depth & size perception Flashcards
what does the term ‘depth’ customarily signify
judgements of an object’s 3D spatial properties (e.g. how wide and what sort of general 3D configuration it has) &/or its relative separation (distance from the fixation plane i.e. horopter or distance from an object from another object in the fov) from other objects or surfaces in the field of view, with ‘distance’ reserved for absolute judgements of how far away an object is from us, the viewer.
what does the visual system make up a variety of
cues (source of information)
what do cues judge
the depths and distances of objects from us
what are the two types of cues
- monocular cues: available to one eye alone
- binocular cues: based on different views of the two eyes
what does the horizontal separation of our 2 eyes result in
our eyes to be in slightly different positions in our heads which then have slightly different views of the world
name the two categories of monocular perceptual cues
relative depth & absolute distance
list the points useful for relative depth (as monocular perceptual cues)
- relative image size
- textural gradients
- interposition/occlusion
- light & shade
- motion parallax
list the points useful for absolute distance (as monocular perceptual cues)
- retinal image size
- texture gradients
- linear perspective
- atmospheric perspective (outdoors)
- height-in-scene
which multiple monocular perceptual cues are useful for both relative depth & absolute distance
relative image size & textured gradients
which monocular perceptual cues don’t tell us much about absolute distance
- interposition/occlusion
- light & shade
- motion parallax
what are all the multiple monocular perceptual cues known as (except motion parallax), and have been exploited by artists for millennia for depict 3D in 2D images
learned pictorial cues
give an example of relative image size
two balloons, larger one looks closer and smaller one looks further away as the larger one forms a larger image on the retina
why is retinal image size perceived this way
because retinal image size is known from learned experience to be proportionally related to the viewing distance
what is textural gradient based on
systemic reductions in the relative size of the individual texture elements, which are assumed to be identical
give an example of textural gradient
circles at the top which are also smaller, seem further away and circles at the bottom which are larger and shows more detail, seem closer by
what is interposition/occlusion
where one object blocks/occludes the view of the one (or more) further behind it, so it is assumed to be nearer
e.g. the visual system does not assume it is a circle with crescents above it, stacked on top of each other, it assumes that they’re all circles
give an example of light & shadow
light coming from the left shows the concavity of a crater as it is in shadow i.e. it is not flat
what gives off the effects of atmospheric perspective
because the distant objects have lower contrast and a blue tinge due to scatter (short wave light) in the atmosphere, it seems further away in the scene
what is height in scene
things lower in the scene seem nearer than things that are higher in the scene
what is not a learned pictorial cue, i.e. is not in conscious awareness
motion parallax: formed by head motion
how does motion parallax: formed by head motion work
e.g. when the head moves/turns to the right, the far object also moves to the right with the observer, but nearer objects moves to the left, against the observer (doesn’t matter where your fixated)
what is motion parallax generated by
self motion i.e. motion of the observer
what is the problem with monocular cues
they’re ambiguous and require assumptions
give an example of a problem with retinal image size
it is an ambiguous distance cue, unless the physical size of the object is already known (familiar) e.g. are the two balloons at different distances, or are they actually the same distance but different sizes…..? they are actually the same distance but different sizes, as they are against a flat screen (can be at different distances if you know the circumstances at which you are viewing these images, otherwise you can’t tell)
swell as ambiguity being associated with retinal mage size, what else can it be associated with and how
shadowing
the assumption is that the source of illumination is from above i.e. creating this shadowing effect which shows convex surfaces as bulging out where the shadow is at the bottom and concave surfaces as dips where the shadow is at the top, as is nearly always the case in indoor or outdoor scenes, but now if you invert the image, the concave impressions are now protruding & convex are dips, when the illuminant appears from below…
so changes from direction of source of illumination = ambiguous depth cue
what is binocular cues to distance & depth generated by
the horizontal separation between the two eyes, such that each contains a different image of the visual scene as viewed from its own vantage point
what circumstance are the differences between two images, most pronounced
at near (up to 10m from us) viewing distance, but are absent at far/infinity (beyond 10m from us) as the differences between images disappear between the two eyes
as a consequence of near objects within 10m being most pronounced, and absent beyond 10m, when do binocular cues to distance & depth operate most effectively
for objects situated within the viewers peri-personal (within arms length) & immediate extra-personal space (beyond arms length)
what are the two cues useful for binocular perspective
- vergence angle = absolute distance
- retinal disparity = relative depth
what is retinal disparity = relative depth a measure of
the different in images between two eyes = good for depth perception
what is vergence angle = absolute distance
between the two eyes, good for defining absolute distance from viewer
what are the two sources of non-retinal information about the angle of vergence between the two eyes which is made available to the visual system
- sensory: proprioreceptive (proprioreceptors inside the EOMs) feedback from the extra-ocular muscles signalling their state of contraction
- motor: corollary discharge/efference copy from the command signals sent to the extra-ocular muscles (from divergence to convergence part of the brain, commands to extra ocular muscles)
what does the sensory non-retinal information about the angle or vergence between the two eyes tell us
how much the muscles are contracting e.g. when looking at near, our medial rectus will be quite strongly contracted, which signals to the brain which tells us we’re looking at a near object
what is the amount of vergence required to bi-fixate a target linearly related to
it’s distance (angle of vergence is linearly related to the distance we’re looking at) which is used as a distance cue
why is a copy of efferent demand sent to the visual system
to let it know we’re converging = corollary discharge,
so the same signals sent to EOMs are sent to the visual system
describe the experiment that was done to examine the effect of altering vergence cues on absolute distance judgements, on normal control subjects
- subjects viewed the same target object at 8 different distances ranging from 20-100cm from their eyes, in a well-lit box containing multiple monocular cues (textured walls, height-in-scene etc)
- they randomly wore plano (control) or 5 prism dioptre lenses, either base in or base out
- base in prism increases divergence, so target appears further away (overshoot)
- base out prism increases convergence, so target appears nearer (undershoot)
- px pointed with a stick outside the box, to indicate the target’s distance: 4 repeats each trial, 8 subjects
results: despite the wealth of alternative monocular cues, their judgements were systematically biased by the vergence specified distance information (so did use vergence to judge how far the target was)
in terms of binocular depth perception, what is the origin of visual perception
geometry, separation between the 2 eyes generates horizontal (retinal) disparities between the 2 retinal images
in terms of binocular depth perception, how is perception generated in the visual cortex
- stereopsis ‘solid vision’
- primary v1 & most extra striate visual areas
- single cells ‘tuned’ to different horizontal disparities = disparity detectors
- advantage of two eyes that work well together is it enhances depth discrimination (stereo acuity)
what is stereo acuity
the smallest angular disparity of images between the two eyes that can be resolved
what is stereo acuity in most adults
10-20 seconds of arc or 1/360 of a degree (= really small angle)
with this degree of stereo acuity these are the distances (d) from fixation that can be resolved at different viewing distances (D)
up to how many metres is regarded peri-personal space
up to 0.5 metres
in our peri-personal space, what differences of depth can we resolve
less than 0.2 of a mm
- 0.1m viewing distance = 0.007mm distance from fixation
- 0.2m viewing distance = 0.030mm distance from fixation
- 0.5m viewing distance = 0.190mm distance from fixation
what is stereo acuity discrimination threshold
the differences in depth from fixation point that you can actually resolve
how many mm of difference in depth can be resolved at a viewing distance of 50 metres or more
2000mm/2m
what is a difference in depth of 2000mm that can be resolved at a viewing distance of 50m equivalent to
monocular vision
therefore theres no difference to determine depth beyond 10m, if have one or two eyes
what does the visual system do when multiple cues to distance & depth are available to us (which is true for most viewing distances)
the visual system evaluates the available evidence & then places the most weight (confidence) on the one (or those) that is/are the most reliable, giving us the best type of information
this does not mean that individual monocular or binocular cues are redundant, because even when normally reliable cues are available in isolation from all others, distance & depth judgements are mis-perceived
when is the mis-perception of distances & depth quite systematic
in ‘isolated’ or ‘reduced’ cue conditions
how is the mis-perception of distances & depth perceived when theres only one or a few cues available to their distance & depth
near objects are perceived as farther away from the viewer & has having greater depth (=overestimated) than they really are
&
far objects are seen as closer & having less depth (underestimated) than they really are when theres only one or a few cues available to their distance & depth
what is the phenomenon of the mis-perception of distance and depth when theres only one or a few cues available & what does it imply
contraction bias
implies that our internal representations of 3D space in the brain are systematically compressed in space
what was the experiment used to determine contraction bias for distance
same experiment used to examine the effect of altering vergence cues on absolute distance judgements, on normal control subjects, but box this time was completely black and the target was hanging in isolation in the box (only isolated distance cues were available)
- vergence: only cue available when using both eyes
- retinal image size: only monocular cue available when using one eye, to know how far the image was
- objects further than 100cm was perceived to be nearer
- objects nearer than 60cm was perceived to be further away than they really were
what is size constancy
the visual system is remarkably good at compensating for changes in retinal image size with viewing distance…since the two change in direct proportion…yet perceived size is virtually independent of distance
due to the visual system being quite good at compensating for changes of retinal image size with viewing distance, what can it ignore
the things that we think are implausible, (e.g. tiny people in the scene which is unrealistic) from past experiences, and notice things that are plausible/realistic
explain the mood illusion
when only retinal image size is regarded, it shows a ~ and true size of the moon in the sky, however cue conflicts can fool size perception, causing the moon to look too big which is an illusion created by other distance cues e.g. height and scene as the moon is too close to the horizon, it looks too big (an illusion) due to a conflict in different cues (retinal image size should show moon as small but based on height and scene, it should be big..therefore visual system is trying to work out size by retinal image size, compared to other cues)
what is the muller-lyer illusion of fooling size perception:interference effects
the horizontal line with the orientation of the fins which stick out (concave lens look) makes the horizontal line look longer, whereas it is actually the same size as the horizontal line which has the fins that point in (convex lens look) but both are same size
what is the ebbinhaus or titchener circles illusion of fooling size perception:but not the hand
the circles which have the smaller circles around it is perceived as larger than the circle with the larger circles around it, (both are the same) but people grasp the two central circles in exactly the same way
which stream is immune to the illusion of a central circle with smaller circles around it compared to a central circle with larger circles around it, but the hand can grasp both circles in the exact same way
the dorsal-action control stream
what can a lesion to the ventral stream/inferior occipital-temporal lobe cause/be associated with and what does this indicate
prosopagnosia and NOT form agnosia
indicates lesion was more lateral & anterior to area LOC
which can only effect 1 hemisphere & it’s opposite hemifield
what does the fact that a lesion to the ventral stream/inferior occipital-temporal lobe, which is associated with prosopagnosia and not form agnosia, indicates a lesion was more lateral & anterior to area LOC, which can only effect 1 hemisphere & it’s own opposite hemifield conclude
each cortical (inferior occipital temporal) lobe must be in charge of its own is constancy mechanism for the opposite hemifield, and patients with these symptoms are aware of their problem
what are the symptoms of lesions to one of their cortical (inferior occipital temporal) lobe, that patients are conscious that they have
- micro-opsia: familiar objects appear shrunken in size
- macro-opsia: familiar objects appear to be enlarged
which brodmann areas are close to the LOC
18 & 19
what area can a stroke cause a lesion to which is in brodmann areas 18 & 19 i.e. close to the LOC
watershed area = between inferior MCA & PCA territories
if the left occipital lobe is unaffected by a lesion to the LOC, what is the effect caused
right side of images is smaller my 15% i.e. px has micropsia due to right side lesion
