Week 5 Summary - Brain and Behaviour PDF

WEEK 5: Perception Week 5 Objective 1: Explain what psychophysics is and define, with examples, these key terms: • • • thresholds—absolute threshold; just noticeable difference; magnitude methods—Fechner’s 'classical' methods; signal detection theory laws—Weber’s law, Fechner’s laws, Stevens’s la...

WEEK 5: Perception Week 5 Objective 1: Explain what psychophysics is and define, with examples, these key terms: • • • thresholds—absolute threshold; just noticeable difference; magnitude methods—Fechner’s 'classical' methods; signal detection theory laws—Weber’s law, Fechner’s laws, Stevens’s law Psychophysics describes the relationship between the physical energy in the environment and the psychological experience of that energy. The absolute threshold is the smallest amount of physical energy a sensory system can detect. The smallest difference between stimuli that we can detect is called the difference threshold, or the just-noticeable difference (JND). The weaker the stimuli, the easier it is to detect small differences between them. For example, it is easier to detect a weight difference between two light envelopes than between two similarly heavy boxes. According to Weber’s law, the smallest detectable difference in stimulus energy is a constant fraction of the intensity of the stimulus. JND = KI, where K is the constant fraction and I is the intensity. • • • There are separate Ks for different types of sensory input. The smaller the value of K, the more sensitive a sense is to a stimulus difference. Weber’s constants vary among people and, in general, we tend to become less sensitive to stimulus differences the older we get. Weber’s law does not hold when stimuli are very intense or very weak. It does hold for simple and complex stimuli. Magnitude estimation is how our perception of stimulus intensity is related to the actual strength of the stimulus. The perception of magnitude is not absolute but relative. Our experience of one stimulus depends on its relationship to others. • Fechner’s law says that constant increases in physical energy will produce smaller increases in perceived magnitude. For example, it only takes a small increase in volume to make a soft sound seem twice as loud, but it will take an incredible increase in volume to make a rock band seem twice as loud. This assumption applies to most, but not all stimuli. • Stevens’s power law for magnitude estimation works for a wider array of stimuli. It includes a factor that takes into account the differential sensitivity of various sensory systems. Signal detection theory is a mathematical model of how a person’s sensitivity and response criterion combine to determine decisions about whether a near-threshold stimulus has occurred. For example, researchers may manipulate the response criterion by altering expectations. If a person is more expectant of a signal, his or her response criterion will lower – he or she will say that a signal has occurred more readily. Each response to a series of signal variations is placed into one of four categories, and the resulting pattern of responses is analysed. • • • • When no signal is presented, but the participant decides that there was a signal anyway, the error is called a false alarm. When a signal occurs but it is not detected, the error is called a miss. When a signal occurs and the participant detects it, the response is called a hit. Not reporting a signal when none was given is called a correct rejection. Week 5 Objective 2: Outline the key features of the constructivist, computational and ecological approaches to perception. 1. The computational model tries to determine the computations that a computer would have to perform to solve perceptual problems to explain how complex computations of the nervous system turn raw sensory stimulation into a representation of the world. The computational approach focuses on the nervous system’s manipulations of incoming signals. 2. The constructivist approach argues that perceptual systems construct a representation of reality from fragments of sensory information. This representation is strongly influenced by learning, expectations, and inferences from past experiences, including culture. 3. According to the ecological approach, most perceptual experience comes directly from the environment rather than from interpretations or expectations. Stimuli directly give most of the information needed to make sense of the world. Week 5 Objective 3: Explain what is meant by perceptual organisation and give examples of figure-ground separation and the Gestalt grouping principles. Perceptual organisation is the task performed by the perceptual system to determine what stimuli go together to form an object. Your perceptual system can organise unconnected elements into objects by creating imaginary connecting lines called subjective contours. Figure-ground Organisation. When faced with complex stimuli, perceptual systems automatically pick out certain features, objects, or sounds to emphasise. This is called figure-ground discrimination. Figure is the emphasised features, and ground is the less meaningful background. • The figure has meaning, stands in front of the rest, and always seems to include the contours or edges that separate it from the less relevant background. • Drawings that can be reversed between figure and ground, reversible figures, show that perception is not only an active process, but also a categorical one. Grouping Perceptual systems group certain elements in the environment together, more or less automatically. Gestalt psychologistsargue that people perceive sights and sounds as organised wholes. Gestalt principles that describe these grouping tendencies are: • • • Proximity: the closer the objects or events are to one another, the more likely they are to be perceived as belonging together. Similarity: similar elements are perceived to be part of a group. Continuity: sensations that appear to create a continuous form are perceived as belonging together. • • • • Closure: people tend to fill in missing contours to form a complete object. Texture: stimuli that have the same texture (for example, oriented along the same directions) tend to be grouped together. Simplicity: people group stimuli to provide the simplest interpretation of the world. Common fate: objects that are moving in the same direction at the same speed are perceived as a group. Week 5 Objective 4: Explain how monocular, binocular and oculomotor cues (information) are used to gauge the distance to feature or object. • • • In ocular accommodation, the muscles surrounding the lens either tighten (to make the lens more curved for focusing on close objects) or relax (to flatten the lens for focusing on more distant objects). Information from the muscles is relayed to the brain and helps create distance perception. Eye convergence involves each eye rotating inward to project the image of an object on each retina. Information about the rotation goes to the brain and the greater the rotation, the closer the object is perceived to be. Retinal disparity (binocular disparity), the difference between the two retinal images of an object (one from each eye), provides distance cues. This difference decreases with increasing distances. Depth can be created by showing each eye a separate photograph of a scene, each taken from a slightly different angle. Researchers suggest that seeing an object at some point in space creates a spatial model in our minds, a model that remains intact even when immediate depth cues are removed. Week 5 Objective 5: Describe basic processes in colour and motion perception. Perception of motion uses cues that make use of optical flow, or the changes in retinal images across the entire visual field. Looming, a rapid expansion in the size of an image so that it fills the retina, is automatically perceived as an approaching stimulus and not an expanding object. If the brain determines that movement of the eyes and head account for all of the movement of images on the retina, then the outside world is perceived as stable, not moving. Wavelengths and Colour Sensations The sensation produced by a mixture of different wavelengths of light is not the same as that produced by separate wavelengths. Most colours are a mixture of light of different wavelengths. The psychological dimensions of colour sensation roughly correspond to the physical properties of light. • • • Hue is the essential ‘colour’, determined by the dominant wavelength in the mixture of the light. Black, white, and grey are not considered hues because no wavelength predominates in them. Colour saturation is related to the purity of a colour. A colour is more saturated and more pure if a single wavelength is more intense than other wavelengths. Pastels are colours that have been desaturated by the addition of whiteness. Brightness refers to the overall intensity of all the wavelengths making up light. Week 5 Objective 6: Describe the concept of perceptual constancy and give examples of colour, size, shape, colour/brightness and constancy Perceptual Constancy Perceptual constancy is the perception of objects as constant in size, shape, colour, and other properties despite changes in their retinal image. Size Constancy According to the computational view, size constancy occurs as objects move closer or farther away because the brain perceives the change in distance and automatically adjusts the perception. The perceived size of an object is equal to the size of the retinal image multiplied by the perceived distance. Shape Constancy Due to shape constancy an object appears the same even though the shape of its retinal image changes. The brain automatically integrates information about retinal images and distance as movement occurs. Expectations about the shape of objects also play a role in shape constancy. Shape constancy can also inhibit our ability to detect changes to objects. Brightness Constancy Because of brightness constancy, no matter how the amount of light striking an object changes, its perceived brightness remains relatively constant. The brightness of an object is perceived in relation to its background. Week 5 Objective 7: Explain what is meant by top-down and bottom-up processing Two types of processing are involved in recognition: bottom-up processing and top-down processing. Top-down processing is guided by knowledge, expectations, and other psychological factors. Top-down Processing People use their knowledge in making inferences or ‘educated guesses’ to recognise objects, words, or melodies, especially when sensory information is vague or ambiguous. The context in which stimuli occur creates an expectancy, shaping later perceptions. Top-down processing often aids us by allowing identification to occur before processing of features is complete, or when features are missing or distorted, but it can also sometimes lead us to erroneous conclusions. Bottom-up processing relies on specific, detailed information elements from the sensory receptors that are integrated and assembled into a whole. Bottom-Up Processing Certain cells, feature detectors, respond to selected features of a stimulus. The stimulus is analysed into basic features before these features are recombined to create the perceptual experience. Features that are subject to separate analysis include orientation in space, colour, motion, and corners. In face recognition, not all features are equally weighted. We tend to rely on large-scale features, such as hair and head shape, to recognise people. Week 5 Objective 8: Explain the key processes involved in attention. Attention is the process of directing and focusing certain psychological resources to enhance perception, performance, and mental experience. Attention directs our sensory and perceptual systems toward certain stimuli, selects specific information for further processing, ignores or screens out unwanted stimuli, allocates the mental energy to do the processing, and regulates the flow of resources necessary for performing a task or coordinating several tasks at once.