Limits...
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.

Show MeSH

Related in: MedlinePlus

Thresholds (60% criterion) for the correct detection of the probe as a function of masker level, all in dB SL (relative to a guinea-pig behavioural audiogram). A (102,25)-ms condition. B (50,50)-ms condition derived from plots of Figure 3A and B. Circles illustrate results for multi-units and triangles for single units. The solid black lines represent the minimum thresholds at each level across the population of units. The grey lines illustrate a slope of one. The top histograms illustrate the proportion of single (grey bars) and multi-units (white bars) for which it was not possible to determine a threshold in each 20 dB range of levels. The histograms to the left of panels A and B illustrate the distribution of probe thresholds in the absence of a masker.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2914239&req=5

Fig4: Thresholds (60% criterion) for the correct detection of the probe as a function of masker level, all in dB SL (relative to a guinea-pig behavioural audiogram). A (102,25)-ms condition. B (50,50)-ms condition derived from plots of Figure 3A and B. Circles illustrate results for multi-units and triangles for single units. The solid black lines represent the minimum thresholds at each level across the population of units. The grey lines illustrate a slope of one. The top histograms illustrate the proportion of single (grey bars) and multi-units (white bars) for which it was not possible to determine a threshold in each 20 dB range of levels. The histograms to the left of panels A and B illustrate the distribution of probe thresholds in the absence of a masker.

Mentions: The previous analysis considered the effect of a masker on probe threshold within each single unit. However, physiological differences between neurons, in their receptive fields and thresholds (Fig. 2), mean that detecting a probe of a given frequency may not be mediated by the same neuron or groups of neurons at all masker levels. Potentially, we are able to select out that sub-population of neurons that has the lowest threshold in each set of conditions. In this case, we should not consider the threshold shift within in a single neuron, but rather the absolute threshold for a single masker condition. Figure 4 shows the data from Figure 3, but with probe thresholds and masker levels expressed relative to the guinea pig audiometric threshold (Fay 1988; this reduced the variability across neurons when compared in dB SPL because the auditory periphery varies in its sensitivity to different frequencies). The histograms to the left of panels A and B indicate the distribution of unmasked thresholds for the two stimulus conditions. Probe detection thresholds were estimated from the neurometric functions as the probe level at which the function reaches 60%.FIG. 4.


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)

Thresholds (60% criterion) for the correct detection of the probe as a function of masker level, all in dB SL (relative to a guinea-pig behavioural audiogram). A (102,25)-ms condition. B (50,50)-ms condition derived from plots of Figure 3A and B. Circles illustrate results for multi-units and triangles for single units. The solid black lines represent the minimum thresholds at each level across the population of units. The grey lines illustrate a slope of one. The top histograms illustrate the proportion of single (grey bars) and multi-units (white bars) for which it was not possible to determine a threshold in each 20 dB range of levels. The histograms to the left of panels A and B illustrate the distribution of probe thresholds in the absence of a masker.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2914239&req=5

Fig4: Thresholds (60% criterion) for the correct detection of the probe as a function of masker level, all in dB SL (relative to a guinea-pig behavioural audiogram). A (102,25)-ms condition. B (50,50)-ms condition derived from plots of Figure 3A and B. Circles illustrate results for multi-units and triangles for single units. The solid black lines represent the minimum thresholds at each level across the population of units. The grey lines illustrate a slope of one. The top histograms illustrate the proportion of single (grey bars) and multi-units (white bars) for which it was not possible to determine a threshold in each 20 dB range of levels. The histograms to the left of panels A and B illustrate the distribution of probe thresholds in the absence of a masker.
Mentions: The previous analysis considered the effect of a masker on probe threshold within each single unit. However, physiological differences between neurons, in their receptive fields and thresholds (Fig. 2), mean that detecting a probe of a given frequency may not be mediated by the same neuron or groups of neurons at all masker levels. Potentially, we are able to select out that sub-population of neurons that has the lowest threshold in each set of conditions. In this case, we should not consider the threshold shift within in a single neuron, but rather the absolute threshold for a single masker condition. Figure 4 shows the data from Figure 3, but with probe thresholds and masker levels expressed relative to the guinea pig audiometric threshold (Fay 1988; this reduced the variability across neurons when compared in dB SPL because the auditory periphery varies in its sensitivity to different frequencies). The histograms to the left of panels A and B indicate the distribution of unmasked thresholds for the two stimulus conditions. Probe detection thresholds were estimated from the neurometric functions as the probe level at which the function reaches 60%.FIG. 4.

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
Related in: MedlinePlus