Dear Fatima and List, I can think of two control sounds that might 'be generated from normal speech, retain more or less of the spectral features of the normal speech, yet is not pseudo-word like'. One is the amplitude-modulated noise that has been used for a while now in Robert Zatorre's lab (and others): you simply modulate white-noise by the amplitude of the speech signal. You come up with something that has very similar amplitude waveform as the original signal, but not the spectral content. This is a very 'low level' control, and it might not do the job of keeping some of the spectral features. So another possibility, which we used recently in a neuroimaging study of voice perception, is to use 'scrambled speech'. Here, the signal is transformed in Fourier space, then for each window of the FFT phase and amplitude components are randomized (phase with phase and amplitude with amplitude), and an inverse FFT is performed. You end up with a sinal which has the same energy as the original one, and a very similar waveform (depending on the size of the FFT window, a very important parameter). It is very similar to the scrambling used in the object recognition litterature, and in fact the spectrogram of these scrambled stimuli looks like an visual scramble of the original spectrogram. Yet the spectral structure is also dramatically modified, perhaps less than for the AM-noise though. Attached are a sample of speech and of its scrambled version. Hope this helps. Pascal BELIN, PhD Neuropsychology/Cognitive Neuroscience Unit Montreal Neurological Institute McGill University, 3801 University Street Montreal, Quebec, Canada H3A2B4 phone: (514) 398-8519 (8504) fax: (514) 398-1338 http://www.zlab.mcgill.ca/ On Wed, 28 Feb 2001, Fatima Husain wrote: > Dear List, > > Sorry to barge into an interesting discussion, but - > My lab wants to image subjects listening to normal and modified speech. > We are trying to investigate semantic memory. > The modified speech is generated from normal speech, retains more or less > of the spectral features of the normal speech, yet is not pseudo-word > like. The reason some of us don't like pseudo words is that, many subjects > treat words and pseudo-words in the same manner, activating a mental > lexicon and semantic memory (what we are interested in). > > We looked at reversed speech and low-pass filtered speech. But these were > either too foreign-language-like (according to naive listeners) or too > much like humming. > > I would be grateful if someone would suggest suitable methods of > modifying speech to fulfill our (somewhat vague) requirements. > > Thanks, > --fatima > > fhusain@cns.bu.edu > fthusain@helix.nih.gov >
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