Effects of continuous tone exposure on frequency representations in A1
Rat pups were reared in continuous pure tones (3.5- or 7-kHz; n=3 for each group) from P7–P35. A1 was subsequently electrophysiologically mapped in fine detail immediately after sound exposure cessation. Naïve rats (n=4) reared under standard housing conditions were also mapped at the same ages. As shown in (left; Group III), A1 in naïve rats consisted of iso-frequency bands oriented approximately orthogonal to a rostro-caudal frequency-representation gradient. Cortical neuron tuning curves recorded in most sites (93%) within A1 were V-shaped, with a clear CF defined at the low threshold peak (). In exposed rats, by contrast, tuning curves recorded from a large number of sites (49% for 3.5-kHz exposed rats and 46% for 7-kHz exposed rats) within A1 were flat-peaked or multipeaked (). Comparison of bandwidths measured at 20 dB above threshold (BW20) showed that tuning curves recorded in exposed rats were more broadly tuned than were those of naïve rats (; ANOVA with post hoc Student-Newman-Keuls test, all p<0.05–0.001), except for lowest CF category centered at 1.75-kHz (ANOVA, p>0.1). However, there was no significant difference in BW20s between rats exposed to 3.5- and 7-kHz (ANOVA with post hoc Student-Newman-Keuls test, all p>0.05).
Fig. 1 (A) Representative characteristic frequency (CF, kHz) maps and tonal receptive fields recorded from A1 of rats exposed to continuous 3.5-kHz (Group I) or 7-kHz (Group II) tones during the critical period, and from naïve rats (Group III). The color (more ...)
At first glance, the A1 area tuned to higher frequencies in exposed rats appeared to be larger, while the area tuned to lower frequencies appeared smaller, when compared to naïve rats (, left; Group I or II vs. III). To verify that observation, we compared the percentage of A1 area that was tuned to different frequencies in each group. As shown in , the percentage of A1 area in exposed rats were larger than in naïve rats at CF categories centered at 14- and 28-kHz, but was smaller at CF categories centered at 1.75-, 3.5-, and 7-kHz (χ2=21.7, p<0.006). This result was further confirmed by quantitative comparisons of distribution for all CFs obtained from different groups (). Significant rightward shift of CF distribution for exposed rats compared to naïve rats (Kruskal-Wallis with post hoc Bonferroni’s test, both p<0.001) indicates decreased areas of representation for lower frequencies, but increased representational areas of higher frequencies, induced by continuous pure-tone exposure. Again, there were no significant differences in the percentages of A1 areas tuned to different frequencies and in CF distribution between rats exposed to 3.5- and 7-kHz (Kruskal-Wallis with post hoc Bonferroni’s test, p>0.05).
We also quantified the precision of A1 tonotopicity for different groups by calculating a tonotopic index. As shown in , average indices in exposed rats were significantly larger than in naïve rats (ANOVA with post hoc Student-Newman-Keuls test, both p<0.01), showing that less ordered tonotopic maps resulted from early sound exposure. Although the average index in rats exposed to 3.5-kHz tones was also larger than in rats exposed to 7-kHz tones, that difference did not quite reach statistical significance (ANOVA with post hoc Student-Newman-Keuls test, p>0.05).
Cortical response thresholds across all frequencies () and the latencies () recorded from exposed rats, however, were not significantly different from that recorded from naïve rats (ANOVA, all p>0.05), indicating normal peripheral hearing of these rats after moderate sound exposure.
Average response thresholds at different CF categories (A) and average latencies (B) recorded from different groups of rats. Bin size =1 octave. Error bars represent means ± SE.
We further compared A1 recruitment functions for different groups of rats. These recruitment functions measure the percentage of the map area activated by a tonal stimulus at a specific frequency and intensity, rather than solely focusing on the preferred frequency determined at the threshold (i.e., CF) for each recording site. It is clear that A1 recruitment functions obtained from exposed rats were very substantially different from that obtained from naïve rats, and marked by an increase in the percentages of A1 area activated by high frequency but a decrease in the percentage of A1 areas activated by low frequency at intensity ranges of ~20–70 dB SPL (, Groups I or II vs. III). presents the average percentage of each map active for ¼-octave-wide range of frequencies centered on low- (2.5-kHz), middle- (9.9-kHz), and high- frequency (19.8-kHz) tones of increasing intensity. Two-way ANOVA showed that the proportional area of A1 activated by low frequencies was smaller in exposed rats, but was larger in middle and high frequency domains, when compared to naïve animals (all p<0.0001).
Fig. 3 (A) Average percentage of A1 that was activated by tones of varying frequency and intensity, in rats exposed to continuous 3.5-kHz or 7-kHz tones, and in naïve rats. (B) Average percentage of each map active for a ¼-octave-wide range of (more ...)
Delayed closure of the critical period resulting from continuous tone exposure
An earlier study from our laboratory showed that continuous broad-band noise exposure delayed the end of the critical period for A1 development (Chang and Merzenich, 2003
). To determine the impacts of continuous pure-tone exposure on the critical period window, a subset of previously continuous-tone-exposed rats were exposed to pulsed 7-kHz tone pips for two weeks, i.e., rats experienced an immediately shift in exposure from continuous tones to modulated tones at P35. These double-exposed subgroups of rats were defined as Group IV (n=3, exposed to 3.5-kHz pure tones during the critical period) or V (n=3, exposed to 7-kHz pure tones during the critical period), respectively. In addition, another group of age-matched naïve rats was also exposed to pulsed 7-kHz tones over the same epoch (P36–P49; Group VI, n=5). Cortical fields of Group IV, V and VI rats were then mapped and compared with age-matched non-sound-exposed rats (Group VII, n=3).
In the CF maps for rats in Groups IV or V, the area tuned to 7-kHz frequency was enlarged, when compared to that of age-matched naïve rats (, Group IV or V vs. VII). Such tone-specific enlargement resulting from mere exposure to sound stimuli characterizes “critical period” plasticity. This distortion is also illustrated in , where CFs of all recording sites from each groups of rats are plotted against a normalized tonotopic axis. Examination of the CF distribution reveals an over-representation of sites tuned to 7-kHz, and a relative under-representation of sites tuned to immediately lower or higher frequencies, in both Groups IV and V, when compared to either naïve-exposed (presumably no longer in the critical period) or age-matched naïve non-exposed rats (, Group IV or V vs. VI or VII). Note that the maps and CF distributions in Group VI rats were substantially like that of naïve rats (, Group VI vs. VII).
Fig. 4 (A) Representative CF maps obtained from Groups IV, V, VI and VII rats. Group IV rats were exposed to continuous 3.5-kHz tones from P7–P35, then pulsed 7-kHz tones for a subsequent 2-week-long period (P36–P49). Group V, rats were exposed (more ...)
To quantitatively characterize effects of pulsed tone exposure on A1 frequency representation of different rat groups, the percentages of A1 areas representing each frequency range were averaged within the same experimental group, and the differences in percentages between exposed and naïve rats were plotted (). Average percentages of A1 areas tuned to 7 kHz ± 0.25 octave in Groups IV and V were very significantly increased compared to naïve rats (unpaired t test, both p <0.001). Average percentages of A1 areas tuned to lower or higher frequencies in these two groups, however, were reduced although the differences at some CF categories did not reach the statistical significance (, Groups IV and V). As expected, the distribution of average percentage of A1 area tuned to different frequency ranges in Group VI was not different from that of naïve rats (, Group VI; unpaired t test, all p> 0.05).
We also compared average BW20s of tuning curves recorded from different rat groups. Although average BW20s in groups IV and V appeared to be larger than in naïve rats, no significant difference was found among different groups (; ANOVA, p>0.05).