1. Introduction

"Classic" view:

    (note that this approach is focusing on the image, while we are focusing on behavior)

(2) The representation is formed through modular analysis.
     multiple representations of the retinal image - what are they doing?
            a) parallel pathways
            b) hierarchical stages of analysis

            a) parallel pathways
                component representations - each carries information about a particular aspect of the image
                 (helping in grouping, preceeding segmentation)
           independent analysis of visual attributes: luminance, texture, color, disparity, movement, orientation

           evidence is coming from:

                                (1) physiology and anatomy of monkey visual cortex
                                       single cell technique:
                                               receptive field location, size, structure
                                               receptive field structure: orientation in V1, motion in MT, color in V4, objects in IT....
                                               topographic maps - economic connectivity across space & features

                               (2) neuropsychology: cortical color- and motion-blindness, agnosias

                                                (1) and (2) leading to the concept of 2 parallel cortical pathways:
                                                                                                        V1-V2-V4-IT(Br21): object identification
                                                                                                        V1-V2&V3/VP-V5-P(Br7): movement and position

                                (3) psychology: e.g., visual search: color, orientation, size, curvature
 

            b) hierarchical levels
                                 - LGN to V1 to all other areas
                                        P/M to ventral/dorsal
                                 - within V1, hierarchy of complexity in RF structure
                                 - road map of cortical organization based on anantomy of
                                                        ascending projections: upper layers to layer 4
                                                        descending projections: lower layers to other than 4
 
 

Here is a great site to study again:
http://www.med.uwo.ca/physiology/courses/sensesweb/

2. Parallel channels in the primate geniculostriatal pathway

3. What are the various extrastriate areas doing?

• Two main processing streams
• VENTRAL (V1-V2-V4-IT) : object vision
• DORSAL (V1-V2&V3/VP-V5-P) : spatial vision
• Roots of the theory:
• I(nfero)T(emporal) lesions produce inability to recognize objects; P(osterior parietal) lesions produce a deficit in spatial vision.
• Gerald Schneider (1960): visual function might be devided into identity vs localization functions


  click on the figure to get the full roadmap by Ungerleider, 1995

4. How does Parvo/Magno map onto Ventral(V1-V2-V4-IT)/Dorsal(V1-V2&V3/VP-V5-P)?

• Ventral/dorsal are a continuation of parvo/magno
• color-coding is the most striking difference along the geniculostriatal pathway
• magno system is colorblind, therefore pure color-contrast stimuli (equiluminant) should reveal what is its function
• conclusion: parallel channels for form, color (WHAT in the ventral) and for movement and depth (WHERE in the dorsal stream).




demonstrating color & luminance contrast
 
 
 

5. Problems

• lesion studies do not support exact mapping  (e.g., stereopsis in the ventral stream)
• psychophysical studies do not always support exact mapping (e.g. multiattribute stimuli)
• color-selectivity in dorsal neurons
• problems with equiluminance (positionsal uncertainty, residual luminance cues, contrast gain-control mechanism is not working
• parvo/magno input to cortex is not clearcut
• dorsal/ventral streams are interconnected, and further subdivided
• dorsal/ventral streams do not end in parietal and temporal cortex, they continue into frontal lobe
• dorsal and ventral frontal areas retain their specialization, and do not collapse into one
• there is more.....can you come up with some?

6. What are the functions of vision again?

• Evolutionary perspective
• there is modularity in non-mammalian vertebrates in terms of visuo-motor behavior (e.g. different modules for escape and orienting behavior) - less is know about such modules in mammals because research is focused on perceptual/cognitive aspects
• "acting" on the world is just as important as "recognizing" it
• current research is focused on the "input", however, the "output" should also tell us about function
• Suggestion: ACTION and PERCEPTION are two functionally and neurally distinct systems
• representational requirements are different:

ACTION	PERCEPTION
size  shape  direction  distance	shape  color  texture  shading  transparency
veridical	constancies (size, color, shape, brightness)
on-line  fast, robust	memory requirement is large
ego-centric	allocentric

Can you come up with some more distinctions between the two hypothetical systems?
How could these be tested experimentally?
 

• supported by physiological, neuropsychological and psychophysical findings


Can you list some experimental findings or observations that might support the theory?