Evidence from Lesions: Agnosia

• Lesions (especially in the left hemisphere) of the inferior temporal cortex lead to disorders of memory for people and things

• recognition and identification are impaired– prosopagnosia is a specific

kind of agnosia: inability to recognize faces

• explicit (conscious) decisions about object features are disrupted

Ventral lesions can decouple awareness from action

• Goodale and Milner – Patient DF

• Patient could not indicate the orientation of a slot using her awareness

• Patient could move her hand appropriately to interact with the slot

– whether visually guided or guided by an internal representation in memory

Ventral lesions can decouple awareness from action

• Single dissociation of action from conscious perception

• Dorsal pathway remained intact while ventral pathway was impaired

• Dorsal Pathway seems to guide motor actions, at least for ones that need spatial information

• Activity within the Dorsal Pathway seems not to be sufficient for consciousness

Lesions of “Retinostriate” Pathway

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

• Identified using perimetry• note macular sparing

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“Retinocollicular” Pathway independently mediates orienting

• Rafal et al. (1990)

• subjects move eyes to fixate a peripheral target in two different conditions:– target alone

Retinocollicular Pathway independently mediates orienting

• Rafal et al. (1990)

• 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)

• Residual vision in cortical blindness has been called “blindsight”

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 is 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

• 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

• Information represented in dorsal pathway guides behaviour but doesn’t support awareness