Social and Functional Pressures in Vocal Alignment: Differences for Human and Voice-AI Interlocutors

Abstract

Increasingly, people are having conversational interactions with voice-AI systems, such as Amazon's Alexa. Do the same social and functional pressures that mediate alignment toward human interlocutors also predict align patterns toward voice-AI? We designed an interactive dialogue task to investigate this question. Each trial consisted of scripted, interactive turns between a participant and a model talker (pre-recorded from either a natural production or voice-AI): First, participants produced target words in a carrier phrase. Then, a model talker responded with an utterance containing the target word. The interlocutor responses varied by 1) communicative affect (social) and 2) correctness (functional). Finally, participants repeated the carrier phrase. Degree of phonetic alignment was assessed acoustically between the target word in the model's response and participants' response. Results indicate that social and functional factors distinctly mediate alignment toward AI and humans. Findings are discussed with reference to theories of alignment and human-computer interaction.

Full text

Social and functional pressures in vocal alignment:
Differences for human and voice-AI interlocutors
Georgia Zellou and Michelle Cohn
UC Davis, Phonetics Lab, Department of Linguistics, Davis, CA, USA
Abstract
Increasingly, people are having conversational interactions
with voice-AI systems, such as Amazon’s Alexa. Do the same
social and functional pressures that mediate alignment toward
human interlocutors also predict align patterns toward voice-
AI? We designed an interactive dialogue task to investigate
this question. Each trial consisted of scripted, interactive turns
between a participant and a model talker (pre-recorded from
either a natural production or voice-AI): First, participants
produced target words in a carrier phrase. Then, a model talker
responded with an utterance containing the target word. The
interlocutor responses varied by 1) communicative affect
(social) and 2) correctness (functional). Finally, participants
repeated the carrier phrase. Degree of phonetic alignment was
assessed acoustically between the target word in the model’s
response and participants’ response. Results indicate that
social and functional factors distinctly mediate alignment
toward AI and humans. Findings are discussed wit h reference
to theories of alignment and human-computer interaction.
Index Terms: vocal alignment, human-computer interaction,
social vs. functional pressures
1. Introduction
Interacting talkers systematically align toward the acoustic-
phonetic patterns of each other’s speech to sound more alike.
This phenomenon is known as vocal alignment (also phonetic
imitation) and has been well reported in the literature [1]–[3].
There is also evidence of vocal alignment toward speech
generated by non-human entities: people align toward the
speaking rate [4] and amplitude [5] of synthetic computer
voices, and even toward speech patterns of modern voice-
activated artificially intelligent (voice-AI) systems (e.g.,
Apple’s Siri, Amazon’s Alexa [6]–[10]). That we see an
application of this human behavior toward technology is in
line with the ‘Computers as social actors’ theory (CASA, [11],
[12]), which posits that people’s behavior during interactions
with technology mirrors their behavior toward humans.
Above and beyond documenting the presence of alignment
toward computers and voice-AI, a growing body of work has
suggested that the magnitude of speech alignment may differ
by interlocutor: individuals tend to show weaker vocal
alignment toward voice-AI, relative to human interlocutors
[6], [8], [9]. Conversely, others have found that people adopt
the lexical choices [13] and syntactic structures [14] for the
computers to a greater extent than for human interlocutors.
Why do we see these conflicting results? Aside from this
alignment occurring at different linguistic levels (e.g.,
phonetic vs. syntactic), one possibility is that modern voice-AI
has different functional and social pressures in communication
than computer systems or avatars. For one, the primary way
we communicate with voice-AI systems is using speech, a
uniquely human form of communication. Also, unlike
computer systems in the past, voice-AI systems have
improved text-to-speech (TTS) synthesis [15] and automatic
speech recognition (ASR) abilities; therefore, the functional
pressures may be lessened. This might explain mixed findings
of less alignment toward voice-AI, but greater alignment
toward computer systems. On the other hand, modern voice-
AI systems exhibit greater social cues, such as having names
(e.g., “Alexa”) and apparent genders. There is even evidence
that humans engage with voice-AI in for purely social
purposes, for example having short, non-utilitarian
conversations with “chatbots” [16]. Therefore, another
possibility is that humans will respond to apparent sociality in
voice-AI more similarly as they would toward another human.
An alternative possibility, following the ‘Uncanny Valley of
the Mind’ framework [17], [18] is that as cues of human-
likeness near ‘real’ human levels, it can trigger feelings of
uneasiness or ‘uncanniness’ toward robots.
The current study investigates the nature of human-AI and
human-human interaction by examining whether degree of
vocal alignment is different based on social and functional
pressures within a verbal exchange. Moment-by-moment, the
dynamics of an interaction vary: speakers may become more
animated and emotionally expressive (social) or make errors
that require correction (functional). Parametrically
manipulating these factors across interlocutors (human vs. AI)
can pinpoint differences, and similarities, in the way humans
engage with voice-AI, relative to humans, and can shed light
more broadly on the nature of human-AI interaction.
Furthermore, this study can contribute to understanding the
mechanisms of vocal alignment, as driven by social dynamics
(§1.1.) and/or pressures to improve intelligibility (§1.2.).
1.1. Social factors
Synchrony between interlocutors is thought to serve as ‘social
glue’. ‘Communication Accommodation Theory’ (CAT [19])
proposes that alignment is used to foster social closeness: the
gender [1], regional affiliation [20], attractiveness [1], and
emotional state [7] of interlocutors predicts alignment. In the
present study, one social dimension is whether the interlocutor
is human or AI; here, as in prior work, we might predict less
alignment toward voice-AI, relative to humans [6], [8].
Further, the current study manipulates within speaker
social dynamics by including an expressive interjection in
some model talker utterances. Interjections, or emotively used
words such as “Yipee!” or “Darn!”, are conventionalized
phrases. Interjections often provide no additional linguistic
meaning to an utterance; their function is purely social in that
they convey the speaker’s emotional or cognitive state [21],
[22]. There is also some evidence that individuals are sensitive
to these interjections, even when they are produced by voice-
{gzellou, mdcohn}@ucdavis.edu
Copyright © 2020 ISCA
INTERSPEECH 2020
October 25–29, 2020, Shanghai, China
http://dx.doi.org/10.21437/Interspeech.2020-13351634

AI: [7] found that participants aligned more to Amazon’s
Alexa voice when they shadowed interjections realized in
hyper-expressive prosody, relative to Alexa’s neutral prosody.
Therefore, in the current study, we utilize these expressive
interjections, not as target words, but as additions to responses
made by the model talker as a way to increase the socio-
communicative force of the utterance. If increased
expressiveness modulates vocal alignment, we predict that
participants will align toward the interlocutor to a greater
extent when the interlocutor’s response contains an expressive
interjection (e.g., “Super! I think I heard boot”), relative to
when they do not (e.g., “I think I heard boot”). Yet,
interjections add no semantic content to the utterance. If the
addition of these expressive-encoding items does not add to
the sociality of the interaction, we predict no difference in
alignment patterns when they are present or not.
With respect to social factors, we might also predict
different alignment patterns toward the human and voice-AI
model talker as a function of the socio-communicative
expressiveness of the interaction. For one, modern-day voice-
AI can play a social role for humans. As previously
mentioned, voice-AI systems are increasingly assuming more
human-like qualities (e.g., more realistic voices, better speech
recognition, etc.). Thus, one prediction is that alignment
toward AI will increase with interactions containing
interjections, paralleling what we expect for alignment toward
human interlocutors. This would support computer
personification frameworks, e.g., CASA [11]. Alternatively,
displays of socio-expressiveness might be negatively
perceived by participants. This would support observations of
an ‘uncanny valley’ in people’s behavioral responses toward
technology which take on near-human qualities [17]. For
example, [18] observed that conflicting cues to human-
likeness in a non-human entity violates people’s expectations
of technology and leads to feelings of uneasiness or
discomfort toward voice-AI. Therefore, we might predict that
when the AI model talker displays highly expressive socio-
communicative responses, participants will diverge, doing the
opposite of how we expect them to behave toward the human
interlocutor in more expressive interactions (converge).
1.2. Functional factors
Other accounts propose that alignment serves a functional,
intelligibility-driven role: to help a speaker be better
understood by their listener by matching their linguistic
representations for better mutual intelligibility [23], [24]. For
example, participants who actively imitate novel speech
patterns later display improved recognition of that speech
(unfamiliar accent [25]; disordered speech [26]). Furthermore,
[27] assessed alignment between dyads completing a map
task. They found that when participants were giving
information during the task, the ‘giver’ displayed greater vocal
alignment than the ‘receiver’. These results lead us to predict
that speakers will actively align toward the speech of another
talker when there is strong pressure to be more intelligible.
Functional pressures are also very much present in
interactions with technology: people adopt the lexical choices
[13] and syntactic structures [14] of a computer system during
interactions to a greater extent than toward a human under
similar conditions. These findings suggest that linguistic
behavior toward AI is distinct from that toward human
interlocutors, in some cases triggering less alignment toward
voice-AI (speech alignment) and in some cases more
alignment toward computers (lexico-syntactic alignment). One
possibility is that the social and functional pressures in the
different paradigms from prior studies led to greater alignment
toward the human or the technological interlocutor. A more
formal investigation into which of these factors leads to more
or less alignment toward voice-AI, relative to humans, can
inform our understanding of the mechanisms of vocal
alignment and the dynamics of human-computer interactions.
For one, we expect people to align in different ways
toward human and voice-AI model talkers as a function of
differing intelligibility pressures in the interaction. Voice-AI
systems are often used for practical and utilitarian tasks, with
the user usually in the ‘giver’ role when producing an
utterance (e.g., “set a timer”, “play a song”, “tell me the
weather”). Thus, we might expect that vocal alignment
patterns toward AI will be more intelligibility-motivated,
relative to those toward a human, reflecting the utilitarian
purpose of voice-AI as task-oriented interlocutors. This would
explain studies comparing alignment toward computers and
humans which find greater alignment toward the computer
[14] where the task was goal-oriented: the bias toward greater
alignment toward technology could be explained by stronger
functional pressures.
Thus, the current study examines whether a correct or
incorrect response from the interlocutor influences patterns of
phonetic imitation. If feedback about intelligibility influences
alignment, we predict that a trial containing an uncertain
response from the interlocutor, that they have not with
certainty understood the target word, will elicit more robust
phonetic imitation by participants than trials where the
interlocutor correctly responds. For our predictors, we expect
to see an effect of Correctness on degree of alignment.
1.3. Present study
The present study examines identical communicative
interactions in a laboratory setting, varying social and
functional factors, across human-human and human-AI
interactions. To our knowledge, no prior work investigates the
relative weight of these social and functional factors within an
interaction with a single interlocutor; most vary the
interlocutor as a way of assessing social variables (e.g.,
gender, attractiveness, [1]). Doing so allows us to probe both
social and functional pressures that may differentially, or
similarly, affect human-human and human-AI interaction.
2. Methods
2.1. Participants, Stimuli, and Procedure
Participants (n=54; 27 F) were native English speakers (mean
age=20.2 years old, sd=2.4) recruited from the UC Davis
subject pool. None reported having any hearing impairments.
Target words were 16 low frequency CV C items selected
from [1]: bat, boot, cheek, coat, dune, hoop, moat, pod, tap,
toot, tot, weave, cot, soap, deed, sock. Both model talkers
(human and AI) produced the target words in a ‘correct’ frame
(“I think I heard sock”) and one in an ‘incorrect’ frame (“I
think I heard sock or sack”), where the distractor was a
minimal pair differing in vowel backness (order of the target
word and minimal pair were counterbalanced across trials and
differed across interlocutors). For the human model talker, a
female native English speaker recorded all utterances in a
sound attenuated booth with a head-mounted microphone. For
the voice-AI model talker, we generated recordings by default
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female Alexa voice (US-En) using the Alexa Skills Kit. Both
talkers produced all sentences in their correct and incorrect
forms in a neutral speaking style, as well as all introductions,
voice-overs, and final responses.
Both model talkers also produced 16 interjections (e.g.,
“Yipee!”) in a hyper-expressive manner, balanced by valence
(8 positive, 8 negative). These were produced naturally by the
human talker; for the Alexa voice, emotionally expressive
interjections recorded by the Alexa voice actor, or
‘Speechcons’, were added to the TTS output using speech
synthesis markup language (SSML) tags (a limitation of TTS
is that emotion is otherwise difficult to synthesize).
Participants sat in a sound-attenuated booth, wearing a
head-mounted microphone and headphones, facing a computer
screen. Subjects first were presented a screen introducing them
to the two model talkers: an Amazon Echo device named
“Alexa” and a female human named “Melissa”, with images
of them. Then, participants produced baseline productions of
target words, reading a carrier sentence containing the target
word, “The word is __”. Subjects completed two baseline
blocks, where order of sentences were randomly presented.
Next, participants completed either the AI or Human
shadowing blocks (order counterbalanced between subjects).
Instructions were given at the beginning of each shadowing
block. The interlocutors (AI, Human) were introduced (“Hi!
I’m Melissa. I’m a research assistant in the Phonetics Lab.” /
“Hi! I’m Alexa. I’m a digital device through Amazon.”) and
went through a voice-over example. During each trial, the
target words occurred in pre-scripted dialogues between the
participant and the model talker. Each trial contained multiple
turns: 1. Initial turn: participants saw a phrase containing the
target word printed on the screen and read it aloud (e.g., “The
word is weave.”). 2. Interlocutor response turn: The
interlocutor provided a response, telling them what she
‘heard’. 3. Participant shadowing response turn: Participants
repeated the sentence a second time. 4. Interlocutor
concluding turn: The interlocutor provided final feedback
(e.g., “Great!”, “Thanks, got it!”, “Perfect!”, etc.; randomly
presented).
Subjects completed two interlocutor blocks, where trials
were manipulated to vary in the social and functional
properties of Interlocutor response turns. To manipulate
intelligibility pressures, there were two correctness
conditions: In 50% of trials, the interlocutor responded with
the correct target word (e.g., “I think I heard weave.”) while in
50% of trials the interlocutor did not correctly identify the
target word with certainty (e.g., “I’m not sure I understood. I
think I heard weave or wove.”). (Note that while incorrect
responses differed from correct responses in information
structure, the former eliciting ‘corrective focus’, our critical
prediction is that there are differences in alignment across
interlocutor types for a given response type.)
To manipulate socio-expressiveness, there were two
expressiveness conditions: In 50% of trials, the interlocutor
responses took the form described above; in the other 50%, the
interlocutor response contained an emotionally expressive
interjection that corresponded to the intended sentiment of the
response: either a positive interjection (e.g., “Yipee! I think I
heard sock.”) for correct turns, or a negative interjection (e.g.,
“Darn! I’m not sure I understood. I think I heard sock or
sack.”) for incorrect turns. Within each block, correctness and
expressiveness trials were intermixed and randomly presented.
There were 128 trials (16 items, 2 correctness conditions,
2 expressiveness conditions, 2 interlocutors).
Figure 1 presents a sample trial and the four response
conditions for turn 2, varying by expressiveness and
correctness.
Figure 1: Sample dialogue of a trial.
2.2. Acoustic Analysis
Interlocutor and participant recordings were force-aligned
with FAVE [28] and segment boundaries were hand-corrected
by phonetically-trained research assistants. Vowel duration
was measured for each target word vowel from the model
talker response turn (turn 2) and the participant’s response
(turn 3). We assessed degree of alignment with Difference in
Distance (DID) = |baseline-model|-|shadowed-model| [1]. This
relative difference score reflects overall alignment, taking into
account baseline similarity between participants and model
talkers. Positive DID values indicate alignment toward, while
negative values indicate divergence from, the model talker,
relative to participants’ baseline productions.
2.3. Statistical Analysis
We modeled DID duration values in a linear mixed effects
model. The model was run in R using the lmer() function in
the lme4 package [29]. Fixed effects included Model Talker (2
levels: AI, human), Correctness (2 levels: correct, incorrect),
Expressiveness (2 levels: regular, expressive). The model
included all two-way interactions, as well as the three-way
interaction, between predictors. Predictors were sum-coded.
By-participant and by-item random intercepts and by-
participant random slopes for each main effect, each two- and
three-way interactions were also included.
3. Results
There was a significant main effect of Correctness [β=-1.52,
t=-2.64, p<0.05]: overall, participants displayed greater
alignment when the model talker produced an uncertainty
response than for a correct response. Correct responses made
by the Human, and all responses made by the voice-AI,
triggered less alignment to the model talkers’ productions.
There was no effect of Expressiveness (p=.14).
Furthermore, the model computed a significant three-way
interaction between Model talker, Expressiveness, and
Correctness [β=-2.1, t=-4.3, p<0.001]. Figure 2 displays this
interaction. For one, speakers align more toward voice-AI in
Incorrect trials that include the expressive interjection (e.g.,
“Darn! I’m not sure I understood. I heard beat or boot.”).
Meanwhile, they also display less alignment toward the human
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in Correct trials without interjections (e.g., “I’m not sure I
understood...”). No other interactions were observed.
Figure 2: Vowel duration DID scores (means and standard
errors of the mean) to AI and Human model talkers’ target
words, by Correctness and Expressiveness.
4. Discussion
The current study investigates how social and functional
factors mediate vocal alignment toward a human and voice-AI
interlocutor in an interactive shadowing task. Overall, we
observe that functional pressures do play a role in predicting
degree of alignment during this task: participants displayed
greater alignment when the model talker responded with
uncertainty about the correct target word than when the model
talker responded unequivocally with the correct target word.
This observation supports theories that functional pressures in
an interaction influence speech alignment [23], [24].
Table 1 summarizes the interaction between social and
functional factors and interlocutor type on alignment patterns.
Table 1: Summary of alignment patterns seen in this study.
Incorrect
(+ functional pressure)
Correct
(- functional pressure)
Regular
(less social)
Converge to human
Diverge from AI
No alignment
(divergence)
Expressive
(more social)
Converge to AI
No alignment to human
Converge to human
Diverge from AI
First, we observe converge toward the human interlocutor
under two conditions: when there is pressure to be intelligible
but with a non-expressive response and with a sociall y
expressive response with no pressure to be intelligible. These
are also the conditions where we observe the greatest
divergence from the voice-AI interlocutor. That we see these
simple factors leading to convergence toward the human, but
divergence from voice-AI, is contrary to prior reports of
greater alignment toward computers than toward human
interactors [13], [14]. Yet, it does align with recent work
reporting less alignment toward voice-AI systems, relative to
naturally produced human voices [6], [8], [9]. The observation
of the same factors leading to different patterns of alignment
toward humans and voice-AI does not support predictions
made by theories of computer personification, e.g. CASA
[11], that people automatically behave toward technology as a
person. Rather, we observe distinct patterns of speech
alignment that people apply toward voice-AI systems.
Additionally, and further in support of distinct vocal
behavior toward human and voice-AI interlocutors, we
observe that participants do align toward voice-AI only with
additive social and functional pressures. Thus, one
interpretation for this is that in order for functional pressures
to trigger alignment toward voice-AI, the system needs to
display even more social signals than for a human interlocutor.
This is not what we would expect from an ‘Uncanny Valley’
hypothesis for human-computer interaction [18]: that people
feel discomfort when technology displays near-humanlike
behavior. We expected highly emotive responses from the AI
system, containing expressive interjections, to be perceived as
off-putting, or uncanny, by participants. Yet, these interactions
received the most alignment for the voice-AI. This also aligns
with recent work showing greater alignment toward these
interjections during word shadowing [7] and improved user
ratings during conversational interactions with a chatbot that
produced these interjections compared to when it did not [30].
Thus, contra an ‘Uncanny Valley’ hypothesis, people respond
positively to socially-expressive utterances from voice-AI.
[23] argue that alignment during interactive dialogue is
automatic and facilitates comprehension by converging
interlocutors’ linguistic representations. Our findings do not
support such a strong stance for how functional pressures
modulate alignment. Both the type of interlocutor and the
socio-communicative force of the interaction mediate how
functional pressures influence alignment.
Future work could also examine how other linguistic
factors mediate alignment toward voice-AI. For example, the
current study used low frequency words, which are shown to
be more susceptible to exposure. Comparing imitation of high
versus low frequency items across interlocutor types could
reveal differences in representational factors at play during
these interactions, cf. [2]. Also, in the current study we
presented comprehension errors with minimal pairs differing
in vowel backness. Future work examining how varying types
of phonological confusions trigger different types of alignment
patterns across interlocutor types could also tease apart what
linguistic-communicative pressure influence speech
alignment. Other phonetic variables (e.g., formant frequencies,
pitch, and intensity) could also be explored in future work.
Investigations of vocal alignment toward voice-AI can
present novel tests and theoretical extensions to human-AI
interaction frameworks. As speech becomes a more dominant
mode of interfacing with technology, understanding how
voice-AI systems influence human language patterns will be
more important. Humans often interact with voice-AI in the
more functional, ‘giver’ role. Our findings suggest this could
lead to more alignment toward voice-AI, if it is expressive.
The findings from the current study also have implications for
voice user interface design. For one, based on a greater degree
of alignment, including expressive interjections in interactions
with AI appears to improve user responsiveness and
interactive behavior toward the voice-AI interlocutor.
Furthermore, this study suggests that the communicative
success of the interaction (e.g., presence of ASR errors) might
dynamically interact with apparent sociality of the system, and
shape the extent users apply human-human speech rules to
their interactions with voice-AI.
5. Acknowledgements
This material is based upon work supported by the National
Science Foundation SBE Postdoctoral Research Fellowship
under Grant No. 1911855 to MC.
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