Re: Monaural vs diotic stimulus presentation

[Erick Gallun <gallun@xxxxxxxxxxxxxxxxxxxxx>]

> When designing a psychophysical study of a monaural phenomenon, that
> is, one in which binaural information does not play a part, I've
> noticed that almost everyone presents their (headphone) stimuli
> monaurally. I was wondering if anyone can give me good reasons for
> not presenting the stimuli diotically, that is, the same stimulus
> presented to both headphone channels.
> Specifically, I'm designing a study on temporal gap detection, using
> noise markers of various bandwidths. I'm interested in relative
> performance as a function of the noise marker composition. So, if the
> absolute gap detection thresholds across the board were lower with
> diotic than monaural presentation, due to some sort of summation
> effect, it wouldn't matter to me.
I think you will find that your subjects do better with diotic than
monaural stimuli.  I presented data at ARO in february showing better
performance with diotic than monaural presentation in an intensity
increment detection task (as have others with quite different tasks -
such as signal detection in noise).  My current thought on this (and
I'd be happy to get feedback on it) is that signals with no external
noise are improved more by diotic versus monotic presentation since
the noise at the two ears is mostly the result of variability in
neural firing, which is uncorrelated between the two ears.  Neural
variability has a small effect when there is binaurally correlated
external noise in the stimulus, since the variability in the signal
due to the noise is dominated by the external noise - in other words,
the signals at the two ears are highly correlated and thus the second
produces very little additional information. Neural variability is
most apparent when it is the only noise source present -  for
example, when the signal is a pure tone..  In that case, it results
in essentially independent samples of the signal.  In fact, my data
suggested a "square root of two" improvement, which is the prediction
from independent noise sources.

Since you are not studying a diotic phenomenon, it shouldn't matter
if performance is elevated in all conditions.  And, as you suggest,
subjects prefer diotic to monaural presentation.  I would be
interested to hear if there is a better reason - aside from saving
money on earphones.

erick gallun
grad. student
hafter lab, UC Berkeley