Speech production and perception: Learning and memory

1. Introduction

1.1. Perturbation and sensorimotor learning

Picture the situation of taking a photo of beautiful lakeside scenery and accidently dropping your camera into the water. Despite your misfortune, you are lucky and can spot the camera within what appears to be a reachable distance at the lake bottom. Hastily, you try to retrieve the camera but grab a few times beside it before you can actually take hold of it. Or even worse, you realize that the bottom that appeared reachable lies in fact much deeper below the water surface. In the described example, the coordination between your visual input and your hand movements is disrupted by the visual distortions caused by the different reflective angles between the water and the air. The fact that you can eventually grab the camera after a few attempts, assuming the lake bottom is indeed reachable by hand, provides evidence for the flexibility of the human sensorimotor system which is able to adapt for the visual perturbations and to find alternative motor strategies to reach the intended goal.

The same is mostly true for mechanical and auditory perturbations of speech. That is, when you later recall you tale of bad luck to a friend during the conference dinner, and you get upset about the unreasonable repair costs of your camera, you might speak with a mouth full of food. In this case, your articulators’ movements might be impeded by pieces of food which will force you to find alternative strategies to intelligibly articulate the words you intend to utter. Or, in another scenario, you may have to increase the loudness of your voice to compensate for the loud conversation happening at the table next to yours.

1.2. Outcome variability in perturbation studies of speech

Despite the general ability of speakers to reorganize their motor strategies to retain the acoustic make-up of the intended speech sounds under aggravated conditions, the outcome of adaptation processes in speech exhibits high inter-individual and inter-study variability. For instance, Gay, Lindblom and Lubker (1981) examined participants’ productions of vowels when a bite block was inserted between their teeth. The authors found that speakers were able to adapt to these static perturbations with very little or no practice and produce acoustic outputs equivalent to their unperturbed speech. However, in a study by Savariaux, Perrier and Orliaguet (1995) when speakers’ lips were blocked with a tube during the production of the French [u]; only six out of 11 speakers were able to partially compensate for the labial perturbation and only one speaker compensated completely by changing the constriction location from a velo-palatal to a velo-pharyngeal region. The remaining four speakers did not compensate at all. Similar variability of the experimental outcomes is also observed across other articulatory perturbation studies, e.g., by Baum & McFarland (1997), Jones & Munhall (2003), and Brunner, Hoole & Perrier (2011). To explain this variability, Savariaux et al. (1995) suggest that the varying degree of adaptation among participants is due to “speaker-specific internal representation of articulatory-to-acoustic relationships”.

One of the more recent hypotheses put forward to explain the outcome variability observed in perturbation studies is the idea by Lametti, Nasir and Ostry (2012) that speakers have individual preferences for articulatory or auditory feedback to control their speech production. To empirically evaluate their claim, Lametti et al. (2012) investigated participants in different experimental conditions where the authors either perturbed participants’ jaw trajectories without altering their speech acoustics, or perturbed their auditory feedback, or applied both types of perturbation simultaneously. The authors found a negative correlation between the amount of articulatory and auditory adaptation which means that speakers who adapted to articulatory perturbations, adapted to auditory alternations to a lesser degree.

Another explanation for partial compensations was provided by Katseff, Houde & Johnson (2012) who suggest that these are the result of speakers’ attempts to integrate the altered auditory signal with the normal somatosensory signal that speakers receive during a perturbation experiment. Similar to other authors (e.g., Sato, Schwartz & Perrier, 2014), Katseff et al. (2012) assume that vowel targets are defined as regions in a multidimensional acoustic-somatosensory space. That is, when during auditory perturbation the acoustic parameters of speakers’ speech are diverted from the target, speakers will compensate for the acoustic error. However, their compensation will stop when the discrepancy between the auditory and somatosensory signals becomes too large. Katseff et al. (2012) support their view by the observation that in their study of F1 perturbation the relative compensation magnitude decreased from 100 % for 50 Hz perturbations to 40 % for 250 Hz perturbations. An analogous finding was previously made by MacDonald, Goldberg & Munhall (2010) for F1 and F2 perturbation.