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
Example neurometric and rate-level functions of five cortical neurons to forward masked tones. Different rows correspond to different units. Panels in the left column illustrate the neurometric functions. Panels in the right column illustrate the corresponding mean spike count in response to the probe as a function of probe level for the same unit. Within each panel, different lines indicate different masker levels, in dB SL (re. guinea-pig audiogram), as indicated in the inset. The line marked with ‘multiplication symbol’ illustrates the responses when no masker was presented. Text on the right of the panels indicates whether the unit is a single (SU) or multi-unit (MU—see text), its characteristic frequency, the frequency of the tones and their duration in ms: (masker duration, probe duration). There was no silent gap between the masker and the probe in these examples.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2914239&req=5

Fig2: Example neurometric and rate-level functions of five cortical neurons to forward masked tones. Different rows correspond to different units. Panels in the left column illustrate the neurometric functions. Panels in the right column illustrate the corresponding mean spike count in response to the probe as a function of probe level for the same unit. Within each panel, different lines indicate different masker levels, in dB SL (re. guinea-pig audiogram), as indicated in the inset. The line marked with ‘multiplication symbol’ illustrates the responses when no masker was presented. Text on the right of the panels indicates whether the unit is a single (SU) or multi-unit (MU—see text), its characteristic frequency, the frequency of the tones and their duration in ms: (masker duration, probe duration). There was no silent gap between the masker and the probe in these examples.

Mentions: Figure 2 shows the neurometric functions (left column) and the corresponding spike rate-count functions (right column; shows the mean number of spikes for each stimulus presentation) for five example units: 3 multi-units (MU – recordings in which the shape of the action potential varies considerably and most likely reflects the activity of a cluster of nearby neurons) and two single units (SU—recordings where the shapes of the action potentials are very similar and most-likely originate from a single neuron). Each line represents a different masker condition (‘multiplication symbol’ corresponds to the no-masker condition). Changes in both the neurometric and rate-level functions caused by the presence of a masker were different for different units. For some units the addition of a masker (lines without symbols in Figure 2; masker levels are indicated in the insets) produced a shift of the neurometric and rate-level functions towards higher levels as the level of the masker increased (Fig. 2A, B and C). This corresponds to the distribution shift shown in Figure 1C when the level of the masker is increased. Neurometric functions were in some instances very similar to spike count functions (Fig. 2A and B), whilst in other cases there were clear qualitative differences (Figs. 2C, D and E), such as a change in the gradient of one of the functions (Fig. 2D). Often these differences were attributable to a ceiling effect when the neurometric functions approached 100%, corresponding to conditions when spike count distributions did not overlap at all. However, not all units reached the maximum performance (100% correct responses), even when the neurometric function saturated (Fig. 2A, B and E). Failing to reach the maximum indicates that the spike-count distributions (see Fig. 1C) to the probe and no-probe conditions overlap each other partially even at high probe levels.FIG. 2.


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)

Example neurometric and rate-level functions of five cortical neurons to forward masked tones. Different rows correspond to different units. Panels in the left column illustrate the neurometric functions. Panels in the right column illustrate the corresponding mean spike count in response to the probe as a function of probe level for the same unit. Within each panel, different lines indicate different masker levels, in dB SL (re. guinea-pig audiogram), as indicated in the inset. The line marked with ‘multiplication symbol’ illustrates the responses when no masker was presented. Text on the right of the panels indicates whether the unit is a single (SU) or multi-unit (MU—see text), its characteristic frequency, the frequency of the tones and their duration in ms: (masker duration, probe duration). There was no silent gap between the masker and the probe in these examples.
© Copyright Policy
Related In: Results  -  Collection

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

Fig2: Example neurometric and rate-level functions of five cortical neurons to forward masked tones. Different rows correspond to different units. Panels in the left column illustrate the neurometric functions. Panels in the right column illustrate the corresponding mean spike count in response to the probe as a function of probe level for the same unit. Within each panel, different lines indicate different masker levels, in dB SL (re. guinea-pig audiogram), as indicated in the inset. The line marked with ‘multiplication symbol’ illustrates the responses when no masker was presented. Text on the right of the panels indicates whether the unit is a single (SU) or multi-unit (MU—see text), its characteristic frequency, the frequency of the tones and their duration in ms: (masker duration, probe duration). There was no silent gap between the masker and the probe in these examples.
Mentions: Figure 2 shows the neurometric functions (left column) and the corresponding spike rate-count functions (right column; shows the mean number of spikes for each stimulus presentation) for five example units: 3 multi-units (MU – recordings in which the shape of the action potential varies considerably and most likely reflects the activity of a cluster of nearby neurons) and two single units (SU—recordings where the shapes of the action potentials are very similar and most-likely originate from a single neuron). Each line represents a different masker condition (‘multiplication symbol’ corresponds to the no-masker condition). Changes in both the neurometric and rate-level functions caused by the presence of a masker were different for different units. For some units the addition of a masker (lines without symbols in Figure 2; masker levels are indicated in the insets) produced a shift of the neurometric and rate-level functions towards higher levels as the level of the masker increased (Fig. 2A, B and C). This corresponds to the distribution shift shown in Figure 1C when the level of the masker is increased. Neurometric functions were in some instances very similar to spike count functions (Fig. 2A and B), whilst in other cases there were clear qualitative differences (Figs. 2C, D and E), such as a change in the gradient of one of the functions (Fig. 2D). Often these differences were attributable to a ceiling effect when the neurometric functions approached 100%, corresponding to conditions when spike count distributions did not overlap at all. However, not all units reached the maximum performance (100% correct responses), even when the neurometric function saturated (Fig. 2A, B and E). Failing to reach the maximum indicates that the spike-count distributions (see Fig. 1C) to the probe and no-probe conditions overlap each other partially even at high probe levels.FIG. 2.

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