Lecture 11 Perception Flashcards
Philosophers of perception sometimes distinguish between three kinds of perceptual experience:
•Accurate (veridical) perception of the world
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•Illusion (nonveridicalperception of the world)
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•Hallucination (in which one has a perceptual experience but, nonetheless, having it does not amount to perceiving the world).
Visual illusions
•In other words, your perception of an illusion has more to do with how your brain works, and less to do with the optics of your eye.
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- An illusion is “a mismatch between the immediate visual impression and the actual properties of the object
- An ”optical illusion” means something actually IN the eye, not most illusions
What happens when you “see stars”
(also a pilot’s visual illusion example)
after a blow to the head, neurons get fired up, eye perceives it as light , but it’s the firing of neurons
•Pilots might encounter visual illusions while in flight, such as a false horizon, or when landing, such as a narrow runway. (Trained to accommodate).
How do illusions give us info about how the brain functions?
- The brain is a limited structure with limited resources, including its number of neurons, wires, and neuronal connections.
- in some cases, illusions may be due to the brain’s need to take “shortcuts”, ie. the brain might need to quickly give more importance to some features in a visual scene than others.
- Also give us insight into how people with visual issues may feel (agnosia, etc)
types of visual illusions
- Impossible Objects (Penrose stairs) – knows it’s 2D, but interprets as 3D,
- Colour Illusions (dress illusion, - some may assume daylight, some may assume artificial light [unconsciously]
- Colour illusions – cataracts can also affect
- Photoshop pros are maybe used to adjusting luminance: see the dress as blue and purple more often
- Theory of colour constancy (incl. depth and shadow)
- Spinning dancer (Multistableobject) – nothing to compare her to, no depth cues
- Wavy lines – curvature blindness illusion (all curved, evolutionary biproduct?-we are quicker to pick up straight edges…big theory of curvature blindness…has it helped us?)
- Ambiguous illustrations
- Optical Art Images – theories – receptors on retina tire?
Impossible objects
An impossible object (also known as an impossible figure or an undecidable figure) is a type of optical illusion. It consists of a two-dimensional figure which is instantly and subconsciously interpreted by the visual system as representing a projection of a three-dimensional object.
Ex. Penrose stairs
what is colour constancy
Relative colour, when object is illuminated w light containing short, med, long wavelengths, and part of a scene, not in isolation
Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red. This helps us identify objects.
what are floaters
proteins suspended in the vitreous humour/body casting shadows on the retina
Multistable objects
spinning dancer:
nothing to compare her to, no depth cues
Curvature Blindness illusion
all curved, evolutionary biproduct?-we are quicker to pick up straight edges…big theory of curvature blindness…has it helped us?
the brain’s visual system may default to seeing corners when there ambiguity over whether a line is a smooth curve or not
It is only seen against a medium grey background however, suggesting that what matters is that the color of the wavy lines shifts from being lighter than the background, to being darker than it.
•Optical Art Images – theories – receptors on retina tire?
The illusion is triggered by eye movements. Even when you stare at a still object, your eyes dart around. Normally, your brain can tell the difference between your eyes moving and an object moving. But because of the strong contrasts and shapes in the illusion, your brain gets confused. Your motion sensors switch on, and the image seems to turn.
Your brain evolved to focus attention on movement because it can be a sign of danger. But in the case of this image, the movement you see is an illusion created in your mind!
The “beer goggle effect”:
- Alcohol impairs a person’s ability to detect facial symmetry. Charlisetheron– the most symmetrical. Facial symmetry=attractive. Women affected by alcohol on facial symmetry
- Ample research shows that symmetry — the degree to which both sides of an individual’s face match — is an advertisement of good genes.
- Subsequently, the idea goes, good genes make for a better potential mate.
- But once alcohol enters the picture, the ability to accurately discern symmetry begins to break down.
Who wrote “The man who mistook his wife for a hat”, who was protagonist, what was it about (and what disorder)?
•One of Oliver Sacks’smost famous books is about the case study of Dr. P.
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•Visual agnosia is a rare condition caused by damage to the brain’s occipital or parietal lobes, characterized by an inability to recognize and identify objects or persons.
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“Not only did Dr. P. increasingly fail to see faces, but he saw faces when there were no faces to see: genially, Magoo-like, when in the street he might pat the heads of water hydrants and parking meters, taking these to be the heads of children; he would amiably address carved knobs on the furniture and be astounded when they did not reply”.
Who coined Agnosia?
What is it, requirements for diagnosis
coined by Froid
- Describes a “lack of knowing” (recognition) that can occur in any sensory modality.
- Person who demonstrates agnosia must demonstrate:
- A deficit in recognition that is not due to impairments in sensation, attention, language, or general intelligence. It is an impairment of perception.
Apperceptive Visual Agnosia
•The inability to recognize an object due to a problem perceiving. - cannot unify an object based on it’s parts
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•Person will display the ability to see contours and outlines when shown an object, but they experience difficulty if asked to categorize objects; unable to group visual sensations into a unified percept.
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•If a usual object is displayed in an unusual orientation, the person will have trouble identifying it.
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•Typically results due to damage to one hemisphere, specifically damage to the posterior sections of the right hemisphere.
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Farah’s Classification of Apperceptive Agnosia
- 4 types
- Narrow apperceptive agnosia – inability to recognize, match, copy or discriminate between elementary visual forms.
- Dorsal Simultanagnosia– inability to appreciate a whole stimulus despite being able to recognise its parts.
- Ventral Simultanagnosia– like the dorsal form but not as severe; patients can negotiate the environment and perform dot counting tasks.
- Perception categorization deficit – an impairment in experiment induced recognition which is not seen in everyday life.
Computational theory of apperceptive agnosia:
- Based on concept of axis transformation.
- Suggests that a 3D representation of an object is obtained by determining the objects minor and major axes, using them to identify
- Eg. Seeing a tennis racket from the side supplies information from the major axis (its length) from which we then use to make inferences about other aspects of the racket (eg. Width of handle).
- Tasks in which the subject has to identify objects from an unusual perspective are therefore thought to be difficult because a major axis has been obscured or foreshortened.
- Apperception agnosia which is attributed to axis transformation = transformational agnosia.
Associative Visual Agnosia
Symptoms, examples, from damage to _________
- The inability to associate visual forms with meaning. (cannot name it). They see it, but can’t connect to a name
- People can replicate (by drawing) what an object is, but they cannot link it to a meaning; iename it.
- Eg. Might only be able to describe a stethoscope as “a long cord with a disk on the end”.
- Associated with damage to both the right and left hemispheres at the occipitotemporal border.
- Not a memory issue eg, see an anchor, say it’s a brake for ships
- Catdog would be real to them? What…
Name 2 theories of associative Agnosia
Shared Systems Theory:
Independent Systems Theory:
Shared Systems theory of associative agnosia
Shared Systems Theory:
- There is one system responsible for mediating the meaning of all stimuli.
- Agnosia is therefore proposed to be a problem of connecting the output of perceptual analysis with the patient’s general store of knowledge.
- Thought to be due to a breakdown in the transmission of info from the perceptual to semantic systems.
Independent Systems Theory of associative agnosia
Independent Systems Theory:
- Suggests that processes such as visual object recognition are independent of other processes eg. Speech/comprehension of words.
- Suggests a form of modularity in which some psychological processes are dissociable and independent of one another.
- Evidence for this theory comes from studies in which impairments in visual processes do not accompany verbal impairments and vice versa.
Category Specific Visual Agnosia
Category-specific agnosias are differential impairments in subject knowledge or recognition abilities pertaining to specific classes of stimuli, such as living things vs. non-living things, animate vs. inaminate things, food, metals, musical instruments, etc. Some of the most common category-specific agnosias involve recognition impairments for living things, but not non-living things, or human faces, as in prosopagnosia. This type of deficit is typically associated with head injury or stroke, though other medical conditions have been implicated, such as, herpes encephalitis.
Implication of category specific visual agnosia
implies that the brain organises info into modules defined by certain categories.
criticisms of category specific visual agnosia
- Stimuli used in dissociation studies have been criticised eg. Drawings of living things are more complex than drawings of non living things.
- Written words may also be more familiar than the objects they represent.
- When these factors are controlled, the dissociations seem to disappear.
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•Is it it’s own separate category? An ongoing question. Maybe it’s just a form of visual agnosia
