Different Pathways , Different Processes

1. Different Pathways , Different Processes

2. Retinocollicular vs. Retinostriate • Recall that 10% of optic nerve gets routed through the Superior Colliculus (SC) • What does it do? • SC contributes to control of eye movements (saccade vector maps) • SC and Pulvinar contribute to orienting of attention

3. Superior Colliculus • Superior Colliculus (SC) contributes to control of eye movements • a saccade is a rapid eye movement • SC contains a retinotopic map of possible saccade vectors • Activation of particular cells guides eyes to specific location left right

4. Superior Colliculus • Superior Colliculus (SC) contributes to control of eye movements • a saccade is a rapid eye movement • SC contains a retinotopic map of possible saccade vectors • Activation of particular cells guides eyes to specific location Does SC contribute to orienting even when “main” visual pathway is disrupted? left right

5. Lesions of Retiostriate Pathway • Lesions (usually due to stroke) cause a region of blindness called a scotoma • Identified using perimetry • note macular sparing X

6. Retinocollicular Pathway independently mediates orienting • The theory is that the retinocollicular pathway continues to operate despite lesions in the retinostriate pathway • note this is somewhat counterintuitive in that it predicts people should be able to orient to visual objects that they can’t “see”

7. Retinocollicular Pathway independently mediates orienting • Weiskrantz (1986) • subject fixates at centre • target appears in periphery and subject saccades to target • in control condition, no light appears • Importantly, both conditions appear the same to subject!

8. Retinocollicular Pathway independently mediates orienting • Prediction: subject should be able to orient accurately to the target even when it is in the blind field

9. Retinocollicular Pathway independently mediates orienting • Subject was able to orient with relatively good accuracy up to about 25 degrees

10. Blindsight • Intact retinocollicular pathway mediates eye movements despite V1 lesions • Might it also orient attention? The theory is that it does.

11. Retinocollicular Pathway independently mediates orienting • Rafal et al. (1990) • Prediction: visual stimuli in scotoma should interfere with (i.e. distract) orienting to stimuli in the good field • subjects move eyes to fixate a peripheral target in two different conditions: • target alone

12. Retinocollicular Pathway independently mediates orienting • Rafal et al. (1990) • Prediction: visual stimuli in scotoma should interfere with (i.e. distract) orienting to stimuli in the good field • subjects move eyes to fixate a peripheral target in two different conditions: • target alone • accompanied by distractor

13. Retinocollicular Pathway independently mediates orienting • Rafal et al. (1990) result • Subjects were slower when presented with a distracting stimulus in the scotoma (359 ms vs. 500 ms)

14. Retinocollicular Pathway independently mediates orienting • Blindsight patients have since been shown to posses a surprising range of “residual” visual abilities • better than chance at detection and discrimination of some visual features such as direction of motion • These go beyond simple orienting - how can this be?

15. Retinocollicular Pathway independently mediates orienting • Recall that the feed-forward sweep in not a single wave of information and that it doesn’t only go through V1

16. Retinocollicular Pathway independently mediates orienting • Recall that the feed-forward sweep in not a single wave of information and that it doesn’t only go through V1 • In particular, MT seems to get very early and direct input

17. Retinocollicular Pathway independently mediates orienting • The theory is that direct connections from the retinocollicular pathway to MT mediate residual vision for moving stimuli • Giaschi et al (2003): tested patient with bilateral V1 lesions • since birth • little or no visual awareness (aware of some fast moving stimuli) • striking “blindsight” capabilities

18. Retinocollicular Pathway independently mediates orienting • Prediction: • moving dots compared to stationary dots should show activation of MT despite ablated V1

19. Retinocollicular Pathway independently mediates orienting • Prediction: • moving dots compared to stationary dots should show activation of MT despite ablated V1

20. Retinocollicular Pathway independently mediates orienting • Prediction: • moving dots compared to stationary dots should show activation of MT despite ablated V1 • Result: • NO. Moving dots activated various other brain regions including auditory areas • Why? • visual system develops to make optimal use of what it’s given • this man’s visual system developed without V1 in a manner unlike anyone else’s

21. Dorsal vs. Ventral Pathways • agnosia vs. ataxia • Goodale and Milner

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