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PSYC201 Lecture 1: Visual Depth PerceptionLearning Objectives  Access and interpret sources outside of that provided by the given material;  Independently read and effectively interpret research papers and journal articles;  Use empirical research to support scientific theory;  Effectively manag...

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PSYC201 Lecture 1: Visual Depth Perception Learning Objectives  Access and interpret sources outside of that provided by the given material;  Independently read and effectively interpret research papers and journal articles;  Use empirical research to support scientific theory;  Effectively manage your time to prepare for coursework and exam deadlines.  Demonstrate knowledge of key concepts, evidence, and theory covered in the module (whether by lectures or selected reading materials);  Appreciate and explain how empirical enquiry contributes to the understanding of cognition;  Present and discuss research findings from cognitive psychology in an informed and grounded way. Vision and Where it Comes From (Evolutionary Arms Race, Cambrian Period, Present Day)  Vision is the main sensory system that we have and how we experience everything  Vision is just one source of information that tells us about the world that we live in  Vision helps us navigate through the world  The sun is a massive ball of plasma which emits electromagnetic radiation into our entire solar system, and the fraction of that energy (light) that is directed towards earth allows for all life on Earth  As these waves (from the sun) envelope our atmosphere and descend upon the surfaces of the earth, many are absorbed in the various materials occupying the earth’s surface providing the vast majority of our planet’s energy. o Only a fraction of the light penetrates through our atmosphere and most of it is reflected off. o A lot of that light gets absorbed into the earth, and is the majority of the energy that supports life on earth o However, not all of these waves are absorbed by every surface they contact. o Many of these waves are reflected, depending on the frequency of the wave and the reflective properties of the surface of which they were in contact with  During the Cambrian period, organisms developed the ability to sense these reflected electromagnetic waves (from the sun)  The detection of these waves signalled to the organism the presence or absence of the object from which the waves were reflected  This ability was initially supported by a single cell or small group of cells that contained a protein that changed its chemical composition after being in contact with electromagnetic waves of certain frequencies (these are called eye spots)  This ability eventually became an effective mechanism to detect the presence of potential predator or prey organisms and is thought to have triggered the Cambrian explosion (Parker, 2003), which refers to a massive amount of diversification and complexity in organisms that occurred in a relatively short period of time around 542 million years ago.  Presumably, in addition of an organism detection system fuelled the evolutionary arms race as organisms’ adaptions continuously selected for protective or predatory adaptations in others

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In present day these electromagnetic sensing systems are no longer a small bit of cells on the organism’s exterior; they are complex organs with lenses and mirrors, containing different receptor types that differentially respond to several different frequencies o Their shapes, sizes and structures are specialised for detecting certain types of reflected waves, which occur in the organism’s ecological niche o In the majority of organisms, they are the primary source of sensory information o Eyes have become one of the triumphs of evolution and have allowed organisms to gather considerable amounts of information about their environments, which has allowed organisms to gather considerable amounts of information about their environments, which has allowed them to take advantage of several resources in which they may not have had access to otherwise o Eyes are prevalent across the entirety of the animal kingdom, which has resulted in an impressive diversity and complexity in vastly different lightsending organs ranging from bioluminescent fish in the unfathomably dark depths of the oceans, to birds of prey in the sky. The evolutionary arms race allowed for the ability to hunt prey, but also created demands to flee and find ways to hide from predators

What is the eye doing? How much light is located where?  An aperture is an opening, hole or gap in the eye  The eye evolved over time to detect the amount of light at given locations  You must become concave to detect where the light is coming from o Can only hit the opposite side of the eye o Can tell left and right apart o General sense of where the light is coming from  However, the more concave, the more sensitive you get (increased directional sensitivity) and it tells you exactly where in detail the light is coming from o Problem – more concavity causes:  Less light allowed to come into the eye  Only a tiny aperture  Light getting blocked by external elements of the eye o Lots of aperture when less concave  Concavity allowed for the eye to detect the directionality of the light to be determined by blocking light from other directions on certain portions of the retinal  The eye evolved to have a fluid filled cavity allowing for the eye to have a spherical shape and thus, a very small, pin hole aperture (increasing concavity and thus the ability to detect directionality) o A cornea evolved (a rounded area of the eye) o If light hits the cornea it is going to reflect light differently to if light hits other areas of the eye  This allowed the aperture to be larger to let more light in, whilst also allowing for directional sensitivity (as it is going to reflect light differently depending on which area of the rounded surface it hits)  The fluid filled cavity also provided nourishment for optical structures

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However, the tiny aperture limits the amount of light that can enter the eye, only allowing for dim images. Then a tissue evolved to cover and protect the eye and the lens developed in the eye, which allowed for an increase in the aperture (this is the cornea) Because the lens is spherical, it bends light differently depending on where the light is coming from allowing for directionality detection with a larger aperture The lens also allows for light to be focused on the retina, which increases the resolution of the image

Observing Evolution Through the Animal Kingdom  Eyes come into focus from primitive light-capturing devices in invertebrates to our camera type eye with a lens. As eyes evolve, visual acuity increases  Eyes began as a smattering of cells on the top of a surface (an eye spot) and became a complex eye which tells us exactly how much light is exactly where Purpose  As a result, the eye allows you to detect if something is coming or going (tells you where something is going to be)  Vision allows you to be able to see the future (tell you what is going to happen before it does – like a Crystal ball)  Organisms were no longer floating around bumping into other organisms that could be food/ predators  Eyes allowed for the avoidance of predators and the detection of possible prey Stimulus: Retinal Projection  Objects project an image on the back of the retina  The fovea (in the back of the eye) provides the most detailed vision due to most photoreceptors (tight, compact photoreceptors that stimulate light)  Example regarding image on the right: o Light bounces off the candle and is projected onto the back of the retina and we get an image of the light (almost like taking a picture) o If you have a stimulus on the back of the retina you have different objects that project at different sizes on the back of the retina o Depth perception  Two candles of the same size  The only information we have about the candle is the image and the size of the image of the candle on the back of the retina  But if the candle is closer it is going to project a larger image on the back of the retina than a candle that is further away Inverse Projection Problem

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The inverse projection problem is the task of determining the object responsible for a particular image on the retina We have three different things o Distal stimulus – the object out there in the environment o Proximal stimulus – retinal image o Percept – what we experience when we look at the world All the information that we have to develop our percept is our retinal image which is 2D and upside down o What we perceive is closer to the distal stimulus that the proximal stimulus, which is a 2D image on the back of the retina We are able to take the retinal image and create a percept. But the percept looks more like the distal stimulus (the object out there in the environment) than the retinal image We have a very sparse amount of information from the retinal image, but were able to have a percept that looks more like a distal stimulus The visual perceptual system is able to recreate what is in the environment with information that information specifying what is out there in the environment is not able to do so and this is done so by the Inverse Projection Problem: o The size, distance, orientation, and shape of the stimulus is ambiguous in the proximal stimulus o The distal stimulus has constant properties as does the percept, but the proximal stimulus does not o As you move closer to the stimulus it gets bigger on your retina, and vice versa o The proximal stimulus’ shape will also change as you move or it moves, whereas you perceive it as constant o So the problem here is how does perception resemble the distal stimulus more than the proximal stimulus, on which perception is based? Diagram explained o The perceptual system must solve- 2 images (one big and far away, one small and closer) which project the same image size on the retina the ambiguity in the proximal - One image of a different orientation (diagonal), but it projects the same retinal image stimulus in order for the percept to resemble the distal stimulus There is an infinite amount of objects that can project the same size retinal We still need distance or size image -

The same object can project an infinite number of retinal images despite appearing to be stationary and the same size The retinal image tells you that you’re seeing an infinite number of different objects of different sizes the size of the object you can calculate

Emmert’s Law: Need Distance or Size  If you have distance and retinal image size, and vice versa  Your perceptual system needs to know how far away something is or the retinal size to calculate the size of the object Size Distance Invariance Hypothesis  If an object projects the same size visual angle and it is further away, it must be bigger  If an object projects the same size visual angle and is closer, it must be smaller  Retinal angle = visual angle

Getting Depth: Solutions  Depth perception is the process of determining size or distance  Depth cues are used to make inferences about how big or far away something is  There are two different types of depth cues, primary and pictorial depth cues  Primary depth cues: o Near space (close differences – arm’s reach)  Primary depth cues are used for objects close to us, within an arm’s reach in distance o Primary depth cues are cues derived from the anatomy/physiology of the eye (what the perceptual system knows about the anatomy of the eye) o There are 3 different primary depth cues: 1. Accommodation – bending of the lens (due to tension of the muscle)  Accommodation is the tension of the muscle that changes the focal length of the lens of eye. Thus, it brings into focus objects at different distances. This depth cue is quite weak, and it is effective only at short viewing distances (less than 2 meters) and with other cues.  The lens is an organ in the eye than bends light so that light focuses on the back of the retina o The lens will bend light more or less depending on where it is coming from  The lens bends light reflected from a stimulus so that it lands on the same area on the back of the retina  The lens bends more for objects that are closer compared to objects that are further away (stays flat)  The amount that the lens has to bend is used as a cue as to how far away the object is o If the lens doesn’t have to bend very much, then the perceptual system assumes that the object is further away o The retinal image and distance can then be used to determine the size of the object  However, this depth cue only works for close objects because all far objects (more than arms reach away) require very little bending of the lens (the difference being either so miniscule or absent and hence cannot be detected) o Distance vision – light rays from distance objects are parallel, so do not need much refraction to focus properly o Close vision – light rays from closer objects diverge and need much refraction to focus properly 2. Binocular disparity (stereopsis)  Binocular disparity is the difference in the image the back of the retina  Stereopsis (depth perception) is the visual ability to perceive the world in three dimensions (3D) - length, width, and depth -

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which then allows a person to judge where an object is relative to him or her. Stereopsis is the ability to determine depth from the difference in the image on the back of the retina o When an object is closer to you, there is a larger difference in location of the object on the retinal image of the left and right eye o When the object is further away, there is a smaller difference in the location of the object on the retinal image of the left and right eye Retinal disparity is based on the idea that you have two eyes in your head o We have 2 eyes because they provide different information about the world (seeing more to either side – expand the field of view) and it allows us to differentiate depth  It gives you information about how far away objects are because the location of an object on the back your right eye will be different to the location of an object on the back of your left eye (your perceptual system averages the locations from both eye) Objects that are further away project to different areas on the back of the retina Closer objects are more peripheral and further objects are more nasal At the cinema 3D glasses at the movies project different images in the two eyes to imitate retinal disparity. Early on they did this using colour filters (blocking out wavelengths of colour, sacrifices colour perception in each depending on the wavelength is filtered out), now they use polarisation (modern), wavelengths have a frequency and amplitude on a 2D plane o Wavelengths can also differ on their orientation (polorisation) o Same wavelength of light in different orientations (vertical vs horiztontal) o Newer ways of using polarisation to have retinal disparity to give you 3D illusions in movies without sacrificing colour perception is by filtering out different types of polarisation of light  E.g. left eye filter out vertical wavelengths, in the right eye it will filter our horizontal wavelengths that allows you to project a different image on the left eye than projected on the right eye (modify to determine depth as they are slightly different)

Depth order – if the difference between the 2 eyes of the location of objects is large or small  If its large – its closer  If its small – its further away  Colour perception is not sacrificed o Convergence (eyes angling inwards – amount of foveation)  Depth perception relative  Amount of rotation required to foveate on an object can specify it’s relative distance  Fovea in the back of the eye, is the area which you have the most acuity in terms of detail, and it is the only area where we have colour  Fovea – when you look at an object you move your eyes together so the target lands on the fovea of each eye o When the object is close you have to move the eyes quite a bit to position the object onto the back of the fovea in each eye o For an object that is further away you don’t have to close in the eyes as much (move the eyes)  E.g. cross-eye  Parallel lines  Only works in near space as you don’t have to move the eye much to meet a central target for a further away object (lines are already parallel – cannot determine how far away an object is)  Depth perception absolute  D = (e/2)* tan(b) (Secondary) Pictorial depth cues o How the perceptual system figures out how big something is or how far away it is based on perspective and Euclidian geometry o Artists use this to portray depth in art o Near and far space (used mostly for far space which is more than an arms length away) o Pictorial depth cues are derived from inferences made from what out perceptual system knows about perspective and Euclidean relationships (what the perceptual system knows about geometry and linear perspective) o These are depth cues that can be created on a 2D canvas  Linear perspective  All parallel lines converge on the horizon  Things above the horizon converge downward  Things below the horizon converge upward  Horizon is defined by your eye height  Can tell if things are taller or smaller than you (size)  Vanishing point  Determining size  Horizon ratio 

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Things below the horizon more upward in your visual field as you move  Things above the horizon move downward  Things taller than you are above the horizon  Things shorter than you are below the horizon  A/B = constant  Can determine how far away something is o Can use the information of how tall an item is in respect to our eye height to determine how far away it is and thus how big it is. If we can calculate how big it is then we can determine how far away it is  Determines distance Texture gradient  Another way to determine size  Objects of equal size occlude the same amount of texture as their base  If you have a flat ground plane that has a consistent texture gradient, then you can use the relative proportion of the amount of texture that the object covers as its base to figure out how big it is Height in plane  Objects higher in one field of view that are on the ground are typically further away  If an object is further away and projects the same visual angle, it must be larger  If it has the same retinal image and is further away, it must be bigger  Determines size Familiar size  Through experience we know how big some things are, and we use that to interpret depth, but this is not always right 

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Field of View/ Binocularity Trade Off  Binocular overlap allows for the perception of depth, but this comes at a cost as the larger degree of overlap between the two eyes decreases the field of view o More overlap decreases the field of view o If there is only a small overlap between the eyes then you only get depth for the small area of overlap o The more overlap, the less you can see from each direction (more depth perception) o Little overlap = little depth perception  For prey animals, field of view is more important than depth perception  For predators, depth perception is more important than field of view  Trade off o The more overlap the better the depth perception, but the less field of view (the amount that you can see around your body)

o The less overlap you have, you have worse depth perception, but a better field of view o Different animals have different amount of overlaps  Prey vs predator  Prey need to all around them, especially behind them (they need a better field of view, so this compensates their depth perception)  Predator needs depth so they can see how far away their prey is (therefore they compensate their field of view)  More likely to need to chase the prey, so depth perception is more important Why do we have two eyes that close together?  We needed to have depth for tools and navigating through wooded environments  However, the need for binocular overlap significantly decreased our field of view  As a result, we had a small field of view and were being picked off by birds of prey and other apex predators  So this one hypothesis why humans are a social species  With lots of us looking in several directions, we could detect predators and retain ou...