Detection and direction-discrimination of diotic and dichotic ramp modulations in amplitude and phase

When the source of a tone moves with respect to a listener's ears, dichotic (or interaural) phase and amplitude modulations (PM and AM) are produced. Two experiments investigated the psychophysical characteristics of dichotic linear ramp modulations in phase and amplitude, and compared them with the psychophysics of diotic PM and AM. In experiment 1, subjects were substantially more sensitive to dichotic PM than diotic PM, but AM sensitivity was equivalent in the dichotic and diotic conditions. Thresholds for discriminating modulation direction were smaller than detection thresholds for dichotic AM, and both diotic AM and PM. Dichotic PM discrimination thresholds were similar to detection thresholds. In experiment 2, the effects of ramp duration were examined. Sensitivity to dichotic AM and PM, and diotic AM increased as duration was increased from 20 ms to 200 ms. The functions relating sensitivity to ramp duration differed across the stimuli; sensitivity to dichotic PM increased more rapidly than sensitivity to dichotic or diotic AM. This was also reflected in shorter time-constants and minimum integration times for dichotic PM detection. These findings support the hypothesis that the analysis of dichotic PM and AM rely on separate mechanisms. © 2003 Acoustical Society of America

Local minimal realisations of trained Hopfield networks

A methodology for investigating the invariant structural characteristics..

Effect of a noise modulation masker on the detection of second-order amplitude modulation

Amplitude modulation waveforms can contain complex patterns of modulation frequency and depth that are characteristic of many biologically relevant sounds. To investigate the mechanisms involved in the processing of such patterns, we measured detection thresholds for second-order amplitude modulation (AM), a sinusoidal AM in which AM depth varies with time at frequency fm′. Second-order AM generates sidebands in the modulation spectrum on either side of the frequency components introduced by the first-order AM. Previous masking studies suggested that a distortion product located at fm′ contributes to the detection of second-order AM. This hypothesis was tested by masking the putative distortion product using a noise modulation masker centred on (1) the second-order modulation frequency (fm′=2 Hz) and (2) the first-order modulation frequency (fm=16 Hz). The second-order AM was applied to a 5-kHz pure-tone carrier. Increasing the depth of a 2-Hz-wide noise modulator masker centred on 2 Hz had little effect on detection thresholds for second-order AM, but increased detection thresholds for 2-Hz first-order AM six-fold. Increasing the depth of an 8-Hz-wide noise modulator masker centred on 16 Hz increased detection thresholds for both first- and second-order AM three-fold. These results show that the detection of the second-order AM, when fm′ is 2 Hz, is not dependent on the detection of modulation at fm′ but is dependent on the detection of modulation components centred on fm

The relative order of a class of recurrent networks

Three types of recurrent network configurations have been proposed since they enable adequate description of temporal behaviour. The concept of relative order has been introduced so as to provide a framework for analysing such network configurations. It has been demonstrated that, excluding pathologies, each configuration is of relative order unity. Therefore it follows that an inverse exists for each type of configuration (Tsinias and Kalouptsidis, 1983; Hirschorn, 1979)

Second-order modulation detection thresholds for pure-tone and narrow-band noise carriers

Modulation perception has typically been characterized by measuring detection thresholds for sinusoidally amplitude-modulated (SAM) signals. This study uses multicomponent modulations. "Second-order" temporal modulation transfer functions (TMTFs) measure detection thresholds for a sinusoidal modulation of the modulation waveform of a SAM signal [Lorenzi et al., J. Acoust. Soc. Am. 110, 1030-2038 (2001)]. The SAM signal therefore acts as a "carrier" stimulus of frequency f(m), and sinusoidal modulation of the SAM signal's modulation depth (at rate f(m)') generates two additional components in the modulation spectrum at f(m)-f(m)' and f(m)+f(m)'. There is no spectral energy at the envelope beat frequency f(m)' in the modulation spectrum of the "physical" stimulus. In the present study, second-order TMTFs were measured for three listeners when f(m) was 16, 64, and 256 Hz. The carrier was either a 5-kHz pure tone or a narrow-band noise with center frequency and bandwidth of 5 kHz and 2 Hz, respectively. The narrow-band noise carrier was used to prevent listeners from detecting spectral energy at the beat frequency f(m)' in the "internal" stimuli's modulation spectrum. The results show that, for the 5-kHz pure-tone carrier, second-order TMTFs are nearly low pass in shape; the overall sensitivity and cutoff frequency measured on these second-order TMTFs increase when f(m) increases from 16 to 256 Hz. For the 2-Hz-wide narrow-band noise carrier, second-order TMTFs are nearly flat in shape for f(m) = 16 and 64 Hz, and they show a high-pass segment for f(m) = 256 Hz. These results suggest that detection of spectral energy at the envelope beat frequency contributes in part to the detection of second-order modulation. This is consistent with the idea that nonlinear mechanisms in the auditory pathway produce an audible distortion component at the envelope beat frequency in the internal modulation spectrum of the sounds. (C) 2001 Acoustical Society of America

Toward biocompatible nuclear hyperpolarization using signal amplification by reversible exchange: Quantitative in situ spectroscopy and high-field imaging

[Image: see text] Signal amplification by reversible exchange (SABRE) of a substrate and parahydrogen at a catalytic center promises to overcome the inherent insensitivity of magnetic resonance. In order to apply the new approach to biomedical applications, there is a need to develop experimental equipment, in situ quantification methods, and a biocompatible solvent. We present results detailing a low-field SABRE polarizer which provides well-controlled experimental conditions, defined spins manipulations, and which allows in situ detection of thermally polarized and hyperpolarized samples. We introduce a method for absolute quantification of hyperpolarization yield in situ by means of a thermally polarized reference. A maximum signal-to-noise ratio of ∼10(3) for 148 μmol of substance, a signal enhancement of 10(6) with respect to polarization transfer field of SABRE, or an absolute (1)H-polarization level of ≈10(–2) is achieved. In an important step toward biomedical application, we demonstrate (1)H in situ NMR as well as (1)H and (13)C high-field MRI using hyperpolarized pyridine (d(3)) and (13)C nicotinamide in pure and 11% ethanol in aqueous solution. Further increase of hyperpolarization yield, implications of in situ detection, and in vivo application are discussed