PERCEPTION OF LINGUISTIC RHYTHM BY NEWBORN INFANTS
McRoberts, 1996 for a critique), we have taken extra care to
blind the experimenter with respect to the condition and to
reduce direct interventions during the test to a minimum. For
this purpose, the experiment has been programmed on a PC
in such a way as to be maximally automatized.
Experimental conditions. Babies are randomly assigned
to the control or to the experimental group. In the habitua-
tion phase, they are exposed to 10 sentences uttered by two
speakers of one language. In the test phase, babies from the
control group hear 10 new sentences uttered by the other two
speakers in the same language, whereas babies from the ex-
perimental group hear 10 new sentences uttered by two new
speakers in the other language. The language presented in
the habituation phase is counterbalanced across subjects, re-
sulting in four experimental conditions. Assignment of the
subjects to the different conditions is managed by the pro-
gram and withheld from the experimenter.
Procedure. The test takes place in a sound-attenuated
booth, with only the experimenter and the baby inside. The
experimenter sits outside of the infant’s field of vision and
wears a sound-proof headset playing masking noise. This
noise consists of four superimposed streams of all the experi-
mental sentences playing continuously, in order to optimally
mask the stimuli. The baby is seated in a reclining chair,
and is presented with a pacifier fixed on a flexible arm. The
air pressure in the pacifier is measured by a pressure trans-
ducer, amplified and transmitted to the computer via an ana-
log/digital board. The computer detects sucks and computes
their relative amplitude.
During the first two minutes, the baby sucks in silence
and the computer calculates a high-amplitude (HA) thresh-
old, such that 75% of the sucks have an amplitude above
the threshold. Subsequently, only HA sucks are considered5.
The habituation phase then starts. Each HA suck may elicit
one sentence, subject to the condition that only one sentence
is played at once, and observing an inter stimulus interval of
at least 500 ms. Sentences are played in a random order, di-
rectly from the hard disk of the computer, by two loudspeak-
ers placed in front of the baby. The habituation phase goes
on until the habituation criterion is met: it consists in a min-
imum 25% decrease in the number of HA sucks per minute
for two consecutive minutes, compared with the maximum
number of sucks previously produced in 1 minute6. When
the criterion is met, the computer switches to the test phase,
which lasts for 4 minutes. Test sentences are played in the
same conditions as the habituation sentences.
Delay and rejection conditions. Other factors may inter-
fere with the test and may make it necessary to delay the shift
to the test phase or simply to discard the baby’s data. We
have tried as much as possible to have these decisions made
automatically by the computer, on the basis of the sucking
pattern and of indications given by the experimenter. When
the baby loses the pacifier, starts crying, or falls asleep, the
experimenter needs to take appropriate action. When doing
so he presses a special key on the keyboard, indicating the
occurrence of an event interfering with the baby’s sucking.
The most critical period in the test consists of the two min-
utes before and the two after the shift, which are used for the
statistical analyses. Itis important to ensure that during those
four minutes, (a) no interference has occurred, (b) the baby
was awake and sucking, and thus heard enough sentences.
The computer thus implements the following conditions:
Delay:
if some interference was signaled during the 2 min-
utes preceding the shift,
OR if the baby didn’t hear any sentence during any
one of those 2 minutes,
then the shift is delayed for at least one minute, and the
habituation phase goes on.
Rejection:
if some interference was signaled during the 2 min-
utes following the shift,
OR if the baby didn’t hear any sentence during any
one of those 2 minutes,
OR if the habituation phase has already lasted for 20
minutes,
then the test is discontinued and the baby’s data are
discarded.
In addition, the experimenter himself may make the decision
to discontinue the test, (a) if the baby refuses the pacifier, (b)
if he/she doesn’t keep awake or suck enough, (c) if he/she
keeps crying or being agitated.
Considering that (a) the experimenter is not aware of the
baby’s experimental condition, (b) he can’t hear the stimuli
during the test, (c) decisions concerning the shift to the test
phase and the acceptability of a baby’s data are as automatic
as possible, we regard it as an unlikely possibility that the
experimenter have a significant influence (be it conscious or
unconscious) on the baby’s behavior, leading to biased ef-
fects.
Participants
Experiments took place at the Maternite Port-Royal,
Hδpital Cochin in Paris. Participation was solicited from the
mothers after birth, during their stay at the maternity hospi-
tal. One of the parents’ informed consent was obtained, and
experiments were run with the agreement of the CCPPRB
Paris-Cochin (the hospital’s ethics committee). Babies were
pre-selected on the basis of their medical files, according to
the following criteria:
Age between 2 and 5 days old;
Gestational age greater than or equal to 38 weeks;
Birth weight greater than or equal to 2800 grams;
5 Eliminating the weaker 25% of the sucks helps increasing the
signal/noise ratio (Siqueland & DeLucia, 1969).
6 The first minute of the phase is not taken into account for the
determination of the maximum. Additional conditions impose that
this maximum is at least 20 HA sucks per minute, and that the ha-
bituation phase lasts at least 5 minutes.