Academic journal article Memory & Cognition

Effects of Number of Items and Visual Display Variability on Same-Different Discrimination Behavior

Academic journal article Memory & Cognition

Effects of Number of Items and Visual Display Variability on Same-Different Discrimination Behavior

Article excerpt

We explored college students' discrimination of complex visual stimuli that involved multiple-item displays. The items in each of the displays could be all the same, all different, or diverse mixtures of some same and some different items. The participants had to learn which of two arbitrary responses was correct for each of the displays without being told about the sameness or differentness of the stimuli. We observed a general improvement in discrimination performance-a rise in choice accuracy and a fall in reaction time-as the number of icons in the display was increased, even when the participants had been trained from the outset with displays containing different numbers of items and when smaller numbers of items were not randomly distributed but grouped in the center of the display. The participants' discrimination behavior also depended on the mixture of same and different items in the displays. Striking individual differences in the participants' discrimination behavior disclosed that people sometimes respond as do pigeons and baboons trained with a similar task. This and previous related research suggest that variability discrimination may lie at the root of same-different categorization behavior.

In daily life, people are confronted with many decisions about whether multiple events are the same as or different from one another. We buy a shirt styled differently from those we already own, yet we purchase the same brand of tomato sauce. We order a meal in a restaurant differing from yesterday's breakfast, yet we read the same morning newspaper. Despite the ubiquity of same-different discrimination in adaptive action, the psychological science of such discriminations is still in its infancy (Wasserman, Young, & Cook, 2004).

Some situations afford easier or harder same-different discriminations. If we were asked whether two or more items are the same as or different from one another, we might imagine that a report of "different" would more readily be prompted by the collection AB than by the collection ABCDEFGH; after all, less information processing and fewer memory resources should be required when the number of nonidentical items is small than when the number is large. Indeed, people make more errors and respond more slowly when enumerating large collections of items than when enumerating small collections of items (Kaufman, Lord, Reese, & Volkmann, 1949; Taves, 1941; Trick & Pylyshyn, 1994). It therefore seems reasonable to believe that if we were to judge the sameness or differentness of a set of items, this judgment would be easier with a small set than with a large set. But as we will soon see, this reasonable belief may not always accord with the behavioral evidence.

Comparative Psychology of Same-Different Discrimination

Humans are not alone in having to inspect and judge whether sets of stimuli are the same as or different from one another; nonhuman animals too must be able to discriminate between same and different stimulus sets. Pigeons, for example, might readily take flight with a flock of birds from the same species but be cautious and refrain from taking wing with a flock of birds from different species.

Early efforts to teach pigeons to report whether two visual items-the smallest number that is needed to make a same-different discrimination-are the "same" as or "different" from one another (e.g., Edwards, Jagielo, & Zentall, 1983; Santiago & Wright, 1984; Wright, Santiago, Sands, Kendrick, & Cook, 1985; Wright, Santiago, Urcuioli, & Sands, 1983) were not especially successful. This observation prompted some theorists to conclude that a same-different discrimination requires a relational judgment that is beyond the ken of pigeons and other nonhuman animals (Mackintosh, 2000). But it is also possible, and perhaps even easier, for organisms to make same-different discriminations with sets of more than two items.

Over the past 10 years, research in our Iowa laboratory has been done to investigate the pigeon's discrimination of visual arrays comprising multiple items, usually 16 in number (see Figure 1). …

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