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Forward masking estimated by signal detection theory analysis of neuronal responses in primary auditory cortex.

Alves-Pinto A, Baudoux S, Palmer AR, Sumner CJ - J. Assoc. Res. Otolaryngol. (2010)

Bottom Line: This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation.However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically.Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

View Article: PubMed Central - PubMed

Affiliation: MRC Institute of Hearing Research, Science Road, University Park, Nottingham, Nottinghamshire, UK. ana@ihr.mrc.ac.uk

ABSTRACT
Psychophysical forward masking is an increase in threshold of detection of a sound (probe) when it is preceded by another sound (masker). This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation. Studies in the auditory nerve and cochlear nucleus using signal detection theory techniques to derive neuronal thresholds showed that in centrally projecting neurons, increases in masked thresholds were significantly smaller than the changes measured psychophysically. Larger threshold shifts have been reported in the inferior colliculus of awake marmoset. The present study investigated the magnitude of forward masking in primary auditory cortical neurons of anaesthetised guinea-pigs. Responses of cortical neurons to unmasked and forward masked tones were measured and probe detection thresholds estimated using signal detection theory methods. Threshold shifts were larger than in the auditory nerve, cochlear nucleus and inferior colliculus. The larger threshold shifts suggest that central, and probably cortical, processes contribute to forward masking. However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically. Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

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A and B Difference between the mean spike rate when no masker was presented and in the same time window after the presentation of the masker. A illustrates results for the (102,25)-ms condition and B for the (50,50)-ms condition. The probe was not presented in order to evaluate the effect of the masker alone. The histograms on the right hand side of the plots illustrate the distribution of the spike count differences. Negative values indicate persistence.
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Fig8: A and B Difference between the mean spike rate when no masker was presented and in the same time window after the presentation of the masker. A illustrates results for the (102,25)-ms condition and B for the (50,50)-ms condition. The probe was not presented in order to evaluate the effect of the masker alone. The histograms on the right hand side of the plots illustrate the distribution of the spike count differences. Negative values indicate persistence.

Mentions: We also estimated the growth of masking slopes in individual units by fitting a straight line to the individual growth of masking functions. Slopes varied between about 0 (i.e. no threshold shift in the presence of the masker) and 1.23 dB/dB, with a mean slope of 0.49 for the (100,25)-ms condition and 0.41 for the (50,50)-ms condition (panels C and D in Fig. 3). This compares with a mean of 0.26 dB/dB (max: 0.5 dB/dB) in the AN (Fig. 7 in Relkin and Turner 1988) and 0.36 dB/dB (max: 1.46 dB/dB) in the IC (Fig. 5 in Nelson et al. 2009). The difference in the masker–probe interval used in the cortical and IC experiments is likely to contribute to the differences in slope reported. In effect, several psychophyscial studies have shown that the slope of the growth of masking function decreases as the interval separating the masker from the probe increases (Jesteadt et al. 1982; Moore and Glasberg 1983; Widin and Viemeister 1979). Furthermore, a recent study of the effects of different pulse rates in the transmission of temporal information to the auditory cortex, through cochlear implant stimulation of the auditory nerve in anesthetised guinea-pigs (Kirby and Middlebrooks 2010) observed growth of masking slopes near 1 at masker offset and shallower slopes at masker–probe intervals of 64 ms (Fig. 8 in Kirby and Middlebrooks 2010).


Forward masking estimated by signal detection theory analysis of neuronal responses in primary auditory cortex.

Alves-Pinto A, Baudoux S, Palmer AR, Sumner CJ - J. Assoc. Res. Otolaryngol. (2010)

A and B Difference between the mean spike rate when no masker was presented and in the same time window after the presentation of the masker. A illustrates results for the (102,25)-ms condition and B for the (50,50)-ms condition. The probe was not presented in order to evaluate the effect of the masker alone. The histograms on the right hand side of the plots illustrate the distribution of the spike count differences. Negative values indicate persistence.
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Related In: Results  -  Collection

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Fig8: A and B Difference between the mean spike rate when no masker was presented and in the same time window after the presentation of the masker. A illustrates results for the (102,25)-ms condition and B for the (50,50)-ms condition. The probe was not presented in order to evaluate the effect of the masker alone. The histograms on the right hand side of the plots illustrate the distribution of the spike count differences. Negative values indicate persistence.
Mentions: We also estimated the growth of masking slopes in individual units by fitting a straight line to the individual growth of masking functions. Slopes varied between about 0 (i.e. no threshold shift in the presence of the masker) and 1.23 dB/dB, with a mean slope of 0.49 for the (100,25)-ms condition and 0.41 for the (50,50)-ms condition (panels C and D in Fig. 3). This compares with a mean of 0.26 dB/dB (max: 0.5 dB/dB) in the AN (Fig. 7 in Relkin and Turner 1988) and 0.36 dB/dB (max: 1.46 dB/dB) in the IC (Fig. 5 in Nelson et al. 2009). The difference in the masker–probe interval used in the cortical and IC experiments is likely to contribute to the differences in slope reported. In effect, several psychophyscial studies have shown that the slope of the growth of masking function decreases as the interval separating the masker from the probe increases (Jesteadt et al. 1982; Moore and Glasberg 1983; Widin and Viemeister 1979). Furthermore, a recent study of the effects of different pulse rates in the transmission of temporal information to the auditory cortex, through cochlear implant stimulation of the auditory nerve in anesthetised guinea-pigs (Kirby and Middlebrooks 2010) observed growth of masking slopes near 1 at masker offset and shallower slopes at masker–probe intervals of 64 ms (Fig. 8 in Kirby and Middlebrooks 2010).

Bottom Line: This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation.However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically.Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

View Article: PubMed Central - PubMed

Affiliation: MRC Institute of Hearing Research, Science Road, University Park, Nottingham, Nottinghamshire, UK. ana@ihr.mrc.ac.uk

ABSTRACT
Psychophysical forward masking is an increase in threshold of detection of a sound (probe) when it is preceded by another sound (masker). This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation. Studies in the auditory nerve and cochlear nucleus using signal detection theory techniques to derive neuronal thresholds showed that in centrally projecting neurons, increases in masked thresholds were significantly smaller than the changes measured psychophysically. Larger threshold shifts have been reported in the inferior colliculus of awake marmoset. The present study investigated the magnitude of forward masking in primary auditory cortical neurons of anaesthetised guinea-pigs. Responses of cortical neurons to unmasked and forward masked tones were measured and probe detection thresholds estimated using signal detection theory methods. Threshold shifts were larger than in the auditory nerve, cochlear nucleus and inferior colliculus. The larger threshold shifts suggest that central, and probably cortical, processes contribute to forward masking. However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically. Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

Show MeSH