Read this paper

• Chellazi et al. (1993) Nature 363 Pg 345 - 347

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

left right

• 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?

Lesions of Retinostriate Pathway

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

• Identified using perimetry• note macular sparing

X

Retinocollicular Pathway Might Independently Mediate 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 Might Independently Mediate 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 Might Independently Mediate 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

• Result:– Hemodynamic activity was in various other non-visual areas

Retinocollicular Pathway independently mediates orienting

• Interpretation:– Patient’s brain underwent profoundly different development

trajectory early in life

– Functionally “remapped