The most direct examination of perceptual learning is provided by investigating the effects of practice. In so-called detection tasks the observer is required to detect the presence or absence of a selected stimulus. For example, effects of practice on visual acuity were studied by requiring observers to detect simple orientation (left or right) in a row of leaning letters; e.g., . Practice tended to lower acuity thresholds, defined as the lowest intensity of illumination at which each observer could detect the orientation. Or, observers were asked to say when they just could see that an approaching pair of parallel bars was double. With practice they continued to report seeing the narrow space between the bars at increasing distances. Such improvements suggest that sensitivity to simple (unidimensional) stimuli is not immutable, being modifiable through practice.

Improvement is not limited to simple variables. In one visual-search procedure, subjects scanned a long list of letters to find a single letter that appeared only once. Search time was reduced by a factor of 10 following extensive practice, after which 10 different letters could be detected as quickly as a single letter. Practice effects with complex targets also have been studied. In one experiment, two rows of figures were displayed on each trial, one with four simple outlines of geometrical figures, the other containing three complicated figures. Subjects were to guess or detect which one of the simple figures was concealed (embedded) in all three of the complex figures. Again, ability to identify the correct simple figure improved with practice.

Tasks involving absolute judgment require much more of the observer than does simple or complex detection. For example, he may be asked to estimate the diameters of circular targets numerically (e.g., in inches or centimetres). In a similar study, two groups of subjects made absolute judgments of widely varying distances outdoors, both before and after interpolated activity. One group spent the interpolated period estimating a large number of other distances, none the same as in the original series. The other group spent the interval on unrelated paper-and-pencil work. In the first (extra-practice) group, judgments became more accurate and less variable than among the pencil-and-paper workers. Increased precision following practice also has been reported for absolute judgments of odour intensities, and of multidimensional visual (colour) and auditory stimuli. Improvement with practice is observed even when the subject remains uninformed of his accuracy; correcting him seems to confer slight benefit.

Many studies have failed to establish a clear basis for observed improvements in altered perceptual sensitivity or discriminability. For example, better performance on an acuity test may result from adopting a new criterion of visual doubleness or from learning how to use characteristics of blur to infer slant among leaning Es. Such uncertainties cloud the theoretical and practical significance of much available data.

U.S. psychologist William James (1842–1910) probably introduced the notion that practice in labelling stimuli can alter their discriminability. Indeed, sometimes vague visual forms that are distinctively named are easier to discriminate (acquired distinctiveness). If several such stimuli have the same verbal label, discriminability may be reduced (acquired equivalence).

Labelling effects in the laboratory have been discouragingly fragile, however, and factors that favour them are poorly understood. Perhaps labelling affects one’s efforts to discover distinguishing characteristics of stimuli. Having him learn distinctive labels may encourage him to analyze sensory features more fully. Or it may be that he begins to perceive a compound stimulus that includes the visual form and its associated label. If labels differ, the presumed compound stimuli are different, and discrimination should be enhanced. These hypotheses express both the discovery and enrichment theses.

Effects of perceptual assumptions

According to one version of the enrichment thesis, exposure to recurrent regularities among stimuli prompts one to assume specific relationships between the environment and his sensory experience. For example, one learns that a continuous sequence of projective transformation (e.g., the circular profile of a dinner plate seems to become elliptical) is associated with changing positions of the object in view, or that continuous symmetrical expansion of the retinal image is associated with approach. In addition, one presumably learns to make assumptions about what is called reality; e.g., despite alterations in retinal image, one perceives the plate to stay the same size. Psychologists Adelbert Ames, Jr., and Egon Brunswik proposed that one perceives under the strong influence of his learned assumptions and inferences, these providing a context for evaluating sensory data (inputs). In keeping with enrichment theory, Brunswik and Ames contended that sensory stimuli alone inherently lack some of the information needed for mature, adaptive perceiving; enrichment was held necessary to reduce ambiguity.

Much of the evidence for the contention that all perceiving is modified by one’s assumptions comes from investigations in which most of the visual, everyday stimuli are eliminated. Often, the subject may view an isolated target in total darkness or look at a motionless display while keeping his head steady. To show that learned assumptions about physical size affect perceived distance, the observer may be asked to judge how far he is from a rectangle of light displayed against total darkness. He is told at one time that the rectangle is a calling card; at another it is called a business envelope. His assumptions about these objects in relation to the size of his retinal image are invoked as prompting him to say that the “envelope” looks more distant than does the “calling card.” Dramatic examples of this effect were invented by Ames, including his famous distorted room (see Figure 3).

Ames held that perceiving under unusual conditions (e.g., in a dark room) follows the same principles that govern more ordinary experience. The special conditions are said to permit experimental scrutiny of the same processes that are so difficult to examine under ordinary, uncontrolled conditions.

An opposing view is that such perceptual assumptions and inferences operate only under specific experimental conditions. It is asserted that only when commonly available sources of information are eliminated is the subject forced to rely on assumptions.

In the tradition of Helmholtz, Ames and Brunswik seemed to liken perceiving to reasoning, although not as a conscious process. They held that perceptual assumptions, once established, are influenced only slightly by logic. Although the floor and ceiling of the distorted room are sloped and all windows are of different size, it projects the same retinal pattern as a normal room; and a naıve subject will report that he sees an ordinary room. But even after he explores the room he remains likely to say it looks rectangular as before, despite his new information. Comparable observations have been reported for a variety of situations. Familiarization or instruction seems to have little effect on long-established perceptual assumptions.

Psychoanalytic theory explicitly calls for motivational influences on such functions as memory, thinking, and perceiving. In particular, the theory is concerned with unconscious motives and conflicts and with unconscious defenses (such as repression) used to control them. According to the psychoanalytic hypothesis, there should be wide perceptual variation among individuals in response to stimuli that have motivational significance. At any rate, a host of experiments have been designed to show that perceiving is indeed subject to unconscious influences.

In some studies, for example, it seemed that so-called obscene words flashed on a screen had to be exposed longer than apparently neutral (control) words before their meaning could be perceived. In the other studies, children of poor families have been found to overestimate the size of coins as compared with the judgments of children of richer families. One major problem with such research lies in finding or creating appropriate experimental and control stimuli. Considering differences in the use of language, for example, it is most unlikely that what once were widely called obscene words would currently evoke the conflicts and defenses of more than a few subjects.

Assuming suitable stimuli can be found, an even more serious problem arises around the interpretation of the subjects’ behaviour; for example, do people really find it more difficult to recognize obscene words or are they simply reluctant to admit recognition? Problems of this sort have plagued researchers, and unambiguously interpretable experiments in this field are most difficult to produce. The hypothesis of such individual influences as motivation on perception remains appealing and viable, but unproved.