Unit 3, topic 2 - visual perception Flashcards
describe sensation
Sensation refers to the bottom up process by which our sense, like vison, detect or receive environmental stimuli and relay it to the brain.
Reception, transduction and transmission
Light rays reflecting off an object, enter the eye through the cornea –> aqueous humor –> pupil–> focuses the light onto retina.
The image on a retina is upside down, back to front and blurred.
describe perception
Perception refers to the top down process by which our brain organises and interprets sensory information into something meaningful.
Photoreceptors, rods and cones in the retina detect and convert light energy to electrical impulses, that travel through the nerve fibres of the retina to the optic nerve –> visual cortex –> brain interprets image.
describe the first step of VP
Reception
Light enters the eye – photoreceptors (light sensitive cells called rods and cones) detect visual stimuli.
Cones (6.5 million of them) - responsible for bright light and colour vision and have high visual acuity (details).
Rods (125 millions of them) - responsible for low light, are sensitive to black and white and have low visual acuity (lack detail).
describe the 2nd step of VP
Transduction
Electromagnetic energy (light) is converted by the receptor cells into electrochemical nerve impulses. Photoreceptors are organised into groups that form receptive fields.
describe the 3rd step of VP
Transmission
Receptor cells send the nerve impulses to the primary visual cortex where specialised receptor cells respond as the process of perception begins.
describe the 4th step of VP
Selection
Stimulus that is attended to is “perceived.”
The primary visual cortex has cells called feature detectors that specialise in detecting different aspects of an image. Eg. Shapes, length, angle, direction and movement.
describe the 5th step of VP
Organisation
Our brain “makes sense” of incoming information by using visual perceptual principles, gestalt principles and depth cues. Perception occurs in different parts of the brain:
Object identification: occurs in the temporal lobe.
Facial recognition: occurs int eh part of the temporal lobe known as the fusiform gyrus.
Spatial orientation is processed in the parietal lobe.
describe the 6th step of VP
Interpretation
Perception occurs when the brain weaves all the information together including information already stored in memory such as past experiences, values, context and motivation.
what does the cornea do
Cornea
The cornea covers the front of the eye and helps to focus incoming light.
what does the aqueous humor do
Aqueous/ anterior chamber
After light passes through the cornea, it travels through a watery fluid called the aqueous humor. The aqueous humor circulates throughout the front part of the eye and maintains pressure inside the eye and also provides nurtrition for the eye.
what does the iris do
Iris
Coloured part of the eye.
As light conditions change, the iris may dilate to make the pupil bigger or constrict to make the pupil smaller. - this allows more or less light into the eye.
what does the lens do
Lens
After light travels through the pupil, it must pass through the lens. It is responsible for focusing light to the retina.
what does the vitreous body do
Vitreous body
After being focused by the lens, light passes through the centre of the eye on its way to the retina. The eye is filled with a clear, jelly-like substance called the vitreous body.
what do the retinal vessels do
Retinal vessels
The retinal blood nourish the inner layers of the retina.
what does the retina do
Retina
The retina is a thin, sensitive tissue lining the back of the eye that acts much like a film in a camera. Light must be properly focused onto the retina, and the surface of the retina must be flat and smooth and in good working order to produce a clear image.
In the retina, rods and cones (light sensitive cells) receive and organise visual information. The retina sends this info via the optic nerve to the brain, enabling you to see.
what does the macula do
Macula
Centre/bulls eye of retina. It contains a high concentration of photoreceptor cells which convert light into nerve signals. The high concentration of photoreceptor cells allows us to see fine details such as newsprint.
At the centre of the macula is the fovea, the site of our sharpest vision.
what does the choroid do
Choroid
Behind the retina, a layer of blood vessels called the choroid supplies oxygen and nutrients to the outer layers of the retina.
what does the sclera do
Sclera
The sclera is the white part of eye that is composed of tough, fibrous tissue that protects the inner workings of the eye.
what does the optic nerve do
Optic nerve
The optic nerve is a bundle of nerve fibres which carries visual information from the eye to the brain.
what are all of the disorders caused by inherited biological disorders
Strabismus (cross-eyed)
Amblyopia (lazy eye)
Myopia (near-sightedness) and Hyperopia (far-sightedness)
Astigmatism
Colour vision deficiency/blindness
Retinis pigmentosa
what is strabismus
Strabismus (cross-eyed) - failure of eyes to maintain proper alignment due to poor muscle control. May result in double vision, eye strain, headaches.
inherited
what is amblyopia
- eye fails to achieve normal visual acuity, even with corrective eye wear.
(lazy eye)
inherited
what is Myopia (near-sightedness) and Hyperopia (far-sightedness)
- light doesn’t focus to retina, causing far (myopia) or near (hyperopia) objects to appear blurry. Treatment: glasses/contacts/laser eye surgery.
inherited
what is Astigmatism
– caused by cornea not being perfectly spherical which results in blurry vision at all distances.
inherited
what is retinis pigmentosa
– degenerative disease of the retina causing night blindness and gradual loss of peripheral vision.
inherited
list the disorders caused by biological ageing
Presbyopia
Floaters
Glaucoma
Age-related macular degeneration
what is presbyopia
– a condition that develops when the lens progressively loses the ability to bend and focus light rays onto the retina. Irreversible but can be treated by wearing eyeglasses or contact lenses.
what are floaters
– gel-like clumps in vitreous fluid that appear as specs in our vision. Harmless but can be annoying.
what is age related macular degeneration
– one of the leading causes of vision loss, starting around 50 yrs. The build-up of grainy macular deposits causes inflammation and degeneration of photoreceptor cells. Irreversible.
what is Glaucoma
– lens becomes cloudy due to protein breakdown resulting in blurry vision. Progressive disease can be treated with surgery.
what is dichromatism
Dichromatism – people with dichromatism are missing 1 of 3 cone pigments.
Affects more males (8%) than females (1%).
Cause – sex linked inheritance (X-chromosome) is the most common cause.
a person can only perceive two of the three primary colours
EG; Red/green colourblind.
what is achromatopsia
Achromatopsia (monochromatism) - no functioning cones, therefore people can only see in shades of lightness (white, grey or black), have poor visual acuity and are usually sensitive to bright lights.
Prevalence: 1 in 10 million.
Causes: Cerebral achromatopsia (caused by damage to brain – stroke to visual cortex) OR congenital – hereditary.
what is perceptual set
Perceptual set – is a bias to perceive certain aspects of sensory data and to ignore others.
identify the factors that influences a persons perceptual set
Past experiences/ knowledge
Context
Motivation
Emotion
what were the results from Minturn and Bruner and what was the year of the study.
Minturn and Bruner (1951) showed that the middle figure in a group of letters and numbers is more likely to be perceived as a letter.
describe emotion as a factor influencing perceptual set
– how we perceive a stimulus can be influenced by our own emotions. Happy emotions = broad focus, negative emotions = narrow focus.
describe motivation as a factor influencing perceptual set
– often we will see what we want to see. For example, if driving and hungry we might see a food ahead sign.
describe context as a factor influencing perceptual set
refers to the environment or situation a stimulus is perceived in. The mind often follows a sequence or will focus on the things that appear out of context.
describe past experience/knowledge as a factor influencing perceptual set
influence how we perceive things. Consequently, people can interpret the same stimuli in different wats.
list the visual principles
Gestalt principles
Perceptual constancies
Depth perception
describe perceptual constancies
the tendency to maintain a stable perception of a stimulus, even while the properties (eg. Size or shape) of the image on the retina may change.
Size constancy: the constant perception of an object’s size, even though the size of the image on the retina alters.
Shape constancy – when we see a door at various slants we perceive it as a rectangular shape even though the retinal image is a trapezoid.
what are the gestalt principles
Based on the principle that ‘the whole is greater than the sum of its parts’.
Gestalt principles help organise, interpret and make sense of visual stimuli. They include:
Figure-ground organisation
Closure
Similarity
Proximity
define each gestalt principle
Figure-ground organisation – images are organised into the central object of attention (figure) and a background (ground) separated by a contour line. This is the first principle that develops in infants.
Closure – when an incomplete object or image is perceived as being complete.
Similarity – humans tend to group stimuli that have visual appearance (eg. Pattern, shape, size or color).
Proximity – humans tend to perceive elements that are close to each as related. Proximity is so essential to our perception that is stronger than other features like shape or colour.
what are the binocular depth cues and define them
Binocular depth cues
Retinal disparity – the difference in the position of our eyes, known as retinal disparity provide different perspectives of the same image. This stereoscopic vision allows our brain to compare and contrast the images to determine depth. The more different the two mages, the closer the object will appear to the viewer.
Convergence – the process by which muscles of the eye turn inwards to track objects close to the face. The brain measures how much the eyes have turned in order to determine distance.
what are the monocular depth cues and define them
Monocular depth cues
Accommodation (similar to convergence) - your
brain measures the tension of the ciliary muscles (which change the shape of the yes lens to focus light onto the retina) to determine distance. The greater the tension, the closer the object is.
Pictorial depth cues:
Linear perspective – parallel lines appear to converge in the distance.
Interposition – objects in the distance are partially obscured by objects in the foreground.
Texture gradient – texture of objects in the foreground are seen in greater detail than those further away.
Relative size – objects producing a larger retinal image are closer than objects further away.
Height in visual field – objects closer to the horizon are further away.
contrast perceptual illusions and optical illusions
Perceptual illusions (subjective interpretations of physical information) reveal important insights into how we construct our representations of the world.
An optical illusion occurs when our perception of a visual stimulus differs from reality.
It is relatively consistent from person to person, and over time.
It is caused by psychological factors as opposed to other perceptual distortions due to biological factor
describe the ponzo illusion
In the Ponzo illusion the upper horizontal line is generally perceived to be longer than the bottom line even though realistically both line are of equal length.
This is due to the pictorial depth cues of linear perspective and heigh in visual field.
describe the Muller-Lyer illusion
In the Muller-Lyer illusion, two lines are of equal length appear different in length.
The line ending with the feathered tails is generally perceived as being a longer line than the one ending with arrow heads.
carpeted world theory and perceptual comprise theory try to explain this phenomenon
what is the world carpeted theory
Carpentered world hypothesis: proposed by Richard Gregory argues that the illusion is caused by ‘misapplied size-constancy theory’ due to our experiences with angles and straight lines in our everyday environment. For example: buildings provide us with environmental cues that lead us to interpret the arrow heads as the outer corner of a building (closer to us), and the feathered tails to look like the inside of a corner (further away from us).
Evidence to support this comes from research in Africa from Zulu people
Individuals that did not live in urbanised areas were not fooled by the illusions.
Individuals that did live in urbanised areas were fooled by the illusions.
what is the perceptual comprise theory
Perceptual comprise theory: proposed by Ross Day. According to him both lines create the same image on the retina. The different tails on the end of the lines create open figures that our brains apply the Gestalt principle of closure to. This creates a ‘solid’ image, shown by the blue lines. Because of the perceptual compromise made, we see the line length as an average between the original (black) and perceived (blue) lines. As a result the arrow head ended lines look much shorter than the feather headed ones.
Evidence for this is that even when you change the ends of the lines to different things with similar positionings the illusion still exists.
what is the Ames room illusion
The Ames room causes people to perceive objects of same or similar size as greatly different.
This illusion demonstrates exploits our brain familiarities with the shapes of rooms. Consequently, we maintain shape constancy at the expense of size constancy.
This is due to the unique shape and features of the room and the restriction of vision to a small peep hole that can only be looked through by one eye.
Room is trapezoid shape (not square).
Floor and ceiling is sloped.
Windows, doors and floor are manipulated to hide the slope of shape of the room.
Peep whole only allows for monocular vision limiting the depth cues that can be used.
what are impossible figures
Impossible figures, are images that have been manipulated to exploit reliance on perceptual cues. This causes us to see images that should not be possible.
Generally 2D images that are perceived 3D.
Images that seem to have no beginning or end.
what are examples of impossible figures
Examples of impossible figures:
The Penrose stairs created by Lionel S. Penrose.
The Penrose stairs created by Escher in 1960.
The impossible cube created by Escher in 1958.
The impossible trident created by Schuster, published in 1964.
what are ambiguous figures and what is an example.
A type of optical illusion that exploits visual similarities, I.e one imaged can be perceived in different ways (but only one at a time).
This is thought to be due to our brains reverting back to gestalt principles, which fill in missing pieces of an image.
Both impossible figures and ambiguous highlight the interconnecting nature of visual perception with multiple brain regions and body systems.
Example: vase or two faces.
summarise the Deregowski (1972) study
Deregowski (1972) review included his own research that built on and evaluated the previous findings of Hudson (1960).
Hudson’s (1960) cross-cultural studies included south African Bantu factory workers, unskilled Zambian workers and primary school children.
Hudson used a variety of different images to determine pictorial perception, particularly whether individual’s were 2D or 3D perceivers.
Similarly, Deregowski (1972) used some of these as well in his own tests that investigated different pictorial depth cues (relative size, height horizon and interposition).
They wanted to determine the extent to which pictorial perception relies on learning.
Do people from different cultures perceive pictures in the same way.
Are pictures a universal means of communication.
What was Hudsons first experiment
Hudson experiment 1.
Participants were shown this picture and asked questions such as what is the man doing? What is closer to the man etc. - questions that would indicate whether participants could correctly interpret the image as 3D or 2D.
Participants that correctly interpreted the depth cues were classified as 3D, otherwise 2D.
African adults and children found it difficult to perceive depth cues in the images.
what was hudsons 2nd experiment
Hudson experiment 2.
Participants were given sticks and clay to model the images.
Most subjects from western cultures see figure A as a 3D object, but when the figure is rotated 45 degress (figure B), they see it as being flat.
African participants from a variety of tribes almost always saw the image as being flat, with the two squares in the same plane.
what was hudsons 3rd experiment
Hudson experiment 3.
Participants had to draw the image after viewing it for 30 seconds.
2D perceivers completed the image more quickly than 3D perceivers who found it difficult to see the trident clearly.
what was hudsons 4th experiment
Hudson experiment 4.
Participants were asked which picture they preferred and why.
The split elephant drawing (left) was preferred by African children and adults to the top-view perspective drawing (right).
what were the conclusions of the Deregowski experiment? and what was the year of the study
1972
Deregowski (1992) - conclusions
To some degree those of African descent found it difficult to pick up on depth cues in pictures.
This suggests that pictorial depth cues are not influenced just by biology and that culture appears to impact visual perception.
Through education and cultural norms people are able to learn how to interpret images.
Thus, pictures cannot be used as a universal language.
what were the limitations of Deregowski (1972) study
Deregowski (1972) limitations
Considered unfair as materials were unfamiliar to participants.
Much evidence is anecdotal and could be unreliable.
An interpreter had to be used to talk with participants so there is a possibility for error.
Methods are slightly ethnocentric – biased to the cultural expectation of the researcher.
