Spreading Activation Within Semantic Categories - American ...

Journal ol Experimental Psychology: General 1975, Vol. 104, No. 3, 234-240

Spreading Activation Within Semantic Categories: Comments on Rosch's "Cognitive Representations of Semantic Categories" Elizabeth F. Loftus University of Washington In the letter-matching task of Posner and Mitchell (1967), a subject looks at a pair of letters and decides whether they are the same or different. The typical finding is that subjects take longer to respond to name-identical pairs such as Aa than to physically identical pairs such as AA. In a variation of this procedure, Beller (1971) presented his subjects one of the pairs of letters to be matched in advance and found that this advance information (or prime) shortened the reaction time for both physical and name matches. This result supports other data suggesting that when a subject is provided in advance with part of the information needed to make a timed response, his reaction time upon reception of the remainder will be reduced (cf. Cohen, 1969; LaBerge, Van Gelder, & Yellott, 1970). Rosch's (1975) procedure is a variation of Beller's in which subjects matched pairs of words or pictures instead of letters. In Experiments 2, 3, 5, 6, 7, and 8, Rosch's subjects were shown two category members (e.g., orange, pear) and their task was to press a same key if both members of the pair belonged to the same natural category and, if not, to press a different key. In some cases, the pair to be judged was preceded or accompanied by a priming stimulus. This priming stimulus was a category name, such as fruit, which informed the subject that at least one of the two stimuli he would see would be a fruit. In Experiments 4 and 5, physical identity instructions were used: The subject pressed a same key if the stimuli

were physically identical (e.g., orange, orange] and pressed the different key if not. Again, in some cases a prime category preceded or accompanied the stimulus to be judged. Several of the major results of this series of studies can be briefly summarized: (a) For physically identical pairs presented with category-same instructions, an advance prime such as fruit facilitated the reaction time to good (typical, common) category members such as orange, but slowed the reaction time to poor (atypical, uncommon) members such as watermelon (Experiments 2, 6, and 7). A simultaneous prime was ineffective (Experiment 3). (b) For same-category and different pairs presented with categorysame instructions, both an advance prime and a simultaneous prime facilitated the reaction time to all category members (Experiments 2, 3, and 6). (c) With physical identity instructions, priming was ineffective (Experiment 4). Several other results were evident (e.g., there were differences between words and pictures), but these will not be discussed. Following is a summary of the major conclusions that were drawn: 1. For physically identical pairs, a category prime contains information used in the perception or encoding of good members of the category, but the prime is sufficiently unlike the poor members so that it interferes with their perception. This conclusion is based on the following logic: The prime influenced the reaction time to physically identical pairs when presented in advance of, but not simultaneously with, the pairs. Thus, This paper benefited from many discussions with the representation generated by the prime colleagues. I particularly wish to thank A. Collins, had to affect the actual perceptual encoding W. Cole, and C. MacLeod. Requests for reprints should be sent to Elizabeth of the stimulus pair. Because the prime F. Loftus, Department of Psychology, University facilitated the reaction time to good memof Washington, Seattle, Washington 98105. bers, it must have contained some of the 234

SPREADING ACTIVATION WITHIN SEMANTIC CATEGORIES information used to perceive those members. Because the prime depressed the reaction time to poor members, it must have interfered with their perception. 2. For same-category and different pairs, a category prime facilitates decision processes that occur after the stimuli are available to the subject, rather than facilitating the encoding of the stimulus itself. The logic of this conclusion is that because the prime was equally effective when presented simultaneously as when presented in advance, it must be that the relevant information in the prime acts on decision processes regarding the stimulus after the stimulus is available. Based on her overall pattern of results (and on her theoretical biases), Rosch argued that a category prime generates a representation about which a great deal can be said. It is not a physical code or a list of category members; neither is it a list of features necessary and sufficient for an item to belong to the category. Rather, it is some relatively abstract meaningful structure that resembles good members of the category and not poor members. How, then, does the prime affect a subject's behavior in a same-different task such as the one used throughout Rosch's set of studies? Although no model is specifically proposed for the physically identical pairs, it is asserted that the category prime influences the encoding of the stimulus. A model is, however, specifically proposed for samecategory and different pairs. According to the model, the subject (a) encodes the stimulus pair, (b) retrieves the superordinate category name for each member of the pair, (c) decides whether the two category names are the same, and (d) responds same if they are the same, otherwise different. If no prime is presented, the subject performs Step B above by searching for a category name in semantic memory. If a prime is presented, the subject need not search for the category but rather simply verify whether each member of the stimulus pair is a member of the prime category. With the simple assumption that verification is faster than search, a uniformly beneficial effect of prim-

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ing is expected. Note that the prime is proposed to operate after the stimulus pair is presented, not on its encoding. Two problems with Rosch's research have led me to an alternative interpretation of her major findings. The first is conceptual: Why should a category prime influence the encoding of physically identical pairs (e.g., orange, orange} and not the encoding of other pairs (e.g., orange, pear} ? The second problem is methodological: Some of the instances that Rosch included in her studies are peculiar. In this article, I suggest that Rosch's major findings can be explained in terms of a spreading-activation theory of semantic processing and that new theorizing is unwarranted. I shall begin with a more critical examination of some of Rosch's categories and instances. All of Rosch's experiments used the same nine natural categories: fruit, bird, vehicle, vegetable, sport, (carpenter's) tool, toy, furniture, and weapon. Among other criteria, to be included in the research a category could not crosscut a large number of other taxonomic structures, as does the category of food. Rosch seems to realize that toy actually crosscuts other categories, but it is included nevertheless. (The reader can verify that toy did crosscut other categories by noting that over one third of the rated instances of toy also belonged to other semantic categories and in many cases are more commonly considered members of other categories. For example, car and truck, both rated as toys, are more commonly thought of as vehicles). Perhaps Rosch felt that, in her experiments, toy would not present a problem if the particular items chosen from that category were unequivocally toys both in name and picture. However, two of the toys actually used in her experiments, skates and tricycle, are about as typical in the vehicle as in the toy category. One of the sport instances used, jump rope, is more typical as a toy than a sport. There are also problems with at least two of the vegetable instances used: pumpkin and tomato. A pumpkin is more typically considered a fruit (perhaps through its association with pies), whereas a tomato technically is a fruit.

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These examples illustrate an important point: Many of the items used in Rosch's experiments belong to more than one of her categories and some of them were actually more typical of other categories. This can be expected to have important consequences for experiments in which these items are used. On trials which utilize these poor category members, a category prime may actually be priming the wrong set of exemplars. Thus, sport may be expected to facilitate the matching of football, football because football is a highly common, closely related member of the sport category. But sport may not facilitate jump rope, jump rope because a jump rope, to many subjects, is a toy. A SPREADING-ACTIVATION THEORY OF SEMANTIC PROCESSING A number of current theories of semantic memory regard it as a highly complex network of concept nodes, with one concept connected to another by a number of links (cf. Collins & Quillian, 1972). In several versions (e.g., Rumelhart, Lindsay, & Norman, 1972), the links in the network are not undifferentiated but represent a relation between two concepts, for example, a superordinate-subordinate relation. In at least one version (Quillian, 1967, 1968) a memory search is initiated as follows. In response to a stimulus the search begins at the node for each concept specified by the stimulus and involves tracing out in parallel along the links from these nodes. A spread of activation is thought to expand continuously, first to all of the nodes linked to the initial concept node, then to all of the nodes linked to each of these nodes, and so on. At each node reached in this process, an activation tag is left which specifies the starting node and the immediate predecessor. When a tag from another starting node is encountered, an intersection between the two nodes has been found. By following the tags back to both starting nodes, the path that led to the intersection can be reconstructed. When an intersection has finally been found, it is necessary only to evaluate

the path to decide if it satisfies the constraints imposed by the task. Priming involves the 'same tracing process that is involved in memory search. When a concept is primed, activation tags are spread by tracing an expanding set of links in the network to some unspecified depth. When another concept is subsequently presented, it has only to make contact with one of the tags left earlier to find an intersection. Collins and Loftus (Note 1) have extended the theory in part by adding additional processing assumptions, thereby enabling the theory to account for widely different empirical results. One such assumption is that when a concept is processed, activation spreads out along the paths of the network in a decreasing gradient. Thus, activation is like a signal from a source that becomes more attenuated over time and distance. A second assumption is that the more properties two concepts have in common, the more links there are between the nodes of these concepts and the more closely related are the concepts. It follows that a concept will prime a closely related concept to a greater extent than it will prime a concept that is less closely related. A final assumption is that recently activated concepts are more readily accessible than nonactivated concepts. If the total amount of activation is limited, then the activation of one concept by another closely related concept may make a third, distant concept temporarily less accessible. To illustrate these assumptions, suppose Figure 1 is a portion of a hypothetical memory structure. According to spreadingactivation theory, priming with sport would quickly activate football and baseball, with which it is closely related, and, to a lesser degree, would activate fishing. Whether it activated jump rope would depend on the closeness of sport and jump rope. For some subjects, the relationship may be sufficiently close that jump rope receives a modicum of activation from sport; however, for other subjects (especially those who regard a jump rope as a toy), jump rope may be sufficiently unrelated to sport and thus receive no acti-

SPREADING ACTIVATION WITHIN SEMANTIC CATEGORIES vation from it. In fact, if the activation of category members reduces the accessibility of the less good members, jump rope may become less accessible than it might have been had sport not been presented. SPREADING ACTIVATION AND THE SAME-DIFFERENT TASK The spreading-activation theory, with a few additional assumptions, can easily accommodate the experimental findings of Rosch. Consider a model for the sequential processes by which a subject performs the task of deciding whether two items belong to the same natural category. How does a subject decide that football, football or jump rope, hunting are from the same category? A model, which I shall refer to as the three-step model, is presented in Figure 2. As the name indicates, the model shows that three major steps are included. In Step 1 the subject encodes the stimulus pair. In Step 2 he determines whether the two members match physically. If they do, he executes a same response, otherwise he pro-

FIGURE 1. Schematic representations of concept relatedness in a hypothetical fragment of semantic memory. (Shorter lines represent closer relatedness.)

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STIMULUS PRESENTATION

Stepl

Step 2

Step3

ENCODE STIMULUS

{

DETERMINE WHETHER THERE IS A PHYSICAL MATCH

Yes

iNo RETRIEVE SUPERORDINATE CATEGORIES AND DETERMINE WHETHER THEY MATCH

FIGURE 2. A three-step model for same-different categorization tasks.

ceeds to Step 3. In Step 3 he retrieves the superordinate categories for each member of the stimulus and decides whether they match.1 If they match, a same response is made, and if they do not, a different response is executed. Consider how the three-step model works when the subject is presented with a physically identical stimulus pair in an experiment involving category-same instructions. How does the subject respond same to football, football? First he must encode the stimulus. Differences in the encoding times would be expected for primed and unprimed stimuli. Among stimuli that are unprimed, good members might be encoded faster than poor members, if for no other reason than they tend to be more frequent, have fewer letters, and so on. As for the stimuli that are primed, good primed members might be encoded even faster than the good unprimed members, because, according to spreading-activation theory, they would receive a good deal of activation from the category prime and would be more readily accessible. Further, the prime may produce a reduction in the accessibility of the poor members. This follows from the assumption that activation of some concepts reduces the accessibility of others. By this 1 In a variation of this model, iStep 3 begins during the execution of Step 2 and is simply not terminated if physical identity is detected. None of the critical predictions of the model would change under this assumption.

ELIZABETH F. LOFTUS

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..Primed Unprimed

Expected Encoding Time

I

High Low Level of Goodness of Stimulus FIGURE 3. Expected encoding time as a function of level of goodness of example for physically identical stimuli that are primed and unprimed.

argument, the prime may hinder the accessibility of poor members relative to those same members that are unprimed. Thus, if we plot expected encoding time as a function of level of goodness of the stimulus, we might expect the interaction shown in Figure 3. After the stimulus is encoded and a physical match is detected, the subject must still execute a same response. Assuming that response execution is not dependent on the type of stimulus, the interaction between priming and level of goodness evident in Figure 3 should be evident in the total reaction times for physically identical stimuli. Thus, the critical interaction is in evidence in the predictions from spreading-activation theory. Consider how the three-step model works when the subject is presented with two instances that do not match physically. The subject is presumed to proceed to Step 3. What occurs in Step 3 depends on whether the stimulus pair was preceded by a prime or not. If a prime was presented, the subject verifies whether the two instances are members of the prime category. If both members belong to the category, the subject executes a same response, otherwise he responds different. If a prime is not presented, the subject must essentially answer the question, Do these two instances have a superordinate in common? or Do these two instances belong to any one of the nine categories being presented in this experiment? He responds same if they do and different if they do not,

In the same-different task, differences in stimulus encoding times again would be expected for primed and unprimed stimuli. Among stimuli that are unprimed, good members might be encoded faster than poor members, for reasons stated earlier. Good primed members would be encoded faster than good unprimed members due to spreading activation from the prime, whereas poor primed members might be slowed by the activation from the prime. The plot of expected encoding time as a function of level of goodness for the same-category pairs is thus expected to look similar to the plot for physically identical pairs, which is shown in Figure 4a. After encoding, stimuli that do not match physically undergo a later step which is influenced by the presence or absence of a prime. If a prime is presented, the subject verifies whether the two instances are members of the prime category. The verification of a good category member is known to be faster than the verification of a poor category member

Expected Encoding Time

_. Primed • Unprimed

High Low Level of Goodness of Stimulus

Expected Encoding plus Retrieval Time High Low Level of Goodness of Stimulus FIGURE 4. Graph (a) shows expected encoding time as a function of levels of goodness of example for nonphysically identical stimuli that are primed and unprimed. Graph (b) shows expected times for encoding (Step 1) and superordinate retrieval (Step 3), for stimuli that are primed and unprimed.

SPREADING ACTIVATION WITHIN SEMANTIC CATEGORIES (Rips, Shoben, & Smith, 1973; Rosch, 1973). The verification of a poor category member must be a particularly difficult task when the category member more readily belongs to another category, as in the case of jump rope and sport. When a prime is not presented, the subject must produce a superordinate that the two instances have in common.2 This process can be assumed to take longer than the verification process and may be presumed to take even more time for poor category members. Thus, adding corresponding verification or production times to the encoding times in Figure 4a would raise both the primed and unprimed functions (raising the latter function somewhat more). This result is shown in Figure 4b. The beneficial effect of priming for both good and poor category members is evident in the predictions from spreadingactivation theory. In sum, the spreading-activation theory accounts for several of Rosch's important results that occur in experiments with category-same instructions. It accounts for the interaction between priming and level of goodness for physically identical stimuli, and it accounts for the consistent beneficial effect of priming for nonphysically identical stimuli. Whereas Rosch claims that priming influences only the encoding of physically identical stimuli and influences only the later stages for other stimuli, the present account assumes that priming influences the encoding of all types of stimuli.

Spreading Activation and Other Same-Different Results The simultaneous prime. A simultaneous prime was found to have no effect on physically identical pairs but to have the same effect as an advance prime for nonphysically identical pairs. If we assume that when the prime is presented simultaneously it does not affect stimulus encoding but still 2 Smith, Rips, and Shoben (1974) have proposed a reasonable model for how this process might occur. Suppose a subject is presented with the pair doll, truck. After encoding the pair, he retrieves the superordinate name for the first member, determining that a doll is a toy. Next he confirms that a truck is a toy.

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affects the later stages, this result is consistent with the theoretical notions offered in this article. For physically identical pairs, an advance prime is thought to influence only encoding, and thus a simultaneous prime would not be expected to be effective. For nonphysically identical pairs, an advance prime is thought to influence both encoding and subsequent stages (as shown in Figure 4), and thus a simultaneous prime would still be expected to be effective. Physical identity instructions. When the subject is requested to respond same only to physically identical stimuli, a prime is not effective. Rosch argues convincingly that with such instructions the subjects process the stimuli less deeply and less meaningfully (at least differently) than in the experiment with category-same instructions. If stimuli must be processed to the level of meaning in order for one concept to activate another, then it would be expected that priming would be ineffective in a task in which stimuli are not processed to the level of meaning. REFERENCE NOTE 1. Collins, A. M., & Loftus, E. F. A spreadingactivation theory of semantic processing. Manuscript submitted for publication, 1975.

REFERENCES Beller, H. K., Priming: Effects of advance information on matching. Journal of Experimental Psychology, 1971, 87, 176-182. Cohen, G. Pattern recognition: Differences between matching patterns and matching descriptions to patterns. Journal of Experimental Psychology, 1969, 82, 427-434. Collins, A. M., & Quillian, M. R. How to make a language user. In E. Tulving & W. Donaldson (Eds.), Organisation of memory. New York: Academic Press, 1972. LaBerge, D., Van Gelder, P., & Yellott, J. A cueing technique in choice reaction time. Perception and Psychophysics, 1970, 7, 57-62. Posner, M. I., & Mitchell, R. F. Chronometric analysis of classification. Psychological Review, 1967, 74, 392-409. Quillian, M. R. Word concepts: A theory and simulation of some basic semantic capabilities. Behavioral Science, 1967, 12, 410-430. Quillian, M. R. Semantic memory. In M. Minsky, (Ed.), Semantic information processing. Cambridge, Mass.: M. I. T. Press, 1968.

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Rips, L. J. Shoben, E. J., & Smith, E. E. Semantic distance and the verification of semantic relations. Journal of Verbal Learning and Verbal Behavior, 1973, 12, 1-20. Rosch, E. On the internal structure of perceptual and semantic categories. In T. Moore (Ed.), Cognitive development and the acquisition of language. New York: Academic Press, 1973. Rosch, E. Cognitive representations of semantic categories. Journal of Experimental Psychology: General, 1975, 104, 192-233.

Rumelhart, D. E., Lindsay, P. H., & Norman, D. A. A process model for long-term memory. In E. Tulving & W. Donaldson (Eds.), Organisation of memory. New York: Academic Press, 1972. Smith, E. E., Rips, L. J., & Shoben, E. J. Semantic memory and psychological semantics. In G. H. Bower (Ed.), The psychology of learning and motivation (Vol. 8). New York: Academic Press, 1974. (Received January 27, 197S)