By Fourier's theorem 1 , signals can be decomposed into a sum of sinusoids of different frequencies. This is especially relevant for hearing, because the inner ear performs a form of mechanical Fourier transform by mapping frequencies along the length of the cochlear partition. An alternative signal decomposition, originated by Hilbert 2 , is to factor a signal into the product of a slowly varying envelope and a rapidly varying fine time structure. Neurons in the auditory brainstem 3-6 sensitive to these features have been found in mammalian physiological studies. To investigate the relative perceptual importance of envelope and fine structure, we synthesized stimuli that we call 'auditory chimaeras', which have the envelope of one sound and the fine structure of another. Here we show that the envelope is most important for speech reception, and the fine structure is most important for pitch perception and sound localization. When the two features are in conflict, the sound of speech is heard at a location determined by the fine structure, but the words are identified according to the envelope. This finding reveals a possible acoustic basis for the hypothesized 'what' and 'where' pathways in the auditory cortex 7-10 .Combinations of features from different sounds have been used in the past to produce new, hybrid sounds for use in electronic music 11,12 . Our aim in combining features from different sounds was to study the perceptual relevance of the envelope and fine structure in different acoustic situations. To synthesize auditory chimaeras, two sound waveforms are used as inputs. A bank of band-pass filters is used to split each sound into 1 to 64 complementary frequency bands spanning the range 80-8,820 Hz. Such splitting into frequency bands resembles the Fourier analysis performed by the cochlea and by processors for cochlear implants. The output of each filter is factored into its envelope and fine structure using the Hilbert transform (see Methods). The envelope of each filter output from the first sound is then multiplied by the fine structure of the corresponding filter output from the second sound. These products are finally summed over all frequency bands to produce an auditory chimaera that is made up of the envelope of the first sound and the fine structure of the second sound in each band. The primary variable in this study is the number of frequency bands, which is inversely related to the width of each band. A block diagram of chimaera synthesis is shown in Fig. 1 Listening tests with speech-noise chimaeras showed that speech reception is highly dependent on the number of frequency bands used for synthesis (Fig. 2). When speech information is contained solely in the envelope, speech reception is poor with one or two frequency bands and improves as the number of bands increases. Good performance (>85% word recognition) is achieved with as few as four frequency bands, consistent with previous findings that bands of noise modulated by speech envelope can produce good speech reception with ...

We develop an objective, noninvasive method for determining the frequency selectivity of cochlear tuning at low and moderate sound levels. Applicable in humans at frequencies of 1 kHz and above, the method is based on the measurement of stimulus-frequency otoacoustic emissions and, unlike previous noninvasive physiological methods, does not depend on the frequency selectivity of masking or suppression. The otoacoustic measurements indicate that at low sound levels human cochlear tuning is more than twice as sharp as implied by standard behavioral studies and has a different dependence on frequency. New behavioral measurements designed to minimize the influence of nonlinear effects such as suppression agree with the emission-based values. A comparison of cochlear tuning in cat, guinea pig, and human indicates that, contrary to common belief, tuning in the human cochlea is considerably sharper than that found in the other mammals. The sharper tuning may facilitate human speech communication.T he mammalian cochlea acts as an acoustic prism, mechanically separating the frequency components of sound so that they stimulate different populations of sensory cells. As a consequence of this frequency separation, or filtering, each sensory cell within the cochlea responds preferentially to sound energy within a limited frequency range. In its role as a frequency analyzer, the cochlea has been likened to a bank of overlapping bandpass filters, often referred to as ''cochlear filters.'' The frequency tuning of these filters plays a critical role in our ability to distinguish and perceptually segregate different sounds. For instance, hearing loss is often accompanied by a degradation in cochlear tuning, or a broadening of the cochlear filters. Although quiet sounds can be restored to audibility with appropriate hearing-aid amplification, the loss of cochlear tuning leads to pronounced, and as yet largely uncorrectable, deficits in the ability of hearing-impaired listeners to extract meaningful sounds from background noise (1).The bandwidths of cochlear filters have been measured directly in anesthetized, non-human mammals by recording from the auditory-nerve fibers that contact the sensory cells (2). Filter bandwidths in humans, however, must be determined indirectly from noninvasive measurements. Traditionally, such studies have relied on psychophysical (i.e., behavioral) measures of filter bandwidth based on the phenomenon of masking; that is, the ability of one sound to interfere with, or ''mask,'' the perception of another. Strong masking is interpreted as indicating that frequency components of the masker fall within the passband of the cochlear filter whose output is used to detect the signal. Interference then occurs because both signal and masker stimulate an overlapping group of sensory cells. Since the pioneering work of Harvey Fletcher (3), filter bandwidths have been obtained by measuring listeners' thresholds for detection of a pure tone in background noises with particular spectral characteristics. These tone th...