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Two- tone unmasking and suppression in a forward-masking situation Robert V. Shannon 1976

Two- tone unmasking and suppression in a forward-masking situation Robert V. Shannon 1976. Spring 2009 HST.723 Theme 1: Psychophysics. Unmasking and Goals.

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Two- tone unmasking and suppression in a forward-masking situation Robert V. Shannon 1976

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  1. Two- tone unmasking and suppression in a forward-masking situation Robert V. Shannon 1976 Spring 2009 HST.723 Theme 1: Psychophysics

  2. Unmasking and Goals Unmasking – addition of another frequency component to the masker signal resulting in decrease of threshold of signal detection. Author’s assumption: Unmasking is caused by suppression. Masking is caused by excitation. Goals: Investigate masking and unmasking as a function of: Time : τ, tv (Forward vs. Simultaneous Masking) Frequency: fp Intensity: Lv – Lp, Lv & Lp Phase

  3. Procedure: Two- alternative forced-choice method Masker: frequency: fp = 1 kHz, fv = variable duration: 500ms intensity: Lp = 40 dB SPL and Lv = 60 dB SPL Signal: frequency: fp = 1 kHz, fv = variable duration: 20ms intensity: Lp = 40 dB SPL Total of 5 subjects: RS, PP, SK, MD and MC.

  4. Temporal Parameters: Simultaneous vs. Forward masking Simultaneous masking: • symmetric about 1000 Hz • most masking is at 1000 Hz Forward masking: • most masking is at 950 Hz • at 1150 Hz amount of masking decreased to -6 dB = unmasking *Subject MC showed no unmasking Note: The baseline (i.e. 0dB) is the amount of masking produced by the 1kHz masker alone (not the absolute threshold)

  5. Temporal Parameters: Duration of the fv component Stimulus: • duration of fp is 500 ms • duration of fv varied from 20ms to 500 ms • fv always occurred at the end of the masker Minimum tv necessary for unmasking to occur?

  6. Temporal Parameters: Duration of the fv component (cont.) Variability across subject: • for RS unmasking at tv = 100 ms • for MC unmasking at tv = 250 ms

  7. Intensity Parameters: Intensity Difference Lv – Lp Stimulus: • Lp = 40 dB SPL is fixed • Lv = 30 - 70 dB SPL (Effect of intensity level of variable frequency masker component onto the masking and unmasking pattern)

  8. Intensity Parameters: Intensity Difference Lv – Lp (cont.) • for Lv = 30 dB SPL no unmasking • for Lv = 40 – 70 dB SPL: • - masking at 950Hz increased from 7 to 17 dB • - unmasking increased from 2 to 7 dB • - maximum of unmasking is at fv=1150 Hz 3. No unmasking for fv<fp (unmasking for this condition might occur for Lv > 70 dB)

  9. Intensity Parameters: Overall Masker level Lv and Lp Stimulus: • Lv - Lp = 20 dB SPL is fixed • Lp = 30 -70 dB SPL • Lv = 50 -90 dB SPL • for fv > fp amount of unmasking is almost the same for all Lp and Lv levels • for fv <fp unmasking does not appear until Lv = 50 dB SPL, it then monotonically increases

  10. Signal frequency and duration Stimulus: • fp is 1, 2, 4 and 6 kHz Subject MC similar masking patterns for all frequencies Subject MD showed no unmasking for 1 and 2 kHz

  11. One-tone masker, Phase and Square Gating One-tone masker Stimulus: fv alone Results: 1. Amount of masking produced by two-tone masker is never less then amount of masking produced by the variable masker component alone. 2. Unmasking is not observed in case where the baseline is an absolute threshold Phase Stimulus: initial phase between fp and fv in the masker were varied. Result: phase had no impact on unmasking Square gating Stimulus: signals with abrupt on and off transitions rather then smooth. Result: The existence of unmasking does not depend on whether or not the masker and signal are turned on and off abruptly or smoothly.

  12. Summary Unmasking is not observed at absolute thresholds. It is only observed when the masking at the signal frequency is raised by the presence of a masker signal. When fv < fp unmasking is dependent on only, Lv, the intensity of the variable masker component. When fv > fp unmasking is dependent on, Lv – Lp, the difference between the intensities of the two masker tones. The variable masker component must be present 40 – 100 ms for unmasking too be observed. Unmasking was interpreted as a result of the variable masker component suppressing the masker component at the signal frequency. If this interpretation is valid then the characteristics of the unmasking of the signal may be used to infer the characteristics of suppression in the masker.

  13. Q&A

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