Re: Definition of masking

[Chris Plack <cplack@xxxxxxxxxxx>]

Response to Rich and Jont.

Richard J. Fabbri wrote:

>>For example, we shouldn't call the increase in threshold for a tone
>>following a masker "forward masking" unless we can be sure that
>>the process is one of swamping rather than, say, adaptation.
>        ... Yes.
>        ... Masking is a "swamping" interaction and, as such, requires
>        all signal elements to be present during the same time period.

Yes, but remember that also applies to "internal" respresentations of the
stimuli: just because two stimuli are not physically present at the same
time, does not preclude a simultaneous interaction between their neural
representations.

>        ... An effect succeeding a stimulus (not during the stimulus)
>        reveals a different process and may, in fact, be an adaptation,
>        possibly a consequence of middle ear (AGC?) time constants:
>        I agree with this masking qualification.
>

Actually, I am reasonably convinced myself that the mechanism underlying
forward masking is not, primarily, neural adaptation. I think forward
masking is due to a simultaneous internal interaction, that we should call
masking. If you're interested, have a look at:

Plack, C.J., and Oxenham, A.J. (1998). "Basilar-membrane nonlinearity and
the growth of forward masking," J. Acoust. Soc. Am. 103, 1598-1608.


Jont Allen wrote:

>An important question is: What is the physical mechanism of the masking
>noise. Is it external noise (i.e., due to roving the signal), or internal
>noise (i.e., neural noise).
>
>In a recent paper (JASA, Dec. 1997, pp 3628-3646), we argue that under
>many conditions the noise is internal, and due to neural noise. At least
>it is "Poisson like" since the variance of the signal is equal to the signal
>mean. In the case of signals like tones, that are not stochastic, the
>ONLY noise is internal noise. When working with wideband noise signals, it
>is less clear where the noise comes from. Is it due to the internal
>representation
>(yes, we say), or is it due to the stochastic nature of the stimulus?
>This would apply for the JND of wideband noise, for example (Miller, 1947)


Presumably, if there _is_ some external variability then this will add to
the internal variability, raising thresholds. Does this follow?

In some situations, for example, a 10 ms probe tone being masked by a 100
Hz-wide narrowband noise, the fluctuations in the noise must be the main
cause of masking. Also, intensity discrimination tasks in which the
pedestal level is roved from trial to trial or even interval to interval
may increase the external noise such that it becomes the main component
limiting performance.

I can see that internal noise could be a major factor in most masking
situations, however.


Bests, Chris









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