Object vision and spatial vision two cortical pathways

Object vision and spatial vision: two cortical pathways Mishkin, Ungerleider & Macko (1983) Presented by Joel Tan

Question • What is the functional organization of the extrastriate cortex? • More specifically: Are there separate streams for processing object information / location information?

Alternatives (1) • Visual functions are exclusively in striate cortex (Lashley) • Extrastriate cortex has visual functions • Solved! Lashley loses

Alternatives (2) • Extrastriate cortex has unified visual functions • Extrastriate cortex is divided in functionally and anatomically separate “streams” • Solved! There are streams

Alternatives (3) • There is a tectofugal stream that guides action and a geniculostriate stream that guides perception. (Evidence from birds) Pohl & Schneider (70 s) • There is a dorsal stream for space perception and a ventral stream for object perception (Evidence from non-human primates) Ungerleider & Mishkin (80 s) • There is a dorsal stream that guides action and a ventral stream that guides perception (Evidence from humans) Goodale & Milner (90 s)

Logic • Lesioning: The method of double dissociation. • Double dissociation “establishes” the correspondence between a particular function and a brain region. (1: 1) • Example: Lesioning Area A impairs function C but not function D. Conversely, lesioning Area B impairs function D but not C. Hence, we associate A with C and B with D.

Method • Compiling a large database of lesioning studies in rhesus monkeys, using the method of double dissociation.

Results – Ventral for Objects Lesioning the ventral stream led to deficits in object discrimination but not object localization.

Results – Dorsal for Space Lesioning the dorsal stream led to deficits in landmark discrimination but not object recognition.

Interpretation In primates, extrastriate cortex consists of two anatomically and functionally separate streams!

Interpretation Dorsal stream: Spatial vision Ventral stream: Object vision

Interpretation – arising issues • Integration: • If the two functions are separate, how does the brain recombine them into a coherent percept? • This is called the “binding” problem. • Particularly tricky if there is more than one object in visual world (as usual) • Is this the function of consciousness?

Problems • Species: How do the results generalize • Alternatives: Doesn’t rule out perception for action vs. perception hypothesis. • Lesioning is not a bulletproof method: Often unspecific and unprecise, the problem of diaschisis, unaccounted for side-effects.