Auditory profile analysis of harmonic signals

Macaque thresholds for detecting increases in intensity: effects of formant structure

Macaque monkeys, like humans, are more sensitive to differences in formant frequency than to differences in the frequency of pure tones (see Sinnott et al. (1987) J. Comp. Psychol. 94, 401-415; Pfingst (1993) J. Acoust. Soc. Am. 93, 2124-2129; Prosen et al. (1990) J. Acoust. Soc. Am. 88, 2152-2158; Sinnott et al. (1985) J. Acoust. Soc. Am. 78, 1977-1985; Sinnott and Kreiter (1991) J. Acoust. Soc. Am. 89, 2421-2429; for summary, see May et al. (1996) Aud. Neurosci. 3, 135-162). In the discrimination of formant frequency, it appears that the relevant cue for macaque monkeys is relative level differences of the component frequencies (Sommers et al. (1992) J. Acoust. Soc. Am. 91, 3499-3510). To further explore the result of Sommers et al., we trained macaque monkeys (Macaca fuscata) to report detection of a change in the spectral shape of multi-component harmonic complexes. Spectral shape changes were produced by the addition of intensity increments. When the amplitude spectrum of the comparison stimulus was modeled after the /ae/ vowel sound, thresholds for detecting a change from the comparison stimulus were lowest when intensity increments were added at spectral peaks. These results parallel previous data from human subjects, suggesting that both human and monkey subjects may process vowel spectra through simultaneous comparisons of component levels across the spectrum. When the subjects were asked to detect a change from a comparison stimulus with a flat amplitude spectrum, the subjects showed sensitivity that was relatively comparable to that of human subjects tested in other investigations (e.g. Zera et al. (1993) J. Acoust. Soc. Am. 93, 3431-3441). In additional experiments, neither increasing the dynamic range of the /ae/ spectrum nor dynamically varying the amplitude of the increment during the stimulus presentation reliably affected detection thresholds.

Sources of variation in profile analysis. II. Component spacing, dynamic changes, and roving level

Profile-analysis experiments have typically employed static profiles with constant frequency components spaced at equal intervals along a logarithmic frequency axis. Most periodic, naturally occurring stimuli, however, have components that are harmonically related and vary dynamically in time. One goal of these studies was to determine whether amplitude-increment detection thresholds are different in dynamic, harmonically spaced profiles compared to those for static-log profiles, and why such differences might exist. A second goal was to determine the impact of roving levels (within-trial variation of level). Thresholds for static-log profiles were, on average, 8.7 dB lower than for static-harmonic profiles. A traditional filter-bank model could not account for this result. No consistent effect of dynamic contour (an exponential rising frequency glide) was observed. Thresholds were consistently poorer by 4 to 7 dB when the level was roved, but the differences in thresholds among the different profiles varied little. It is proposed that the higher thresholds observed in static-harmonic profiles may be accounted for by the more intense pitch strength associated with the harmonic profiles.

Complex tone processing in primary auditory cortex of the awake monkey. II. Pitch versus critical band representation

Noninvasive neurophysiological studies in humans support the existence of an orthogonal spatial representation of pure tone frequency and complex tone pitch in auditory cortex [Langner et al., J. Comp. Physiol. A 181, 665-676 (1997)]. However, since this topographic organization is based on neuromagnetic responses evoked by wideband harmonic complexes (HCs) of variable fundamental frequency (f0), and thus interharmonic frequency separation (deltaF), critical band filtering effects due to differential resolvability of harmonics may have contributed to shaping these responses. To test this hypothesis, the present study examined responses evoked by three-component HCs of variable f0 in primary auditory cortex (A1) of the awake monkey. The center frequency of the HCs was fixed at the best frequency (BF) of the cortical site. Auditory evoked potential (AEP), multiunit activity, and current source density techniques were used to evaluate A1 responses as a function of f0 (=deltaF). Generally, amplitudes of nearly all response components increased with f0, such that maximal responses were evoked by HCs comprised of low-order resolved harmonics. Statistically significant increases in response amplitude typically occurred at deltaFs between 10% and 20% of center frequency, suggestive of critical bandlike behavior. Complex tone response amplitudes also reflected nonlinear summation in that they could not be predicted by the pure tone frequency sensitivity curves of the cortical sites. A mechanism accounting for the observed results is proposed which involves mutual lateral inhibitory interactions between responses evoked by stimulus components lying within the same critical band. As intracortical AEP components likely to be propagated to the scalp were also strongly modulated by deltaF, these findings indicate that noninvasive recordings of responses to complex sounds may require a consideration of critical band effects in their interpretation.

Effect of temporal position, proportional variance, and proportional duration on decision weights in temporal pattern discrimination

Two experiments investigated how listeners allocate their attention to different segments of a temporal pattern. The experiments allowed a direct test of the predictions of the Proportion of Total Duration (PTD) rule and the Component Relative Entropy (CoRE) model. Listeners had to decide whether two sequences of nine tones had the same or different temporal patterns (tone duration = 25 ms, tone frequency = 1000 Hz). A sequence's temporal pattern was determined by the time intervals between each tone's offset and the next tone's onset. On same trials, the time intervals at corresponding temporal positions in the two sequences were identical. On different trials, the corresponding time intervals were randomly varied. Listener attention to different temporal positions within a sequence was assessed by calculating the decision weights at each position. The results supported the CoRE model and were inconsistent with the PTD rule. Manipulating the mean of the time intervals within the sequence had no consistent effect on the pattern of weights (or on overall performance), indicating that listener attention was not affected by either the proportion of total duration or the perceptual salience of a longer or shorter time interval. However, manipulating the variance of the time intervals had a significant effect: the highest weight was given to the highest variance segment. This weighting strategy leads to better performance because higher variance segments are more diagnostic of whether the sequences are the same or different.

Psychoacoustical evaluation of the pitch-synchronous overlap-and-add speech-waveform manipulation technique using single-formant stimuli

This article presents two experiments dealing with a psychoacoustical evaluation of the pitch-synchronous overlap-and-add (PSOLA) technique. This technique has been developed for modification of duration and fundamental frequency of speech and is based on simple waveform manipulations. Both experiments were aimed at deriving the sensitivity of the auditory system to the basic distortions introduced by PSOLA. In experiment I, manipulation of fundamental frequency was applied to synthetic single-formant stimuli under minimal stimulus uncertainty, level roving, and formant-frequency roving. In experiment II, the influence of the positioning of the so-called "pitch markers" was studied. Depending on the formant and fundamental frequency, experimental data could be described reasonably well by either a spectral intensity-discrimination model or a temporal model based on detecting changes in modulation of the output of a single auditory filter. Generally, the results were in line with psychoacoustical theory on the auditory processing of resolved and unresolved harmonics.