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

Unmasking and Goals Unmasking – addition of another frequency component to the masker signal resulting in decrease of threshold of signal detection. Goals: Investigate masking and unmasking as a function of: Time : τ, t v (Forward vs. Simultaneous Masking) Frequency: f p Intensity: L v – L p, L v & L p Phase Author’s assumption: Unmasking is caused by suppression. Masking is caused by excitation.

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

Temporal Parameters: Simultaneous vs. Forward masking Note: The baseline (i.e. 0dB) is the amount of masking produced by the 1kHz masker alone (not the absolute threshold) 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

Temporal Parameters: Duration of the f v component Stimulus: duration of f p is 500 ms duration of f v varied from 20ms to 500 ms f v always occurred at the end of the masker Minimum t v necessary for unmasking to occur?

Temporal Parameters: Duration of the f v component (cont.) Variability across subject: for RS unmasking at t v = 100 ms for MC unmasking at t v = 250 ms

Intensity Parameters: Intensity Difference L v – L p Stimulus: L p = 40 dB SPL is fixed L v = dB SPL (Effect of intensity level of variable frequency masker component onto the masking and unmasking pattern)

1.for L v = 30 dB SPL no unmasking 2.for L v = 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 f v =1150 Hz Intensity Parameters: Intensity Difference L v – L p (cont.) 3. No unmasking for f v 70 dB)

Intensity Parameters: Overall Masker level L v and L p Stimulus: Lv - L p = 20 dB SPL is fixed L p = dB SPL L v = dB SPL for f v > f p amount of unmasking is almost the same for all L p and L v levels for f v <f p unmasking does not appear until L v = 50 dB SPL, it then monotonically increases

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

One-tone masker, Phase and Square Gating One-tone masker Stimulus: f v 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 f p and f v 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.

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 f v < f p unmasking is dependent on only, L v, the intensity of the variable masker component. When f v > f p unmasking is dependent on, L v – L p, 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.

Q&A