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Different Pathways , Different Processes PowerPoint PPT Presentation


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Different Pathways , Different Processes. 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.

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Different Pathways , Different Processes

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Different Pathways , Different Processes


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


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


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


Lesions of Retiostriate Pathway

  • Lesions (usually due to stroke) cause a region of blindness called a scotoma

  • Identified using perimetry

  • note macular sparing

X


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”


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!


Retinocollicular Pathway independently mediates orienting

  • Prediction: subject should be able to orient accurately to the target even when it is in the blind field


Retinocollicular Pathway independently mediates orienting

  • Subject was able to orient with relatively good accuracy up to about 25 degrees


Blindsight

  • Intact retinocollicular pathway mediates eye movements despite V1 lesions

  • Might it also orient attention? The theory is that it does.


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


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


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)


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?


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


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


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


Retinocollicular Pathway independently mediates orienting

  • Prediction:

    • moving dots compared to stationary dots should show activation of MT despite ablated V1


Retinocollicular Pathway independently mediates orienting

  • Prediction:

    • moving dots compared to stationary dots should show activation of MT despite ablated V1


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


Dorsal vs. Ventral Pathways

  • agnosia vs. ataxia

  • Goodale and Milner


05-31

W. W. Norton


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W. W. Norton


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W. W. Norton


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W. W. Norton


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W. W. Norton


05-10

W. W. Norton


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