9. The Space Sense I 2223.pptx

The Space Sense I Introduction. Objectives and Readings. Monocular visual projection. Binocular visual projection. The horopter. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Define oculocentric and egocentric visual direction. • Define the concept of the pri...

The Space Sense I Introduction. Objectives and Readings. Monocular visual projection. Binocular visual projection. The horopter. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Define oculocentric and egocentric visual direction. • Define the concept of the principle visual direction. • Describe Hering’s law of identical visual directions and how it was determined. • Define the terms corresponding and disparate retinal points and their relation to the horopter. • Define the terms crossed and uncrossed disparity. • Define Panum’s fusional area. SFO1004 • Levin, Leonard A. (2011) Adler's physiology of the eye: clinical application. Chapter 36 Binocular Vision (ebook). – Pgs. 677-679 (visual direction) – Pgs. 679-681 (retinal correspondence) – Pgs. 683-685 (binocular disparity) • https://webvision.med.utah.e du/book/part-viii-psychophysi cs-of-vision/space-perception/ Sarah J Waugh© • Web Activity 6.2DrBinocular D Spatial Localisation Objective space • The physical locations, with respect to the observer, of objects in the environment (i.e., the real world). Subjective space • The perceived locations, with respect to the observer, of objects in the environment. SFO1004 • We use these ideas when we measure eye alignment. • We compare the physical test arrangement (objective space) to what the patient perceives (subjective space). Dr Sarah J Waugh© The subjective visual space – Law of direction • An object viewed by an eye is imaged onto the fovea. • Each retinal element has its own sense of direction (in space). • The specific subjective direction of a retinal element has been called retinal local sign of the receptor. SFO1004 • Stimulation of the fovea gives rise to the principal visual direction. • The fovea is the dominant sense of visual direction. • Our sense of direction is organised around the principal visual direction. • Although we are discussing retinal local signs it is important to note that the sense of direction is ultimately cortical in nature (i.e. in the brain!). Dr Sarah J Waugh© Visual Direction oculocentric • Localisation of objects involves both a direction and a distance. • We can specify the direction of an object with respect to the fovea – we call this the oculocentric direction (or eye centred direction). • In Fig A, the oculocentric direction has changed but the object has not moved. • In Fig B, the oculocentric direction has not changed but the object has moved as well as the eye. SFO1004 Both eye and object moved Stationary object – eye moves F’ F A F ’ F B Dr Sarah J Waugh© Visual Direction egocentric • The egocentric direction is the direction in relation to one’s self. That is the direction with respect to the body (head). • Egocentric direction is dependent on retinal location, eye movements and body displacement. SFO1004 • Visual information passing from the retina to the visual cortex (visual part of the brain) is integrated with eye movement information and body position. • A number of factors contribute to this knowledge including: – Muscular stretch receptors in the head and neck – Vestibular apparatus in the ear (that indicate changes to head position and acceleration) Dr Sarah J Waugh© • Stimulation of the fovea gives rise to the principal visual direction. • Visual directions of all other stimulated retinal points associated with objects at different angular locations in space are experienced in relation to the principal visual direction (i.e., right, left, up, down etc.). SFO1004 Principal visual direction Principal Visual Direction F Dr Sarah J Waugh© LOOK HERE • How can we be sure that stimulating the fovea in each eye gives the same (identical) visual directions. • Hering proposed an experiment to prove this was the case. • He viewed a target through a window with his right eye (left eye was closed), locating a point on the window through which he could see the target (house). He now looked through his other eye, through the same point on the window and located a target in the distance (tree). • When both eyes were opened, the two objects Hering, 1879, From Ogle, 1950 The Law of Identical Visual Direction appeared to be superimposed (i.e. single), as they had the identical visual directions. • This is Hering’s Law of Identical Visual Direction. SFO1004 Dr Sarah J Waugh© Binocular projection and correspondence • Under binocular viewing of a target, both eyes have the same oculocentric direction as shown. • In this case, the two principal visual directions (one from each fovea) are identical. • What do we perceive when 2 (one in each eye) identical visual directions are stimulated (at the SFO1004 F F Dr Sarah J Waugh© Correspondence and the cyclopean eye • When identical visual directions are stimulated, one in each eye, we see a single object. • Moreover, it is as though we are seeing the object(s) through a single, centrally located eye, sometimes referred to as the cyclopean eye (after the mythical cyclops). SFO1004 F F Dr Sarah J Waugh© The concept of the cyclopean eye • The cyclopean eye, situated in the centre between our two eyes, is an imaginary concept that is helpful in understanding of binocular directions. SFO1004 F Cyclopean projection F 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© Corresponding retinal points • When two retinal elements (one in each eye) are stimulated and give rise to the same visual direction, they are said to be corresponding. • Within the binocular visual field, each element in one eye has a corresponding element in the other eye (so when stimulated they give rise to identical visual directions). F F Cyclopean projection SFO1004 Dr Sarah J Waugh© Corresponding retinal points • As corresponding points give rise to the same visual direction, the object is seen as single (i.e. ‘fused’). • Retinal elements which are not corresponding are known as disparate points. • Stimulation of disparate points does not give rise to identical visual directions and double vision or diplopia occurs. F F Cyclopean projection SFO1004 Dr Sarah J Waugh© The longitudinal horopter horopter • We could examine the binocular visual space by locating objects in the visual field such that their images would fall on corresponding retinal points. • If we then joined these points in space, they would form the horopter. It represents a map of retinal correspondence. • The horopter formed in this way is referred to as the longitudinal horopter (there is also a vertical horopter). SFO1004 F F Dr Sarah J Waugh© The geometrical horopter • The fixation point (P) and point Q fall on geometrically corresponding points in the 2 retinae. • A geometric model of the horopter can be constructed assuming the eyes are perfectly spherical and there is perfect symmetry. • The horopter becomes a circle, representing a model of corresponding points. • Known as the Vieth-Muller circle. SFO1004 Vieth – Muller circle P α1 F Q α2 F Vieth, 1818; Muller, 1826 Dr Sarah J Waugh© The empirical (measured) horopter • Measurements of the horopter using different criteria have been made. – Measurements based on identical visual direction. – Measurements of the region of single binocular vision (i.e. the range of fusion). – Measurements of optimal stereoscopic depth judgements. SFO1004 • All methods show that the horopter is not a circle (as predicted from the geometry of ViethMuller). • The shape of the horopter varies with viewing distance. • The horopter can be useful in understanding processes of binocular vision. Dr Sarah J Waugh© Demonstration – crossed and uncrossed diplopia SFO1004 Dr Sarah J Waugh© Crossed Diplopia (Heteronymous) Uncrossed Diplopia (Homonymous) Fixation Fixation F Seen by RE F Seen by LE F Seen by LE F Seen by RE Corresponding retinal points and fusion • We have observed that for an object to be seen as single, under binocular viewing, its image must fall on corresponding retinal points. • However, the condition of requiring exactly corresponding points is not true. • Panum (1858) showed that fusion (single vision) is still possible despite SFO1004 not stimulating exactly F F Dr Sarah J Waugh© Demonstration – fusion and diplopia SFO1004 Dr Sarah J Waugh© Panum’s Fusional Area • Panum (1858) recognised • Therefore, binocular single that single vision was vision is not confined to possible with the the immediate vicinity of stimulation of disparate the horopter but extends points and proposed that for a small distance in for any point in one front of and behind it. retina there is a circle or • This region of binocular area of points on the single vision is known as other retina, the Panum’s fusional area. stimulation of which will Beyond the limits of lead to fusion of the two Panum’s fusional area monocular inputs. objects appear double. 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©