The neurobiology of rhyme judgment by deaf and hearing adults: an ERP study

Abstract

We used electrophysiology to determine the time course and distribution of neural activation during an English word rhyme task in hearing and congenitally deaf adults. Behavioral performance by hearing participants was at ceiling and their ERP data replicated two robust effects repeatedly observed in the literature. First, a sustained negativity, termed the contingent negative variation, was elicited following the first stimulus word. This negativity was asymmetric, being more negative over the left than right sites. The second effect we replicated in hearing participants was an enhanced negativity (N450) to nonrhyming second stimulus words. This was largest over medial, parietal regions of the right hemisphere. Accuracy on the rhyme task by the deaf group as a whole was above chance level, yet significantly poorer than hearing participants. We examined only ERP data from deaf participants who performed the task above chance level (n = 9). We observed indications of subtle differences in ERP responses between deaf and hearing groups. However, overall the patterns in the deaf group were very similar to that in the hearing group. Deaf participants, just as hearing participants, showed greater negativity to nonrhyming than rhyming words. Furthermore the onset latency of this effect was the same as that observed in hearing participants. Overall, the neural processes supporting explicit phonological judgments are very similar in deaf and hearing people, despite differences in the modality of spoken language experience. This supports the suggestion that phonological processing is to a large degree amodal or supramodal.

Figure 1

Schematic representation of sequence of events during the rhyme judgment task.

Figure 2

Distribution of electrodes used to collect ERP data. Unfilled electrodes were not included in the analysis. The two most lateral columns of electrodes (black), in the left and right hemispheres were combined in the analyses. Similarly, the two most medial columns of electrodes (dark grey) in the left and right hemispheres were also combined. This consolidation of the data resulted in two levels of the lateral/ medial factor.

Figure 3

Left and right hemisphere waves are plotted together over frontal electrodes. Plots show ERP responses in (A) deaf (performing above chance level) and (B) hearing subgroups (n=9 each group) to the first stimulus (S1s) collapsed across rhyming and non-rhyming trials. There was no significant difference between subgroups in hemispheric asymmetry of the CNV.

Figure 4

Right, medial parietal sites are displayed. Plots show ERP responses in (A) deaf (performing above chance level) and (B) hearing subgroups (n=9 each group) to the second stimulus (S2s). Enhanced negativity of the waveform between 300-600ms in response to non-rhyming words is visible in both groups. The difference between subgroups within this time window failed to reach significance (p=.08).