Fusion and the Acquisition of s-nasal Clusters

Rebecca HansonUniversity of Calgary

1. Introduction

In this paper, I consider an integrated approach to the study of child language and

examine the interaction of various aspects of the developing phonology, including both prosodic

and melodic structure. The fusion hypothesis I propose incorporates information from both

areas in order to explain a child’s acquisition of s-nasal onset clusters. The specific problem

addressed is the emergence of a temporary voicing contrast among the nasal segments, with

voiceless nasals appearing as substitutes for the s-nasal clusters. Because this contrast is non-

target, it is difficult to explain using prevailing models of feature geometry. Thus, in the course

of my analysis I propose some revisions to the traditional representation of nasal segments.

The organization of the paper is as follows. I begin by summarizing the theoretical

assumptions which form the backbone of my analysis, followed by a survey of relevant material

from the literature. In section 3, I present and analyze data on s-nasal clusters from one

normally-developing child. I also propose that fusion, a process well-attested in adult languages,

can also apply in child languages. In section 4, the fusion analysis is tentatively extended to s-

clusters involving other sonorants; then, several problems I encountered in section 3 are noted

along with possible solutions. I conclude with a summary of the paper and proposals for further

research.

2. Background

2.1 Theoretical Assumptions

2.1.1 Feature Hierarchy

Following many phonologists (Clements, 1985; Rice & Avery, 1991; Brown, 1997;

Piggott, 1992), I begin with the assumption that features are not unordered bundles, but are

hierarchically arranged in a UG-constrained structure. This feature hierarchy, or feature

geometry, is responsible for all existing phonological systems, though no one language will

exploit the entire structure. A feature geometry model, then, must be able to account not only

for what processes do occur in human language, but also for those that don’t, and why. As

proposed by Clements (1985) and modified in Rice & Avery (1991), the feature hierarchy is

composed of two major node types: organizing nodes and content nodes. Organizing nodes are

dominated by the Root node; they organize related content nodes1. Content nodes have actual

articulatory status, such as Labial or Spread Glottis, and dominate secondary content nodes or

terminal features. An example of a feature hierarchy is given in (1), taken from Rice & Avery

(1995).

The nodes and features are hierarchically related to each other by dependency; for

example, a terminal feature is said to be a dependent of the node immediately dominating it.

Thus, in (1) the feature [coronal] is a dependent of the Place node, and [lateral] is a dependent of

the Oral content node.

Following Avery & Rice (1989) I assume that phonological processes are restricted in the

hierarchy to three operations: spreading, delinking, and fusion. While the details of spreading

are not immediately relevant here, the general conditions are given for reference in (2); for

further information see Avery & Rice (1989; henceforth A&R).

(2) a. Spreading can occur only if a structural target is present-prevents node generation

b. A feature or node can spread only to an empty position-ie. spreading cannot trigger delinking

Fusion plays a central role in section 3.2. A&R define fusion as "an operation which

takes identical primary content nodes and fuses them provided that the nodes are non-distinct;

i.e. both nodes do not dominate different secondary nodes." Fusion is either right-headed or left-

headed, depending on the position of the triggering segment, and is driven by the Obligatory

Contour Principle (McCarthy 1986; Yip 1988, cited in A&R) to reduce two identical adjacent

elements to one. The outline of a general fusion process, taken from A&R, is given in (3):

Note that only the secondary features of the head are maintained.

2.1.2 Acquisition is Structure Building

If the Feature Hierarchy is, as I assume, a reasonable model of phonological

representation, then it follows that any proposed hierarchy must also account for child language;

it must be obedient to the same learnability and continuity considerations that are important to

all areas of child language theory. There are two main theoretical approaches to the acquisition

of the feature geometry: the Pruning Hypothesis and the Building Hypothesis (Brown, 1997).

The Pruning Hypothesis assumes that the Universal Grammar (UG) supplies a fully

differentiated structure, with representations for any contrast found in any language. As the

child realizes that not all possible contrasts are used in the ambient language, the unnecessary

structure is "pruned" away. According to the Building Hypothesis, however, UG supplies only

minimal structure, which is elaborated in response to contrasts detected in the input. Global

uniformity results from the constraint in UG on the general order of acquisition (ie. Organizing

nodes must be acquired before Content nodes, which must be acquired before any of their

dependent nodes or features); variability results from the fact that the child is free to begin

acquisition at any Organizing node, and may continue by elaborating either within or outside

that node. That is, acquisition is constrained in the general order, but is free in the specifics.

Like Brown (1997) I adopt the Structure Building Hypothesis, and assume that an adult

phonology is achieved by elaborating the minimal structure to allow the necessary contrasts. An

important part of this assumption is that the hierarchy is expanded in a step-by-step fashion, or

monotonically; new structure is added only when necessary to represent a contrast used in the

ambient language. The representation will therefore be underspecified, and only the features

involved in a contrast are specified underlyingly. In addition, I assume that the features

themselves, as well as the organizing and content nodes, are monovalent (A&R; Brown, 1997)

and that a contrast can be maintained by the presence versus absence of a feature or node, rather

than through plus/minus values.

2.1.3 Context-sensitive Radical Underspecification

In my analysis I will be working from the assumption that Context-sensitive Radical

Underspecification is an acquisition phenomenon2. Context sensitivity was proposed by Dinnsen

(1996a, 1996b) when he encountered continuity violations in other underspecification

approaches to child data. He examined longitudinal data, and concluded that the default setting

of a feature can depend on its position in the syllable. Specifically, he found that the features

[voice] (Dinnsen, 1996a) and [continuant] (Dinnsen, 1996b) both seem to exhibit context effects.

Other child phonologists (Velleman, 1996; Macken, 1996; Ingram, 1996) have noted the

influence of syllable structure on features. The context sensitivity of [voice] is important here,

for determining when the voicing contrast is actually acquired; until the child consistently

produces target-like voicing in both onset and coda position, I assume that [voice] is not yet

contrastive.

2.1.4 The Distinctive Feature Hypothesis

In order to determine more accurately the features that belong to Amahl’s Underlying

Representation, I will make use of Ingram’s (1996) Distinctive Feature Hypothesis, given in (4)

below.

(4) The Distinctive Feature HypothesisAny feature assigned to a child’s representation must be present in the target phonemes.

Or, conversely, a feature cannot be assumed to be part of the child’s representation of a

particular segment, unless that feature is present in all the target phonemes for which that

segment is used. Thus, for a child who uses [d] for /b,d,g/ and [t] for /p,t,k/, the only distinctive

feature active is [voice], since that is the only feature consistent with all target phonemes. This

hypothesis guarantees that compatibility considerations will be obeyed, and will form an

important basis for my analysis.

2.1.5 Minimal Word

Fee (1995, 1996) and Demuth (1996) propose four stages of prosodic development;

Stage II, the Minimal Word Stage, is specifically relevant here. The Minimal Word (or Wdmin)

is supplied by UG as a bimoraic Foot. Fee (1996) proposes three substages within Stage II,

given below in (5).

(5) Stage IIa: bisyllabic core syllables (Wdmin= µµ)Stage IIb: coda consonants appear (Wdmin= µµ)Stage IIc: long vowels and complex vowels emerge (Wdmin= µµ)

The data in Smith (1973) indicates that Amahl is producing bimoraic monosyllables (µµ)

until about stage 10, when complex onsets appear. Amahl’s elaboration of prosodic structure

will be central to the analysis in section 3.

2.2 Literature Review

Rice & Avery (1995) were among the first to apply their feature hierarchy model,

originally developed for adult languages, to the acquisition process. Many phonologists have

since taken advantage of child data to assess the various hierarchies; in this paper I will refer

specifically to Brown (1997) and Ingram (1996), both of whom have worked with R&A’s model

and have discussed its strong and weak points. Brown did extensive cross-linguistic research

into the behavior of laterals, and her conclusions will prove valuable to my discussion of nasals.

She incorporates several feature geometries, including Rice &Avery (1991) and Piggott (1992),

into a unified model which, it turns out, provides a solution to a concern Ingram (1996) raises

about the Rice & Avery model; namely, that many children begin acquisition with a nasal vs oral

distinction. As Brown points out, R&A predicts that the sonorant/obstruent contrast must come

before a nasal/oral sonorant contrast. Consequently, Brown revises the R&A structure of the

Sonorant Voice (SV) node: the bare SV, instead of being realized as a nasal, is realized as an

approximant3; [nasal] is a phonological feature dependent on the SV node. The hierarchy, with

relevant revisions, is given in (6), though I will propose further revisions later on.

I have retained the simplified Laryngeal node from (1). Vocalic and Lateral have both

been removed from the hierarchy as a result of the revisions.

1 For the criteria that determine whether a proposed node can validly be considered part of the universal feature geometry, see McCarthy (1988; cited in Brown, 1997)

2 Given that there are several processes such as word-final devoicing of obstruents, it is possible that context sensitivity extends beyond acquisition into fully developed languages.

3 This designation addresses another difficulty with R&A, that approximants cannot be represented.

The aspects of this model which are most relevant to my discussion are the SV node and

the Airflow node. The SV node is argued for in Rice & Avery (1991), Piggott (1992), and Rice

(1993); the internal structure of the Airflow node, however, is admittedly uncertain at this point.

Brown (1997) makes use of the Airflow node in her discussion of laterals. She concludes,

based on data from both child and adult languages, that [lateral] is not a phonological feature.

Instead, laterality is the phonetic result of a combination of features. She distinguishes between

sonorant and non-sonorant laterals ([l] and [] respectively), and proposes distinct

representations of each:

For a motivation of these structures see Brown (1997); however I would like to draw

attention to one statement she makes: "The two representations are homologous in that the

features [approximant] and [continuant] can be said to be functionally equivalent" (p.31). In

other words, either feature can be used to signal continuous airflow through the oral cavity. I

suggest that a further parallel can be drawn, involving nasals.

The proposal that nasals can be represented in two ways is offered by Piggott (1992).

Research into the cross-linguistic behavior of nasality led him to propose that [nasal] could be a

dependent of either the SV node or what he calls the Soft Palate (SP) node4. My proposal is

very similar, but instead of adding an SP node to the hierarchy in (6), I will make use of the

Airflow node. Briefly, I suggest that continuancy can be either oral or nasal depending on the

direction of the air flow (which is, incidentally, determined by the action of the soft palate5),

therefore nasals do not actually involve a total obstruction of the air. It is for this same reason

that Ladefoged & Maddieson (1996) avoid using the term ‘nasal stop’ to refer to nasals.

3. Data and Analysis

3.1 Data

Consider the data in Figure 1 (taken from Smith, 1973).

target production stage target production stage‘Smith’ [mit] 1 ‘snow’ [nu:] 14

[mit/mit] 15 [no:] 15[mit] 16 [nu] 21[smis] 25

Figure 1. Target versus production in sN clusters for Amahl

Amahl consistently follows the same route in acquiring s-nasal clusters (henceforth

referred to as sN clusters). He begins by apparently deleting the /s/ altogether; then, around

Stage 15 he replaces the cluster with a voiceless nasal, developing a non-ambient contrast

between voiced and voiceless nasals. The result is near-minimal pairs like those in (8).

(8) a. [nu:] ‘new’ b. [m:(r)] ‘more’[no:] ‘snow’ [m:l] ‘small’

The contrast remains in use from Stage 15 to about Stage 25, which is when he begins to

consistently produce sN clusters. It is interesting to note that the target-like production of /sN/

coincides with the loss of a voicing contrast among the nasals.

3.2 Analysis

4 Kenstowicz (1994) also indicates [nasal] as a dependent of the SP node.

5 Piggott (personal communication) does suggest that in English, [nasal] is SP dependent.

Considering that many English children develop this temporary voicing contrast, it is

surprising how difficult it is for feature geometries to account for it. Using the model given in

(6), several problems arise, beginning with the awkwardness of representing a voiceless nasal.

The voiced counterpart is easily modeled by the existence of [nasal] dominated by the SV node.

Voicelessness might be obtained by delinking the source of the voicing, the SV node, but that

would clearly also result in the loss of nasality. Or, alternatively, the contrast could involve the

addition of the Laryngeal node with an accompanying voiceless designation. However, this

solution seems very arbitrary, with no apparent motivation.

Perhaps the problem lies not with the feature geometry, but with the traditional view of

nasals as stops. Recall from section 2.2 that, while nasals do entail a total obstruction of oral

airflow, there is still continuant airflow through the nasal cavity. If it is true that nasals thus

share some properties with continuants, there should be evidence in adult languages. In fact,

Piggott (1992) cites at least two languages, Igbo and Applecross Gaelic, in which fricatives are

targets of nasal spreading. According to the conditions in (2), for fricatives to be targets of nasal

spreading, both [nasal] and [continuant] must be dependents of the same node.

Based on this possibility, consider the following revision of the Airflow node:

The elaboration under ‘cont’ (continuant) reflects the possible choice between oral and

nasal continuancy. I will assume that the defualt is ‘oral’. Because nasality is no longer solely

dependent on the SV node, it is now possible to represent voicelessness by simply adding the

Laryngeal voiceless feature. Note that this representation is only motivated when nasals and

fricatives pattern together, so in order to consider the representation in (9) we must determine if

that patterning occurs with Amahl.

At first, it appears that this is not the case; there are no instances in which a lone nasal

substitutes for a fricative, or vice versa. In fact only the instance where a nasal appears for a

target fricative is in /sN/ clusters. However, according to the Distinctive Feature Hypothesis,

that means that the nasal must bear the features of both /s/ and /N/: continuancy, nasality, and

voicelessness. At least in this context, then, suppose the following representation of nasals:

While we now have a possible representation for the voicing contrast seen in (8), we still

can’t explain why the contrast appears only in the case of /sN/ clusters, or why the contrast is

only temporary.

In order to get a clearer idea of what process is involved here, it would be helpful to

consider some of the general aspects of Amahl’s acquisition that could be playing a role. We

have already seen (section 2.1.5) that he produces only Minimal Words until stage 10; that is,

until then he does not allow branching onsets. Clusters involving /s/ do not appear until around

stage 26. The (obstruent) voicing contrast, reflected by the target-like presence of voiceless

onsets and voiced codas (the marked values according to context-sensitive underspecification),

emerges around stage 12 or 13. Labial continuants appear in stage 15, coronals not until stage

22. The relevant contrasts and the stages at which they are acquired, are summarized in Figure

2:

Stage Contrast Gained Sample sN production10 branching onset clean-->[gli:n] N13 obstruent voicing pig-->[pig] N15 (oral) continuancy foot-->[fut/ ut] N/N22 coronal continuant /s/ see-->[si:] N26 s-clusters stop-->[st p] SN

Figure 2: Corresponding contrasts and sN productions for Amahl

Considering this information about Amahl’s phonology, I propose the following

explanation for his /sN/ acquisition.

Before stage 15, Amahl is able to represent sonorants and stops, but not continuants. It is

not surprising, then, that an sN-cluster would be realized as a nasal while an s+stop cluster is

realized as a stop. Thus, based on the proposal so far, we would expect any cluster involving

continuants -- including s-clusters -- to emerge when the oral continuants are acquired at stage

15. Instead, this is exactly when the voicing contrast among nasals appears. What is the

connection?

At this point, I refer to Brown’s (1997) proposal that [lateral] is not phonological feature,

but the phonetic realization of certain configuration of features (see (7) above). Suppose that

nasality could similarly result from the combination of a bare SV node plus continuant nasal

airflow (the SV node is required to account for the fact that singleton nasals are always voiced).

In that case, before stage 15 the only difference between /s/ and /N/ (ignoring place features) is

that /N/ has an SV node, as in (11).

At stage 15, the emergent obstruent voicing contrast means that /s/ must be specified for

Laryngeal features; also, the oral continuant /f/ has appeared by now, so the direction of airflow

must be designated, as in (12):

Recall that bare ‘continuant’ is realized as oral. I have not specified Place features, but

note that the first oral continuant to appear is a labial. Thus, although the oral/nasal continuant

contrast is now emerging, the coronal fricative has not yet been acquired. This leads to an

explanation of the voiceless nasals, for the following reason.

Suppose that, in order to fill his Minimal Word template while remaining as faithful as

possible to the target features, Amahl has made use of fusion as a repair strategy. That is, rather

than simple truncation (deleting the unacquired coronal fricative), he fuses the identical Airflow

nodes for as long as necessary; until /s/ is acquired. That would explain why s-clusters do not

appear after /s/ itself appears; I propose that it also explains why the voicing contrast in nasals is

temporary, and why it is restricted to target sN clusters, as illustrated by the fusion process

described below.

Recall the structures of /s/ and /N/ before Stage 14. Neither has any feature below

[continuant], so fusion can occur, though it is vacuous. At this point the headedness of the

fusion cannot be determined. The process at this point is shown in (13).

At the introduction of the Laryngeal voicing distinction (Stage 15), fusion results in a

structure that has both Laryngeal voicing features and Sonorant Voicing; the SV node delinks in

this situation6. Because the emergence of oral continuants now requires that /s/ be specified as

oral, the headedness of the operation becomes apparent: the resulting nasal continuant

(phonetically voiceless) indicates that fusion is right-headed (RH), as in (14).

(14) Fusion at Stage 15

At stage 25, the coronal continuant is now fully distinct from other continuants. The oral

continuancy must now be specified, with the result that the [continuant] nodes for /s/ and /N/ are

now distinct and can no longer fuse. This blocked fusion is represented in (15).

6 At this point I cannot explain why the SV node would delink in the presence of Laryngeal features. It may have something to do with the phonetic impossibility of combining (laryngeal) voicelessness with (sonorant) voicing. Or, the reason could involve the nature of the voicing for each: the laryngeal state is a specific property of obstruent voicing, but in sonorants, voicing is a property of the general configuration of the entire vocal tract (Ladefoged, 1996).

(15) Blocked fusion at Stage 25

Together, then, numbers (13) through (15) above supply a phonologically motivated,

continuous account of both the emergence and the disappearance of the voicing contrast in

nasals.

4. Discussion

4.1 Generalizing the Analysis

The analysis I have given above involves the fusion of sN clusters into a single segment,

with the result being a sort of "greatest common denominator" of features. After considering

whether this analysis can deal with other clusters, I would like to point out some of the

difficulties with the fusion proposal in general.

Since nasals traditionally belong to the class of sonorants (even though I have suggested

that [nasal] is not a dependent of the SV node for English) it is likely that other sonorants would

also show the same pattern of acquisition. There is some indication that this is the case.

Consider the data in Figure 3.

/sl/ cluster stage /sw/ cluster Stage

‘sleep’-->[wi:p] 2 ‘sweet’-->[wi:t] 6

-->[li:p] 11 -->[vit/wi:t] 15

-->[i:p] 15 -->[wi:t/fi:t] 16

-->[sli:p] 25 -->[fi:t] 17

-->[fi:t/swi:t] 25

Figure 3. Target versus production in /sl/ and /sw/ clusters for Amahl

It is interesting to note the similarities between these s+sonorant clusters and the sN

clusters in Figure 1. In all cases, the substitution pattern is the same: until stage 14, the /s/

disappears; at stage 15 a voiceless sonorant is produced; and finally, at around stage 25, the s-

cluster emerges. While a detailed analysis of the acquisition of each cluster would take me

beyond the scope of this paper, it seems very possible that the same process used for /sN/

clusters could account for the acquisition of all s+sonorant clusters, at least for Amahl. There are

two factors, however, that could make an analysis difficult.

First, regarding the /sl/ clusters, it is not easy to tell exactly when the lateral itself is

acquired. As I have mentioned, Brown (1997) proposes that [lateral] is not a phonological

feature, but is the phonetic result of both a bare Place node and either the SV node (sonorant [l])

or the Airflow node (non-sonorant []). This structure is often produced by spreading

operations7, making it difficult to discern whether a surface lateral is contrastive, or simply the

result of a process like consonant harmony.

The other difficulty involves the representation of approximants. There is no general

consensus in the literature regarding this. If we assume, with Brown, that they are the realization

of a bare SV node, how can we explain the alternation between /w/ and the oral continuant /f/?

Perhaps a closer look at the acquisition patterns of clusters involving approximants could help

identify the representation of these segments. Also, note that both nasals and laterals could

possibly be the phonetic result of a hierarchy configuration that involves the Airflow node.

Perhaps the representation of approximants is similar.

7 See Brown (1997) for a discussion of this phenomenon.

Bibliography

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4.2 Problems with the Analysis

Although it shows promise in dealing with s-clusters, there are several difficulties with

the fusion analysis presented here, and these problems must be resolved before the proposal can

be considered viable. One source of awkwardness involves the fusion itself: in the above

analysis, the identical Continuant nodes of two adjacent segments (both constituents of the

syllable onset) merge to yield a single segment. The problem with this analysis is that it

apparently involves the unaccountable disappearance of a Root node. To be consistent with

A&R’s description of fusion, given in section 2.1, the /sN/ fusion should instead be represented

as follows:

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& Linguistic Theory, 10, 33-37Rice, K. & Avery, P. (1991). On the relationship between laterality and coronality. In C.

Paradis & J.F. Prunet (Eds.) Phonetics and Phonology: The Special Status of Coronals, Vol. 2, (pp.101-124). San Diego, CA: Academic Press.

Rice, K. & Avery, P. (1995). Variability in a deterministic model of language acquisition: A theory of segmental elaboration. In J. Archibald (Ed.) Phonological Acquisition and Phonological Theory (pp.1-22). Hillsdale, NJ: Lawrence Erlbaum.

Rice, K. (1993). A re-examination of the feature [sonorant]: The status of ‘sonorant obstruents’. Language, 69, 308-344.

Smith, N. (1973). The Acquisition of Phonology. Cambridge, MA: Cambridge University Press.

Velleman, S. (1996). Metathesis highlights feature-by-position constraints. In B. Bernhardt, J. Gilbert & D. Ingram (Eds.), Proceedings of the UBC International Conference on Phonological Acquisition (pp.173-186). Somerville, MA: Cascadilla Press.

Yip, M. (1988). The OCP and phonological rules: a loss of identity. Linguistic Inquiry, 19, 65-100

Because there are two Root nodes, the result of fusion is technically a geminate, and

should surface as a phonetically long nasal (unless, for some unclear reason, both Root nodes

attach to a single timing slot). This would be very difficult to detect, and it is impossible to tell

from the data if it is in fact the case. Alternatively, the Avery & Rice designation might be too

strict; perhaps, in certain circumstances, Organizing nodes can fuse. Itô & Mester (1993)

propose the fusion, in Japanese, of adjacent Root nodes over a syllable boundary. It may be

possible, then, that the /sN/ fusion proposed here actually involves the fusion of Root nodes.

Further consideration of the implications of such a proposal would be needed, especially

regarding the restrictions on when Root fusion is allowable.

Another difficulty with the analysis in section 3.2 is that it predicts that s-clusters should

emerge very close to the stage when /s/ is acquired. Figure 2, however, reveals that several

stages intervene. At present, I cannot account for the delay. It is possible that other factors

could be at work: if /sN/ clusters are relatively rare in English, input effects could contribute to

their late acquisition. On the other hand, the delay might be caused by English constraints on

onset consonant clusters, specifically that these clusters must agree in voicing. /sN/ clusters are

exceptions to this constraint, and it may take the child longer to identify them as such. If that is

the case, it would be helpful to examine data from a language in which there is no constraint on

voicing agreement.

5. Conclusion

The acquisition of nasal segments is an area of child phonology that has not received

much attention. The approach followed in this paper involves the interaction of many aspects of

phonological acquisition, including prosodic structure, voicing and continuancy contrasts, the

role of sonorant voicing in nasals, and, to some extent, place contrasts. All of these areas, I

propose, have contributed to the output form produced in place of target sN onset clusters. The

patterns I observed in Amahl’s acquisition led me to revise a traditional assumption about nasals;

namely, I dispute their designation as stops and suggest that they share properties with

continuant segments. This revision allowed the proposal of fusion as a strategy for the

representation of sN clusters.

In this paper, I have examined a very limited range of data; therefore my proposals are

tentative and only intended as a possible starting point for further study. Specifically, the

problems I have noted regarding the fusion analysis need to be resolved. Cross-linguistic data

on the acquisition of sN clusters should be helpful in this regard, especially if languages that

contrast voicing in nasals are included. Further research should also include an evaluation of the

fusion hypothesis in different contexts, such as other continuant-sonorant clusters (e.g. /fl/ and

/fr/ in English), and coda clusters involving nasals.