Visual Perception

Visual Perception 1Sensation Stimuli from the outside world is detectedby sensory receptors found in out oursensory organs. Sensation is the stimulation of our sensoryorgans and the transmission of informationto the central nervous system.

2Selective AttentionStimulation of the sense organs is automaticMultiple sense organs are being stimulated at any timeWe only focus on select stimuli at any one time- this is known as selective attention3Perception While the sense organs gather the information (sensation), perception is the name given to the process during which the brain organises and interprets the information from the sense organs. Perception givens meaning to the sensations. 4Sensation and PerceptionSensation is the same for everyone (except the visually impaired). Perception is not the same for everyone. List the following: Your favourite colourYour favourite vegetableYour favourite muscial instrumentYour favourite perfume/cologneThe temperature at which you like to eat soup

5Thresholds Absolute threshold- the smallest or weakest level of a stimulus required for detection Differential threshold- the smallest or weakest change in a stimulus that is required for the sensory system to detect a change

Note: The differential threshold is also known as the just noticeable difference (JND)6The Visual Perceptual System While there are 5 sensory systems we will focus only onthe visual perceptual system.

The visual sense organ (the eye)collects information in the form ofelectromagnetic radiation- light!7The electromagentic spectrumThe entire range of electromagnetic energy is known as theelectromagnetic spectrum. The human eye however can only detect a small portion of this known as the visible spectrum.

8Wavelength, frequency and amplitudeElectromagnetic energy travels in waves and therefore has different wavelengths, frequecies and amplitudes. We see different wavelengths/freuencies as different colours. We see different amplitudes as different intensities of light. 9Structures of the Human Eye

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11More on the lensCiliary muscles change the shape of the lensDistant objects = flat and elongated lensNear objects = round bulging lensRemember the lens aims to focus the image onto the retina.

12More on the lensThe lens also refracts (bends) light. This allows the image to converge (come together) on the retina. The light bends (refracts) as it passes through the bent surface of the lens. The light closest to the ends of the lens are bent the most while the light traveling through the middle is bent the least. The result is an upside-down image.

13More on the retinaThe receptor cells of the eye are found in the retina. They are known collectively as photoreceptors. Photo = lightThere are two types of photoreceptors: rods and cones. The area of the retina where the optic nerve connects to the retina is known as the blind spot because it is absent of any rods or cones. 14Find your blind spot

15RodsMore sensitive to light therefore assist vision at nightPresent in large numbers on outer regions are retina. Therefore they play a large role in peripheral vision. Perceive shades of greyApprox 120 million in number 16ConesSensitive to colour therefore involved in colour visionMost effective in bright lightConcentrated in the centre region of the retinaResponsible for visual acuity (high def)The fovea is a spot in the retina that contains only cones. It is on this spot that an image is focused to enable the best visual acuity (best detail). Approx 8 million in number17RodsNight and low light visionShades of greyFound on outside edge of retinaPlay a large role in peripheral vision More numerous overallConesWork best in bright lightColour visionFound in centre of retinaFovea used for high acuity; contains cones only 18Structure of the Retina

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20From eye to brain- the visual perceptual process

21The Visual Perceptual ProcessThe visual perceptual process is a sixstage process that describes how wesee and interpret visual stimuli. Whilethere are six stages, it is a continuousprocess22

23Visual Perceptual PrinciplesVisual perceptual principles guide the way in which visual information is organized, grouped together and given meaning- selection, organisation and interpretation phase of the visual perceptual process. They are like a set of instructions that enable us to organize and interpret visual information in a reliable and meaningful way. These principles are used involuntary and instantly by the brain allowing for efficient interpretation and understanding.

24Visual Perceptual PrinciplesThere are three broad categories of these principles:

Gestalt PrinciplesDepth perception principlesConstancies 25Gestalt PrinciplesSuggest that we group individual elements together to see the whole- we do not perceive the individual elementsThe whole is greater than the sum of its parts There are four Gestalt principles: Figure-groundClosureProximitySimilarity 26Figure- groundThe visual system simplifies a scene into a figure- the focus of out attention- and the ground which is everything else- the background. This helps us quickly identify the most important information in scene.

27ClosureWhen a figure is incomplete, ourvisual perceptual system willcomplete the picture so that we seea whole.

28ProximityObjects close to each other are perceived as belonging to the same group.

29SimilarityObjects that are similar in appearance are perceived as belonging to the same group.

30Gestalt Principles and Advertising

31Depth Perception PrinciplesRefers to out ability to judge distance- remember that the image projected on the retina is two dimensional but our world is three dimensional. We use cues in the from both our body and from our environment to judge how near or far an object is from usThere are two types of depth cues: Primary Depth CuesSecondary Depth Cues 32Primary Depth CuesInternal cues/signals provided by the body

There are three primary depth cues: Retinal disparityConvergenceAccomodation 331. Retinal Disparity Our eyes are positioned approximately 7 cm apart. Therefore each eye receives a different image. The brain merges the two images together and the amount of retinal disparity (difference) acts as a depth cue.

The more different the images, the closer the object. The more similar the images, the further away the objects are. Beyond 10 meters, retinal disparity does not act as a depth cue because the images are virtually the same.342. ConvergenceThe eyes generally turn in to focus an object on the retina. Information on the position of the eye and muscle tension acts as depth cues. If more inward turning of the eye, the closer the object. Convergence works for objects up to 7m from the eye. After this, the eyes are parallel.

353. AccomodationAccommodation refers to the lens changing shape to focus on an object. The muscles contract and the lens bulges to focus on near objects. The muscles relax and the lens elongates to focus on far objects. The information about the shape of the lens and the action of the muscles is sent to the brain where is assists depth perception

Accommodation is effective for objects up to 3m from the viewer.

Accomodation is the only monocular (one eye only) depth cue. 36Secondary Depth Cues

37Secondary Depth CuesSecondary depth cues are cues to depth that are provided by the environment.

There are five secondary depth cues: 1. Linear Perspective2. Interposition3. Texture Gradient4. Relative Size5. Height in the Visual Field 381. Linear PerspectiveTwo parallel lines merging together give the perception of depth.

392. InterpositionProvides a cue for depth when two or more obejcts overlap each other. The object behind is perceived as being further away

403. Texture GradientSurfaces that are less detailed are perceived as being further away than objects that include fine detail.

414. Relative Size An object that projects a large image on the retina is perceived to be closer than an object that casts a small image on the retina. Most effective with familiar objects

425. Height in the Visual Field Uses the horizon line as a cue for depth and distance. Objects drawn closer to the horizon are perceived as being more distant than objects further from the horizon.

43Constancy PrinciplesThe images projected onto the retina are dynamic and changing- as an object moves, the image on the retina changes Even though these images are changing we perceive the size, shape and orientation of an object to be constant using the constancy principles. 44Size ConstancyAn object has the same size regardless of the distance from which it is viewed.

We perceive the man in the background as being normal height even though his image is smaller. 45Shape ConstancyThe tendency to see an object as the same shape no matter what angle it is viewed from

The door remains a rectangle even though it is moved and viewed from other angles.46Orientation Constancy The tendency to visually perceive the true position of an object in the environment even though the retinal image of the object may be at a different orientation.

47The Perceptual SetPsychological factors influence how to perceive the world. These factors include past experience, context, culture, motivation and emotion. The way that we perceive and interpret events is subjective and personal. We have a readiness to interpret stimuli to meet our expectations. This is known as a perceptual set- a mental predisposition to interpret stimuli in an expected and predictable way.

How would you interpret this scene? You interpretation is influenced by your perceptual set.

48ProcessingA perceptual set is linked to top-down processing- observing the whole image first and applying existing knowledge to attribute meaning. When we proof read an assignment we may not notice simple spelling mistakes because we are using top-down processing. This is an efficient way of processing information but is not always accurate.

For more accurate interpretations, we need to use bottom-up processing. In bottom-up processing, the individual elements of the stimulus are analyzed and after this a decision is made regarding the nature of the stimuli. A developing child will use bottom-up processing to learn about the world around them. We use bottom-up processing to read a new word.49Past Experience Past experience refers to the situations and events we have encountered throughout our life prior to the present. Each individuals past experiences differ and this contributes to our individual uniqueness. Through experience we form ideas and expectations about the world. Past experiences led to the formation of a perceptual set and hence impact on our interpretation of the world. 50ContextThe context is the situation or setting in which we view a stimulus. The context in which we view a particular stimulus influences our interpretation of the stimulus. The context provides a cue to help your interpretation. With the aid of this cue we can make quicker evaluations and interpretations of information. However while correct on most occasions using cues based on context can lead to mistakes in interpretation. 51What could it be?Your answer would have beeninfluenced by your past experience on yourperceptual set

What could it be now? Your answer shows the influence of context on the perceptual set. 52Visual Illusions 53Muller-Lyer Illusion54Explanation of the Muller-Lyer Illusion- Apparent distance theory 55The Ames Room56Explanation of the Ames Room57