Ventral and dorsal streams in cortex: focal vs. ambient processing/exploitation vs. exploration

摘要

The idea of a dissociation of the visual pathway into two distinct streamsa??ventral and dorsala??that each processes distinct kinds of information is a powerful one. Two proposals along those lines state that the ventral stream processes information about object identity ("what"), whereas the dorsal stream processes information about either object location ("where"; Ungerleider & Mishkin, 1982;) or to perform motor acts ("how"; Goodale & Milner, 1992). Both proposals are influential but contradicted by recent data (e.g. ventral stream is involved in where/how computations; the dorsal stream is involved in "what" computations). We suggest a more robust dichotomy breaking down into 1. a ventral stream sampling high-resolution/focal spaces, and therefore, macularly-biased, and 2. dorsal ambient sampling, and therefore less spatially biased streams. This dichotomy may derive from pressures exerted during evolution by dense receptive surfaces. The idea further hews more closely to the theme of embodied cognition: Function arises as a consequence of an extant sensory underpinning. A continuous, rather than sharp, segregation based on function emerges, and carries with it an undercurrent of an exploitation-exploration dichotomy. Under this interpretation, cells of the dorsal stream, which individually have large receptive fields and poor spatial localization, do not provide information about location but rather of the presence/absence of salient objects in the visual field for exploration. Our model is not exclusive to the primate/hominid visual system but is extendable to the bat auditory system and could provide an evolutionary basis for the development of the fovea and mechanisms for eye tracking in animals. We leverage our dichotomy to explain neuropsychological cases (visual agnosia, optic ataxia), account for the prevalence of multisensory integration in the dorsal rather than the ventral stream under a Bayesian framework, and provide a dynamic component to the ventral-dorsal dichotomy that helps create a unified, seamless perception.