2. Illusions and lesions Flashcards
What does it mean that conscious perception is construction of the senses?
It is not exact replica of outside world! 1/3 of the time we are blind as incoming information is restricted to point of fixation
What are examples of conscious perception being construction of the senses?
1) Array of pink dots disappearing (brain gets bored with unchanging stimuli).
2) Necker cube (only one interpretation at the time).
3) The “dress” argument (same light wavelength, but different assumptions).
4) Chess board
5) Ebbinghaus illusion
anterior
in front, toward the face
posterior
behind, toward the back
superior
above, toward the head
inferior
below, toward the feet
medial
toward the middle
lateral
toward the edge
dorsal
toward the top of the brain or the back of the spinal cord
ventral
toward the bottom of the brain or the front of the spinal cord
saggital
divides into right and left part
coronal
divides into front (anterior) and back (posterior)
axial
divides into dorsal and ventral plane
traumatic brain injury
intracranial injury - injury to brain caused by external force (accidents)
stroke
when blood vessels are clogged, blood cannot pass, regions don’t get the oxygen and die
ischemic stroke
blockage of blood vessel, lack of blood flow to affected area
hemorrhagic stroke
rupture of blood vessel, leakage of blood
tumour
abnormal cells
blinsight - symptoms
cortical blindness - patients claim to NOT see anything in affected field
however, if they are forced to choose what they saw, their responses are above chance
also some brain lesions may lead to disorders of attention/awareness
blindsight - areas affected
occipital lobe, at least V1 affected, effects of damage map contralaterally to the affected visual field, both gray and white matter affected
What can blinsight patients under forced-choice guessing do?
discriminate shapes, colour and orientation
reach for object
accurately position hand for grasping
motion perception
Why blindisight patients can perform force-choice task?
Most of visual information (90%) travels to V1 (which is affected in blindsight).
However, the 10% goes to dorsal stream and superior colliculus. Those regions may be responsible for being able to perform forced-choice task.
Why should we sceptical about studies with blindsight patients?
Blinsight in monkeys -> monkeys have great sound perception (potential confound)
Human patients -> you need to trust patients that they truly don’t see anything (however, in practice, many of those patients are participating in many experiments -> pressure to respond according to their previous performance)
hemispatal neglect - area affected
(right) parietal damage
hemispatial neglect - symptoms
after damage to one brain hemisphere, deficit in attention to and awareness of one side of space is observed
attention seems to be affected - pre-occupied with only one side of the visual field
hemispatial neglect - typical findings
1) drawing half of objects (both in case of copying and spontaneous drawing)
2) disecting the line moved to the side
3) eye-movements ‘‘heatmaps’’ reveal bias towards one hemifield
4) curious finding! upon seeing two houses (one of them on fire - usually the one on the left), patients claim that houses look the same, but they would prefer to live in the house on the right
How does hemispatial neglect support Global Workspace Theory?
attention is nevessary for consciousness
Visual form agnosia
In case of DF it arised due to CO poisoning and damage to LOC (lateral occipital cortex).
What is specific to LOC?
It likes objects (higher activity when seeing real objects vs scrambled ones)
visual form agnosia - DF case
1) cannot copy objects, but can draw them from memory (DF-SPECIFIC finding)
2) she is unable to identify shape of objects
3) she knows what the objects are (+is able to recognize them by color)
What are main findings from slot task for DF?
She struggles with matching (saying how is the thing oriented and matching it).
However, she is good at posting! So even though she is unaware of information, she can use it
What can DF slot task performance inform us about unconscious behaviour?
There seems to be dissociation between perception and action. It may be linked to the fact that ventral stream (affected in DF case) related to problems in perception (matching) but not so much by action
dorsal attention system
top-down attention (FEF - frontal eye fields + IPS - intraparietal sulcus), voluntary attention
ventral attention system
bottom-up attention (TPJ - temporo parietal junction + VFC - ventral frontal cortex); detection in enviornmentally salient stimuli
What similarities are there between DF slot performance and healthy humans?
Perception suffers from illusions, but action is not influenced by illusions.
Healthy subjects participated in Ebbinghaus illusion in which the central disks were either:
perceptually different + physically identical;
or
perceptually identical + physically different
The grip sizes were NOT FOOLED by the illusion.
What can we conclude on the basis of those findings?
Ventral stream -> related to consciousness
Dorsal stream -> related to action which is automatic (unconscious)
However, this idea is simplification! There is crosstalk between streams, so there is just relative difference between processing routes!
What are other ventral stream deficits?
- color agnosia (V4 damage)
- prosopagnosia (face blindenss)
- motion blindness (MT/V5 damage)
apperceptive agnosia
difficulty in assembling the pieces together into meaningful whole
associative agnosia
object perception intact, but there is issue in assigning meaning or identity to it
