Brief Lecture Notes for Unit 4
Taken together, sensation and perception refer to the process of observing and processing information by means of the specific sense modalities. More than the traditional "five senses" exist; six general groupings are the visual, auditory, tactile, proprioceptive, olfactogustatory, and homeostatic.
Contemporary psychologists do not draw a sharp distinction between sensation (the taking in of individual inputs or sensory "building blocks") and perception (the integration, organization, or combination of those inputs into coherent perceptual wholes), even though there are obvious links neurologically between sensation and cortical projection areas, on the one hand, and between perception and cortical association areas, on the other hand (review Unit 3). Three reasons why:
1. In real life, neither sensation nor perception can occur in isolation (apart from the other process).
2. There is no clear-cut, unambiguous dividing line between the two processes, no easy way to say when one ends and the other begins.
3. The traditional model implies that all elements of sensation must end before perception begins, yet some processes that have perceptual elements (involve the combination and integration of information) actually occur outside the CNS itself, inside the transducers.
The nativist-empiricist controversy (review Unit 1) is relevant to the study of perception. Do we learn how to perceive the world (the empiricist view) or are we born knowing how (the nativist view)? More on this issue later in this unit.
For many (though not all) sense modalities, there is a distinction between the distal stimulus (the actual outward object or event in the outer world that is being perceived) and the proximal stimulus (the physical representation of that object or event at or within the transducers). For instance, if you are looking at a distant tree, the tree itself is the distal stimulus; the image of the tree at the back of the eye is the proximal stimulus. In such instances, obviously, both distal and proximal stimuli are necessary for the conscious experience of perceiving the object or event (the percept) to occur. (Based on these concepts, what are two ways to avoid seeing the tree?)
The study of the relationship between physical properties of a stimulus and psychological properties of the resulting percept is known as psychophysics. (Note that the mind-brain issues discussed in Unit 3 are implicated here as well.) Three key issues in psychophysics are stimulus detection, stimulus discrimination, and stimulus scaling.
Stimulus detection deals with the question of absolute threshold, that is, the lowest-intensity stimulus that can barely be detected or observed under normal conditions. (Note the link to the concept of threshold as discussed in Unit 3.) Since thresholds vary depending on circumstances, in practice we can define the absolute threshold as the stimulus intensity that can be detected 50% of the time in repeated trials, or in an even more complex fashion having to do with the ratio between "hits" and "false alarms" (see text for more details).
Stimulus discrimination deals with the question of difference threshold, that is, the smallest difference (e.g., in magnitude) between two stimuli that can just barely be noticed or detected. While, for a given sense modality, there is (subject to the limitations above) one and only one absolute threshold, there are an infinite number of possible difference thresholds, since the size of the difference threshold depends on the magnitude of the stimuli involved. Weber's law says that (approximately and within some limits) the ratio between the difference threshold and the magnitude of the less intense of the two stimuli being compared (the so-called standard stimulus) is a constant for any given sense modality; this ratio is known as the Weber fraction for that sense modality. Weber fractions allow us to compare the sensitivity of different sense modalities. Sense modalities with small (closer to zero) Weber fractions (e.g., vision, approximately 1/50) are highly sensitive to small stimulus differences; relatively speaking, sense modalities with large (closer to 1) Weber fractions (e.g., taste, approximately 1/4) are insensitive to small differences.
Stimulus scaling asks the question, "What amount of change in physical stimulus intensity is necessary to produce a given change in sensory experience?" (For instance, if I want the room to seem twice as bright to a reader, how much of an actual physical change in illumination must I generate to produce that psychological or subjective effect? In general, the answer is not "exactly twice as much light energy"; there is a nonlinear scaling relationship involved.) Stevens' power law mathematically expresses the nature of the scaling relationship. Sensory modalities (like vision) that require maximum responsiveness at low stimulus levels have a low power coefficient; those (like the sense of pain) that require maximum responsiveness at high stimulus levels have a high power coefficient (see graphs in text, on the board, and... if I maintain a healthy relationship with my scanner... on this Web page).
Three sensory phenomena of interest are adaptation, contrast, and constancy. Adaptation refers to changes in the sensitivity of transducers as a result of recent experience, such as your eyes' ability to adjust to the dark. In contrast phenomena, the perceived quality of a stimulus is changed by other surrounding (or recent) stimuli; for instance, lemonade tastes more sour if you drink it right after eating a candy bar. In constancy phenomena, irrelevant changes in the proximal stimulus are ignored; for instance, in size constancy, you correctly can perceive the size of objects at different distances, even though the size of the image on the back of your eye changes as the object moves further away from you.
Sensory mixture can be either intramodal (within a given sense modality) or intermodal (between modalities). The former can be either composite, in which the individual elements are lost (e.g., the visual mixture of different colors of light), or separable, in which they are not (e.g., the notes of a musical chord). In intermodal mixture, some sense modalities (e.g., vision) tend to dominate others (e.g., the vestibular sense).
A key concept in perceptual patterning is acuity, or the ability to perceive two separate stimuli (separated either in space or in time) as being distinct and separate. Acuity is limited by the fact that the brain tends to fill in gaps in perceptual patterns, a fact known as interpolation.
Treisman's theory of perception involves two stages of processing. The first, feature detection, is an automatic, simultaneous, parallel process, while the second, feature integration, requires focused attention and is a stepwise, serial process. (More on this concept in Unit 7.)
Bower's classic experiment on size constancy illustrates difficulties in resolving the nativist-empiricist controversy. In this experiment, young infants were taught to turn their heads to a 1" cube, but not to cubes of other sizes, held at a constant distance of 1' away, using methods to be discussed in Unit 6. Then, the infants were shown cubes of various sizes at a new distance of 3' away. Nativists would predict that (because size constancy is built-in or innate) the babies would correctly turn their heads to the 1" cube regardless of the distance at which it was held from them. Empiricists, believing that size constancy must be learned or acquired through experience, would predict that the babies would incorrectly judge the size of objects on the basis of the proximal stimulus, which would mean that they would turn their heads to a 3" cube held 3' away (do you see why?) The nativist prediction is generaly confirmed, but an empiricist counterargument is that, because newborn infants cannot be used in the study, size constancy may be learned... just very quickly, efficiently, very early in life.
Study Guide
1. What is a sensory modality? Name six major sensory modalities.
2. What are three reasons why contemporary psychologists do not draw a sharp distinction between sensation and perception? Explain.
3. What is the difference between a distal and a proximal stimulus? How is this distinction related to the notion of perceptual constancy?
4. What is psychophysics? How do detection, discrimination, and scaling differ? How do absolute and difference thresholds differ? What is a Weber fraction? Be able to compute Weber fractions in a "story problem" type context.
5. How do adaptation, contrast, and constancy phenomena differ? Give examples of each.
6. Compare and contrast different forms of sensory mixture.
7. How are acuity and interpolation related?
8. Explain, and provide evidence for, Treisman's two-stage theory of perception. How do serial and parallel processing differ?
9. Summarize and critique Bower's experiment, and relate it to the nativist-empiricist controversy in perception.