Chapter 1: Introduction to Perception

Chapter 1: Introduction to Perception

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.

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.

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 - 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.

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 1.5 (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.

Figure 1.6 Look at this drawing first, then close your eyes and turn the page, so you are looking at the same place on the page directly under this one. Then open and shut your eyes rapidly. (Adapted Bugelski & D. Alampay, 1961.)

Figure 1.9 Did you see a “rat” or a “man”? Looking at the more ratlike picture in Figure 1.11 increased the changes that you would see this one as a rat. But if you had first seen the man version (Figure 1.8), you would have been more likely to perceive this figure as a man. (Adapted Bugelski & D. Alampay, 1961.)

Figure 1.11 Man version of the rat-man stimulus. (Adapted Bugelski & D. Alampay, 1961.)

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

Figure 1.7 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. In this example, (a) 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.

Psychophysics - Overview of Methods of Measurement • Qualitative Methods • Describing • Recognizing • Quantitative Methods • 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

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

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.

Classical Psychophysics - continued • Difference Threshold (DL) - smallest difference between two stimuli a person can detect • Same methods can be used as for absolute threshold • As magnitude of stimulus increases, so does DL • Weber’s Law explains this relationship • DL / S = K

Figure 1.14 The difference threshold (DL). (a) The person can detect the difference between a 100-gram standard weight and a 102-gram weight but cannot detect a smaller difference, so the DL is 2 grams. With a 200-gram standard weight, the comparison weight must be 204 grams before the person can detect the difference, so the DL is 4 grams. The Weber fraction, which is the ratio of DL to the weight of the standard is constant.

Table 1.1 Weber fractions for a number of different sensory dimensions

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.)

Quantitative Methods - continued • Magnitude estimation (cont.) • Relationship between intensity and perceived magnitude is a power function • Steven’s Power Law • P = KSn

Figure 1.16 The three functions from Figure 1.15 plotted on log-log coordinates. Taking the logarithm of the magnitude estimates and the logarithm of the stimulus intensity turns the functions into straight lines. (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

Chapter 1: Introduction to Perception