Linguistic versus attentional influences on nonlinguistic categorization in 15-month-old infants

Cognitive Development, 6, 355-375 (1991)

Linguistic Versus Attentional Influences on Nonlinguistic Categorization

in 15-Month-Old Infants

Kenneth Roberts Boys Town National Institute for Communication Disorders in Children

Marianne Jacob Kent State University

Why linguistic input facilitates nonlinguistic categorization is frequently explained in terms of children's attention to uniquely linguistic forms such as words. How- ever, whether this facilitation is rooted in the children's attention to word forms very early in lexical learning has not been examined directly. A previous experiment (Roberts & Cuff, 1989) provided a set of conditions under which 15-month- olds did not successfully categorize in the absence of linguistic input. The two experiments reported here replicate Roberts and Cuff (1989) exactly, with the exception that either language (Experiment 1) or instrumental music (Experiment 2) was provided as accompanying input. Infants in both experiments successfully categorized and significantly increased attention during habituation. Although directly documenting the influence of language on categorization prior to the "vocabulary explosion," this influence does not appear attributable to the presence of word forms. Instead, factors common to language and music (e.g., attention- getting properties or factors influencing attention) may facilitate nonlinguistic categorization at the beginnings of word learning.

The fundamental role of categorization in human functioning is widely recognized (e.g. , Bruner, Goodnow, & Austin, 1956; Lakoff, 1987; Rosch, 1977, 1978; Smith & Medin, 1981). In particular, categorization is recognized as basic to the nature of language and language learning (Bowerman, 1976; Nelson, 1985; Schlesinger, 1977, 1982). Bowerman (1976) characterized language learning as a complex set of interrelated problems in category formation. More

This research was supported by grant RO 1-HD 20109 from the National Institutes of Health, and in part by Biomedical Research Sciences Grant RR07208 from Kent State University.

We would like to thank V. Gathercole, B. Sehick, S. Nittrouer, and especially R. Golinkoff for helpful comments on a previous draft of the manuscript. We would also like to thank M. Domski and R. Richer of the Akron General Hospital Biomedical Engineering Department for help in configuring some of the instrumentation.

Correspondence and requests for reprints should be sent to Kenneth Roberts, Boys Town National Institute, 555 N. 30th St., Omaha, NE 68131 (or by E-mall to [email protected]).

Manuscript received January 24, 1991; revision accepted April 16, 1991 355

356 Kenneth Roberts and Marianne Jacob

recently, she observed that "no acquisitional task puts greater demand on children's skill at categorization than learning to talk. The forms of language are themselves categories, and these fornas are linked to a vast network of categorical distinctions in meaning . . . " (Bowerman, 1989, pp. 28-29). Thus, a basic understanding of the variables influencing categorization and its relationship to linguistic forms should provide important insights into fundamental aspects of language acquisition.

Many child language researchers acknowledge that, in some instances, words map onto prior established nonlinguistic categories (e.g., Bowerman, 1989; Gopnik & Meltzoff, 1984; Johnston & Slobin, 1979; Markman & Hutchinson, 1984; Mervis & Mervis, 1988; Nelson, 1973, 1974; Schlesinger, 1977; Slobin, 1985; Waxman, in press). Some of these same researchers also emphasize (to varying degrees) that linguistic input, particularly input that provides an object label, may facilitate nonlinguistic categorization.

A variety of conditions, both nonlinguistic and linguistic, may facilitate categorization in infants. For example, a number of nonlinguistic, exemplar-related variables have been shown to facilitate the elicitation of nonlinguistic categories: (a) increasing the number of two-dimensional exemplars used to introduce a category when these exemplars exhibit lower perceptual similarity (e.g., Bomba & Siqueland, 1983; Homa & Vosburgh, 1976; Roberts & Cuff, 1989), or using a large number of three-dimensional objects to introduce a category (e.g., Golin- koff & Halperin, 1983; Ross, 1980); (b) targeting a basic-level category, as opposed to a superordinate category (e.g., Daehler, Lonardo, & Bukatko, 1979; Mervis & Crisafi, 1982; Roberts & Cuff, 1989); (c) using category prototypes to introduce the category (Hupp & Mervis, 1982; Mervis & Pani, 1980; Roberts & Horowitz, 1986); and (d) increasing the attribute correlations among exposure exemplars (Younger & Cohen, 1983). In general, the facilitative effects of these exemplar-related variables are explained in terms of the increased information made available about the target category.

Linguistic input can facilitate nonlinguistic categorization in two ways. Ob- ject labels can shape already established categories into concordance with categorical distinctions required by the adult language (e.g., Bowerman, 1989; Schlesinger, 1977); here linguistic input modifies category boundaries. Another way is to help the child detect a category not previously noticed within a specific context; here linguistic input stimulates a child to treat objects categorically (i.e., as similar in kind). A strong form of this latter effect would be the establishment of a new category not previously recognized under any contextual condition (e.g., Bowerman, 1980; Markman & Hutchinson, 1984, Experiment 4). A weaker form would be that, for a category recognized (in the absence of language) within some contexts but not within others, the addition of linguistic input in those other contexts would stimulate categorical responding (Markman and Hutchinson, 1984; Waxman and Gelman, 1986).

However, there is little empirical evidence from early in development that

Linguistic Influence on Categorization 357

directly documents changes in nonlinguistic categorization as a function of linguistic input. Those studies aimed at documenting the facilitative effect of linguistic input upon children's detection of categories, rely primarily on inferring nonlinguistic categorization from the linguistic responses of children who are beyond the beginnings of language learning (Roberts & Horowitz, 1986; Schlesinger, 1982). While a few studies (Bowerman, 1980; Banigan & Mervis, 1988; Mervis, 1987; Mervis & Mervis, 1988) used very young children, this evidence also relies primarily on inferring nonlinguistic categorization from linguistic data (see Gopnik & Meltzoff, 1987, for an exception).

Reflecting this reliance, why linguistic input per se has a facilitative effect on categorization is frequently explained in terms of children's use of uniquely linguistic information, such as that associated with words (e.g., Adams & Bul- lock, 1986; Bowerman, 1980; Harris, Barrett, Jones, & Brookes, 1988; Mark- man & Hutchinson, 1984; Waxman, 1990). This is surely true at some point in development, particularly after the frequently reported "vocabulary explosion," which may reflect fundamental insight by children as to what words do (Gold- field & Resnick, 1990). However, it is unclear as to what degree such uniquely linguistic information may be used by infants prior to the vocabulary explosion; in the presence of language input, detection of nonlinguistic categories may reflect the influence of input-associated factors that are indeed not uniquely linguistic. Many child language researchers would not necessarily reject this possibility; however, the issue is nevertheless frequently left open.

In any case, available studies have not utilized the necessary nonlinguistic input required to separate the influence of linguistic variables such as words from more general cognitive variables associated with both linguistic and nonlinguistic inputs (e.g., attentional influences, acoustic factors, or manner of input presentation). In other words, when linguistic input is made available, it is not yet clear whether the detection of nonlinguistic categories prior to the vocabulary explosion is facilitated by the presence of word forms per se, or by more general information also present in nonlanguage inputs. The present experiments repre- sent a first step in directly examining the facilitative influence of linguistic input upon the detection of a nonlinguistic category prior to the vocabulary explosion. In addition, the effects of the presence or absence of words per se in accounting for this influence are examined.

In doing this, we followed, in its essentials, the generally accepted logic utilized in recent studies of linguistic constraints/biases. In general, these studies utilize a no-input condition in which children fail to treat stimuli in a categorical fashion (i.e., as similar in kind) in the absence of language input. In several experiments, these stimuli are from known basic- (e.g., Markman & Hutchin- son, 1984) or superordinate-level categories (e.g., Waxman & Gelman, 1986). Thus, in these instances, performance in the no-input condition may reflect a failure to categorize under less optimal circumstances, rather than a lack of the target category or an inability to categorize. Categorization performance in the


no-input condition is contrasted with performance in a novel word condition, in which linguistic (labeling) input is provided, with all other conditions maintained as in the no-input condition. Of interest is any change in categorical responding as a function of a single manipulation: the addition of labeling.

Recently, Roberts and Cuff (1989, Experiment 3), as part of a series of experiments on superordinate categorization, reported unsuccessful categorization in 15-month-olds habituated to three different animal exemplars. Using a multiple-habituation procedure (Cohen & Strauss, 1979; Roberts, 1988; Roberts & Horowitz, 1986), 36 infants were habituated to slides of line drawings of a dog, cat, and horse, with visual fixation time as the dependent measure. No linguistic or other input accompanied the visual stimuli. Upon habituation, half of the infants (experimental group) received single-trial, counterbalanced presentations of three novel, within-category stimuli (deer, pig, bird), followed by an out-of-category exemplar (cat') and a posttest checkerboard. The other half (control group) continued to receive single-trial, counterbalanced presentations of the habituation stimuli for the five test trials (no change in the visual stimuli). Categorization was operationally defined as a pattern reflecting no significant differences in looking time between the experimental and control groups to at least one within-category exemplar (generalization), with a significant increase in looking by the experimental infants to the out-of-category exemplar (discrimination). Analyses indicated that infants did not exhibit such a pattern. In particular, infants failed to discriminate the out-of-category exemplar. This failure suggested that these infants did not reliably distinguish instances from nonin- stances of the target category after repeated exposure to three different exemplars; thus, they failed to categorize under these experimental conditions.

This is not to say that 15-month-olds could not form this category under any no-input conditions. A follow-up experiment reported in Roberts and Cuff (1989, Experiment 4) found that if the number of exemplars in the habituation set was increased to six, 15-month-olds responded categorically (see also Golinkoff & Halperin, 1983; Mandler & Bauer, 1988; and Ross, 1980, for evidence of categorization above the basic level in 1 to 2-year-olds). What Experiment 3 and a recent replication under similar conditions (Roberts, 1990) provide is a set of reference conditions, in terms of procedures, visual stimuli, and the absence of attendant input, under which infants do not treat novel test stimuli categorically. These experiments, then, are analogous to a no-input condition. Thus, analogous to constraints/biases studies, the experimental conditions of Experiment 3 in Roberts and Cuff (1989) can be used to examine the influence of a single manipulation. In particular, by maintaining all other conditions unchanged, the addition of different types of input can be directly evaluated in terms of effecting a change in categorization performance. Since exposure to some animals is probable by 15 months of age and, thus, the category may be known under some conditions, the addition of auditory input would test the weaker form of the


facilitative influence of input on category detection (Markman & Hutchinson, 1984; Waxman & Gelman, 1986).

The two experiments reported here replicated Experiment 3 of Roberts and Cuff exactly, with the exception that two types of input were added: Either linguistic labeling (Experiment !) or instrumental music (Experiment 2) was presented whenever an infant fixated the visual stimuli. While instrumental music embodies (or can be made to) many characteristics also found in linguistic input (e.g., modulation of frequency, time, and intensity, and presentation at times of joint attention to an object), specific word forms are absent. Thus, coupled with linguistic input, instrumental music input provides a needed test of whether or not the presence of words per se helps infants to detect a category that was not detected in the absence of auditory input (Roberts & Cuff, 1989, Experi- ment 3). If infants prior to the vocabulary explosion categorize successfully in both experiments, this would suggest that the presence of words in input is not a parsimonious account of the performance change in Experiment 3 of Roberts and Cuff.

EXPERIMENT 1

Method

Subjects. Thirty-six 15-month-old infants ( + / - 7 days) with no known visual problems served as subjects. As in Roberts and Cuff (1989), these infants were recruited from the greater Akron, Ohio area. Maternal reports provided on the day of the session indicated that all infants experienced uneventful gestation and delivery and that all infants had apparently normal hearing and no current ear infections. An additional 7 infants were excluded from the final sample due to fussing (n = I), experimenter error (n = 4), or maternal cuing (n = 2). In both instances of maternal cuing, the mother pointed to certain stimuli during the test phase.

Stimuli

Visual Stimuli. The visual stimuli (see Figure 1, p. 360) were identical to those used in Roberts & Cuff (1989, Experiment 3). The drawings were equated for size. Slides of these line drawings were presented to the infants.

Auditory Stimuli. The auditory stimuli consisted of two intonationally different tokens of the single word animal, and five different carrier phrases containing the word animal embedded at the end of each phrase (see Appendix). In choosing the carrier phrases, mothers of 4 infants ranging in age from 12-15 months were given objects and pictures of both animals and nonanimals. They were asked to interact with their infants using these materials. The five cartier

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To generate the auditory stimuli, a female graduate student produced several tokens of each phrase and of the single word animal in a play situation with a 15- month-old infant. These productions reproduced as closely as possible the into- nation contours used by the mothers. The play situation used pictures of animals and animal toys. Taping was conducted in a pediatric audiological suite. The play situation was tape recorded using a Teac X7 reel-to-reel tape deck and a Realistic microphone. The microphone was suspended from the ceiling to approximately 2 ft above the student and infant.

One noise-free production of each of the seven auditory stimuli (five carrier phrases plus two intonationally different productions of the single word animal) was selected. Three tokens of each of the seven auditory stimuli were then dubbed from the master onto a second tape. These tokens were then spliced together in such a way as to create three stimulus blocks consisting of the seven different stimulus utterances. Each utterance occurred once in each block with order deternained randomly. These blocks were then dubbed randomly in end-to- end fashion to create a continuous 10 min tape of the seven utterances. The interval between each utterance was 1 s.

Setting and Apparatus. Identical to Roberts and Cuff (1989, Experiment 3), infants were seated on their mother's lap in a semidark testing enclosure consisting of dark curtains on three sides and a large free-standing wall (8 ft x 5 ft). A removal board (41 in. x 30 in.) containing a single rear-projection screen (7 in. × 7 in.) was mounted in the free-standing wall. Auditory stimuli were delivered via a Teac X7 tape deck and a Concept Enterprises Coustic-ci speaker. The speaker was centered 2 in. above the projection screen. Mother and infant faced the rear-projection screen from a distance of about 27 in. Infant state and the mother's behavior were monitored via a TV monitor. Observers wearing head- phones through which masking music was played recorded visual fixations using the corneal reflection technique. In this technique, points of light appear in the pupil area of an infant's eye when fixating the screen. Corneal reflections were observed through small holes (J/2 in. diameter) located at the bottom and 4 ~& in. to each side of the projection screen. Observers depressed a single hand-held button whenever an infant was judged to be fixating the screen. The auditory stimuli were delivered at approximately 75 dB contingent on an infant's fixation of the visual stimulus. Contingent delivery was chosen as an approximation of mothers' tendencies to name an object only when the infant is attending to that object (Bruner, 1975; Tomasello & Farrar, 1986). The masking music was provided to prevent observers from hearing the auditory stimuli or any noise associated with pressing the hand-held buttons. Visual stimuli were presented via two Kodak Ectographic III B slide projectors, one for habituation stimuli and the other for test stimuli. Stimulus presentation, timing of fixations and intertrial


intervals, and calculation of habituation criterion were computer controlled by a Zenith 158 microcomputer equipped with a Labmaster laboratory control board (Scientific Solutions, Inc.).

Procedure. The habituation-discrimination procedures with trials under infant control were identical to those used in the Roberts and Cuff (1989, Experi- ment 3) study. All infants were habituated to blocks of three different line drawing exemplars of prototypical animals (dog, cat, horse). These exemplars were presented randomly within blocks of three trials. Mean looking times were computed for each block as the session progressed. A trial consisted of one unlimited look, beginning with an initial fixation of at least 1 s and terminating with a look away of 1 s. The intertrial interval was 2.4 s. The auditory stimuli containing the label anima! were continually presented as long as an infant fixated the visual stimulus. Any look away terminated the auditory stimuli. Thus, infants in this phase were exposed numerous times to three different animals accompanied by linguistic input. Habituation criterion was defined as mean looking within a block that equalled or exceeded a 50% decrement from the largest of the first two block means in the habituation phase.

Each infant was randomly assigned to one of two test groups: experimental (n = 18) or control (n = 18). The test phase for the experimental group consisted of counterbalanced, single-trial presentations of the three within-category exemplars (deer, pig, bird), followed by the out-of-category stimulus (car), and then by a 24 x 24 checkerboard. Because generalized habituation is more likely to occur as an infant sees more test stimuli, the fixed ordering of the out-of-category stimulus is considered a conservative test for categorization (Caron, Caron & Meyers, 1982). The checkerboard was included at the end of testing to control for fatigue or disinterest in looking. The test phase for the control groups consisted of continued counterbalanced presentation of the habituation stimuli (dog, cat, horse) for five additional trials (no change from the habituation phase). The control group provided an indication of baseline responding when no stimulus changes were introduced during the test phase. In both groups, as in habituation, whenever an infant fixated the visual stimulus, the auditory stimulus tape was activated.

Categorization was examined in terms of both a between- and within-groups analysis. The between-groups analysis tested for a pattern of responding in which no significant difference existed between the experimental and control group on at least one of the novel within-category test trials (generalized habituation), but with a significant increase in looking time by the experimental group (discrimination) to the out-of-category (car) stimulus. The within-groups analysis provided a further evaluation of differential treatment of in- and out-of-category exemplars.

For approximately one-third of the infants, two independent observers re-


corded fixations. Observer agreement was sampled every 20 msec. An agreement was defined as both observers recording a fixation. A percentage agreement was calculated for each habituation and test trial. The mean percentage agreement over test trials was 95% in habituation and 97% in the test phase.

Maternal Interview. Mothers were interviewed in detail by the experimenter to determine their infant's possible comprehension of the category label animal. Mothers have been reported to be quite accurate in their reports of their children's word comprehension (Behrend, 1988; Gleason & Weintraub, 1978; Thomas, Campos, Shucard, Ramsay, & Shucard, 1981). The interview method used in this study was an adaptation of the Recognition Method used by Snyder, Bates, and Bretherton (1981). If a mother indicated that her infant did comprehend a particular label, she was asked what her infant did that indicated comprehension, and which situational contexts were involved. The mother was also probed regarding accompanying gestures. In particular, care was taken to determine whether a label was associated with a single, unique object (e.g., a particular dog), or whether a label was associated with several different objects (e.g., several different dogs or perceptually similar nondogs). Comprehension was credited if infants indicated appropriate responses without gestural cues in more than one context, and in association with more than one exemplar.

In all instances, mothers judged that their infants did not comprehend the word animal, with most mothers also noting that they did not use this label with their infants. However, it was frequently reported that infants comprehended several basic level labels for the visual stimuli (e.g., dog~doggie, cat~kitty, cow, horse, or bird). Seventy-eight percent of the mothers indicated that their infant comprehended two or more of the basic level labels for the visual stimuli used in Experiment 1. These reports are consistent with recent studies of the character of maternal labeling (e.g., Blewitt, 1983; Lucariello & Nelson, 1986; White, 1982) and with studies of early lexical knowledge (Benedict, 1979; Hoek, Ingram, & Gibson, 1986).

Results Within each group, graphical inspection of looking times on each test trial revealed a pronounced right skew in the distributions. To equalize the variances and normalize the distributions (Rummel, 1970), a natural log transformation was applied to the looking times in the test phases of each group. All analyses of test data were performed on these transformed scores.

To examine category formation, a Group (experimental vs. control) × Trial analysis of variance (ANOVA) was performed, with trial as a repeated variable. Analyses revealed a significant main effect for group, F(1,34) = 10.66, p = .003, and a significant Group × Trial interaction, F(4,136) = 7.04, p < .001. F tests for simple effects were used to analyze the Group x Trial interaction


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Mean natural logs of looking times at habituation (hab) and during the test phase (T1-T5) of Experiment 1 (linguistic input). In the test phase, the stimulus designations by trial for the experimental group were: TI- Deer, T2- Pig, T3- Bird, T4- Car, T5- Checkerboard. The stimulus designations for the control group over T1-T5 reflect the continued counterbalanced presentations of the habituation stimuli (dog, cat, horse); thus, no change from the habituation phase.

(Bruning & Kintz, 1977). Results indicated significant differences between groups (p < .001) only on the out-of-category trials (Trials 4 and 5; see Figure 2).

Inspection of the graphical representation of the interaction in Figure 2 suggested the possibility of linearly increasing and decreasing patterns of looking in the experimental and control groups, respectively. Orthogonal tests of trend performed separately on each group revealed significant linear trends: Experi- mental group, F(1,34) = 23.81, p < .001, and control group, F(1,34) = 8.80, p = .009. Tests for quadratic, cubic, and quartic trend were nonsignificant in each group.

In the experimental group, a one-way, repeated-measures ANOVA on the mean looking times in the test phase revealed a significant effect, F(4,68) = 6.03, p < .001. Post-hoc Newman-Keuls contrasts revealed significant differences (p < .05) in mean logged looking on the following contrasts: deer-car, deer-checkerboard, pig-car, pig-checkerboard. However, while logged looking to bird versus checkerboard differed (p < .05), looking to bird versus car did not. This suggests, consistent with the between-groups analysis, that infants in the experimental group treated the deer and pig exemplars as different from both the car and checkerboard. The differential treatment of these in- and out-of-


category exemplars has been recently replicated (Roberts, 1990). Additionally, these analyses suggest that the between-groups differences on the out-of-category trials were not simply a reflection of the linear decrease in looking in the control group. However, unlike the between-groups data, these analyses raise the possibility that generalization did not extend to the bird exemplar.

One possible explanation for increased looking to the car is an increased salience or preference associated with this stimulus. However, consideration of recently reported data suggest that this was not the case. Roberts and Cuff (1989, Experiment 3), in an experiment not providing auditory input but that was otherwise identical to Experiment 1, found that looking in the test phase to the within-category stimuli and the out-of-category car did not differ. Moreover, this finding has been recently replicated. After accumulating 90 or 156 s of fixation time to the habituation stimuli used in the present experiment (no accompanying input), 15-month-olds' looking to the within-category stimuli and out-of- category car did not differ when tested as in Experiment 1 (Roberts, 1990).

A second possibility is a perceived mismatch by the infant between the label animal and the out-of-category stimulus (car). However, this assumes that a word-object correspondence was actually achieved for this category. The present data provide no information in this regard. More important is the consideration that the pattern of increased looking was also found using instrumental music (Experiment 2) in which there could be no word-object mismatch. However, even in the event of a perceived mismatch between animal and car, the contra- dictory information must have a reference point. That is, the location of a mismatch on the car trial could only be in reference to the target category. Failure to categorize, or extraction of a different category would place the mismatch, and hence recovery, on different trials. Thus, even the presence of mismatches would not appear to compromise interpretations regarding categorization.

To examine order effects in the test phase, separate one-way repeated- measures ANOVAs were performed on each group. No significant effects of presentation order were found for the within-category stimuli in the experimental group, F(2,34) = .38, p = .68. In contrast, an order effect was found in the control group, F(4,68) = 2.54, p = .047. However, post-hoc Tukey tests (Kep- pel, 1982) of pairwise contrasts among the five test trials revealed no significant differences, although looking to whichever stimulus was presented first tended to be longer than to stimuli presented in the fourth or fifth position (p < . 10). An analysis of trend revealed a significant linearly decreasing trend, F(1,17) = 14.29, p = .001. Not unexpectedly, this suggests for the control group that looking in the test phase decreased reliably from the first to the last stimulus presentation as these infants continued to see and hear the habituation stimuli.

Mean total fixation time in seconds and mean trials-to-criterion for each group during habituation were, respectively, as follows: Experimental, 209.46 (SD =

118.07) and 15.33 (SD = 9.31); control, 164.11 (SD = 88.88) and 11.50 (SD =


4.15). A t test for independent groups revealed no significant differences between groups on either total fixation time, t(34) = 1.30, p = .20, or trials-to-criterion, t(34) = 1.60, p = . 12. Consequently, group differences in the test phase cannot be attributed to differential attention during habituation.

Mean fixation times in seconds at habituation for each group were as follows: Experimental, 7.34 (SD = 6.37); control, 7.17 (SD = 3.61), t(34) = .10, p = .92. This indicates that each group had habituated to nearly identical levels as the test phase began. Thus, group differences in the test phase cannot be attributed to differential looking levels as the test phase began.

As this experiment progressed, there were strong indications that infants were accumulating more fixation time in habituation (M = 186.78; SD = 105.53) than infants in Roberts and Cuff (1989, Experiment 3), which did not incorporate stimulus labeling (M = 92.77; SD = 66.44). A comparison confirmed this, t(70) = 4.52, p < .001. However, infants in Experiment 1 did not differ from those in Roberts and Cuff in the number of trials needed to meet habituation criterion: 13.42 (SD = 7.37) and 14.83 (SD = 7.02), respectively [t(70) = .83, p = .41]. This suggests that the infants in Experiment 1 were looking longer per trial in habituation.

Discussion Both between- and within-group analyses are consistent with categorization, although the within-groups data suggest more restricted generalization. Consis- tent with previous maternal reports (Roberts & Cuff, 1989), most infants comprehended the basic level names for some of the habituation and test stimuli. That these stimuli probably retained conceptual identity independent of infants' generalized habituation across test stimuli suggests that the category formed was in a superordinate relationship to related basic-level categories (Nelson, 1985; Roberts & Cuff, 1989). This successful categorization contrasts with Roberts (1990) and Roberts and Cuff (1989, Experiment 3), an experiment not providing linguistic input, but otherwise identical. A previously reported experiment (Roberts & Cuff, 1989, Experiment 2) demonstrated that 12-month-olds could discriminate between the same habituation and test stimuli used in the present experiment. It is a reasonable assumption that 15-month-olds can also make these discriminations. Further evidence for discriminability is that most infants comprehended the basic-level labels for some of the habituation and test stimuli.

While linguistic input appeared to facilitate nonlinguistic categorization, the reasons for this are unclear. First, it is possible that the presence of consistent word forms (e.g., animal) stimulated infants to search for relevant classification principles. Alternatively, it is possible that the attention increase in Experiment l relative to Roberts and Cuff (1989, Experiment 3) could account for successful categorization. To further examine these possibilities, Experiment 2 substituted an instrumental music segment for the labeling stimuli used in Experiment I.


EXPERIMENT 2

Method

Subjects. Thirty-six 15-month-old infants ( + / - 7 days) served as subjects. Infants were screened in the same manner as in Experiment 1. An additional 3 infants were excluded from the final sample due to fussing.

Stimuli

Visual Stimuli. The visual stimuli used were identical to those in Experi- ment 1.

Auditory Stimuli. The auditory stimuli consisted of an instrumental music segment from a children's song ("Baby Beluga" by Raffi). The segment was 25 s in duration. In the first half of the segment the melody was played by a muted comet. In the second half the melody was repeated, but this time by a clarinet. This segment was dubbed in an end-to-end fashion, with 1 s intervals separating each segment, to form a 15-min stimulus tape.

Procedure. The procedures were identical to Experiment 1, with the exception that reliability was computed by dividing the total time in agreement by the sum of the total time in agreement and disagreement, and multiplying this result by 100. For approximately one-fourth of the infants, two independent observers recorded fixations. Mean reliability per trial in these sessions was 85%.

Maternal Interview. Maternal interviews were conducted in the same manner as in Experiment 1. Of the 29 mothers interviewed, only 2 reported that their infants might comprehend the word animal. As in Experiment 1, it was frequently reported that infants comprehended several basic-level labels for the visual stimuli. Seventy percent of the mothers interviewed reported that their infant comprehended two or more of the basic-level labels for the visual stimuli.

Results and Discussion Within each group, graphical inspection of looking times on each test trial revealed a pronounced right skew in the distributions. As in Experiment l, a log transformation was applied to the looking times in the test phases of each group.

A Group (experimental vs. control) x Trial ANOVA was performed, with trial as a repeated variable. Analyses revealed a significant main effect for group, F(1,34) = 15.42, p < .001, a significant main effect for trials, F(4,136) = 3.57, p = .008, and a significant Group x Trial interaction, F(4,136) = 2.67, p = .035. Post-hoc F tests indicated significant differences between groups on Trial l (p = .02), and on the out-of-category trials (Trials 4 and 5; p < .01; see Fig- ure 3, p. 368).


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Figure 3. Mean natural logs of looking times at habituation (hab) and during the test phase (TI-T5) of Experiment 2 (instrumental music). In the test phase, the stimulus designations by trial for the experimental group were: TI- Deer, T2- Pig, T3- Bird, T4- Car, TS- Checkerboard. The stimulus designations for the control group over T1-T5 reflect the continued counterbalanced presentations of the habituation stimuli (dog, cat, horse); thus, no change from the habituation phase.

As in Experiment I, a one-way repeated-measures ANOVA performed on the test trials of the experimental group revealed a significant effect, F(4,68) = 6.51, p < .01. Post-hoc Newman-Keuls contrasts revealed significant differences ( p < .05) between the mean logged looking to each within-category trial (deer, pig, bird), and both the out-of-category (car) and posttest (checkerboard) trials. There were no other significant differences.

Separate one-way repeated-measures ANOVAs performed on each group revealed no significant effects of presentation order for the within-category test stimuli in the experimental group, F(2,34) = 1.49, p = .24, or the test stimuli in the control group, F(4,68) = .27, p = .90.

Mean total fixation time in seconds and mean trials-to-criterion for each group during habituation were, respectively, as follows: experimental, 123.69 (SD = 41.94) and I 1.50 (SD = 4.02); control, 163.59 (SD = 114.28) and 14.33 (SD = 9.41). A t test for independent groups revealed no significant differences between groups on either total fixation time, t(34) = 1.39, p -- . 17, or trials-to-criterion, t(34) = 1.18, p = .25.

There were no group differences on mean fixation times in seconds at habituation: experimental, 5.45 (SD = 2.83), and control, 4.58 (SD = 2.09), t(34) = 1.05, p = .30. These data indicate that the differences between groups in the test phase cannot be attributed to differential attention during habituation, or differential looking levels as the test began.


Table 1. Mean Total Fixation Times in Seconds During Habituation for Roberts & Cuff (1989, Exp. 3) and Experiments 1 and 2

Experiment M SD

R & C 92.77 66.44 I 186.78 105.53 2 143.64 87.21

In addition, separate 1 x 3 ANOVAs were performed on the total fixation times and trials-to-criterion of the present experiments and of Roberts and Cuff (1989, Experiment 3). Analyses revealed a significant effect for total fixation time, F(2,105) = 10.33, p < .001, but not for trials-to-criterion, F(2,105) = .68, p = .51. Post-hoc analyses (Tukey HSD) revealed significantly greater total fixation times in both Experiments 1 and 2, relative to Roberts and Cuff: p < .001 and p = .04, respectively (see Table I). There was also a nonsignificant tendency for greater total fixation in Experiment 1 as compared with Experiment 2, p = . 10. These findings suggest that the addition of either linguistic or music input significantly increased infants' attention.

Both the between- and within-group analyses suggested successful categorization in the context of a nonlinguistic input. However, while the between-groups analysis suggested discrimination of the within-category deer relative to the control group, the within-group analysis suggested that deer was not treated differently than the other within-category stimuli.

GENERAL DISCUSSION

In the context of both linguistic input and music input, infants successfully categorized at a level of abstraction higher than that of related basic-level categories, when assessed using a measure independent of the input. These results contrast with those of Roberts and Cuff (1989, Experiment 3), an experiment not providing input, but otherwise identical to the present experiments. Thus, these data directly document a positive (facilitative) change in nonlinguistic categorization, prior to the vocabulary explosion, as a function of adding either language or music input. Moreover, these data appear to extend this facilitative effect of input on categorization to an earlier point in development than that indicated by studies focusing on toddlers and preschoolers (e.g., Markman & Hutchinson, 1984; Waxman & Gelman, 1986; Waxman & Kosowski, 1990).

Successful categorization in both experiments suggests that the facilitative influence of linguistic input on the detection of a nonlinguistic category cannot be explained in terms of the presence of words per se, at least under the experimental conditions in effect. Rather, the data raise the possibility that, prior to the


vocabulary explosion, factors associated with both types of input, and thus not unique to language, may provide a more parsimonious account of the change in categorization than that reported in Experiment 3 of Roberts and Cuff.

These data add to the growing evidence that infants have or can acquire categorical knowledge at the beginnings of learning language, and that linguistic input, among other factors, may facilitate the elicitation of this knowledge. This provides further support that fundamental categorization abilities that are basic to many aspects of language learning are available as children begin to learn language (cf. Bowerman, 1980, 1989; Mandler & Bauer, 1988; Nelson, 1973, 1974; Ross, 1980; Sugarman, 1981). In addition, these data clearly suggest what many accounts usually leave open: As it influences the detection of a category prior to the vocabulary explosion, language input may serve as a manifold source of information rather than a uniquely linguistic one. Many language-learning accounts may embrace this conclusion. Nevertheless, our data underscore the need to explore the nature of these manifold sources. Moreover, these experiments illustrate the utility of nonlinguistic inputs as controls to both test the role of uniquely linguistic information and reveal potential nonlinguistic alternatives.

Recent constraints/biases accounts propose that word learning proceeds as unproblematically as it does because children's uniquely linguistic biases reduce the number of hypotheses about word-referent relationships that must be considered. One such bias is that a novel noun refers to a category of the same kind of things. This bias is referred to as the taxonomic assumption (Markman & Hutchinson, 1984; Markman, in press) or the noun-category bias (Waxman & Kosowski, 1990). Reflecting the influence of this bias, children's treatment of objects as similar in kind within a specific context is attributed to the sheer presence of the novel noun (Markman & Hutchinson, 1984; Markman, in press). Thus, the presence of a novel noun is taken to highlight the categorical relationship holding among a set of different objects, at least for children beyond the point of the "nominal insight" and accompanying "vocabulary explosion" (Waxman and Kosowski, 1990). In principle, the existence of a taxonomic assumption or noun-category bias in children who are "post vocabulary explosion" may be well motivated. However, the existence of such a bias and the conditions of its influence prior to the vocabulary explosion remain largely uninvestigated (Waxman & Kosowski, 1990; but see Bauer & Mandler, 1989).

Because our experiments dealt with nonlinguistic categorization rather than word learning per se, our data do not speak directly to the existence of the bias prior to the vocabulary explosion. However, consistent with Bauer and Mandler (1989), the data do suggest that this bias may not be responsible for infants' categorical treatment of target stimuli in these experiments, such that in the absence of nouns categorical treatment would not occur. That is, conditions related to the general nature of auditory input (e.g., attentional properties, manner of presentation, acoustic properties) may exist, such that the highlighting of categorical relations prior to the vocabulary explosion is not uniquely rooted in


the presence of a novel noun, although it may be in children who are past this point. Admittedly, it is possible that this bias may operate under other conditions prior to the vocabulary explosion. Nevertheless, our data highlight the need to specify those conditions under which categorization is influenced by input-related variables of a more general nature, as well as those of a uniquely linguistic nature. These specifications will require manipulation of linguistic and nonlinguistic inputs, as well as the presence versus absence of input. Such manipula- tions will be critical to constructing more detailed accounts of the existence and role of linguistic biases.

Although infants' attention to nouns early in lexical development may not be necessary for language to influence categorization, it remains unclear which of several possible factors associated with both inputs might explain our results. One possible factor is attention. In both experiments, total accumulated fixation time (attention) during habituation significantly increased relative to infants re- ceiving no input (Roberts & Cuff, 1989, Experiment 3). It is possible that infants at this age will solve a categorization problem when sufficient attention is de- voted to the problem.

The possible role of attention is generally consistent with the recent findings of Bauer and Mandler (1989), and Baldwin and Markman (1989). Bauer and Mandler (1989) speculated that the nonverbal reinforcement of categorical responses in their studies served to remind 16-month-olds of or focus their attention on the categorical nature of the task. They speculated that labeling may serve a similar function for older children. Our data appear to point to a similar attentional explanation of categorization success, but without the potential pro- cedural confound of reinforcing categorical responding.

Similar to the present experiments, Baldwin and Markman (1989) reported increased attention to objects by 10 to 14-month-old infants when objects were labeled. They speculated that increased attention may play an important role in stimulating infants to establish word-object relations. However, exactly how an increase in attending might lead to establishing word-object correspondences was left open. One possibility raised by the present data that deserves future consideration is that, early in language learning, increased attention may be a factor (among several) that facilitates nonlinguistic categorization. Consistent with numerous accounts of word learning, the appearance of a prior nonlinguistic category, in turn, may serve as a catalyst to find a word that corresponds to the unlabeled category, that is, to fill a lexical gap (Clark, 1983).

While attention itself may prove ultimately to be an important factor, one must also consider that alternative factors, although affecting attention, may be more directly responsible for the observed influence on categorization. One candidate is the degree of correlation between the input and an infant's fixation of a visual stimulus. In the present experiments the correlation was perfect. That is, the input was presented only when an infant was fixating the visual stimulus; termination of the fixation also terminated the input. This parallels the tendency


of mothers to label objects during periods of jo int attention rather than during periods of split focus (e.g. , Ninio & Bruner, 1978; Tomasello & Farrar, 1986). Because the ability to detect relationships between co-occurring stimuli or events is ubiqui tous in humans (Alloy & Tabachnik, 1984), it seems plausible that infants may be sensit ive to the degree of correlation between their at tending to an object and the presence of input, as they are to correlated attribute information (Younger & Cohen, 1983). One manifestat ion of processing correlation information might be increased attention. A further possibili ty is that the modulat ion of acoustic characteristics (e.g. , frequency, t iming, and intensity modulat ions) common to both "motherese" and music may be responsible for increased attention (e .g . , Fernald, 1985; Fernald & Kuhl, 1987). Future experiments directed toward the role of these factors should provide more detailed insights into why linguistic input influences nonl inguis t ic categorization, and the condit ions under which specific variables exert their influence.

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APPENDIX

Carrier Phrases Used as Linguistic Input in Experiment 1

1. See the an ima l?

2. L o o k at the an imal .

3. W h e r e ' s the animal '?

4. T h a t ' s an an imal .

5. T h e r e ' s an an imal .

6. An ima l .

7. A n i m a l .

Linguistic versus attentional influences on nonlinguistic categorization in 15-month-old infants

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