Cortical hemispheric asymmetries are present at young ages and further develop into adolescence

Abstract

Specialization of the auditory cortices for pure tone listening may develop with age. In adults, the right hemisphere dominates when listening to pure tones and music; we thus hypothesized that (a) asymmetric function between auditory cortices increases with age and (b) this development is specific to tonal rather than broadband/non-tonal stimuli. Cortical responses to tone-bursts and broadband click-trains were recorded by multichannel electroencephalography in young children (5.1 ± 0.8 years old) and adolescents (15.2 ± 1.7 years old) with normal hearing. Peak dipole moments indicating activity strength in right and left auditory cortices were calculated using the Time Restricted, Artefact and Coherence source Suppression (TRACS) beamformer. Monaural click-trains and tone-bursts in young children evoked a dominant response in the contralateral right cortex by left ear stimulation and, similarly, a contralateral left cortex response to click-trains in the right ear. Responses to tone-bursts in the right ear were more bilateral. In adolescents, peak activity dominated in the right cortex in most conditions (tone-bursts from either ear and to clicks from the left ear). Bilateral activity was evoked by right ear click stimulation. Thus, right hemispheric specialization for monaural tonal stimuli begins in children as young as 5 years of age and becomes more prominent by adolescence. These changes were marked by consistent dipole moments in the right auditory cortex with age in contrast to decreases in dipole activity in all other stimulus conditions. Together, the findings reveal increasingly asymmetric function for the two auditory cortices, potentially to support greater cortical specialization with development into adolescence.

Keywords: EEG; beamformer; development; evoked response; hemispheric specialization; right dominance; source localization; tonal sound.

Figure 1

Mean cortical auditory evoked potentials (CAEP) at the vertex (Cz) and global field power (GFP) for the Child Group and across the two Adolescent Groups are shown in (a) and (b), respectively. CAEP and GFP waveforms evoked by stimulation of the left or right ear are depicted in blue and red lines, respectively. The waveforms evoked by click‐trains are shown in the top row. In the Child Group, monaural click‐trains and tone‐bursts evoked immature CAEP waveforms at Cz consisting of iP1 and iN2 peaks. Across the Adolescent Groups, mean Cz waveforms evoked by click‐trains and tone‐bursts exhibited mature CAEP waveforms containing 2 positive (P) and 2 negative (N) peaks: P1, N1, P2, and N2. In all groups, GFP waveforms showed peaks corresponding to the each peak in the Cz waveform. (c) Topographic maps at the iP1 peak latency. (d) Topographic maps at P1, N1, and P2 peak latencies in the Adolescent Groups. Tone‐evoked surface potentials are similar between the stimulated ears at all peak latencies, while click‐trains evoked different surface potential patterns between the stimulated ears

Figure 2

(a) Grand average pseudo‐Z maps of the 11 participants in the Child Group for the iP1 time window indicate the strongest cortical activity in the auditory cortex contralateral to the stimulated ear for left and right monaural click‐trains and tone‐bursts. (b) Significantly larger dipole moments were measured in the auditory cortex contralateral to the stimulated ear for click‐trains (left ear stimulation: p = .03; right ear stimulation: p = .035). For tone‐bursts, contralateral activity was higher than ipsilateral for left ear stimulation (p = .03) but activity was bilateral (not significantly different between left and right) for right ear presentations (p = .283) (*p < .05). (c) Peak dipoles occurred at earlier latencies in the right than left hemisphere for stimuli (collapsed) presented from the left ear (p = .04). By contrast, latency was similar between hemispheres for right ear stimuli (p = .298)

Figure 3

(a) Grand average pseudo‐Z maps of cortical activity indicate that monaural click‐trains evoked contralateral dominant activation in adolescents whereas tone‐bursts evoked right hemispheric dominant activation, regardless of ear stimulated in a similar aged cohort. (b) Click‐train stimulation of the left ear evoked significantly increased activity in the right than left auditory cortex (p = .002), whereas right ear stimulation evoked bilaterally symmetric cortical activation (p = .332). Averaged across three time windows, dipole moments were significantly larger in the right auditory cortex than the left auditory cortex in response to tone‐bursts to either ear (left ear stimulation: p = .014, right ear stimulation: p = .026). (c) No significant effects on latencies were found bar longer latencies in the right cortex ipsilateral to right tone‐burst stimulation during the P2 time window (p = .018) (*p < .05, **p < .01)

Figure 4

(a) In all participants (Bar 1) in the Child Group, cortical lateralization (CL) scores (100 × (dipole moment in right auditory cortex − dipole moment in left auditory cortex)/sum of right and left dipole moments) evoked by left ear stimulation are more positive (increased activity in the right auditory cortex) than by right ear stimulation. This is true for both click‐trains and tone‐bursts. This pattern is more variable in the Adolescent Groups. (b) Mean ± 1 SE CL values are shown for all time windows in the Child Group and both Adolescent Groups; solid lines are significantly different from 0 whereas dashed lines are not significant. In the Child Group, the CL score was significantly more positive (increased activity in right hemisphere) when evoked by the left than right ear stimulation during the iP1 time window (**p < .01). In adolescents listening to tone‐bursts, CL scores were significantly more positive when evoked by stimulation to the left than right ear (**p < .01), but this difference was not significant in the adolescents listening to tone‐bursts

Figure 5

Peak dipole moments are plotted as a function of chronological age in each condition ((a) click‐trains; (b) tone‐bursts). Dipole moments during the iP1 in Child Group were compared with those at each P1, N1, and P2 time windows in the Adolescent Groups (top, middle, and bottom columns, respectively). An exponential regression line using log‐transformed dipole moments in the left and right auditory cortices (visualized by blue triangles and red circles, respectively) are shown by blue and red lines, respectively. Solid and dotted lines indicate significant (p < .05) and non‐significant correlations, respectively. Dipole moments decrease with age for most parameters evoked by left ear stimuli. On the other hand, age‐dependent decreases in amplitude upon right ear stimulation are evoked by click‐trains but not by tone‐bursts in the right auditory cortex