perception- binocular interactions Flashcards
learning objectives
-disparity detection under stereoscopic viewing
-what is bi-stable image
-what is binocular rivalry (BR)
-factors affecting temporal dynamics of BR
geometry of retinal disparity
-look at diagram in slides
-p is fixation target (image) with its image falling on the fovea of each eye
(fovea have high number of cones therefore image falls there)
-Q is then seen in depth in relation to P (so an image of further depth)
relative to P, q is at diff angles at the fovea
-this is disparity- the information is not the same in our 2 eyes
what is the binocular correspondence problem
-to get binocular disparity, the brain needs to determine which features in the right eye correspond to those in the left eye (correct matches) and which do not (false matches)
diagram
from the projection of four similar objects on the retinas there are 16 different possible combinations, four of these are true , the rest are false.
construction of a random dot stereogram
-to create a square floating in the background (uncrossed disparity) corresponding central squares in the two half images are moved outwards
-to create a central square floating in front of the background (crossed disparity) portions of the two half images are moved towards each other (the picture appears to be coming towards you)
-the extent of movement determines the magnitude of depth separation between the square and the background
Jules random dot stereograms and disparity explained
He used huge arrays of black and white squares for the left and right stimuli of stereoscope cards
- the pattern of black and white squares was the same for both stimuli except for an inner region where the block of squares was shifted slightly relative to the surrounding squares in one stimulus.
(slightly diff images for each eyes and bits of image will have disparity between them)
-random squares filled in the voids after shifting
-when viewed in a stereoscope, an inner square of black and white squares appears much nearer than the surrounding black and white squares
-this arises because the inner square is disparate in the two retinas
julesz model for finding dot clusters of the same disparity
-jules proposed a global stereopsis mechanism which selected a match based on most uniform set of disparity measurements (i.e point by point comparison based on brightness and polarity)
one of the theories on how our brain detect disparities :
-we have disparity detecting neurons all way through our visual system
-we have a set of neurons that are responsible for responding to the small disparity and set responsible for large, and larger and larger etc
so multiple neurons tuned to different levels of disparities
-so when you have layers and layers of neurons at different disparity, if the disparity in the image you see matches neurons those neurons will respond maximally.
neural basis of stereopsis
-stereo blindness
-strabismus
-stereo blindness is the inability to perceive depth from retinal disparity
-when the eyes are misaligned (a squint) (strabismus) , the brain supresses the view of one eye
- stereo blindness is often caused by uncorrected strabismus
(disparity detection will not be there)
what is a bistable image
-image which have the possibility of being perceived in two ways
(refer to all examples eg the duck/rabbit, the moving cylinder)
perception of ambiguous figures
(stochastic fashion)
stochastic fashion - appears to be random but in fact is statistically not
-when there are multiple , but mutually exclusive solutions , to an image presented, the perception alters between possible solutions in stochastic fashion.
-for a bi stable image, the perceived image alters between the two possibilities ,in what appears to be a random fashion. the exact moment of switches cannot be predicted but it has specific statistical properties
binocular rivalry
Binocular rivalry is a perceptual phenomenon which occurs when a different image is presented to each eye. Instead of seeing a fusion of the stimuli presented to each eye, the different images alternatively dominate our perceptual experience.
right eye sees one image , left eye see another image
binocular rivalry - effect of seeing different images
-instead of fusion, the brain choose which one it sees, usually youll see the dominant eye image first (the image next to dominant eye)
-essentially images will alternate between the two
rivalry process is attributed to :
-‘high level’ mental operation (Helmholts,james) : if you pay attention to one image you can hold it longer than the other one
-‘low level’ competition between image features (Levelt 1965) - if one image is brighter youll see it first
-or could be a combination of both
temporal dynamics (br images)
-how does our brain switch between the images , is it random or is there a pattern?
-reversals are unpredictable, however they have specific properties
-look at the graph in the slides
-shows the frequency distribution of the durations of individual periods of rivalry dominance. The smooth curve shows the best fit gamma distribution to the obtained data (shown by the dots) Rivalry durations vary irregularly from period to period of rivalry.
(not random so example of stochastic distribution)
factors affecting temporal dynamics
-stimulus strength affects supression duration (eg luminance, contrast, movement, colour)
-attention: dominance duration of a stimulus can be influenced by voluntary attention)
-context can also affect the duration of pre dominance
(these factors can make one image appear more so become dominant)
associated effects of rival stimuli
-visual sensitivity is reduced for the supressed image , except for transient changes
-visual adaptation and aftereffects persist for the supressed image (image we don’t see)
-visual priming during suppression - (no evidence for sematic priming or picture priming)
-visual evoked response (VEPs)
-functional MRI studies show networks of activity for seen and supressed images. BOLD signals related to the dominant states are more pronounced in higher visual areas
(different parts of our brains respond to areas etc respond to faces, buildings) so scans show that when seeing two images (rivalry) with each eye say its face and building, the area of the brain which sees the supressed image is supressed.
s-single unit recording: no evidence for effect of rivalry in LGN
