Read this paper Chellazi et al. (1993) Nature 363 Pg

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 leftright 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