Participants were asked to breath through their mouth or their nose, forcing them to adopt a particular position of their velum (up or down). While breathing in each of these positions, they categorized sounds from an /ada/ to /ana/ continuum. The position of the speech articulators, even though adopted for the purposes of breathing, altered participants' perception of external speech sounds, so that when the velum was down (to breath through the nose), they tended to hear the consonant as the nasal /n/ - a sound necessarily produced with a lowered velum, rather than as /d/ in which the velum must be raised.
If the organs of speech production are moved during speech
perception, that movement can influence how perception
unfolds. For example, if hearing a sound ambiguous between
/aba/ and /ava/ while mouthing (or even imagining) /ava/,
a person will tend to hear the sound as /ava/ (and
contrariwise for mouthing /aba/)
The question of whether (or to what degree) the motor system
influences speech perception is old and strongly contested.
The best known version of this idea is the Motor Theory
of Speech Perception
A somewhat similar view is proposed by
More recent alternatives to the Motor Theory (and its
variants) maintain a role for the motor system, but do not claim
that it is an obligatory component of speech perception. For
example
These motor-helping-hearing theories are quite
similar to the Perception-for-Action-Control Theory
These recent alternatives to the Motor Theory propose a specific mechanism by which the motor system influences speech perception: corollary discharge.1
Corollary discharge is an internal sensory signal generated
by one’s own motor system whenever one acts
Another role of corollary discharge is to tag concurrent
matching external sensations as “self-caused” and so
unworthy of intense perceptual processing
Perceptual capture is a shift in perception caused by the
fact that corollary discharge is an anticipation, and as such
can pull ambiguous stimuli into alignment with the
anticipated percept. Hickok,
In terms of hearing, several studies have found evidence of
anticipations altering the perception of sounds. A compelling
1Corollary discharge is the sensory prediction generated by a
‘forward model’ – which is a system that takes motor commands as
input and predicts the sensory consequences. Efference copy refers
to the motor command received by the forward model, but
sometimes the terms corollary discharge and efference copy are used
interchangeably.
example of this is the “White Christmas” effect
A consequence of theories which propose a perceptual ‘fill
in the gap’ role for corollary discharge is that the position of
the perceiver’s own articulators should matter for what
information gets filled in. For a prediction of the sensory
consequences of an action to be accurate it must take into account
the starting point of the action, as the sensory consequences
of an action can be vastly different depending on where the
effector is starting — think of the tactile sensory difference
between slamming your jaw shut when your tongue is in its
normal resting position vs. extended out between your teeth
(ouch!). Thus corollary discharge is necessarily generated
using the current position of the articulators as the basis for
prediction
Given that the predictions of corollary discharge take into account the position of the effectors, the perceptual channelling discussed above should be sensitive to the positions of one’s own speech articulators (even if one is not speaking). Thus when a person’s velum is down in order to allow breathing through the nose, then the perceptual channelling (or ‘filling in’) done by the motor system should be biased towards a prediction of nasality and the person should thus be more likely to hear an ambiguous external sound as nasal. In the context of this experiment, this means that people should hear an /ada/~/aga/ ambiguous sound more often as /ana/ when they are breathing through their nose in comparison to when they are breathing through their mouth.
The sounds /d/ and /n/ were chosen as these sounds have the same place of articulation and are both voiced and are both stops, differing almost exclusively in whether the velum is up (for /d/ and so no airflow through the nose) or down (for /n/ with airflow through the nose). Thus the primary difference between these sounds is mirrored in the position of the velum for breathing — up for breathing through the mouth, down for breathing through the nose.
The prediction that people should hear more /n/ when their
velum is down for breathing through their nose is similar
to the perceptual capture effect demonstrated in
This experiment is also similar to that of
Participants were asked to breath through their mouth or nose while they categorized sounds as /ada/ or /ana/. The prediction is that when participants are breathing through their nose, the necessarily lowered velum will influence their perception so that they hear the sounds as more similar to the nasal /ana/. In a control experiment, participants categorized /ada/ vs. /aga/ while breathing through their mouth or nose. No difference was predicted for this control experiment.
A female native speaker of standard European French was
recorded saying /ada/ and /ana/. A 10 000 step
continuum between these sounds was created using STRAIGHT
There were two conditions:
In each condition, stimuli were presented using the
staircase method
Each staircase consisted of thirteen reversals with decreasing stepsize after each reversal. The step sizes were: 1250, 1000, 800, 650, 500, 350, 250, 150, 80, 50, 30, 20, 10 (from a 10 000 step continuum). The two interleaved staircases started at points 2400 and 7600 of the continuum.
An abstract example of the layout of a staircase procedure is shown in Figure 1. A sound from one end of the continuum (for example the /ada/ end of the continuum) is played to the participant. If the participant categorizes it as belonging to the category consistent with that end of the continuum (/ada/ in our example), then the computer selects the next sound to be closer to the other (/ana/) end of the continuum. If the participant hears this also as /ada/, the computer chooses the next sound to be even closer to /ana/ and so on until the participant reports hearing /ana/. At this point, the computer reverses direction of sound selection (hence this is called a ‘reversal’) and selects the next sound to be closer to the /ada/ end of the continuum, however the stepsize along the continuum is made smaller (the computer makes smaller jumps along the continuum between sound selections, so that there is more precision). When the participant starts to hear the sound as /ada/ again, the computer reverses again (and again makes the stepsizes along the continuum smaller) moving back toward /ana/. This back and forth continues as the computer homes in on the participant’s boundary between /ada/ and /ana/, changing direction of movement along the continuum and getting more precise with each reversal. This is a robust and relatively quick method of estimating a person’s perceptual boundary between two sounds. The structure of each trial was very simple, participants were presented with an audio stimulus that was somewhat ambiguous between /ada/ or /ana/ and they pressed (with their right hand) a keyboard button (right or left arrow key) to indicate their perception of the sound as /ada/ or /ana/.
Participants were given instructions on the task and familiarized with the software before conducting the experiment. The experiment itself took about 20 minutes for each participant to complete.
The order of breathing conditions (half the participants starting with breathing through the mouth half starting with breathing through the nose) was counterbalanced across participants as was the correspondence of response button (left arrow vs. right arrow on the computer keyboard) to sound.
The experiment was run on the PsychoPy
experimentplatform
Thirty-nine native French speaking participants (31 female, 35 right-handed) were run at Université Paris Descartes (average age 22.28, standard deviation 2.37).
For each breathing position, each participant’s data from the four staircases was submitted to a logistic regression to determine what point on the continuum corresponded to their perceptual boundary between /ada/ and /ana/ (the point at which they would hear the sound equally often as /ada/ and /ana/). These calculated boundaries were the dependent measure for the experiment. As this was a within-subjects design, the individual variability in perceptual boundaries (which are often highly variable between individuals) is not an issue here – each participant served as his/her own control.
As predicted, participants heard significantly more /ana/ when breathing through their nose than when breathing through their mouth, as determined by a paired t-test [t(38) = 2.08, p = .044, d = .33].
In the control experiment (categorizing /aga/ vs. /ada/), no such difference was found between breathing conditions [t(38) = 1.36, p = .18, d = .21]. However, the interaction between experiment and control versions did not reach significance. We believe this is due to a lack of power and are creating a new version of the experiment to address this issue.
Components of our motor systems, known as forward models, constantly predict the sensory effects of our actions — this prediction is corollary discharge. Corollary discharge serves a variety of crucial roles, including providing feedback for actions performed too quickly to use ‘regular’ sensory feedback and tagging self-produced sensations as such, thus preventing sensory confusion. It is this second role that allows corollary discharge to influence the concurrent perception of external sensations.
A recent group of theories
This experiment tested that prediction and has shown that the position of one’s own articulators does influence the perception of the speech — even when the position of the articulators is adopted for a non-speech activity (breathing). These results support theories which argue for a role of the motor system (and corollary discharge) in speech perception and makes a unique contribution in showing that the static position of the articulators can have this effect even when their position is not intended to produce speech.
These results are relevant to the ongoing debate about
embodied cognition — the degree to which the body and motor
control systems are used in cognition. In the realm of
semantic processing of language a similar debate is ongoing
about the degree of motor involvement in the processing of
the meaning of sentences. For example, the Action Sentence
Compatibility effect demonstrates that movements of the arm
are faster when a person reads a sentence implying arm
movements, suggesting that the person’s motor plan for arm
movements was triggered by reading the sentence
Ongoing research is currently exploring the extent of this effect, examining how widespread (in terms of speech sounds) such effects are.
Funding for this project was provided by a United Arab Emirates University Research Start-Up Grant (Perceptual-Motor Linkages in Speech and Cognition - 31h060) to Mark Scott.