Visual Perception

Sensation Stimuli from the outside world is detected by sensory receptors found in out our sensory organs. Sensation is the stimulation of our sensory organs and the transmission of information to the central nervous system.

Selective Attention Stimulation of the sense organs is automatic Multiple sense organs are being stimulated at any time We only focus on select stimuli at any one time- this is known as selective attention

Perception 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.

Sensation and Perception Sensation is the same for everyone (except the visually impaired). Perception is not the same for everyone. List the following: 1.Your favourite colour 2.Your favourite vegetable 3.Your favourite muscial instrument 4.Your favourite perfume/cologne 5.The temperature at which you like to eat soup

Thresholds 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)

The Visual Perceptual System While there are 5 sensory systems we will focus only on the visual perceptual system. The visual sense organ (the eye) collects information in the form of electromagnetic radiation- light!

The electromagentic spectrum The entire range of electromagnetic energy is known as the electromagnetic spectrum. The human eye however can only detect a small portion of this known as the visible spectrum.

Wavelength, frequency and amplitude Electromagnetic 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.

Structures of the Human Eye

More on the lens… Ciliary muscles change the shape of the lens Distant objects = flat and elongated lens Near objects = round bulging lens Remember the lens aims to focus the image onto the retina.

More on the lens… The 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.

More on the retina… The receptor cells of the eye are found in the retina. They are known collectively as photoreceptors. Photo = light There 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.

Find your blind spot

Rods More sensitive to light therefore assist vision at night Present in large numbers on outer regions are retina. Therefore they play a large role in peripheral vision. Perceive shades of grey Approx 120 million in number

Cones Sensitive to colour therefore involved in colour vision Most effective in bright light Concentrated in the centre region of the retina Responsible 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 number

Rods Night and low light vision Shades of grey Found on outside edge of retina Play a large role in peripheral vision More numerous overall Cones Work best in bright light Colour vision Found in centre of retina Fovea used for high acuity; contains cones only

Structure of the Retina

From eye to brain- the visual perceptual process

The Visual Perceptual Process The visual perceptual process is a six stage process that describes how we see and interpret visual stimuli. While there are six stages, it is a continuous process

Visual Perceptual Principles Visual 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.

Visual Perceptual Principles There are three broad categories of these principles:  Gestalt Principles  Depth perception principles  Constancies

Gestalt Principles Suggest that we group individual elements together to see the whole- we do not perceive the individual elements The whole is greater than the sum of its parts There are four Gestalt principles: 1.Figure-ground 2.Closure 3.Proximity 4.Similarity

Figure- ground The 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.

Closure When a figure is incomplete, our visual perceptual system will complete the picture so that we see a whole.

Proximity Objects close to each other are perceived as belonging to the same group.

Similarity Objects that are similar in appearance are perceived as belonging to the same group.

Gestalt Principles and Advertising

Depth Perception Principles Refers 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 us There are two types of depth cues: 1.Primary Depth Cues 2.Secondary Depth Cues

Primary Depth Cues Internal cues/signals provided by the body There are three primary depth cues: 1.Retinal disparity 2.Convergence 3.Accomodation

1. 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.

2. Convergence The 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.

3. Accomodation Accommodation 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.

Secondary Depth Cues

Secondary depth cues are cues to depth that are provided by the environment. There are five secondary depth cues: 1. Linear Perspective 2. Interposition 3. Texture Gradient 4. Relative Size 5. Height in the Visual Field

1. Linear Perspective Two parallel lines merging together give the perception of depth.

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

3. Texture Gradient Surfaces that are less detailed are perceived as being further away than objects that include fine detail.

4. 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

5. 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.

Constancy Principles The 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.

Size Constancy An 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.

Shape Constancy The 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.

Orientation 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.

The Perceptual Set Psychological 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.

Processing A 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.

Past Experience Past experience refers to the situations and events we have encountered throughout our life prior to the present. Each individual’s 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.

Context The 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.

What could it be? Your answer would have been influenced by your past experience on your perceptual set What could it be now? Your answer shows the influence of context on the perceptual set.

Visual Illusions

Muller-Lyer Illusion

Explanation of the Muller-Lyer Illusion- Apparent distance theory

The Ames Room

Explanation of the Ames Room