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Mintz (2003) described a distributional environment called a frame, defined as the co-occurrence of two context words with one intervening target word. Analyses of English child-directed speech showed that words that fell within any frequently occurring frame consistently belonged to the same grammatical category (e.g., noun, verb, adjective, etc.). In this paper, we first generalize this result to French, whose function word system allows patterns that are potentially detrimental to a frame-based analysis procedure. Second, we show that the discontinuity of the chosen environments –i.e., the fact that target words are framed by the context words– is crucial for the mechanism to be efficient. This property might be relevant for any computational approach to grammatical categorization. Finally, we investigated a recursive application of the procedure and observed that the categorization is paradoxically worse when context elements are categories rather than actual lexical items. Item-specificity is thus also a core computational principle for this type of algorithm. Our analysis, along with results from behavioral studies (Gómez, 2002; Gómez and Maye, 2005; Mintz, 2006), provide strong support for frames as a basis for the acquisition of grammatical categories by infants. Discontinuity and item-specificity appeared to be crucial features.
Grammatical categories such as noun, verb, and adjective are the building blocks of linguistic structure. Identifying the categories of words allows infants and young children to learn about the syntactic properties of their language. Thus, understanding how infants and young children learn the categories of words in their language is crucial for any theory of language acquisition. In addition, knowledge of word categories and the syntactic structures in which they participate may aid learners in acquiring word meaning (Gleitman, 1990; Gleitman, Cassidy, Nappa, Papafragou and Trueswell, 2005; Landau and Gleitman, 1985).
In their introductory text on syntactic theory, Koopman, Sportiche and Stabler (2003) describe the main concepts that allow linguists to posit syntactic categories: “a category is a set of expressions that all ‘behave the same way’ in language. And the fundamental evidence for claims about how a word behaves is the distribution of words in the language: where can they appear, and where would they produce nonsense, or some other kind of deviance.” These observations are fundamentally at the core of the notions behind structural linguistics in the early 20th century (Bloomfield, 1933; Harris, 1951), namely, that form-class categories were defined by co-occurrence privileges. Maratsos and Chalkley (1980) advanced the proposal that children may use distributional information of this type as a primary basis for categorizing words. In the past decade, a number of studies have investigated how useful purely distributional information might be to young children in initially forming categories of words (Cartwright & Brent, 1997; Mintz, 2003; Mintz, Newport, & Bever, 2002; Redington, Chater, & Finch, 1998). Employing a variety of categorization procedures, these investigations demonstrated that lexical co-occurrence patterns in child-directed speech could provide a robust source of information for children to correctly categorize nouns and verbs, and to some degree other form-class categories as well.
One challenge in forming categories from distributional cues is to establish an efficient balance between the detection of the especially informative contexts and the rejection of the potentially misleading ones. For example, in (1), that cat and mat both occur after the suggests that the two words belong to the same category. However, applying this very same reasoning to example (2) would lead one to conclude that large and mat belong to the same category (see Pinker, 1987, for related arguments).
To address the problem of the variability of informative distributional contexts, the procedures developed by Redington et al. (1998) and Mintz et al. (2002) took into account the entire range of contexts a word occurred in, and essentially classified words based on their distributional profiles across entire corpora. While in (1) and (2), the adjective large shares a preceding context with cat and mat, in other utterances it occurs in environments that would not be shared with nouns, as in (3). Many misclassifications that would occur if only individual occurrences of a target word were considered turned out not to result when taking into account the statistical information about the frequency of a target word occurring across different contexts1.
Mintz (2003) took a different approach. Rather than starting with target words and tallying the entire range of contexts in which they occur, the basis for his categorization is a particular type of contexts which he called frequent frames, defined as two words that frequently co-occur in a corpus with exactly one word intervening. (Schematically, we indicate a frame as [A x B] with A and B referring to the co-occurring words and x representing the position of the target words.) For example, in (3), [the x on] is a frame that contains the word cat; it so happens that in the English child-directed corpora investigated by Mintz (2003), this frame contained exclusively nouns, leading to a virtually error-free grouping together of nouns. Examining many frames in child-directed speech, Mintz demonstrated that in English, frames that occur frequently contain intervening words that almost exclusively belong to the same grammatical category. He proposed that frequent frames could be the basis for children’s initial lexical categories.
One critical aspect of frequent frames is that the framing words—e.g., the and on in the example above—must frequently co-occur. Arguably, co-occurrences that are frequent are not accidental (as infrequent co-occurrences might be), but rather arise from some kind of constraint in the language. In particular, structural constraints governed by the grammar could give rise to this kind of co-occurrence regularity. It is not surprising, then, that the words categorized by a given frequent frame play a similar structural role in the grammar—i.e., they belong to the same category.
Thus, in the frequent frames approach, the important computational work involves identifying the frequent frames. Once identified, categorization is simply a matter of grouping together the words that intervene in a given frequent frame throughout a corpus. In contrast, in other approaches (Mintz et al., 2002; Redington et al., 1998) the crucial computations involved tracking the statistical profile of each of the most frequent words with respect to all the contexts in which it occurs, and comparing the profiles of each word with all the other words. Thus, an advantage of the frequent frames categorization process is that, once a set of frequent frames has been identified, a single occurrence of an uncategorized word in a frequent frame would be sufficient for categorization. Moreover, it is computationally simpler, in that fewer total contexts are involved in analyzing a corpus.
In addition to research showing the informativeness and computational efficiency of frequent frames (in English), several behavioral studies suggest that infants attend to frame-like patterns and may use them to categorize novel words. For example, Gómez (2002) showed that sufficient variability in intervening items allowed 18-month-old infants to detect frame-like discontinuous regularities, and Gómez and Maye (2005) showed that this ability was already detectable in 15-month-olds. This suggests that the resources required to detect frequent frames is within the ability of young infants. Second, Mintz (2006) showed that English-learning 12-month-olds categorize together novel words when they occur within actual frequent frames (e.g., infants categorized bist and lonk together when they heard both words used in the [you X the] frequent frame).
Although frequent frames have been shown to be a simple yet robust source of lexical category information, the analyses have been limited to English. One goal of the present paper is to start to test the validity of frequent frames cross-linguistically. To this end, in Experiment 1, we test the validity of frequent frames in French, a language which presents several potentially problematic features for the frame-based procedure.
An additional goal was to characterize the core computational principles that make frequent frames such robust environments for categorization. To this end, in Experiment 2 in both French and English, we compare frames with other types of contexts that are at first sight very similar to frames in terms of their intrinsic informational content and structure: [A B x] and [x A B]. Interestingly, despite the similarity of these contexts to frames, they yielded much poorer categorization. The results of this experiment suggest that co-occurring context elements must frame a target word.
Finally, in Experiment 3 we investigated the consequences of a recursive application of this frame-based procedure, again with French and English corpora. Specifically, we performed an initial analysis to derive frame-based categories, then reanalyzed the corpus defining frames based on the categories of words derived in the initial analysis. A somewhat counterintuitive finding was that the recursive application of the frame-based procedure resulted in relatively poor categorization. This finding suggests that computations based on specific items—words—as opposed to categories, is a core principle in categorizing words, at least initially.
This first experiment investigates the viability of the frequent frames proposal for French. Several features of the language suggest that frequent frames may be less efficient in French than in English. For example, English frequent frames heavily relied on closed-class words, such as determiners, pronouns, and prepositions. In French, there is homophony between clitic object pronouns and determiners, le/la/les, which could potentially give rise to erroneous generalizations. For instance, la in ‘la pomme’ (the apple) is an article and precedes a noun, whereas la in ‘je la mange’ (I eat it) is a clitic object pronoun and precedes a verb. There are also a greater number of determiners, which could result in less comprehensive categories. For instance, French has three different definite determiners, le/la/les, varying in gender and number, that all translate into the in English. Finally, constructions involving object clitics in French exclude many robust English frame environments, e.g. [I x it], a powerful verb-detecting frame in English, translates into [je le/la x] in French, which is not a frame. Do French frequent frames nevertheless provide robust category information, as in English?
The analysis was carried out over the Champaud (1988) French corpus from the CHILDES database (MacWhinney, 2000). This corpus is a transcription of free interactions between Grégoire (whose age ranges between 1;9.18 and 2;5.27) and his mother. Only utterances of the mother were analyzed, comprising 2,006 sentences. This is the largest sample available to us for which the age of the child is in the range of the English corpora analyzed by Mintz (2003). Those corpora contained on average 17,199 child-directed utterances, so the present corpus is an order of magnitude smaller. Thus, this experiment provides a test of the robustness of the frequent frames approach, in addition to a test of the cross-linguistic viability.
The corpus was minimally treated before the distributional analysis procedure was performed: all punctuation and special CHILDES transcription codes were removed.
We ran Cordial Analyseur over the corpus. This software developed by Synapse Développement (http://www.synapse-fr.com) maps each instance of a word with its syntactic category relying on supervised lexical and statistical strategies. The resulting categorization of words was used as the standard for evaluating the categories derived using frequent frames.
Syntactic categories included: noun, pronoun, verb, adjective, preposition, adverb, determiner, wh-word, conjunction and interjection2. (The word group designates a set of words that are grouped together by the distributional analysis.) Table 1 provides details about the distribution of the categories across the corpus. In Table 1 and throughout this paper, we use type to refer to a particular word and token to refer to a specific instance of the word in the corpus.
Every frame was systematically analyzed from the corpus, where a frame is an ordered pair of words that occurs in the corpus with an intervening target word (schematically: [A x B], where the target, x, varies). Utterance boundaries were not treated as framing elements, nor could frames cross utterance boundaries. The frequency of each frame was recorded, and the intervening words for a given frame were treated as a frame-based category. The frame-based categories were then evaluated to determine the degree to which they matched actual linguistic categories, such as noun and verb.
In order to obtain a standard measure of categorization success, comparable to prior studies, we computed accuracy and completeness scores. These measures have been widely used for reporting in other studies (e.g., Cartwright & Brent, 1997; Mintz, 2003; Mintz et al., 2002; Redington et al., 1998). Pairs of analyzed words were labeled as Hit, False Alarm or Miss. A Hit was recorded when two items in the same group came from the same category (i.e. they were correctly grouped together). A False Alarm was recorded when two items in the same group came from different categories (i.e. they were incorrectly grouped together). A Miss when two items from the same category ended up in different groups (i.e. they should be grouped together but were not).
As equation 1a shows, accuracy measures the proportion of Hits to the number of Hits plus False Alarms (i.e. the proportion of all words grouped together that were correctly grouped together). Completeness measures the degree to which the analysis puts together words that belong to the same category (as equation 1b shows, it is calculated as the proportion of Hits to the number of Hits plus Misses). Both measures range from 0 to 1, with a value of 1 when the categorization is perfect.
Two scoring conditions were available for each measure depending on whether word tokens or word types were considered. By default, we will report results for the type condition.
Departing from Mintz (2003), we elected to first evaluate all frames and their corresponding word categories, even if the frames were relatively infrequent. In subsequent analyses, like Mintz, we then established a frequency threshold to select a set of frequent frames and corresponding word categories to evaluate.
For each set of frame-based categories, 1000 sets of random word categories were arbitrarily assembled from the corpus; these random categories were matched in size and number with the actual frame-based categories they were to be compared with. Mean accuracy and completeness obtained from these 1000 trials provided a baseline against which to compare the actual results and were used to compute significance levels, using the ‘bootstrap’ or ‘Monte Carlo’ method. For instance, if only 2 out of 1000 trials matched or exceeded the score obtained by the algorithm, that score was said to significantly exceed chance level, with a probability of a chance result being p=0.002 (2 out of 1000).
Frame-based categories contained mainly nouns and verbs. Specifically, in the largest frame-based categories—the 20 categories containing at least 10 different types—48% of the types were nouns, and 41% were verbs. This is not a surprise since nouns and verbs constitute 75.6% of the types in the corpus. Interestingly, the frame statistics are similar even if calculated in terms of tokens: although nouns and verbs together only constitute 38.3% of the tokens in the whole corpus, 37% of the tokens captured by the frames were nouns and 46% were verbs.
Rather than applying an a priori threshold to select a set of frequent frames to evaluate (Mintz, 2003), we first evaluated performance iteratively on a successively larger number of frame-based categories. That is, we first assessed categorization by evaluating the largest frame-based category (by type), then the two largest categories, then the three largest categories, etc. Essentially, at each successive iteration we relaxed the criterion for determining whether or not a given frame defined a category.3 Figure 1 reports accuracy for such sets of groups: from left to right the number of groups increases as the criterion for category size is relaxed. Figure 2 reports completeness for the same sets of groups (the set with only one group being trivially complete).
Accuracy remains at ceiling for groups classifying 15 different types or more and is overall significantly better than chance for every set of groups represented here (p<.01, see section ‘comparison to chance categorization’ above for an explanation of how this probability was estimated). Behavioral data from Gómez (2002) suggest that internal variability makes distant dependencies easier to recover for 18 month-old infants. As a result, these groups, classifying many different types, may well also correspond to the most salient frames psychologically.
Unsurprisingly, completeness decreases quickly as the number of groups taken into account increases: for instance, since several frames consistently capture nouns, each of them leads to an independent group of nouns and completeness suffers from this situation (e.g., many pairs of items coming from different groups happen to be pairs of nouns, which adds to the number of misses). Nevertheless, completeness is overall significantly above chance (p<.01).
The previous analysis showed accuracy and completeness over a range of possible frame-based categorizations, as a function of the size of the resulting categories. We then analyzed a more limited set of frames based on a frame-frequency criterion, similar to Mintz (2003). We selected frames that both grouped together more than .5% of the types present in the corpus -i.e., 11 types- and accounted for more than .1% of the tokens -i.e., occurred more than 18 times. This leads to the 6 frequent frames described in Table 2; overall they classified 172 tokens from 99 types which accounted for 2236 tokens of the corpus. Thus, this sample of frames accounted for 31% of the nouns and verbs of the corpus (12% of all the tokens).
This classification was perfectly accurate: each frame selected words from only one category. While framing elements were function words, they categorized target words: specifically, three groups of nouns, and three groups of verbs. Token and type completeness was .34 and .33, respectively (see Table 4). Again, these scores are significantly above chance level (p<.01). Interestingly, each group of nouns formed by these frames corresponds to a different subcategory (feminine, plural and masculine nouns respectively); this is not surprising given that the frames categorizing nouns included determiners, which, in French, mark grammatical gender. While it may be tempting to conjecture that frames could capture finer-grained categories more broadly, none of the frames capturing verbs favored a particular sub-type of verb (such as transitive or intransitive).
This analysis extends previous results described by Mintz (2003) for English child-directed speech: Here we showed that in French, frequent frames delimit accurate groups of content words. It is striking that these results hold for French, which has a more varied and ambiguous function words system than English. Recall the potential difficulty introduced by the fact that all definite determiners, in French, are homophonous to clitic object pronouns (le/la/les). While determiners typically precede nouns or adjectives, object clitics typically precede verbs. To estimate the extent of the problem, we tallied the number of times that le/la/les occurred as determiners vs. clitic objects: we observed that le, la and les occurred as determiners 802 times in our corpus and 145 times as object clitics. These numbers indicate the ambiguity faced by any simple mechanism attempting to categorize a content word on the basis of the immediately preceding function word. In contrast, the frequent-frames mechanism did not suffer at all from the ambiguity: le, la and les appeared as left-framing elements of the three most frequent noun-detecting frames, and those frames did not contain verbs (or words from any other category). Thus, the additional constraint of co-occurrence with the right framing element efficiently disambiguated the ambiguous function words. Note that, in principle, a verb could occur in many successful noun-detecting frame, such as [le x de] in (4).
However, this type of construction, though grammatical, is absent from the corpus and arguably extremely rare in child directed speech. This illustrates the strength of the co-occurrence restriction imposed by the frame structure that reduces dramatically the syntactic ambiguity of single function words.
In addition, a significant portion of the corpus was accounted for by a very restricted sample of frames. This result strongly argues in favor of the plausibility of the mechanism very early on: identifying even a few of the most frequent frames may allow infants to categorize many of the nouns and verbs they encounter.
One explanation as to why words from categories other than noun and verb were not captured by frequent frames here is that the corpus we analyzed was relatively small. For example, the corpora analyzed in Mintz (2003) contained, on average, over 14,000 child-directed utterances; some frames in those analyses contained other classes, like adjectives, determiners, and prepositions. We expect that if larger corpora are analyzed, frame-based categories will successfully group together words from other syntactic categories as well.
Overall, then, the potentially problematic characteristics of French do not appear to be problematic in practice for a frequent-frames approach to early word categorization.
In this experiment, results from the frequent frames analysis are compared with results from analyzing similar environments: front contexts and back contexts. Front contexts are ordered pairs of contiguous words that categorize following words -[A B x] – and back contexts are ordered pairs of contiguous words that categorize preceding words [x A B]. In many respects, these environments are similar to frames: they all involve trigrams in which the co-occurrence of two words serves as the context for categorizing the third. The main difference is that frames are discontinuous and categorize intervening words, whereas front and back contexts feature two contiguous contextual elements. This experiment thus provides an indication of the importance of the particular context selected by a frame, versus contexts that are formally and computationally similar on all other dimensions. As we mentioned earlier, some English frames found by Mintz (2003) translate into one of these new environments: e.g., [I x it] translates into [je le/la x] in French. It could be that front contexts are just as useful in French as corresponding frame contexts in English. Alternatively, it could be that the discontinuity inherent in the frame context is important for capturing category regularities cross-linguistically. Experiment 2 addresses this question, using both French and English corpora.
Procedures and analysis methods similar to those in Experiment 1 were used here. The primary difference was that in Experiment 2, categories were formed on the basis of two new types of environments - front contexts of type [A B x] and back contexts of type [x A B]. The analysis was run over the same French corpus as in Experiment 1, and over the English corpus, Peter, from the CHILDES database (files Peter01.cha to Peter12.cha; Bloom, Hood & Lightbown, 1974; Bloom, Lightbown & Hood 1975). This corpus is one of the largest corpus investigated in Mintz (2003), and provides syntactic labeling for evaluating the analysis outcome.
Groups resulting from the alternative contexts contain many more types than frame-based groups: for instance in French, 19 front contexts and 14 back contexts classify more than 20 types whereas no frame does. This already suggests that these contexts are qualitatively different.
Applying the previous frequency threshold, frequent contexts in the French corpus were defined as the contexts occurring at least 18 times and classifying more than 11 types. For the English corpus, we kept the 45 most frequent contexts, just as in Mintz (2003)4. Results are given in Table 3: in French, accuracy for frequent front and back contexts is .30 and .25; in English, it is .52 and .29. These scores are significantly above chance level (p<.01) though far below results from frames. Completeness is .050 and .057 for frequent front and back contexts in French; .056 and .046 in English. Except for front contexts in English, these results are at chance.
Front contexts lead to slightly better results than back contexts. This asymmetry may reflect the fact that both French and English are right recursive languages so that function words generally precede the words they control. This explains why the co-occurrence of two adjacent words imposes a stronger syntactic constraint to the following words than to the preceding words.
Crucially, for English and French, frames yield much better categorization than the two continuous environments. The superiority of frames may be symptomatic of a syntactic consequence of the discontinuity that characterizes their structure. Specifically, we propose that the frequent co-occurrence of two words is indicative of a syntactic pattern that the two words together regularly exemplify, and thus the words are likely to be relatively close structurally in such situations. This would strongly constrain the structural relationship of the intervening target word and the framing elements. In contrast, in the case of fronting or back contexts, the preceding or following positions would be relatively unconstrained with respect to the target word. To illustrate this idea, Figure 3 shows a range of structural relationships between a target word and front or back contexts, as well as the two basic structural schemas for frames. The possible structures are much restricted for frames compared to front/back contexts. Intuitively, this should result in greater uniformity in the target position’s syntactic category for frames compared to the other contexts.
Thus, the advantage of frames over the alternative contexts examined in this experiment could be explained by the types of syntactic structures that are likely to be involved. These results suggest that discontinuity is a crucial property of frequent frames for purpose of categorization
In the next experiment we test the effects of recursively applying the frequent frames analysis to a corpus. Specifically, we use the groups formed by an initial application of the procedure as the new framing elements, rather than specific words. This natural generalization may allow us to capture more abstract regularities, for example, verbs are found between two pronouns, as opposed to verbs are found between ‘he’ and ‘it’.
The frequent frames mechanism as investigated so far could yield initial category knowledge that could serve as a basis for detecting new frame-like contexts with the re-application of the categorization procedure. For instance, if the words I and you have been categorized together, it may be reasonable to consider them as equivalent in terms of their role in defining frames and to obtain a single group from the frames [I x it] and [you x it]. This would be a highly desirable outcome, as it could consolidate separate frame-based groups belonging to the same linguistic category (for example, the frames [I x it] and [you x it] both contain verbs), thus making frame-based categories even more informative linguistically (but see Mintz, 2006, for an alternative consolidation proposal).
The same French and English corpora used in Experiment 2 were analyzed here. In the first phase of the procedure, we performed a frequent frame analysis as described in Experiment 1. In the second phase, we ran the procedure again, but allowed the groups produced in the first phase to participate in recursive frames. For example, suppose that the first application of frequent frames grouped together I and you in one frame (F1), and this and that in another (F2). Then, the utterance I saw this would trigger the categorization of the intervening word saw in three different recursive-frame-based groups: [F1 X this] (which groups together any word intervening between a word previously categorized in F1 and this), [I X F2] (idem) and [F1 × F2] (which groups together words intervening between a word categorized in F1 and a word categorized in F2).
The results from the first phase are reported in Experiment 1 and Mintz (2003). Thus, we focus on the recursive application of the procedure.
The groups derived from this procedure contain a large number of tokens and types. Furthermore, they capture words from a wider variety of categories, including function words. Establishing a frequency criterion for frames as before5, accuracy for frequent recursive frames is .37 in French and .25 in English: these scores are much lower than those obtained from simple frames (see Table 4) though still better than chance.
These lower results may be due to some artifact in the recursive application of the frequent-frame mechanism. Firstly, there were a few miscategorization errors within groups derived from item-based frames: these errors may add noise to the recursive step. Secondly, the actual groups provided by the first application of the mechanism may not be optimal for purposes of recursive categorization. For instance, because they mainly capture content words, which may impose fewer constraints on their neighbors than function words do. To evaluate the asymptotic performance of the recursive frames mechanism, we assumed perfect categorization in the previous steps, and used actual syntactic categories to establish recursive frames. We call this analysis “asymptotic” because it is analyzing the limit of the performance of recursive frames, given an ideal prior categorization of framing elements.
The asymptotic manipulation did not improve the results of the recursive analysis. Even under these highly idealized circumstances, these environments provided groups with very low accuracy (see the last column of Table 4)6. To give an example, the most frequent environment is [Verb x Noun], its accuracy is .14. Sentences (5) to (8) respectively exemplify the occurrence of a determiner, an adjective, a preposition and a noun within this context.
These results show that the frequent-frames categorization procedure does not benefit from a recursive application, even under the idealized assumption that a first application provided a complete and error-free categorization. In other words, the information captured by frequent-frames that is relevant for lexical categories is fundamentally item-specific: frequent frames provide a better categorization when they involve specific words rather than their syntactic categories.
At first sight, this might seem like a counterintuitive result. After all, if grammars are organized around categories, shouldn’t the category of the target word be predicted by the surrounding categories at least as well as by the surrounding words themselves? We speculate that allowing categories to define frames eliminates one of the powerful features of frequent frames, namely, that frequently co-occurring items in a frame configuration are a symptom of a linguistically “stable” local environment. In other words, the frequent co-occurrence of two given items may reflect a given syntactic structure; if the framing elements are allowed to vary among whole groups of words, it may well be that the different instances of the co-occurrences involve different syntactic relations and thus impose different constraints on the intervening words. For instance in sentences (5) to (8), the particular properties of the framing words — either the preceding verb, which could accept different constructions, or the noun, which could occur without a determiner or as a compound –– may account for the variety of grammatical categories that can intervene.
The analyses reported in this paper extend previous results described by Mintz (2003) in English: frequent frames help recover accurate syntactic categories in child-directed French as well. This is so, despite the fact that the function word system of French offers particular challenges to categorization based on frequent frames: e.g., the increased number of determiners, and homophony between determiners and object clitics. In addition, we identified two core computational principles that should be particularly useful for any mechanism relying on context to categorize words into syntactic categories. Firstly, discontinuous frames of the type [A x B] provide a much more efficient categorization than continuous contexts of the type [A B x] or [x A B], even though the quantity of information is formally the same in both context types. Discontinuous environments may be more constraining because of general syntactic properties of languages. Thus, line (2) in Figure 3 shows that when a specific syntactic relation holds between two non-adjacent words (as in frequent frames), the intervening syntactic position is highly constrained. It is very likely that the success of the frequent frames algorithm derives from this type of local syntactic patterns. In contrast, line (1) in Figure 3 shows that pairs of adjacent words do not constrain their surrounding environments in the same way: a wide range of syntactic structures can fit a string of two adjacent words. Secondly, the recursive analysis presented in Experiment 3 shows that the distributional analysis is maximally efficient when the framing elements A and B are specific items rather than syntactic categories.
Both these principles fit well within a psychologically plausible acquisition model. For instance, infants at the start of the acquisition process already have access to specific items but not yet to established categories. It is then an unexpected bonus that item-specificity leads to better categorization than an analysis where the framing elements are syntactic categories, even perfect ones. Further, we suggested that a reason for this seemingly paradoxical finding is that recursive frames defined by open-class categories can select specific contexts that reflect a variety of structures, whereas frequently co-occurring words in a frame configuration reflect a much more stable structure. The words grouped by the recursive frames are hence more likely to be from different categories than are words grouped by lexically based frames.
How would the frequent frames algorithm fit within a more global view of early lexical and syntactic acquisition? To start with, the computation of frequent frames relies on a prior segmentation of the speech stream into words. There is now converging evidence that word segmentation is efficiently mastered by the age of 10 to 16 months (depending on word types, see e.g. Jusczyk, Houston & Newsome, 1999; Nazzi, Dilley, Jusczyk, Shattuck-Hunagel, & Jusczyk., 2005). Infants may thus start compiling frequent frames during the first half of their second year of life. Congruent with this hypothesis, Gomez & May (2005) showed that 15-month-olds were already able to detect non-adjacent dependencies of the type [A x B] (that is, the necessary computational pre-requisite to frequent frames), and Mintz (2006) showed that even 12-month-olds categorize together nonwords that appear within the same frequent frames (see also Höhle et al., 2004, for evidence that 15-month-old German infants exploit determiners to recognize valid noun contexts for novel words).
At this point, infants would possess frame-based categories, containing words that typically ‘behave the same’, in that they belong to the same syntactic category. However, even when accuracy is high, there are typically several frame-based categories for each syntactic category. For instance, several different frames pick out nouns, several others pick out verbs. Learners would thus need to merge frame-based categories to obtain more comprehensive categories. Several possible strategies can be used to that end, such as grouping together frame-based categories that share one of their framing elements as well as some of their categorized words (see Mintz, 2003, 2006, for a fuller discussion of possible merging mechanisms).
Let us assume that learners successfully merged frame-based categories to obtain more comprehensive categories. Before they can use these categories to constrain their lexical and syntactic acquisition, they would need to label them. That is, they would need to identify which of these categories correspond to nouns, verbs, adjectives, etc…One way to do this would be to identify the syntactic category of a few words referring to concrete objects and events. For instance, if infants are able to acquire the meaning of a few frequent nouns referring to concrete objects (Gillette, Gleitman, Gleitman, & Lederer, 1999), they may then be able to classify as nouns all the words that occur within the same distributionally-defined category (even if these other nouns are not frequent themselves, or do not refer to concrete objects). On this view, then, distributional information in the form of frequent frames accomplishes the categorization work, and the first-learned words start the category-labeling process.
Such a process would be expected to occur sometimes before the age of 2, as recent experimental evidence suggests that infants are able to exploit the syntactic context in which a non-word occurs to infer something about its meaning. For example, 23-month-old French infants interpret a novel verb as referring to an action (Bernal, Lidz, Millotte & Christophe, in revision), and 24-month-old American infants interpret a novel preposition as referring to a relationship between objects (Fisher, Klingler & Song, 2006). We have shown that the frequent frames in speech to learners of either French or English provides distributional information that would allow them to converge on the relevant categories within this time frame.
This paper investigated the cross-linguistic validity of the frequent frame mechanism for syntactic categorization in French and English. This constitutes the initial step in testing the cross-linguistic viability of this account of how children may initially categorize words. As part of this investigation, we discovered several characteristics that might make frequent frames a particularly robust context: discontinuity and item-specificity.
Future work should address its generalizability to other, typologically varied, languages. For instance, it remains to be shown that frequent frames would also be efficient in languages with more flexible word order such as Turkish. These languages are very rich in functional elements, but they appear as bound morphemes, not as words, as in English or French. Mintz (2003) has suggested that a generalized analysis that operated on morphemes rather than words might capture the relevant regularities in languages with freer word order and richer morphology. Ultimately, a successful distributional theory of word categorization will have to consider the word (or morpheme) segmentation process that precedes it (e.g., see Christiansen & Onnis, this issue). That process essentially defines the units over which a categorization mechanism can initially operate. It could be that functional morphemes in Turkish, for example, are readily segmented by the same mechanism that segments words in English and French. A frequent frames analysis could then operate on stems and affixes, rather than open and closed class words. That level of analysis would likely result in much more stable patterns than would be available at the level of words, in languages with rich morphology and more flexible word order.
On the other hand, some languages, such as Cantonese, are said to make limited use of function words. Given that frequent frames rely heavily on function words in the two languages studied so far, how would the frequent frame analysis fare in languages like Cantonese? A preliminary analysis of Cantonese child-directed speech suggests that frequent frames still provide useful information, with an accuracy around .80; importantly, discontinuity proved to be a crucial property, just as in French and English.
Further cross-linguistic research is necessary to address these questions, and to further test the validity of this account of early category learning. These studies will also shed light on whether the core computational principles advocated here provide the same benefits when analyzing typologically different languages
The work presented in this paper was supported by a grant from the French ministry of Research to Anne Christophe (ACI n° JC6059), by a PhD grant from the French Ministry of Research to Emmanuel Chemla and Savita Bernal, and by a grant from the National Institutes of Health (HD 040368) to Toben Mintz. The research was also supported by a grant from the Agence Nationale de la Recherche (‘Acquisition précoce du lexique et de la syntaxe’, n° ANR-05-BLAN-0065-01), and by the European Commission FP6 Neurocom project. We would also like to acknowledge the personal help and support we benefited from Emmanuel Dupoux, Christophe Pallier, Paul Smolensky and Dominique Sportiche
1Mintz et al. and Redington et al. also incorporated more distributional positions into their analysis than just the immediately preceding word, e.g., the following word, words that were two positions before or after, etc. However, the addition of contexts does not, a priori¸ make the potential for misclassifications go away.
2Another set of analyses relied on a set of categories where pronouns and nouns were collapsed into a single category, as in previous distributional investigations; results were extremely similar.
3Although category size is not directly based on frame frequency, the number of types occurring within a frame is correlated with the frequency of the frame. We chose to organize the presentation of the evaluation metrics by category size simply for clarity. Below, we analyze categorization using a specific frame-frequency threshold.
4We considered other thresholds to make the new results match in number of tokens or types categorized, or number of groups obtained with groups obtained from frequent frames; results were similar.
5Again we ran the analysis with different thresholds and found no relevant difference.
6In an attempt to improve the categorization of the asymptotic application of the recursive algorithm, we also ran it with finer-grained categories for verbs, with three subclasses : auxiliaries, finite, and infinitive verbs. This manipulation did not change the results.