Abstract
Current evidence supports the view that the visual pulvinar of primates consists of at least five nuclei, with two large nuclei, lateral pulvinar ventrolateral (PLvl) and central lateral nucleus of the inferior pulvinar (PIcl), contributing mainly to the ventral stream of cortical processing for perception, and three smaller nuclei, posterior nucleus of the inferior pulvinar (PIp), medial nucleus of the inferior pulvinar (PIm), and central medial nucleus of the inferior pulvinar (PIcm), projecting to dorsal stream visual areas for visually directed actions. In primates, both cortical streams are highly dependent on visual information distributed from primary visual cortex (V1). This area is so vital to vision that patients with V1 lesions are considered “cortically blind”. When the V1 inputs to dorsal stream area middle temporal visual area (MT) are absent, other dorsal stream areas receive visual information relayed from the superior colliculus via PIp and PIcm, thereby preserving some dorsal stream functions, a phenomenon called “blind sight”. Non-primate mammals do not have a dorsal stream area MT with V1 inputs, but superior colliculus inputs to temporal cortex can be more significant and more visual functions are preserved when V1 input is disrupted. The current review will discuss how the different visual streams, especially the dorsal stream, have changed during primate evolution and we propose which features are retained from the common ancestor of primates and their close relatives.
Highlights
This review focuses on the contributions of the visual pulvinar of primates to the two major processing streams that flow from primary visual cortex through early visual areas to targets in temporal and posterior parietal cortex
We suggest instead that it is homologous to the tectum to posterior pulvinar to the middle temporal visual area (MT) complex pathway that forms part of the more complex dorsal stream of primates
To what end? We suggest that the dorsal stream of visual processing in primates is a cortical system for action that pre-dates the emergence of primates and is usually present in a greatly reduced form in tree shrews, rodents, and other mammals as a stream dominated by visual information from the superior colliculus
Summary
This review focuses on the contributions of the visual pulvinar of primates to the two major processing streams that flow from primary visual cortex through early visual areas to targets in temporal and posterior parietal cortex. Classes of ganglion cells of the retina are already specialized for different modes of vision, and these specializations feed into the dorsal or ventral cortical streams This classical view of the two cortical streams was primarily thought to depend on the functionally segregated outputs from primary visual cortex, which is heavily dependent on visual information conveyed through the above mentioned retinogeniculostriate pathways. A relay of visual information from the superior colliculus to the pulvinar and to temporal cortex is a very basic feature of mammalian brains, and MT could have emerged out of part of temporal cortex by changes in connections that made MT less dependent on the superior colliculus and highly dependent on newly evolved inputs from V1 (see [14] for review)
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