This study wants to determine the relationship between spontaneous hand gestures, stuttering and intelligibility in individuals with Down syndrome. One third of these individuals has fluency problems, such as stuttering. Gesture use appears to be a strength in individuals with Down syndrome. While they are able to compensate for their language problems, it is not clear if they also use gestures to compensate for their speech problems. Therefore, this study will observe the impact of their gesture use on the stuttering frequency/severity and on the intelligibility of children with Down syndrome.

This study has three research questions. The first question is: Is there a difference in gesture use between individuals with Down syndrome who stutter and individuals with Down syndrome who do not stutter? The hypothesis is that the children who stutter will make more gestures to compensate for the fluency problems. The kind of spontaneous hand gestures will also be considered. These results will be compared to those of typical developing individuals.

The second research question is: Are stuttering events that are accompanied by a gesture more intelligible than stuttering moments that are not accompanied by a gesture? Research showed that the use of signs has an positive impact on the speech intelligibility of individuals with Down syndrome. Here it is investigated if this is also true for spontaneous hand gestures. In case of better speech intelligibility it is investigated if the gain in intelligibility is caused by how recognizable the gesture is or by the effect of the gestures on speech itself. The effect of different types on the speech intelligibility of the stuttering events will also be investigated. Typically developing individuals who stutter will function as control group.

The third research question is: 'Does gestural priming have an influence on the fluency of children with Down syndrome? Gestural priming is a secondary speech signal that gives feedback to the first speech signal by simultaneously mimicking the first speech signal. In this research a hand puppet will imitate the mouth movements of the participants. Next to that, the speech will be simultaneously be accompanied by beat gestures, meaningless up and downward movements. The hypothesis is that due to mirror neurons, the participants will become more fluent. Mirror neurons are neurons in the brain that can produce a neural basis for fluency by the perception of the second speech signal.

The parents of the individuals with Down syndrome are given a questionnaire to see if their child can participate in the study. A receptive language test is administered to the individuals with Down syndrome to get an indication of their developmental age. An stuttering discrimination test and a stuttering awareness test were also developed for this study, to see if individuals with Down syndrome are able to hear the difference between stuttered and fluent speech and to see if they are aware of their stuttering. Awareness of stuttering might impact their gesture use.

For the first research question, the relationship between gestures on the stuttering frequency/severity is investigated. To collect this information, video's of spontaneous speech will be analyzed. All the participants are filmed by their parents or caregivers. During the spontaneous speech, the individuals talk about anything they want. Multiple video's with different subjects are filmed per participant since stuttering is very variable. For example, if a child is more excited by one topic than by another, it could increase the stuttering frequency. The same procedure is administered to typically developing individuals. The transcription program CLAN is used to transcribed the videos and to code the stuttering events and the gestures. After this, statistical analysis is used to see if individuals with Down syndrome who stutter differ in their gesture use from the individuals who do not stutter. Next to that, the results of the Down syndrome group will be compared to the typically developing group.

For the second research question, the videos of the individuals are used to investigate the effect of gestures on the intelligibility of stuttering events. There are two conditions: (1) Videos of stuttering events with and without a gesture and (2) audio recordings of stuttering events with and without a gesture (4).The audio recordings are identical to the information in the video recordings.

Typically developing adults who are naïeve to the research question are asked to watch or listen the video/audio recordings of the individuals with Down syndrome. For each recording they are asked to transcribe what they thought the individuals with Down syndrome had said and on a 7-point Likert scale they had to indicate how intelligible they found the speech. The intelligibility results are compared between gesture and no gesture and between audio and video.

The same protocol will be done with video/audio recordings of typically developing individuals who stutter and their results will be compared to those of the participants with Down syndrome.

For the third research question, only the group of children who stutter is retained. There are three experimental conditions and one control condition based on the procedure of Snyder, Waddell & Blachet (2016). In the control condition, the investigator shows three pictograms on a laptop that form a sentence, for example, the cat eats cake. The participants have to produce this sentence. There is a training session to make sure the participant understands to goal of the pictograms and can produce the sentences. The actual test condition contains 7 sentences. In the control condition no gestures or priming is used. The experimental conditions are the same, but priming is induced. In the first experimental condition, the participant gets a hand puppet that must make the same mouth movements as she does, but the puppet needs to start moving before the participant starts talking. In the second experimental condition, the participant does the same thing with the hand puppet, but she has to hold it behind a screen, so she cannot see it herself. In the third condition, it is the investigator who simultaneously mimics the participant with the hand puppet. Each participant does all the conditions, but in a different order, so there is control for a saturation effect. Each experimental condition is preceded by a training session where different forms of prompting are used to teach to participants how to work with the hand puppet.

Additionally, since it is unnatural to walk around with a hand puppet, the same experiment is executed but with the use of beat gestures. These are rhythmical gestures that control the parsing of the speech stream. The children produce the sentences again but this time they make simple up and downward beat gestures during their speech or watch the experimenter make these gestures while they speak. The second experimental condition is eliminated in this case, since beat gestures are often made with two hands.

All the conditions are videotaped and transcribed in CLAN. The stuttering frequency is measured for all the conditions and compared between them. If the beat gestures have a fluency inducing effect, than this might be an indication to develop a new stuttering therapy based on spontaneous hand gestures.

Inclusion Criteria for the participants with Down syndrome

• Stuttering (group 1)
• No stuttering (group 2)
• Able to understand and produce a three-word sentence
• Mild to moderate mental disability

Exclusion Criteria:

• Severe mental disability
• No understanding and/or production of a three-word sentence
• Uncorrected visual or auditory impairment

Inclusion Criteria for the typically developing participants

• Stuttering (group 1)
• No stuttering (group 2)
• Able to understand and produce a three-word sentence

Exclusion Criteria:

• No understanding and/or production of a three-word sentence
• Uncorrected visual or auditory impairment
• Other language/speech problems

The groups will be assembled from individuals with Down syndrome and typically developing children who do and do not stutter who live in Flanders. They will be recruited via regular and special education primary schools, via speech language pathologists, via revalidation centra and via the Down syndrome association Flanders.