6. The Space Sense II 2020.pptx

The Space Sense II Introduction. Objectives and Readings. Fusion and Rivalry. Perception of depth and distance. Stereopsis. Factors affecting stereopsis. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Define and understand the terms physiological diplopia and...

The Space Sense II Introduction. Objectives and Readings. Fusion and Rivalry. Perception of depth and distance. Stereopsis. Factors affecting stereopsis. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Define and understand the terms physiological diplopia and fusion. • Describe the phenomenon of binocular rivalry. • Define suppression and be able to give examples of when it occurs. • Describe the various monocular cues to depth perception. • Define and understand the terms stereopsis and stereoacuity. • Describe the factors that affect stereopsis. SFO1004 • Levin, Leonard A. (2011) Adler's physiology of the eye: clinical application. Chapter 36 Binocular Vision (eBook). – Pgs. 683-685 (binocular disparity) – Pgs. 685-690 (stereopsis) – Pgs. 679-681 (suppression) • https://webvision.med.utah .edu/book/part-viii-psychop hysics-of-vision/space-perc Dr Sarah J Waugh© eption/ Fusion • Is the process that invokes both a neurophysiological and perceptual (psychological) response by which the brain combines the two images (one from each eye) into a single percept. • Requires that the images fall on (near) corresponding points in the two eyes (within the limit of Panum’s area). • Panum’s area determines the size of the region in space where there is binocular single vision. • For foveal vision at about 30cm in front of the eyes, Panum’s area ranges from a few cm in front of and behind fixation (about 7-12 min arc) SFO1004 • In the periphery Panum’s area increases (at 5° it is about 24-30 min arc). • So we are less likely to see physiological diplopia in periphery. Dr Sarah J Waugh© Fusion of images (sensory) • The left and right eye images need to be similar (although not necessarily exactly the same). • In the figure, by trying to ‘cross your eyes’, the two images are similar and can be fused into a single percept. SFO1004 Dr Sarah J Waugh© Fusion of images (sensory) • In the figure, fusion of the two circles is more difficult as the colours don’t ‘fuse’ as easily. • There is often an associated oculomotor response (i.e. an associated movement of the eyes). • We sometimes talk about ‘sensory’ fusion and ‘oculomotor’ fusion as separate functions however, the two are linked. SFO1004 Dr Sarah J Waugh© When you don’t fuse – physiological diplopia SFO1004 Dr Sarah J Waugh© When you don’t fuse – binocular rivalry • Binocular rivalry is the process that occurs when fusion of two dissimilar images is attempted. • The objects should still fall on corresponding points. • The perception in rivalry is one of a rhythmic alternation of the separate images. • Trying to fuse two grating patterns as shown in the image leads to binocular rivalry. • The two images do not fuse, rather, the perception is of ‘switching’ between images, one eye then the other. SFO1004 + Dr Sarah J Waugh© When you don’t fuse – suppression • Sometimes the perception is a ‘piecemeal’ or patchwork effect where parts of the images are seen. • But what happens to the one of the images when it is not seen? • When not seen, the image is suppressed, and the process is called (rivalry) suppression. SFO1004 Dr Sarah J Waugh© When you don’t fuse – suppression • Suppression is generally considered to be an inter-ocular (or binocular) process of inhibition. • Information in one eye is inhibited to below the threshold of perception and is not perceived by the suppressed eye. • Suppression is the (partial) absence of sensory fusion. SFO1004 • Suppression is part of the normal binocular process but why does it exist? • It is a helpful adaptation because if it did not occur, we may experience double vision or other unwanted issues. • Which is why suppression can occur in ocular abnormalities such as amblyopia or strabismus. Dr Sarah J Waugh© When you don’t fuse – suppression • Suppression also occurs when we move our eyes quickly from one point to another. • In the video, I am making fast eye movements (called saccadic) from side to side. Can you see my eyes moving? • Now you do the same – hold a mirror close to you so you can see your eyes. The move them from side to side. Can you see them move? If you said yes, are you sure! This is saccadic suppression. SFO1004 Dr Sarah J Waugh© A form of suppression (Troxler effect) can also occur in monocular conditions. SFO1004 Dr Sarah J Waugh© Interval • This is a good time to pause and reflect on the first section of the presentation. • When you are ready, continue to the next series of slides. SFO1004 Dr Sarah J Waugh© Perception of depth – monocular cues Monocular cues Binocular cues • Aerial perspective. • Light and shade • Stereopsis. distribution. • Overlap. • Geometric perspective (converging lines). • Interpretation of size. • Motion parallax. SFO1004 Dr Sarah J Waugh© Perception of depth – aerial perspective • Results from imperfections in the atmosphere that produce light scatter. • The greater the distance away an object, the more it will be affected. • Produces a blur in the contours of more distance objects. • Distance objects can also appear more ‘blue’ because of the light scatter (related to wavelength of light). SFO1004 Dr Sarah J Waugh© Perception of depth – light and shade • This contributes to depth based on the distribution of light and shade. • The patterns are interpreted as though the light source is overhead. • Hence any shading may give an impression of depth. • In the figure making the ovals look either concave or convex in appearance. SFO1004 Dr Sarah J Waugh© Perception of depth – overlap • Overlapped objects appear further away. • The green building overlaps the red building and therefore the green building appears closer. • The bats overlap the green building and appear closest of all. SFO1004 Dr Sarah J Waugh© Perception of depth – geometric perspective • Powerful cue for depth sense. • Progressively smaller angles give appearance of changes in depth (if objects are familiar). SFO1004 A street scene in Olomouc town centre showing how perspective gives a sense of distance and depth. Dr Sarah J Waugh© Perception of depth – interpretation of size • Interplay between geometric perspective and knowledge of object size. • The people in this Lowry painting are drawn larger in the foreground and smaller in the middle of the picture, and therefore the middle drawn appear further away. SFO1004 Dr Sarah J Waugh© Perception of depth – motion parallax • Motion parallax can be a very powerful cue to depth perception. • Objects closer move more quickly than objects further away. • Depth judgements are therefore more accurate with head movements. • This is important to remember when testing depth perception with some tests in the clinic. SFO1004 Dr Sarah J Waugh© Perception of depth – stereopsis • The binocular perception of depth in the absence of monocular cues. • Stereopsis is the binocular perception of relative distances between objects that results from the relative horizontal retinal disparities between the monocular images. • Binocular disparities occur as a result of the horizontal separation of the 2 eyes, which provide a slightly different view of objects. • Only horizontal disparities produce stereopsis. Vertical disparities do not. SFO1004 • The difference in depth of two objects produces ‘retinal disparity’. This is the cue that allows stereoscopic depth perception to occur. Dr Sarah J Waugh© Perception of depth – stereopsis • Stereopsis provides information about relative depth (rather than absolute distance). • We can determine the smallest detectable stereoscopic depth (called stereoacuity) by determining the stereo-angle ‘η’. • Under optimal (laboratory) conditions stereoscopic acuity thresholds of 10 arc sec or less can be recorded. SFO1004 Dr Sarah J Waugh© Conditions required for stereopsis • Must be using both eyes. • At least 2 objects in the visual field. • The objects must produce a retinal disparity (i.e. they stimulate disparate retinal points). • Edges and contours are not required. Can perceive depth in ‘random dot’ stereograms. Central ‘dots’ are shifted (as in this example). The brain correlates the dots and an image is perceived. SFO1004 This Photo by Unknown Author is licensed under CC BY-SA A random dot ‘autostereogram. In order to perceive the image you have to ‘focus’ your attention behind the plane. Dr Sarah J Waugh© Stereopsis – optical defocus • Stereopsis (stereoacuity) is reduced as ocular defocus increases. • The reduction in stereoacuity is greater for monocular than binocular defocus. • The amount of reduction depends on how stereoacuity is measured (i.e. which test is used). SFO1004 Fricke and Siderov, unpublished data Dr Sarah J Waugh© Stereopsis – retinal eccentricity • Stereopsis is present across the retina in the periphery. • Stereoscopic acuity is most acute (i.e. best) at the fovea and decreases in the retinal periphery. • Shown in the figure are data depicting the decrease in stereoacuity (threshold) up to 10° in the periphery. Siderov and Harwerth, 1995 SFO1004 Dr Sarah J Waugh© Stereopsis – clinical testing • There are many different clinical tests available to assess stereopsis and measure stereoacuity. • The tests separate the images to the 2 eyes using different means: – Real depth (Frisby) – Red and green filters (TNO) – Polarising filters (Titmus Fly and Randot) SFO1004 Dr Sarah J Waugh© Questions? • Remember you can ask questions via Brightspace discussion board or the chat function on Teams. SFO1004 Dr Sarah J Waugh©