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Re: By any other name...
To follow Daniel Pressnitzer's message: in bad SNR the presence of the
tone can be difficult to establish, because, well, of the presence of
the noise. Any objective method for deciding whether a tone was there
during the noise segment will run into the absolute limits imposed by
'ideal observer analysis', based on whatever measure you think is
relevant (spike counts, intervals, absolute spike times...) in the
appropriate channels. Thus, at the rather bad SNR in which the
continuity illusion is usually run, an ideal observer would probably be
unable to discriminate between noise+tone and noise alone during the
noise burst with any reliability. It's only the experimenter who may
know with certainty whether the tone was there, not the listener.
Thus, I would strongly support Daniel's idea that the role of the
noise is to make the situation ambiguous, rather than to supply the
information about the presence of the tone. Under these circumstances,
heuristics (such as the 'old+new') probably become important in the
higher-level interpretation of the scene.
For what it is worth, here's my own self-promotion: we did a study
of the neural responses to low-level tones in fluctuating maskers (Las
et al. J. Neurosci. 2005). The parameters were set for studying
comodulation masking release rather than the continuity illusion, but
with this caveat, we found that in cortical neurons responded to the
noise+tone stimulus similarly to their responses to tone alone (strongly
suppressing the firing rate fluctuations that correlated with the noise
envelope). At the same SNRs, this was not the case in inferior
colliculus - changes in firing patterns were enough to signal the
presence of the tone, but the responses were still more similar to the
noise alone responses. Moving to unsupported speculations, we concluded
that the representation of the tone as a separate 'object' was
substantially enhanced in cortex relative to the inferior colliculus.
Maybe this is a neural signature of the perceptual continuity of the
tone within the noise.
Dept. of Neurobiology
The Alexander Silberman Institute of Life Sciences
Edmond Safra Campus, Givat Ram | Tel: Int-972-2-6584229
Hebrew University | Fax: Int-972-2-6586077
Jerusalem 91904, ISRAEL | Email: israel@xxxxxxxxxxxxx
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