In the following analyses, we inves

In the following analyses, we investigated whether lexical
behavior during exposure and age differences
(among the older adults) predict the strength of the
perceptual learning effect. The first analysis focused on
whether differences in the frequency of acceptance of
the odd-sounding items as words during the lexicaldecision
task resulted in differences in the amounts that
people would shift their phoneme categories. To that
end, we investigated whether listeners who more often
judged ambiguous items to be words during the lexicaldecision
task gave more learning-consistent responses
(i.e., more /r/ responses when exposed to the ambiguous
sound in /r/-final words, and more /l/ responses when
exposed to the ambiguous sound in /l/-final words). We
focused on the ambiguous stimuli because these are the
crucial items in the lexical-decision task that were supposed
to induce phonetic category adjustment. In each
age group, the two exposure groups were taken together,
and a new “learning-consistent” category was created
in which the /r/ responses during the phonetic categorization
task among the group of listeners exposed to the
ambiguous sound in /r/-final words and the /l/ responses
of the group of listeners exposed to the ambiguous
sound in the /l/-final words were combined. Moreover,
we only analyzed data from the stimulus steps of interest
(i.e., the most ambiguous steps: 2, 3, and 4).
Percentages of ambiguous items accepted as words during
the lexical-decision task were calculated for each
participant, and used as a fixed predictor of whether
the category response was learning consistent (the dependent
variable, coded as 0 and 1, for not learning
consistent and learning consistent, respectively). Age Group
(younger listener group on the intercept), Stimulus Step, and
Test Block were used as control variables; Subject was the
random factor.
Table 4 displays the parameter estimates in the final
model. Listeners who more often judged an ambiguous item
as being a word in the lexical-decision task gave more
learning-consistent responses during the phonetic categorization
task; that is, they showed stronger perceptual learning
than did listeners who less often judged an ambiguous item
as being a word. These listeners thus seem to have retuned
their phoneme categories more.
Subsequently, the effect of age on the perceptuallearning
effect was investigated among the older participants.
Since hearing loss is a common phenomenon
among older listeners, Hearing Loss was used as a
control variable (centralized to the mean). The dependent
variable was again the “learning-consistent” category,
taking into account only the three most ambiguous
stimulus steps. Age and hearing sensitivity were shown
to be correlated (r = .40, p < .005). To reduce collinearity
in the model, a residual was created for Age
(with Hearing Loss partialed out), which was used as
a fixed predictor. The other control variables were
Stimulus Step and Test Block, and Subject and Items
were the random factors.
Consistent with the age group comparison, in which the
younger listeners initially had a larger perceptual learning
effect than did the older listeners, the perceptual learning
effect in the first block was smaller with increasing age (β =
−0.1142, SE = 0.0385, p < .005). Moreover, the decrease in
the perceptual learning effect over blocks was smaller with
increasing age (β = 0.04473, SE = 0.0133, p < .001),which is
also consistent with the results of the group comparison.
Hearing sensitivity did not modify the size or stability of the
perceptual learning effect, showing that our stimuli were
indeed audible for listeners with hearing loss.