Re: By any other name...

[Israel Nelken <israel@xxxxxxxxxxxxx>]

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.
   Eli

  

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Israel Nelken
Dept. of Neurobiology             

The Alexander Silberman Institute of Life Sciences
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