complexities of visual guidance of action and associated neurological deficits.