Perception Lecture Notes: Secondary Cortical Visual Areas and the What/Where Pathways

Professor David Heeger

Secondary Cortical Visual Areas

Diagram of the areas

There are 20-30 secondary visual areas after V1. Most lie in the occipital lobe. But some are in the parietal and temporal lobes as well.

These areas are identified based on both physiology and anatomy (the acronym we'll use is "FACT"):

• Function (physiology): neurons in different brain areas are selective for different things (e.g., motion in MT).


Functional specialization hypothesis: this is the main hypothesis driving research into secondary visual cortical areas, specific brain area are involved in specific visual functions. Examples: MT is specialized for visual motion perception.  IT is specialized for recognition.  V4 is specialized for color vision.  This kind of approach has been adapted to identify visual cortical areas in the human brain using fMRI, e.g., the video from the neuroimaging lecture showing how moving stimuli "light up" human MT.
 

• Architecture: cytoarchitecture (cell size, cell density, number of layers, density of axons, etc.) revealed through various different stains can change from one visual area to the next, helping to identify the boundaries between distinct brain regions..




Stained slice showing cortical layers
 



Visual areas identified via cytochrome oxidase staining

Cytochrome oxidase is an enzyme used in metabolism. Staining for it gives characteristically different patterns in different visual areas. In V1 it shows puffs or blobs. In V2 it yields a striped pattern. In V3 it yields a dense strip. In MT it yields a dense region.
 

• Connections. An anatomist can use chemicals that are transported by neurons (called tracers). The chemical is injected into one brain area. One then waits a while, then sacrifices the animal and stains for the tracer to see where it ends up. Some tracers are picked up by cell bodies and are transported to regions the cells project to; other tracers are picked up by axon terminals and are transported back to the corresponding cell bodies.
  



Wiring diagram

• Topography, e.g. retinotopy. Each of the separate early visual areas contains a retinotopically organized map of the visual field.




One can measure retinotopy using fMRI in the human brain using the expanding ring and rotating ray patterns we showed in class. The separate retinotopic maps define boundaries between visual areas.

Parietal and Temporal Pathways

There are two distinct streams (parallel pathways). One goes to the parietal lobe and the other goes to the temporal lobe.

What do these two pathways do? Clinical observations have provided us with most of the information on this. One finds different deficits in patients who have lesions in these two different areas. The deficits are very different:

• Parietal lesions lead to deficits in spatial orientation, attention (this is the "where" pathway).
• Temporal lobe lesions lead to deficits in object recognition (thus, this is the "what" pathway).