Lecture 6 Flashcards
Depth and Motion
Depth perception
perception of distance
What is so important about depth perception?
there are no cells in the visual system that tell us whether something is far or near us.
distance needs to be inferred from available information in the retina
binocular cues
visual cues that depend on both eyes
monocular cues
visual cues to distance that are just as effective with one eye as with both
Retinal disparity
the difference in the apparent position of an object as seen by the left and right retinas
images are very different earn they are further away - bigger disparity
the ability to see depth is a…
learning process that is learnt through interacting with the world
convergence
the degree to which the eyes turn in to focus on close objects
the more the muscles contract, then the closer something is to us
linear perspective
parallel lines appear to come together in the distance
texture gradient
a packed together appearance of objects are further away and have finer texture
clarity or bluishness
further away = more blue and less clear
due to light refraction, lights travelled much further
object size
close together objects produce larger images
only works for objects of familiar size
relative height
below the horizon - further away objects has higher bases
above the horizon - closer objects have higher bases
occlusion
objects partially hidden are seen as further away
shadows and shading
bigger shadows, bigger distance from the ground
accommodation
the lens of the eye changes shape to focus on near objects, our brain detects these changes and infers the distance
flat lens - far object
round lens - near object
motion parallax
the difference in speed of movement of images across the retina as you travel
near objects more rapidly across the retina compared to those that are far away
size constancy
heuristic - the ability to perceive depth correctly helps us achieve size constancy
even though the retinal projection becomes smaller when the item moves away, we correctly perceive the items correct physical size
misjudging distance
when we are misled by the cues that usually help is achieve constancy in size and shape, we experience an optical illusion - a misinterpretation of a visual stimulus
misjudging distance leads to misjudging size
we may overestimate distance and therefore overestimate the objects size and can lead to us making incorrect judgments
moon illusion
low in the sky, the moon appears bigger than when its higher
yet when you take a picture, they look the exact same
what can we learn from optical illusions
perception is a constructive process and involves inferences
the brain computes what light is striking the retina but also learns what objects are present and what they are doing
perception is a constructive process and involves inferences
we impose order on patterns
we see 3 dimensions in 2 dimensional drawings
we see optical illusions, as a result of misinterpreted image cues
motion perception
to be able to see we need to see changes in the visual signals
what is motion perception good for?
helps us with survival - animals freeze in presence of predators (think! DEER)
allows us to interact with the environment
allows us to percieve the results of our actions
where is motion in the visual system?
Rods
medial temporal (MT) lobe
in V1, most cells are sensitive to motion and send this information to the MT lobe
MT lobe uses this information to detect motion
Motion blindness
damage to the area MT causes the inability to perceive motion
things appear as static images - objects jump in front go you without having moved
objects don’t have to be moving for us to perceive motion
motion after effects - adapting to a moving direction causes is to see motion on a static display
motion on the retina is not needed for motion perception
corollary discharge theory
retinal slips - an image slips across the retina. either because the object moved or the eyes moves
these retinal slips could be combined with an internal sense of our eyes movements to improve our perception of motion
misinterpreting motion
car next to you is moving, you think its your car so you slam the breaks on
biological motion
motion produced by humans and animals
can convey gender, activity, intention and mood
studied by using point light walkers
brain areas involved in biological motion
activates neurones in are STS - superior temporal sulcus and the FFA - fusiform face area
people with motion blindness can still perceive biological motion
biological motion perception may be related to social behaviour
area STS is connected to the orbito-frontal cortex and amygdala - necessary for recognition of social, biological events such as the recognition of emotional expression and personal intenion
evidence for the idea biological motion perception may be related to social behaviour
impaired biological perception in individuals with autism
above average biological motion perception in Williams syndrome, a disorder associated with highly social behaviour
perception always involves inferences
influenced by experience and context
constancies - retinal images change but our perception remains the same
context effects - retinal image is the same but our perception changes
perception is a constructive process
we can see the world as a coherent 3D place despite
- the physiology of the visual system
- the complexities of the distal world
2 visual pathways
1 helps us to interact with the world, we are unaware of it but every movement we make is fed by information from this pathway
the other pathway helps us recognise what things are
we don’t have to know what something is before we move to deal with it. e.g you hand can ignore what you think your eyes are actually seeing
