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Influence of different envelope maskers on signal recognition and neuronal representation in the auditory system of a grasshopper.

Neuhofer D, Ronacher B - PLoS ONE (2012)

Bottom Line: If AM filters contribute to reduced masking, signal recognition should depend on the degree of overlap between the song envelope spectrum and the noise spectra.Increasing levels of signal degradation in different frequency bands led to similar changes in the spike trains in most neurones.There is no indication that auditory neurones of grasshoppers are specialized to improve the SNR with respect to the pattern of amplitude modulations.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, Humboldt-Universität zu Berlin, Berlin, Germany. neu.dani@gmx.at

ABSTRACT

Background: Animals that communicate by sound face the problem that the signals arriving at the receiver often are degraded and masked by noise. Frequency filters in the receiver's auditory system may improve the signal-to-noise ratio (SNR) by excluding parts of the spectrum which are not occupied by the species-specific signals. This solution, however, is hardly amenable to species that produce broad band signals or have ears with broad frequency tuning. In mammals auditory filters exist that work in the temporal domain of amplitude modulations (AM). Do insects also use this type of filtering?

Principal findings: Combining behavioural and neurophysiological experiments we investigated whether AM filters may improve the recognition of masked communication signals in grasshoppers. The AM pattern of the sound, its envelope, is crucial for signal recognition in these animals. We degraded the species-specific song by adding random fluctuations to its envelope. Six noise bands were used that differed in their overlap with the spectral content of the song envelope. If AM filters contribute to reduced masking, signal recognition should depend on the degree of overlap between the song envelope spectrum and the noise spectra. Contrary to this prediction, the resistance against signal degradation was the same for five of six masker bands. Most remarkably, the band with the strongest frequency overlap to the natural song envelope (0-100 Hz) impaired acceptance of degraded signals the least. To assess the noise filter capacities of single auditory neurons, the changes of spike trains as a function of the masking level were assessed. Increasing levels of signal degradation in different frequency bands led to similar changes in the spike trains in most neurones.

Conclusions: There is no indication that auditory neurones of grasshoppers are specialized to improve the SNR with respect to the pattern of amplitude modulations.

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Related in: MedlinePlus

Comparison of the influence of different envelope maskers on neuronal representation.The graphs show the slopes of the distance curves (as in Fig. 4 E,F) of single cells, which were tested with different envelope maskers. Error bars indicate the mean standard error (s.e.m.) of the linear regression, each point in a graph represents a single neuron. The different cell types are indicated by different symbols (see inset). A) 0–1000 Hz vs. 0–1000 Hz notch, LN: 18, AN: 9; B) 0–1000 Hz vs. 100–500 Hz, LN: 12, AN: 5; C) 0–1000 Hz vs. 200–750 Hz, LN: 7; D) 0–1000 Hz vs. 0–100 Hz, LN: 9, AN: 13; E) 0–1000 Hz vs. 100–200 Hz, LN: 7, AN: 5; F) 0–100 Hz vs. 100–200 Hz, LN: 9, AN: 6; G) 100–500 Hz vs. 200–750 Hz, LN: 7, AN: 1.
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pone-0034384-g005: Comparison of the influence of different envelope maskers on neuronal representation.The graphs show the slopes of the distance curves (as in Fig. 4 E,F) of single cells, which were tested with different envelope maskers. Error bars indicate the mean standard error (s.e.m.) of the linear regression, each point in a graph represents a single neuron. The different cell types are indicated by different symbols (see inset). A) 0–1000 Hz vs. 0–1000 Hz notch, LN: 18, AN: 9; B) 0–1000 Hz vs. 100–500 Hz, LN: 12, AN: 5; C) 0–1000 Hz vs. 200–750 Hz, LN: 7; D) 0–1000 Hz vs. 0–100 Hz, LN: 9, AN: 13; E) 0–1000 Hz vs. 100–200 Hz, LN: 7, AN: 5; F) 0–100 Hz vs. 100–200 Hz, LN: 9, AN: 6; G) 100–500 Hz vs. 200–750 Hz, LN: 7, AN: 1.

Mentions: In general, the investigated cells exhibited a linear increase of spike train distances with increasing levels of envelope degradation Therefore the slopes of the distance curves, as in Fig. 4E,F, could serve as a measure of the impact an envelope degradation had on the neuronal representation of the original female song (see Material and Methods, and [25]). By comparing the slopes of various distance curves we explored whether the different envelope maskers differed in their influence on neuronal signal representation. The graphs in Figure 5 summarize the pair-wise comparisons between these slopes for two envelope maskers each. Specimens of the different cell types are marked with different symbols (see inset). The local interneurones exhibited on average higher slope values than the ascending interneurones. Within a computation level there was no consistent difference between the different cell types investigated. For most comparisons, there were only minor, not significant deviations from the diagonal, indicating a similar impact of different envelope maskers. Table 1 summarizes the p values of the Wilcoxon signed rank test to compare the slopes of the distance curves in response to different envelope maskers. For local interneurons no comparison revealed significant differences between the slopes (taking a Bonferroni correction for multiple testing into account, the p level was set at 0.007). However, the 0–100 vs. 100–200 and the 100–500 vs. 200–750 Hz comparisons just missed this significance level. In both cases, the envelope maskers with higher modulation frequencies yielded lower slopes of the distance curves. Remarkably, even the ascending neurons did not show significant differences between different envelope maskers. This result was unexpected since the ascending neurons were the most likely candidates able to filter out high frequency envelope degradations, in view of the filter properties of their modulation transfer functions [19], [20], [21], see discussion.


Influence of different envelope maskers on signal recognition and neuronal representation in the auditory system of a grasshopper.

Neuhofer D, Ronacher B - PLoS ONE (2012)

Comparison of the influence of different envelope maskers on neuronal representation.The graphs show the slopes of the distance curves (as in Fig. 4 E,F) of single cells, which were tested with different envelope maskers. Error bars indicate the mean standard error (s.e.m.) of the linear regression, each point in a graph represents a single neuron. The different cell types are indicated by different symbols (see inset). A) 0–1000 Hz vs. 0–1000 Hz notch, LN: 18, AN: 9; B) 0–1000 Hz vs. 100–500 Hz, LN: 12, AN: 5; C) 0–1000 Hz vs. 200–750 Hz, LN: 7; D) 0–1000 Hz vs. 0–100 Hz, LN: 9, AN: 13; E) 0–1000 Hz vs. 100–200 Hz, LN: 7, AN: 5; F) 0–100 Hz vs. 100–200 Hz, LN: 9, AN: 6; G) 100–500 Hz vs. 200–750 Hz, LN: 7, AN: 1.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3316687&req=5

pone-0034384-g005: Comparison of the influence of different envelope maskers on neuronal representation.The graphs show the slopes of the distance curves (as in Fig. 4 E,F) of single cells, which were tested with different envelope maskers. Error bars indicate the mean standard error (s.e.m.) of the linear regression, each point in a graph represents a single neuron. The different cell types are indicated by different symbols (see inset). A) 0–1000 Hz vs. 0–1000 Hz notch, LN: 18, AN: 9; B) 0–1000 Hz vs. 100–500 Hz, LN: 12, AN: 5; C) 0–1000 Hz vs. 200–750 Hz, LN: 7; D) 0–1000 Hz vs. 0–100 Hz, LN: 9, AN: 13; E) 0–1000 Hz vs. 100–200 Hz, LN: 7, AN: 5; F) 0–100 Hz vs. 100–200 Hz, LN: 9, AN: 6; G) 100–500 Hz vs. 200–750 Hz, LN: 7, AN: 1.
Mentions: In general, the investigated cells exhibited a linear increase of spike train distances with increasing levels of envelope degradation Therefore the slopes of the distance curves, as in Fig. 4E,F, could serve as a measure of the impact an envelope degradation had on the neuronal representation of the original female song (see Material and Methods, and [25]). By comparing the slopes of various distance curves we explored whether the different envelope maskers differed in their influence on neuronal signal representation. The graphs in Figure 5 summarize the pair-wise comparisons between these slopes for two envelope maskers each. Specimens of the different cell types are marked with different symbols (see inset). The local interneurones exhibited on average higher slope values than the ascending interneurones. Within a computation level there was no consistent difference between the different cell types investigated. For most comparisons, there were only minor, not significant deviations from the diagonal, indicating a similar impact of different envelope maskers. Table 1 summarizes the p values of the Wilcoxon signed rank test to compare the slopes of the distance curves in response to different envelope maskers. For local interneurons no comparison revealed significant differences between the slopes (taking a Bonferroni correction for multiple testing into account, the p level was set at 0.007). However, the 0–100 vs. 100–200 and the 100–500 vs. 200–750 Hz comparisons just missed this significance level. In both cases, the envelope maskers with higher modulation frequencies yielded lower slopes of the distance curves. Remarkably, even the ascending neurons did not show significant differences between different envelope maskers. This result was unexpected since the ascending neurons were the most likely candidates able to filter out high frequency envelope degradations, in view of the filter properties of their modulation transfer functions [19], [20], [21], see discussion.

Bottom Line: If AM filters contribute to reduced masking, signal recognition should depend on the degree of overlap between the song envelope spectrum and the noise spectra.Increasing levels of signal degradation in different frequency bands led to similar changes in the spike trains in most neurones.There is no indication that auditory neurones of grasshoppers are specialized to improve the SNR with respect to the pattern of amplitude modulations.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, Humboldt-Universität zu Berlin, Berlin, Germany. neu.dani@gmx.at

ABSTRACT

Background: Animals that communicate by sound face the problem that the signals arriving at the receiver often are degraded and masked by noise. Frequency filters in the receiver's auditory system may improve the signal-to-noise ratio (SNR) by excluding parts of the spectrum which are not occupied by the species-specific signals. This solution, however, is hardly amenable to species that produce broad band signals or have ears with broad frequency tuning. In mammals auditory filters exist that work in the temporal domain of amplitude modulations (AM). Do insects also use this type of filtering?

Principal findings: Combining behavioural and neurophysiological experiments we investigated whether AM filters may improve the recognition of masked communication signals in grasshoppers. The AM pattern of the sound, its envelope, is crucial for signal recognition in these animals. We degraded the species-specific song by adding random fluctuations to its envelope. Six noise bands were used that differed in their overlap with the spectral content of the song envelope. If AM filters contribute to reduced masking, signal recognition should depend on the degree of overlap between the song envelope spectrum and the noise spectra. Contrary to this prediction, the resistance against signal degradation was the same for five of six masker bands. Most remarkably, the band with the strongest frequency overlap to the natural song envelope (0-100 Hz) impaired acceptance of degraded signals the least. To assess the noise filter capacities of single auditory neurons, the changes of spike trains as a function of the masking level were assessed. Increasing levels of signal degradation in different frequency bands led to similar changes in the spike trains in most neurones.

Conclusions: There is no indication that auditory neurones of grasshoppers are specialized to improve the SNR with respect to the pattern of amplitude modulations.

Show MeSH
Related in: MedlinePlus