Chapter 1: Sensation and Perception

Overview of this Lecture Why study perception? What are the steps in the perceptual process How is perception measured?

Understanding how you perceive the world Why Study Perception? Understanding how you perceive the world Language processing Color vision Multitasking and attention Mindfulness Rehabilitation Counseling Devices to assist people with vision and hearing losses

Your friend Ellen is helping you collect bugs for your science class. Walk in the woods searching for bugs. If you don’t like bugs, imagine Ellen is helping you shop for the latest, coolest sneakers. Science project

Figure 1.2 (a) We take the woods as the starting point for our description of the perceptual process. Everything in the woods is the environmental stimulus. (b) Ellen focuses on the moth, which becomes the attended stimulus. (c) An image of the moth is formed on Ellen’s retina.

The Perceptual Process Stimulus All objects in the environment are available to the observer. Observer selectively attends to objects. Stimulus impinges on receptors resulting in internal representation.

The Perceptual Process - continued Electricity Transduction occurs which changes environmental energy to nerve impulses Transmission occurs when signals from the receptors travel to the brain. Processing occurs during interactions among neurons in the brain.

Figure 1.3 (a) Transduction occurs when the receptors create electrical energy in response to the light. (b) Tranmission occurs as one neuron activates the next one. (c) This electrical energy is processed through networks of neurons.

Figure 1.4 Comparison of signal transmission by cell phone and the nervous system. (a) Cell phone #1 sends an electrical signal that stands for “hello.” The signal that reaches cell phone #2 is the same as the signal sent from cell phone #1. (b) The nervous system sends electrical signals that stand for the moth. The nervous system processes these electrical signals, so the signal responsible for perceiving the moth is different than the original signal sent from the eye.

Figure 1. 5 (a) Ellen has conscious perception of the moth Figure 1.5 (a) Ellen has conscious perception of the moth. (b) She recognizes the moth. (c) She takes action by walking toward the tree to get a better view.

The Perceptual Process Experience and Action Perception occurs as a conscious experience. Recognition occurs when an object is placed in a category giving it meaning. Action occurs when the perceiver initiates motor activity in response to recognition.

Figure 1. 1 The perceptual process Figure 1.1 The perceptual process. The steps in this process are arranged in a circle to emphasize that the process is dynamic and continually changing. See text for description of each step in process.

Knowledge Information that the perceiver brings to the situation. Knowledge can affect a number of steps in the perceptual process. Experience helps you better identify stimuli. Example: Police knowing how to identify evidence. Knowledge

Demonstration Take a look at the next two slides. How do you identify the stimulus? Demonstration

Look at this drawing first, then close your eyes.

Did you see a “rat” or a “man”?

Demonstration 2 Take a look at the next two slides. How do you identify the stimulus? Demonstration 2

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Knowledge What the perceiver brings to the situation. Experience and/or expectation Is perception bottom-up or top-down Knowledge

Two Interacting Aspects of Perception Bottom-up processing Processing based on incoming stimuli from the environment Also called data-based processing Top-down processing Processing based on the perceiver’s previous knowledge (cognitive factors) Also called knowledge-based processing

Perception is determined by an interaction between bottom-up processing, which starts with the image of the receptors, and top-down processing, which brings the observer’s knowledge into play. Both types interact

the image of the moth on Ellen’s retina initiates bottom-up processing; and (b) her prior knowledge of moths contributes to top- down processing.

How do we study perception? Depends on the tools we have available. Early psychologist could only present stimuli and ask subject to respond. Ex. Rat/man demonstrations Psychophysical approach (PP) the stimulus-perception relationship What you see is what you get. How do we study perception?

PP: Psychophysical Approach Present stimulus and get subjects response of perception PH1: Physiological Approach 1 Present stimulus and measure neurons firing (brain response). PH2: Physiological Approach 2 Present stimulus and measure both brain response and subjects response. .

Physiological Response How does the brain respond to visual stimuli? Visual cortex in back of brain. Implant electrodes in visual cortex of cat. Measure response from cells. Nobel prize in 1981 Hubel and Wiesel PH1 in Goldstein book Physiological Response

Newest techniques allow us to scan the brains of awake, human subjects. Subject can tell us what they perceive. Brain scans show us which areas are most active. Assume those areas are important for perception. PH2 in book Brain scans

(PP) links stimuli and perception, (PH1) links stimuli and physiological processes and the (PH2) links physiological processes and perception.

Measuring Perception Begin with psychophysical approaches. First methods available to psychologists. Don’t require brain implant or scanners. Measuring Perception

Psychophysics - Overview of Methods of Measurement Qualitative Methods (identify and sort) Describing Recognizing Quantitative Methods (ratio scales) Detecting Perceiving Magnitude Searching

Qualitative Methods of Psychophysical Measurement Description Indicating characteristics of a stimulus First step in studying perception Called phenomenological method Recognition Placing a stimulus in a category by identifying it Categorization of stimuli Used to test patients with brain damage

Description

Wisconsin Card Sorting Test Where do you sort the top card on deck?

Quantitative Methods - Classical Psychophysics Absolute threshold - smallest amount of energy needed to detect a stimulus Method of limits Stimuli of different intensities presented in ascending and descending order Observer responds to whether she perceived the stimulus Cross-over point is the threshold Hearing Test

Figure 1.12 The results of an experiment to determine the threshold using the method of limits. The dashed lines indicate the crossover point for each sequence of stimuli. The threshold - the average of the crossover values - is 98.5 in this experiment.

Classical Psychophysics - continued Absolute threshold (cont.) Method of adjustment Stimulus intensity is adjusted continuously until observer detects it Repeated trials averaged for threshold

Classical Psychophysics - continued Absolute threshold (cont.) Method of constant stimuli Five to nine stimuli of different intensities are presented in random order Multiple trials are presented Threshold is the intensity that results in detection in 50% of trials.

Figure 1.13 Results of a hypothetical experiment in which the threshold for seeing a light is measured by the method of constant stimuli. The threshold - the intensity at which the light is seen on half of its presentations - is 180 in this experiment.

Is There An Absolute Threshold? There are differences in response criteria among participants Liberal responder - responds yes if there is the slightest possibility of experiencing the stimulus Conservative responder - responds yes only if he or she is sure that a stimulus was present Each person has a different response criterion but the sensitivity level for both of them may be the same Signal detection theory is used to take individual’s response criteria into account.

Figure 1.17 Data from experiments in which the threshold for seeing a light is determined for Julie (green points) and Regina (red points) by means of the method of constant stimuli. These data indicate that Julie’s threshold is lower than Regina’s. But is Julie really more sensitive to the light than Regina, or does she just appear to be more sensitive because she is a more liberal responder?

Quantitative Methods - Modern Psychophysics Magnitude estimation (scaling) Stimuli are above threshold. Observer is given a standard stimulus and a value for its intensity. Observer compares the standard stimulus to test stimuli by assigning numbers relative to the standard.

Modern Psychophysics - continued Magnitude estimation (cont.) Response compression As intensity increases, the perceived magnitude increases more slowly than the intensity. Response expansion As intensity increases, the perceived magnitude increases more quickly than the intensity.

Figure 1.15 The relationship between perceived magnitude and stimulus intensity for electric shock, line length, and brightness. (Adapted from Stevens, 1962.)

Other Measurement Methods Searching for stimuli Visual search - observers look for one stimulus in a set of many stimuli Reaction time (RT) - time from presentation of stimulus to observer’s response is measured

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