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Visual Illusions and Disorders of Vision (Blindsight- impairment to…
Visual Illusions and Disorders of Vision
Blindsight- impairment to conscious visual experience of stimulus in part of visual field. Hemianopias= loss of vision to the left or right side of fixation due to damage in the visual context - not the eye itself. blinsight= some patients can respond to stimuli in 'blind' part of visual field under certain conditions, even though consciously they do not see anything
hemianopia: blindsight- cortical blindness: persuade and Cowey 2008= patient GY presented with an object in upper or lower lower part of his blind visual field. he had topping to the opposite location of object. they found- when in blind area- e responded above chance to the real location o nth object, while being able t do the task fine when in good visual field.
demonstrates that there are different levels at which visual information is processed
agnosia- refers to the inability to recognise people or objects even when basic sensory modalities are intact. visual agnosia- where the person has difficulty recognising objects, faces and or words, auditory agnosia which involves the inability to recognise sounds
somatosensory agnosia- where person has difficulty perceiving objects through tactile stimulation
visual agnosia- a deficit in processing that is restricted to the visual input modality, patients may show impaired object recognition with or without impaired face (proposopagnosia) and or reading (Alexia)
case reports- important critical task, performed by a neurologist. neuropsychological nature- psychomotor speed, attention, memory, executive and visuospatial functions, motor symptoms severity and functional status, understand comorbidity
visual agnosia patient IES - clinical diagnosis of visual object agnosia and prosopagnoisa without Alexia. chromatopsia - colour blinds, no language comprehension or production process, no loss of semantic knowledge and upper right quadrant visual field cut - visual field losss
IES- object processing BORB picture naming, real objects, object naming errors
IES: drawing from memory - stored object knowledge seems to be intact
visual agnosia- inner et al 1991- patient DF could not identify shapes, didn't effect movement.
Goodale et al 1991- when asked to distinguish blocks perceptually - DF couldn't do it, when asked to pick blocks up - did this without problems , but couldn't estimate block size
two processing streams- dorsal system- involved in object localisation (where), ventral system - involved in object identification (wha)
ventral v dorsal streams- Milner and goodale 2006- an abstract representation of the world, stored for future reference, enables us to organise the information in the world and plan future actions
optic ataxia- patient VK Jackson, Archibald , Carey and goodale 1991- can see objects but cant interact with them
evidence from lesions with monkeys- lesions of the posterior parietal cortex= deficits on a spatially demanding 'landmark' task but no affect on object discrimination
lesions of inferior temporary cortex produced deficits discriminating objects on the basis of their visual features but did not affect their performance on a landmark task
ventral and dorsal streams: Milner and goodie 1995- top route is the retinotectal pathway and the bottom route is the retinogeniculate pathway
Milner and goodie 1995-action is a more appropriate term than localisation for the dorsal stream
dorsal stream- acts in real time, guides actions, enables smooth and effective movement
Milner and goodale- it is not the case that the ventral system is not involved in action - it is- but it is involved in actions when memory is involved, time is available for planning, planning which movement to make is necessary and the action is unpractised/awkward
blindsight patients cannot perceive objects which are clearly being processed at some pat of the visual system, agnosia patients can see objects and act accordingly to them but not know what they are
Richard Gregory- visual illusion= to systematic visual and other sensed discrepancies from simple measurements with rulers, photometers, clocks and so on
Gregory- insufficient information provided by the senses for us to make sense of what we are seeing, thus we use our existing knowledge (schemas) to fill in the gaps). perception is an active and constructive process, governed by hypothesis testing- we most often get it right but we sometimes get it wrong leading to a visual illusion
illusions can be categorised into two classes- bottom up physiological illusions- low level physiological mechanisms and top down cognitive illusions- constructivist
Gregory and heard 1979- perceptual system tried to resolve which object the motor belongs to when white tiles meets black. border locking- edge detection in the context of simultaneous spatial and colour registration in the human visual system
photoreceptors (light cells) and lateral inhibition- are activated when a light shines on it, they then inhibit the firing of a adjacent cells to maximise contrast and help in detection of contrast changes.
chevreuil illusion-mach bands- shades of black grey and white. lateral inhibition- casques the mach band illusion, less inhibition form on path appears to brighter= dark area= produces less inhibition as there is less light reflection here resulting in the excitatory response to be larger and so this area appears brighter
why do we see dark patches? intersection cells- more inhibition than cells elsewhere so activate less- making this area seem darker. Foveate= cells in fovea have smaller receptive field so illusion disappears
perceptual system is interested in properties of surface e.g. colour not illumination - tries to factor this out. black surround= low luminance- so perceptual system adjusts accordingly
top down illusions- brain makes inferences about the features that do not correspond to reality
muller ayer illusion- some evidence that people with less familiarity with corners show the effect less- deregowski 1972
further object must be bigger than its retinal image and perceive it as bigger
other size constancy illusions- erroneously using depth information another case of misapplied size constancy
vision tells you about the world not about images- this is just an illusion
jastrow illusion- most people see Lower left face as being infant most of the time- because people view objects from above, with the top side visible, far more often than from below, wit h the bottom visible so the brain prefers the interpretation that the cube is viewed from above
like the necker cube these- rely on stored knowledge about the world are also under some degree of conscious control
illusory figures- kanizsa triangle- illusory contours may be partly accounted for by low level contrasts effects, partly by more cognitive processes inferring the existence of occluding objects
gestalt- essence or shape of an entity complete form
reification- perception of an object as having more spatial info than is presented. constructive or generative aspect of perception- by which the experienced percept contains more explicit spatial information than the sensory stimulus on which it is based
contrast illusions- checkerboard illusion, we use our knowledge of light sources to make inferences about the true colours of the squares.
our visual system modifies this information to provide us with an interpretation that in reality would be correct. our visual system is designed to interpret information in a way that is consistent with reality - vision isn't about seeing light its about seeing things
Gilchrist and radonjic 2012- a identical, b perspective, c blurred.
illusions occur due to the physiology of the eye- bottom up illusions because the eye has to use knowledge about the world to convert a 2D image to a 3D image- size knowledge, light source knowledge