False perspectives on human language: Why statistics needs linguistics

A sharp tension exists about the nature of human language between two opposite parties: those who believe that statistical surface distributions, in particular using measures like surprisal, provide a better understanding of language processing, vs. those who believe that discrete hierarchical structures implementing linguistic information such as syntactic ones are a better tool. In this paper, we show that this dichotomy is a false one. Relying on the fact that statistical measures can be defined on the basis of either structural or non-structural models, we provide empirical evidence that only models of surprisal that reflect syntactic structure are able to account for language regularities.

One-sentence summary: Language processing does not only rely on some statistical surface distributions, but it needs to be integrated with syntactic information.

A sharp tension exists about the nature of human language between two opposite parties: those who believe that statistical surface distributions, in particular characterized using measure like surprisal, provide a better understanding of language processing, vs. those who believe that discrete recursive hierarchical structures implementing linguistic information are a better tool, more specifically, syntactic structures, the core and unique characteristic of human language (Friederici, 2017). In this paper, we show that this dichotomy is a false one. Relying on the fact that statistical measures can be defined on the basis of either structural or non-structural models, we provide empirical evidence that only models of surprisal that reflect syntactic structure are able to account for language regularities. More specifically, our goal is to show that the only kind of surprisal measure that is well correlated with behavioral or brain measures is one which takes into account syntactic structure. We do so by showing that the syntactic surprisal is the only surprisal measure able to distinguish our stimuli in the same way a human listener would do. Crucially, here all confounding factors, including acoustic information, will be factored out distinguishing our study from previous in the field, such as in Frank et al. (2015), Brennan and Hale (2019), Shain et al. (2020).

1. On four different models of surprisal

It is a truism that during language processing the brain computes expectations about what material is likely to arise in a given context. The natural next step from this observation and one that characterizes much work in psycholinguistics is to formulate a hypothesis about the differences in processing load: in general, the less expected a piece of linguistic material is, the more difficult its processing (Taylor, 1953; Goldman-Eisler, 1958). Expectation can be quantified in terms of the information theoretic notion of Surprisal (Attneave, 1959), where the surprisal of a word w in context w_c is defined as:

$\begin{array}{ll}Surprisal(w|w_c)=-logp(w|w_c)& (1)\end{array}$

If a word is highly unlikely in a context, its surprisal will be very high. In contrast, if the word's is highly likely, its surprisal will approach 0.

Surprisal serves as a very useful linking hypothesis between patterns of behavior and brain response on the one hand and a single numerical quantity, namely the probability of a form. And because surprisal does not make explicit reference to linguistic structure, surprisal is often thought to provide an alternative perspective on language processing that avoids the necessity to posit such structure. This view is incorrect, however. Surprisal depends crucially on a particular characterization of a word's probability. Such a characterization, a probability model, may or may not make reference to linguistic structure. In this section, we will describe two dimensions along which language probability models can vary, and then use these dimensions to characterize four distinct probability models. Each of these models can be used as the input to the surprisal equation given above, so that different values of surprisal can result depending on the assumptions behind the probability model (see Figure 1).

FIGURE 1

Figure 1. Models of surprisal. The two dimensions of language models (linear vs. hierarchical structure and word vs. category prediction). A choice in each dimension yields a distinct model of language, from which we can extract probability values.

1.1. Dimension 1: sequences vs. hierarchical structure

Our first dimension concerns the structure that is assumed in the generation of language. The simplest conception views language as a concatenative system. In this view, a sentence is simply a sequence of words generated one after another in a linear fashion. To account for which sentences are well-formed and which are not, constraints are imposed on adjacent elements, or bigrams. For example, in the context preceded by word “the”, a linear model of English will permit words like “cat” or “magazine” to occur, but not “of”. To make a probability language model, we can simply assign a probability to a word w in a given context defined by the previous word w_c, so that the probabilities for all of the words sum to 1 for each context. Given a sufficiently large corpus, we can estimate these probabilities by taking the ratio of the number of occurrences of the context and of the context-word bigram:

$\begin{array}{ll}p(w|w_c)=\frac{count(w_c,w)}{count(w_c)}& (2)\end{array}$

This model can be extended to an n-gram model, where the length of the context is increased to include more material: in an n-gram model, the conditioning context will include n-1 words. A 3-gram model could thus assign a higher probability to “magazine” than “cat” in the context “read the” while doing the reverse in the context “fed the”. A bigram model could not assign distinct probabilities in the two contexts, since the single adjacent word, namely “the”, is identical in both. For this reason, an n-gram model gives a more refined assessment of likelihood as the value of n grows. However, because the number of conditioning contexts expands exponentially with the length of the context, it becomes increasingly difficult to accurately estimate the values of the probability model. A variety of methods have been proposed to integrate the information from longer contexts with information in shorter contexts. We use such a composite model for our model of N-gram surprisal.

Chomsky (1957) famously argued that linear models, were inadequate models of natural language, as they are incapable of capturing unbounded dependencies. To illustrate, consider the likelihood of the word “is” or “are” in context “The book/books that I was telling you about last week during our visit to the zoo”. This will depend on the whether the word “book” or “books” appears in the context. Because the distance between this contextual word and the predicted verb can grow without bound, no specific value of n will yield an n-gram model that can correctly assign probability in such cases.

Chomsky's suggested alternative generates language using a hierarchically organized process. In this way, linearly distant elements can be structural close. One simple model for this involves context-free grammars (CFG), a set of rules that specify how a unit in a sentence tree can be expanded:

S → NP VP

NP → Det N

VP → V NP | V

Det → the | a

N → book | books

V → read | reads

Where S is the sentence, NP is a noun phrase, VP is a verb phrase, Det is a determiner, N is a noun and V is a verb.

Generating a sentence with such a grammar starts at the start symbol S. A rule whose lefthand side matches this symbol is then selected to expand the symbol. Each element of this expansion is in turn expanded with an appropriately matching rule, until the only remaining unexpanded symbols are words. The result of this CFG derivation is a tree-structured object T, whose periphery consists of the words of the sentence that is generated, called the yield of T. A CFG can be used as the basis of a probability model by assigning probability distributions for the possible expansions of each symbol (i.e., a value between 0 and 1 is assigned to each rule, with the values for the rules that share the same lefthand side summing to 1). In such a probabilistic CFG (PCFG), derivations proceed as with CFGs, but the choice of expansions is determined by the probabilities. In PCFGs, the probability of a tree structure is the product of the probabilities of each of the expansions. Because a sequence of words S might be generated by different trees, the probability of S is the sum of the probabilities of all of the trees T with yield S. Hale (2001) shows how to use PCFGs to calculate the surprisal for a word given a context: we take the summed probability of all trees whose yield begins with the context-word (i.e., the prefix probability for context-word) divided by the summed probability of all trees whose yield begins with the context (i.e., the prefix probability for context).

PCFGs of this form suffer from being unable encode dependencies between lexical items: the choice of the verb in a VP is made independently of the choice of the noun in the verb's NP object. A body of work in the literature in natural language processing has addressed this shortcoming by adding 'lexicalization' to a PCFG, and this is the approach we adopt, following (Roark et al., 2009).

1.2. Dimension 2: word vs. category prediction

As already noted, n-gram models with longer contexts suffer from an estimation problem: it is impossible to get accurate estimates of the likelihood of relatively infrequent words in contexts that are defined by sequences of, say, 5 words. We can avoid this problem by incorporating another aspect of abstract linguistic structure: the categorization of words in part-of-speech (POS) classes. We can define a POS n-gram model as one where both the context (and the predicted element are POS (e.g., noun, verb, determiner, etc.,). To compute the surprisal of a word w, then, equation (Attneave, 1959) becomes:

$\begin{array}{ll}{p}_{POS}(w|w_c)=\frac{count(c_c,c)}{count(c_c)})& (3)\end{array}$

where c_c is the POS of the context, and c is the POS of the target word.

This is what we use for our model of POS surprisal.

With a small set of POS labels, the probability values for longer n-grams can be accurately estimated. Note though that POS n-gram model is insensitive to the meaning of individual words, so it will be unable to distinguish the probability of “cat” and “magazine” occurring in any context, as they are both nouns, but could distinguish their likelihood from that of prepositions like “of” or adjectives like “furry”. As a result, this model's predictions for surprisal will differ from those of a word-based surprisal model.

Roark et al. (2009) propose a method for separating between word vs. category prediction in the context of a hierarchy-sensitive probability models. Specifically, for the category predictions, the prefix probability of the context-word sequence omits from the probability of the generation of the word. Following Roark et al., we call the resulting surprisal predictions Syntactic Surprisal. For word predictions, on the other hand, the context includes not only that contributed by the preceding words, but also the structure up to, but not including, the generation of the word. Again following Roark et al. (2009), we call the surprisal values computed in this way Lexical Surprisal.

2. Challenging data

In order to test different types of surprisal models a new set of stimuli has been designed building on Artoni et al. (2020). In that work the neural decoding of linguistic structures from the brain was found in carefully controlled data, where confounding factors such as acoustic information were factored out distinguishing this work from previous in the field such as in Frank et al. (2015), Brennan and Hale (2019), Shain et al. (2020). Specifically, their stimuli involved pairs of sentences sharing strings of two words with exactly the same acoustics (homophonous phrase, hence HP) but with completely different syntax. This strategy was made possible by relying on the properties of the Italian language. HPs could be either a Noun Phrase (NP) or a Verb Phrase (VP), depending on the syntactic structure that is involved. More specifically, HPs contained two words, such as la porta [la'pɔrta]: a first monosyllabic word (e.g., la) which could be interpreted either as a definite article (Eng. “the_fem.sing”) or an object clitic pronoun (Eng. “her”); a second polysyllabic word (e.g., porta) which could be interpreted either as a noun (Eng. “door”), or a verb (Eng. “brings”). The whole HP could be interpreted either as a NP (“the door”) in Pulisce la porta con l'acqua (s/he cleans the door with water) or as a VP (“brings her”) in Domani la porta a casa (tomorrow s/he brings her home) depending on the syntactic context within the sentence where they were pronounced. Crucially, there is a major syntactic difference between NPs and VPs even though they are pronounced in exactly the same way: in NPs the article is base generated on the left; in VPs, instead, the clitic is base generated on the right and it is then moved to the left, a syntactic operation called “cliticization” (Moro, 2016). Indeed, in Artoni et al. (2020) two different electrophysiological correlates have been found in multiple cortical areas in both hemispheres, including language areas, factoring sound out, for NPs and VPs. However, a potential problem remained as to how surprisal could interfere with the measure of syntactic information. In fact, the linguistic material preceding HPs was different in the NPs vs. VPs interpretation, such as in Pulisce la porta (s/he cleans the door) vs. Domani la porta (tomorrow s/he brings). These stimuli have been revised and refined: three novel experimental conditions have been generated by modulating the syntactic context preceding HPs, as followes:

(i) unpredictable HPs (UNPRED): the syntactic context preceding HPs allows both NPs and VPs since it is an adverb. Therefore, the syntactic types of HPs are not predictable at the beginning of the sentence, but only after the HPs: if HPs are followed by verbs (such as in Forse la porta è aperta, “Maybe the door is open”) they realize NPs, otherwise they realize VPs (Forse la porta a casa, “Maybe s/he brings it at home”). Since the lexical context preceding HPs is exactly the same for both NPs and VPs, no differences in the surprisal value can be detected at the HP.

(ii) Strong predictable HPs (Strong_PRED): the syntactic context preceding HPs allows either NPs or VPs (but not both) and, therefore, the syntactic type of HP is predictable at the beginning of the sentence: if HPs are preceded by verbs, they realize NPs (such as in Pulisce la porta con l'acqua, “S/he cleans the door with water”); if HPs are preceded by nouns, they realize VP (such as in La donna la porta domani, “A woman brings her tomorrow”). This was the kind of stimuli exploited in Artoni et al. (2020), where the lexical context preceding HPs was different in NPs and VPs, allowing different surprisal values in the two cases.

(iii) Weak predictable HPs (Weak_PRED): the syntactic context preceding HPs allows both NPs and VPs, as in the unpredictable HPs, thus the first word of the HP (la) could either be an article or a clitic pronoun, but the second word of the HP (porta) can only be analyzed as a noun (door), as in 1^st class predictable HPs, since the temporal adverb introducing the sentence (such as ieri, “yesterday”) requires a past tense whereas the verbal form of the HP displays a present tense (brings) (such as Ieri la porta era aperta, “Yesterday the door/^*brings it was open”). As in the unpredictable class, the surprisal value is eliminated by the lexicon preceding HPs, which is the same for both NPs and VPs (only the morphosyntactic shape of the second HP word forces the interpretation forward the NP).

A total of 150 trials were prepared: 60 for UNPRED-HPs, 30 UNPRED-NPs and 30 UNPRED-VPs, 60 for Strong_PRED-HPs, 30 Strong_PRED-NPs and 30 Strong_PRED-VPs, and 30 for Weak_PRED-HPs, only Weak_PRED-NPs since there cannot be VPs of this type.

3. Statistical analysis

We performed statistical analysis on the surprisal values calculated using the N-gram, Lexical, POS, and Syntactic surprisal of the 5 classes of stimuli (Strong_PRED-NP, Strong_PRED-VP, Weak_PRED-NP, UNPRED-NP, UNPRED-VP) relative to the first and the second word of the HPs. This analysis aimed at identifying the statistical language model that best differentiated between various linguistic stimuli in the same way as a human listener would do (e.g., distinguish Strong_PRED-NP and Strong_PRED-VP but not UNPRED-NP and UNPRED-VP).

Kruskal-Wallis tests revealed significant differences across the surprisal values associated with all five classes for all notions of surprisal. For the nouns and verbs, the difference was significant only for the POS surprisal and the syntactic surprisal. We further investigated these differences using Conover post-hoc tests with Holm-Bonferroni correction. For the articles and clitics, only the syntactic surprisal captured the difference across all three classes of predictable items (p < 0.0001, Figure 2A, top row). The POS and N-gram surprisal values of the articles were lower than those of the clitics (p < 0.05), while the lexical surprisal values of the articles of the Strong_PRED-NP sentences were lower than the lexical surprisal values of the articles of weak_PRED-NP sentences and the clitics of Strong_PRED-VP sentences. For nouns and verbs, both the POS surprisal and the syntactic surprisal showed a difference between all three stimuli classes (p < 0.05, Figure 2A, bottom row). There was no difference between the N-gram surprisal values or lexical surprisal values of nouns and verbs. For the unpredictable items, only the POS surprisal values were different between the articles and clitics and between the nouns and verbs (Figure 2B).

FIGURE 2

Figure 2. Statistical analysis and decoding. (A) Boxplots of the surprisal values for the (strong and weak) predictable items for the articles/clitics (art./cl., top row) and the nouns/verbs (N/V, bottom row). Each column represents a distinct notion of surprisal. (B) Same as (A) but for unpredictable (Unpred) items. (C) Boxplots of the accuracies for the distinct classification tasks using different sets of features. Each data point is the accuracy of 1 fold in a 10-fold cross validation procedure. The red dashed lines are the chance levels. Strong Predictable N vs. V: classification task (i). (Strong and weak) Predictable N vs. V: classification task (ii). Unpredictable N vs. V: classification task (iii). Predictable vs. unpredictable: classification task (iv). For each set of features both the surprisal of the article/clitic and of the noun/verb were considered. The set of features are: ngram – N-gram surprisal; lex – Lexical surprisal; pos – POS surprisal; syn – Syntactic surprisal; tot – all of the above.

We defined four different classification tasks: Strong_PRED nouns vs. verbs (i), predictable (Strong_PRED and Weak_PRED) nouns vs. verbs (ii), UNPRED nouns vs. verbs (iii), and predictable items vs. unpredictable items (iv). For each classification task we trained and validated (10-fold cross validation) one Support Vector Machines (SVM) for each notion of surprisal (i.e., using the values calculated according to the given notion of surprisal as features), and one SVM trained on all surprisal values regardless of the surprisal type, called tot-SVM. For classification tasks (i), (ii), and (iv), the SVMs trained on POS surprisal, Syntactic surprisal, and the tot-SVM reached near 100% accuracy, above the other two classifiers (p < 0.05, Conover post-hoc with Holm-Bonferroni correction). For classification task (iii), tot-SVM and the POS surprisal-trained SVM reached 100% accuracy, while Syntactic surprisal-SVM achieved slightly above-chance accuracy (Figure 2C).

4. Discussion and conclusion

In this paper four different probability models of surprisal have been compared by exploiting the following contrasting factors: words vs. parts-of-speech and sequences vs. hierarchical structures. In order to test these models three experimental conditions have been generated by modulating the surprisal context: those where the phrase was completely unpredictable by the contexts (unpredictable phrases), those where the phrase was immediately predictable by the first word of the phrase (strong predictable phrases), and those where the phrase was predictable only after the second word of the phrase (weak predictable phrases). Notably, all confounding factors, including acoustic information, were factored out distinguishing our work from previous in the field such as in Frank et al. (2015), Brennan and Hale (2019), Shain et al. (2020). We found that only those models combining hierarchical structures and part-of-speech categories successfully distinguished the three classes. On the other hand, surprisal models that only considers sequences of both words and parts-of-speech fail to replicate the expectation associated to the three classes. All in all, our modeling results point to the conclusion that statistical surface distributions are insufficient for capturing subtle distinctions in linguistic patterns.

Conspicuously absent from our discussion of language models are ones based on deep neural networks. Apart from their enormous success in practical tasks in natural language processing [e.g., as seen with the large language models (LLM) underlying systems like ChatGPT (Floridi and Chiriatti, 2020)], such models have also been used to model neural activity during sentence processing via the surprisal values they provide (Goldstein et al., 2022; Heilbron et al., 2022; Russo et al., 2022). On the surface, it would appear that such models belong to the class of linear lexical models (on a par with n-grams), as they do not appear in embody any sort of linguistic abstraction. As such, their success in modeling neural activity would provide a counter-example to the claims in this paper. However, because of their complexity, the factors governing the behavior of such models is quite obscure, and indeed studies of the internal representations of some of these models has found that they do indeed encode linguistic abstractions, incorporating both grammatical categories and hierarchical structure (Lin and Tan, 2019; Tenney et al., 2019; Manning et al., 2020). Yet, because of their complexity, it is virtually impossible to determine the precise role played by such abstractions in the computation of word probabilities, and for this reason we leave these models aside.

Eventually, it is important to note that the work reported here does not take into account brain data: the preliminary goal chosen here, in fact, is rather to determine what properties a statistical model of language needs to have in order to distinguish among different types of linguistic stimuli modulating surprisal. Nevertheless, our research does lead to a better comprehension of brain data as well: for example, the electrophysiological data observed in Artoni et al. (2020), as considered under the novel perspective proposed here, show that for those brain data to be fully understood, syntactic notions must necessarily be included in surprisal models.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

Conceptualization and funding acquisition: AM. Methodology: MG, AC, FA, and RF. Visualization: AC and RF. Supervision, writing—original draft, and writing—review and editing: MG, AC, FA, RF, and AM. All authors contributed to the article and approved the submitted version.

Funding

Ministero dell'Università e della Ricerca (Italy) grant: INSPECT-PROT. 2017JPMW4F_003.

Acknowledgments

The authors would like to thank Silvestro Micera and Claudia Repetto, also involved in PRIN-INSPECT project as heads of a different units, and Stefano Cappa for their precious suggestions.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher's note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/flang.2023.1178932/full#supplementary-material

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