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Why do forward maskers affect auditory intensity discrimination? Evidence from "molecular psychophysics".

Oberfeld D, Stahn P, Kuta M - PLoS ONE (2014)

Bottom Line: Intensity difference limens (DLs) were strongly elevated under forward masking but less with contralateral than with ipsilateral maskers.Higher perceptual weights assigned to the maskers corresponded to stronger elevations of the intensity DL.The effects of masker lateralization are evidence for top-down influences, and the observed positive signs of the masker weights suggest that the relevant mechanisms are located at higher processing stages rather than in the auditory periphery.

View Article: PubMed Central - PubMed

Affiliation: Section Experimental Psychology, Department of Psychology, Johannes Gutenberg-Universität Mainz, Mainz, Germany.

ABSTRACT
Nonsimultaneous maskers can strongly impair performance in an auditory intensity discrimination task. Using methods of molecular psychophysics, we quantified the extent to which (1) a masker-induced impairment of the representation of target intensity (i.e., increase in internal noise) and (2) a systematic influence of the masker intensities on the decision variable contribute to these effects. In a two-interval intensity discrimination procedure, targets were presented in quiet, and combined with forward maskers. The lateralization of the maskers relative to the targets was varied via the interaural time difference. Intensity difference limens (DLs) were strongly elevated under forward masking but less with contralateral than with ipsilateral maskers. For most listeners and conditions, perceptual weights measuring the relation between the target and masker levels and the response in the intensity discrimination task were positive and significant. Higher perceptual weights assigned to the maskers corresponded to stronger elevations of the intensity DL. The maskers caused only a weak increase in internal noise, unrelated to target level and masker lateralization. The results indicate that the effects of forward masking on intensity discrimination are determined by an inclusion of the masker intensities in the decision variable, compatible with the hypothesis that the impairment in performance is to a large part caused by difficulties in directing selective attention to the targets. The effects of masker lateralization are evidence for top-down influences, and the observed positive signs of the masker weights suggest that the relevant mechanisms are located at higher processing stages rather than in the auditory periphery.

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Stimuli.Schematic depiction of a trial. In a two-interval auditory intensity discrimination task, the two target tones (T1 and T2) were combined with two forward maskers (M1 and M2). All tone levels were independently and randomly perturbed. The gray horizontal lines show the mean levels for maskers (µM), standard (µS), and standard-plus-increment (µS+I). The level increment (µS+I−µS = 5 dB in this example trial) was individually selected for percent correct in the range from 70% to 85%. It was presented in either the first or the second interval with equal probability. The dotted rectangles depict the randomly selected tone levels in the example trial (LM1 to LT2).
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pone-0099745-g001: Stimuli.Schematic depiction of a trial. In a two-interval auditory intensity discrimination task, the two target tones (T1 and T2) were combined with two forward maskers (M1 and M2). All tone levels were independently and randomly perturbed. The gray horizontal lines show the mean levels for maskers (µM), standard (µS), and standard-plus-increment (µS+I). The level increment (µS+I−µS = 5 dB in this example trial) was individually selected for percent correct in the range from 70% to 85%. It was presented in either the first or the second interval with equal probability. The dotted rectangles depict the randomly selected tone levels in the example trial (LM1 to LT2).

Mentions: In the present experiment, methods of "molecular psychophysics" [18], also known as perceptual weight analysis or behavioral reverse correlation [26], [27], [28], [29], provided a rich set of behavioral measures that made it possible to test which of the three effects play a role for intensity discrimination under forward masking. In a two-interval intensity discrimination task (see Figure 1), we imposed random trial-by-trial variation on the levels of the maskers presented in the first interval (LM1) and in the second observation interval (LM2). Now assume that the masker presented in interval 1 shifted the representation of the target intensity in interval 1 towards lower values, for instance due to neural response suppression, corresponding to effect A. Also assume that the amount of response suppression increases with the sound pressure level of the masker, as has been reported for auditory nerve neurons [19], [30], [31]. As a consequence, the probability of responding that the louder target tone had been presented in interval 1 should be negatively related to the (randomly varying) level of the masker in interval 1 (LM1). The same negative relation between LM1 and the probability of selecting the first interval would result if the representation of masker intensity entered the decision variable with a negative decision weight (effect C). In contrast, if the masker caused a shift of the representation of target intensity towards higher values [13], [20], or if the representation of masker intensity entered the decision variable with a positive decision weight, then the probability of selecting the first interval would be positively related to LM1. Thus, by quantifying the influence of the variation in masker level on the decision, it is possible to decide which of the potential effects of the maskers (e.g., response suppression versus enhancement) are compatible with the observed responses. The methods used in the present experiment quantify this relation in terms of perceptual weights (formally defined by the w-terms in Eq. (1) in section Results), which should be distinguished from the unobservable decision weights the listener applies when combining the representations of the masker and target intensities into the decision variable, according to the observer model introduced above.


Why do forward maskers affect auditory intensity discrimination? Evidence from "molecular psychophysics".

Oberfeld D, Stahn P, Kuta M - PLoS ONE (2014)

Stimuli.Schematic depiction of a trial. In a two-interval auditory intensity discrimination task, the two target tones (T1 and T2) were combined with two forward maskers (M1 and M2). All tone levels were independently and randomly perturbed. The gray horizontal lines show the mean levels for maskers (µM), standard (µS), and standard-plus-increment (µS+I). The level increment (µS+I−µS = 5 dB in this example trial) was individually selected for percent correct in the range from 70% to 85%. It was presented in either the first or the second interval with equal probability. The dotted rectangles depict the randomly selected tone levels in the example trial (LM1 to LT2).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4061042&req=5

pone-0099745-g001: Stimuli.Schematic depiction of a trial. In a two-interval auditory intensity discrimination task, the two target tones (T1 and T2) were combined with two forward maskers (M1 and M2). All tone levels were independently and randomly perturbed. The gray horizontal lines show the mean levels for maskers (µM), standard (µS), and standard-plus-increment (µS+I). The level increment (µS+I−µS = 5 dB in this example trial) was individually selected for percent correct in the range from 70% to 85%. It was presented in either the first or the second interval with equal probability. The dotted rectangles depict the randomly selected tone levels in the example trial (LM1 to LT2).
Mentions: In the present experiment, methods of "molecular psychophysics" [18], also known as perceptual weight analysis or behavioral reverse correlation [26], [27], [28], [29], provided a rich set of behavioral measures that made it possible to test which of the three effects play a role for intensity discrimination under forward masking. In a two-interval intensity discrimination task (see Figure 1), we imposed random trial-by-trial variation on the levels of the maskers presented in the first interval (LM1) and in the second observation interval (LM2). Now assume that the masker presented in interval 1 shifted the representation of the target intensity in interval 1 towards lower values, for instance due to neural response suppression, corresponding to effect A. Also assume that the amount of response suppression increases with the sound pressure level of the masker, as has been reported for auditory nerve neurons [19], [30], [31]. As a consequence, the probability of responding that the louder target tone had been presented in interval 1 should be negatively related to the (randomly varying) level of the masker in interval 1 (LM1). The same negative relation between LM1 and the probability of selecting the first interval would result if the representation of masker intensity entered the decision variable with a negative decision weight (effect C). In contrast, if the masker caused a shift of the representation of target intensity towards higher values [13], [20], or if the representation of masker intensity entered the decision variable with a positive decision weight, then the probability of selecting the first interval would be positively related to LM1. Thus, by quantifying the influence of the variation in masker level on the decision, it is possible to decide which of the potential effects of the maskers (e.g., response suppression versus enhancement) are compatible with the observed responses. The methods used in the present experiment quantify this relation in terms of perceptual weights (formally defined by the w-terms in Eq. (1) in section Results), which should be distinguished from the unobservable decision weights the listener applies when combining the representations of the masker and target intensities into the decision variable, according to the observer model introduced above.

Bottom Line: Intensity difference limens (DLs) were strongly elevated under forward masking but less with contralateral than with ipsilateral maskers.Higher perceptual weights assigned to the maskers corresponded to stronger elevations of the intensity DL.The effects of masker lateralization are evidence for top-down influences, and the observed positive signs of the masker weights suggest that the relevant mechanisms are located at higher processing stages rather than in the auditory periphery.

View Article: PubMed Central - PubMed

Affiliation: Section Experimental Psychology, Department of Psychology, Johannes Gutenberg-Universität Mainz, Mainz, Germany.

ABSTRACT
Nonsimultaneous maskers can strongly impair performance in an auditory intensity discrimination task. Using methods of molecular psychophysics, we quantified the extent to which (1) a masker-induced impairment of the representation of target intensity (i.e., increase in internal noise) and (2) a systematic influence of the masker intensities on the decision variable contribute to these effects. In a two-interval intensity discrimination procedure, targets were presented in quiet, and combined with forward maskers. The lateralization of the maskers relative to the targets was varied via the interaural time difference. Intensity difference limens (DLs) were strongly elevated under forward masking but less with contralateral than with ipsilateral maskers. For most listeners and conditions, perceptual weights measuring the relation between the target and masker levels and the response in the intensity discrimination task were positive and significant. Higher perceptual weights assigned to the maskers corresponded to stronger elevations of the intensity DL. The maskers caused only a weak increase in internal noise, unrelated to target level and masker lateralization. The results indicate that the effects of forward masking on intensity discrimination are determined by an inclusion of the masker intensities in the decision variable, compatible with the hypothesis that the impairment in performance is to a large part caused by difficulties in directing selective attention to the targets. The effects of masker lateralization are evidence for top-down influences, and the observed positive signs of the masker weights suggest that the relevant mechanisms are located at higher processing stages rather than in the auditory periphery.

Show MeSH
Related in: MedlinePlus