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Blindsight. Patients with scotomas could move eyes to the location of a light flash (Poppel et al., 1973). Case D.B. (Larry Weizkrantz) hemianopic with ability to point to location, detect movement, discriminate orientations, discriminate Xs and Os.
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Blindsight • Patients with scotomas could move eyes to the location of a light flash (Poppel et al., 1973). • Case D.B. (Larry Weizkrantz) hemianopic with ability to point to location, detect movement, discriminate orientations, discriminate Xs and Os. • Location and motion usually detectable. Other things variable including color, flicker depending on the case.
Implicit effects • Respond more quickly to two than one even if one is in the blind field (even for saccades). • Words in the blind field influence interpretation of other words. • Monkeys could reach for stimuli in blind field but didn’t report them in a number or presence/absence task.
Temporal distractors have a greater effect in the blind field. This is consistent with mediation by the superior colliculus.
Awareness varies • Some ‘feel’ onset of stimuli, jagged or smooth feeling (not visual experiences); but intense stimuli may say ‘dark shadow’.
Vision mediated by remaining striate cortex, e.g. scattered light, but how does this explain seeing black things (which don’t scatter light) on white? Or hemidecorticates? (no V1) Imaging shows no activation of V1 when saccades suppressed by stimuli in blind field. Suppresson is seen for temporal hemifield but not nasal (temporal dominates for superior colliculus). Does this mean subcortical areas are unconscious and cortex is ‘conscious’? Explanations of blindsight
Extrastriate mechanism • Some LGN fibers go to extrastriate areas from pulvinar, not just through V1, so can reach MT or V4. This suggests vision without awareness is related to the quality of representation. Need more complete and fully processed representation. • Supported by data that hemidecorticates can’t make motion and color judgements. Need the associative visual areas for these. • Variability, but two main mechanisms can contribute to blindsight performance: subcortical and direct LGN-extrastriate projections.
Apperceptive agnosia • No awareness (explicit knowledge) of shapes and objects, but other visual perception good. • Good implicit responses to objects. Proper grip size, proper slot orientation (not for T objects so still shape limited). • Loss of ventral stream; dorsal (where) is ok and can mediate action programming. This is a privileged role account, since both ventral and dorsal stream code for size and orientation, but only ventral supports awareness.
Associative visual agnosia • Explicit object recognition is impaired despite elementary perception abilities. Copy is ok. • Implicit recognition is shown by turning pictures to correct orientation (doesn’t know what they are). • Can match related pictures in multiple choice but no knowledge of picture. Despite no awareness could judge accuracy! • Multiple choice data support quality of representation account. Degraded representations support performance when choices are limited, but don’t support open-ended tasks.
Unilateral neglect and extinction Indirect tests show awareness
First was a same/different task but it could be solved by simpler comparisons as in the degraded stimulus experiment with normals; also when naming choice limited to two were they are ok. • But another subset of cases can’t do two choice name but are successful on same/different. Longer to respond different if some similarity in the two so may be semantic encoding (still without awareness).
More implicit perception in neglect • Semantic priming: responses on right primed by left side words, even though no explicit perception. • Flaming house versus normal house: preferred normal, but didn’t know the what difference was. But not replicated.
Neglect and extinction explanations • Breakdown in flow between conscious and non-conscious systems. Privileged property explanation (language system is privileged conscious system). • Alternatively, consciousness is a state of integration among systems. Damage weakens or degrades representation so it doesn’t have sufficient influence to create a new global state in which the representation is integrated. Similar to quality of representation explanation. • Farah favors degraded representation or integration account. Not clear that perception is normal (needed for privileged property account). Degraded stimuli can activate semantic representations.
