perception-spatial vision cont and depth perception

Question 1

learning objectives

Answer 1

Monocular cues
-pictorial cues (size ,interposition,lighting and shadows)
-clarity and elevation (including aerial and linear perspective,texture)
-accomodation
-kinetic cues (motion parrallax)

binocular cues
-convergence
-stereopsis

Question 2

continuing from spatial vision
summary

Answer 2

-different frequencies in vision and we call them spatial frequencies
-we said there are patterns of sinosudiol waves that change in contrast over space

Question 3

what does the graph (contrast one) tell us

Answer 3

-tells us how we observe the range of frequencies around us
-as can be seen, in the very high spatial frequencies we are not very good

Question 4

visibility of gratings of different frequencies

Answer 4

-Blakemore and Campbell (1969) adapted observers for several minutes to a high contrast grating of 7 cycles/degree.

-then observer viewed a low contrast test grating and adjusted its contrast till it could just be seen

Question 5

our nervous processing and how it gets tired- helps in understanding depth perception

Answer 5

-sensory system
-if you stare at something for some time, we have nerves that process info and fire and send signals to the brain
-after a while they fatique and get tired and dont perform as well
-so the response because wors and more sluggish
-however you still see what’s around you
-when you take the stimulus away it imbalances the nervous processing
-by looking at the imbalance you can work out how these mechanisms relate to each other
-eg first lecture looking at the dot and red photo and seeing green after

Question 6

visibility of gratings of diff frequencies continued

Answer 6

-Blakemore and Campbell (1969) adapted observers for several minutes to a high contrast grating of 7 cycles/degree.

-then observer viewed a low contrast test grating and adjusted its contrast till it could just be seen

-after staring at it for a while we can remove it and then measure sensitivity to the same thing again or to a slightly different frequency

(can adapt to certain frequency and still measure contrast threshold for other frequencies)

-the experiment was repeated (adaptation followed by test grating) with several different frequencies of test grating
-experiment also repeated with different adaptation grating frequencies

Question 7

visibility of gratings of different frequencies
conclusion
-what does this conclusion mean?

Answer 7

-If you adapt to a certain frequency, your not very good at detecting that afterwards, but your also not good at detecting neighbouring frequencies

-if you go further and further (higher or lower spatial frequency) your sensitivity to those are not affected.

-means that in our nervous system there are channels of processing which are tuned to spatial frequencies, and when you adapt to that frequency your not very good at detecting it, but if you adapt to a certain frequency, a separate channel is not affected

Question 8

multi channel model of CSF
what does the graph show
-implications

Answer 8

-normal human CSF as envelope of several underlying spatial frequency channels
-overall showing contrast sensitivity function

-can be used in diagnostic tests
-can use them in rehabilitation of patients with brain injure (use these mechanisms to see what are the brain pathways that are damaged after stroke etc

Question 9

orientation of gratings experiment
-masking?

Answer 9

-conducted by Campbell and Kulikowski 1966
-using 10 cycles per degree horizontal masking/adapting grating (always present) and variously orientated test gratings flashed on and off for 1 second periods

procedure
-horizontal grating , you make people adapt to it for a few minutes (get them to look at it) and then take it away, then measure the sensitivity to a grating at a different orientation (so it slightly sideways etc) and measure threshold for detection

-the masking grating was also changed in contrast between experiments

(masking occurs when visibility of one image is reduced by the presence of another)

Question 10

campbell and kulikowski experiment findings
as the angle between the test and masking gratings ______, the effect of masking _______
-conclusion

Answer 10

-what they find is that if you adapt to lets say vertical grating then your threshold sensitivity is affected for gratings of vertical or gratings slightly close to vertical, but your sensitivity to horizontal grating is not affected whatsoever.

as the angle between the test and masking gratings decreases the effect of masking increases

(i.e increased contrast of the test grating is needed for it to be seen)

-what that tells us is that our nervous system not only looks at the spatial frequency but also takes orientation into consideration

Question 11

-what does selective adaptation suggest about spatial frequency

-graph from experiment

Answer 11

-we have got tuning in our nervous system to different spatial frequencies and different orientations.
-selective adaptation effects suggest that different neural channels are used to detect different spatial frequencies

-detection of any spatial target depends on responses in sets of neurons tuned to a certain spatial frequency and orientation

Question 12

depth
learning objectives

Answer 12

*Monocular cues

Pictorial cues (size, interposition, lighting and shadows)
Clarity & Elevevation (including aerial & Linear perspective, texture)
Accommodation
Kinetic Cues (motion parallax)

Binocular cues

Convergence
Stereopsis

Question 13

seeing in 3d
what does depth perception register

Answer 13

-the visual system uses optical information in the 2D retinal images to compute object shape

-depth perception registers the distance between object and observer

Question 14

depth cues

Answer 14

sources of depth information are depth cues

Question 15

two categories of depth cues

Answer 15

monocular
binocular

Question 16

-monocular visual depth cues can be provided by…..
-monocular depth cues may be ____ or _____ depth cues
-binocular cues rely on…
-binocular vision allows humans to…

Answer 16

-monocular depth cues can be provided by both eyes or by one eye alone.
-monocular depth cues may be static or motion depth cues

-binocular cues rely on comparison of input from each eye to yield depth information
-binocular vision allows humans to discriminate very small differences in relative depth

Question 17

monocular cues
-pictorial cues
the 3 categories

Answer 17

-pictorial cues have 3 categories
size
interposition
lighting and shadow

Question 18

monocular cues
pictorial cues
size
what is size constancy

Answer 18

-as the distance between object and observer increases , the retinal image becomes smaller
-hence the size of a familiar object provides a depth cue

-perceived size is scaled in terms of perceived distance (size constancy).

-your brain has the concept of size constancy (we use it all the time-we can judge distances etc)

Question 19

monocular cues
pictorial cues
interposition

Answer 19

-when one object occludes part of another, the partially occluded object is perceived as more distant. This is called interposition.
-brain works out what is covered by what
-2 squares example you know the white square is Infront of purple one (so purple is further away)

Question 20

monocular cues
pictorial cues
lighting and shadow

Answer 20

-very important aspect
shadows
-shadows give us a perception of depth
-look at square examples, by looking at a shadow your brain works out whether something is where it is in space in front of you

lighting
our nervous system is hardwired to always assume the lighting comes from above

Question 21

monocular cues
clarity and elevation
-aerial and linear perspective

Answer 21

clarity as a cue to depth
-aerial perspective (distant objects appear less clear than close objects.

-linear perspective is a depth cue provided by line convergence (linear perspective tells us that depth cues should converge in a long distance away)

-linear perspective was discovered by Italian artists during 15th century, and enabled artists to capture depth and 3d perspective on a 2d surface

Question 22

monocular cues
texture

Answer 22

-texture gradients provide depth information (gradient of image size/texture perspective)

-plane in front of you is called frontal plane
-imagine 3 objects at diff distances from you
-the angle between each object is different (the further away the smaller the angle)
- this texture gradient is what your brain uses to figure out depth

Question 23

errors in perceived size are attributed to……

Answer 23

-the brain uses a large array of cues to interpret your position in your surround (weighting to diff cues are different)

-errors in perceived size are attributed to monocular depth cues.

-in the Ponzo illusion, perspective cues imply distance between regions of figure

-these cues affect size judgement of objects located in these regions

Question 24

integration
-various depth cues provide unique information that is processes….
-depth perception is degraded when….

Answer 24

-the amount of weighting you will give to these cues are different, some of them are more reliable , some of them are less and the brain dynamically changes the weighting between these in order to give you perception

-various depth cues provide unique information that is processed additively.

-depth perception is degraded when depth cues provide conflicting information.

Question 25

monocular cues
accommodation

Answer 25

-change of focus when you look at a close up object

-lens in the eye is able to change focus , the muscles relax and lens becomes fatter when you see something close by, if far away the lens stretches and becomes thinner to see long distance

• this information gets sent to brain and so will tell you where something is (close or far)

-accommodation can come from one eye or both eyes (but don’t interreact between two eyes)

Question 26

monocular cues
kinetic cues
(motion cues)
-motion parallax

Answer 26

-cues to depth perception in which motion is used to estimate depth

-motion parallax is relative apparent motion that provides an effective depth cue

-motion parallax can occur by observer movement relative to an object , or when observer is still and the object moves

• motion parallax is a monocular depth cue that causes objects that are closer to you to appear to move faster than objects that are further away

Question 27

range of effectiveness of different depth cues

Answer 27

occlusion - 0-2 meteres,2-20, above 30
relative size - all
accommodation and convergence -0-2
motion parallax - 0-2 and 2-20
relative height- 2-20, above 30
atmospheric perspective - above 30

Question 28

anything higher up in an image your brain assumes is ……

Answer 28

further away
anything lower down your brain assumes is closer

Question 29

binocular cues
convergence

Answer 29

-very strong cue on where things are
-convergence is when both eyes rotate inwards at different angles to focus on an onject

Question 30

binocular cues
disparity / stereopsis

Answer 30

-depth perception that relies on disparities between the views of the left and right eyes is called stereopsis
Binocular disparity is the difference in location of a feature between the rights eye and the lefts eye
-The amount of disparity depends on the depth (i.e., the difference in distance to the two object and the distance to the point of fixation), and hence it is a cue that the visual system uses to infer depth.

Question 31

how to we extract depth information with both eyes? retinal disparity

Answer 31

-fovea at the back is really important in terms of processing info (where we have most receptors at the back of eye)

-when you look at an image it falls on the fovea in each eye (image on them)

-angles at which the images are falling aren’t always the same (eg if you look at something slightly towards the right

-information from each eye is not the same (disparity of information) detecting this disparity will give you the strongest depth cue you can get