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ABSTRACT: In this study we investigate whether stimulus variability affects the auditory steady-state response (ASSR). We present cosinusoidal AM pulses as stimuli where we are able to manipulate waveform shape independently of the fixed repetition rate of 4 Hz. We either present sounds in which the waveform shape, the pulse-width, is fixed throughout the presentation or where it varies pseudo-randomly. Importantly, the average spectra of all the fixed-width AM stimuli are equal to the spectra of the mixed-width AM. Our null hypothesis is that the average ASSR to the fixed-width AM will not be significantly different from the ASSR to the mixed-width AM. In a region of interest beamformer analysis of MEG data, we compare the 4 Hz component of the ASSR to the mixed-width AM with the 4 Hz component of the ASSR to the pooled fixed-width AM. We find that at the group level, there is a significantly greater response to the variable mixed-width AM at the medial boundary of the Middle and Superior Temporal Gyri. Hence, we find that adding variability into AM stimuli increases the amplitude of the ASSR. This observation is important, as it provides evidence that analysis of the modulation waveform shape is an integral part of AM processing. Therefore, standard steady-state studies in audition, using sinusoidal AM, may not be sensitive to a key feature of acoustic processing.
PLoS ONE 01/2012; 7(4):e34668. · 4.09 Impact Factor
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ABSTRACT: Magnetoencephalography (MEG) provides excellent temporal resolution when examining cortical activity in humans. Inverse methods such as beamforming (a spatial filtering approach) provide the means by which activity at cortical locations can be estimated. To date, the majority of work in this field has been based upon power changes between active and baseline conditions. Recent work, however, has focused upon other properties of the time series data reconstructed by these methods. One such metric, the Source Stability Index (SSI), relates to the consistency of the time series calculated only over an active period without the use of a baseline condition. In this paper we apply non-parametric statistics to SSI volumetric maps of simulation, auditory and somatosensory data in order to provide a robust and principled method of statistical inference in the absence of a baseline condition.
NeuroImage 01/2011; 54(2):906-18. · 5.89 Impact Factor
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Speech Communication. 01/2011; 53:903-913.
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ABSTRACT: Previous behavioural studies in human subjects have demonstrated the importance of amplitude modulations to the process of intelligible speech perception. In functional neuroimaging studies of amplitude modulation processing, the inherent assumption is that all sounds are decomposed into simple building blocks, i.e. sinusoidal modulations. The encoding of complex and dynamic stimuli is often modelled to be the linear addition of a number of sinusoidal modulations and so, by investigating the response of the cortex to sinusoidal modulation, an experimenter can probe the same mechanisms used to encode speech. The experiment described in this paper used magnetoencephalography to measure the auditory steady-state response produced by six sounds, all modulated in amplitude at the same frequency but which formed a continuum from sinusoidal to pulsatile modulation. Analysis of the evoked response shows that the magnitude of the envelope-following response is highly non-linear, with sinusoidal amplitude modulation producing the weakest steady-state response. Conversely, the phase of the steady-state response was related to the shape of the modulation waveform, with the sinusoidal amplitude modulation producing the shortest latency relative to the other stimuli. It is shown that a point in auditory cortex produces a strong envelope following response to all stimuli on the continuum, but the timing of this response is related to the shape of the modulation waveform. The results suggest that steady-state response characteristics are determined by features of the waveform outside of the modulation domain and that the use of purely sinusoidal amplitude modulations may be misleading, especially in the context of speech encoding.
European Journal of Neuroscience 10/2010; 32(9):1599-607. · 3.63 Impact Factor
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ABSTRACT: Speech contains complex amplitude modulations that have envelopes with multiple temporal cues. The processing of these complex envelopes is not well explained by the classical models of amplitude modulation processing. This may be because the evidence for the models typically comes from the use of simple sinusoidal amplitude modulations. In this study we used magnetoencephalography (MEG) to generate source space current estimates of the steady-state responses to simple one-component amplitude modulations and to a two-component amplitude modulation. A two-component modulation introduces the simplest form of modulation complexity into the waveform; the summation of the two-modulation rates introduces a beat-like modulation at the difference frequency between the two modulation rates. We compared the cortical representations of responses to the one-component and two-component modulations. In particular, we show that the temporal complexity in the two-component amplitude modulation stimuli was preserved at the cortical level. The method of stimulus normalization that we used also allows us to interpret these results as evidence that the important feature in sound modulations is the relative depth of one modulation rate with respect to another, rather than the absolute carrier-to-sideband modulation depth. More generally, this may be interpreted as evidence that modulation detection accurately preserves a representation of the modulation envelope. This is an important observation with respect to models of modulation processing, as it suggests that models may need a dynamic processing step to effectively model non-stationary stimuli. We suggest that the classic modulation filterbank model needs to be modified to take these findings into account.
European Journal of Neuroscience 10/2009; 30(6):1183-91. · 3.63 Impact Factor
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ABSTRACT: Many experimental studies into human brain function now use magnetoencephalography (MEG) to non-invasively investigate human neuronal activity. A number of different analysis techniques use the observed magnetic fields outside of the head to estimate the location and strength of the underlying neural generators. One such technique, a spatial filtering method known as Beamforming, produces whole-head volumetric images of activation. Typically, a differential power map throughout the head is generated between a time window containing the response to a stimulus of interest and a window containing background brain activity. A statistical test is normally performed to reveal locations which show a significantly different response in the presence of the stimulus. Despite this being a widely used measure, for both phase-locked and non-phase-locked information, it requires a number of assumptions; namely that the baseline activity defined is stable and also that a change in total power is the most effective way of revealing the neuronal sources required for the task. This paper introduces a metric which evaluates the consistency of the response at each location within a cortical volume. Such a method of localisation negates the need for a baseline period of activity to be defined and also moves away from simply considering the energy content of brain activity. The paper presents both simulated and real data. It demonstrates that this new metric of stability is able to more accurately and, crucially, more reliably draw inferences about neuronal sources of interest.
NeuroImage 09/2009; 49(2):1385-97. · 5.89 Impact Factor
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ABSTRACT: Human sensitivity to amplitude modulation has long been of interest to researchers, both in behavioural and neurological measures. Processing of amplitude modulation is implicated in the process of speech perception and sinusoidal amplitude modulation is used extensively to probe the mechanisms involved in encoding this information. The temporal envelope of speech is more accurately described as bursts of modulation rather than continuous modulation and the current work exposes participants to a continuum of modulation waveforms; from sinusoidal to pulsatile. Waveforms were amplitude modulated at 4 Hz and imposed upon a 500 Hz pure-tone carrier. The waveforms were generated using raised-cosine pulses with different half-durations. Half-durations of 8, 16, 24, 32, 64 and 125 ms were used (125 ms producing sinusoidal amplitude modulation at 4 Hz). Stimuli were 240 seconds in duration and responses were collected on a 248 channel whole-head MEG scanner. The frequency domain steady-state response was analysed from each condition in 14 participants, and results confirmed that the response to sinusoidal amplitude modulation was significantly lower than to modulations more representative of those found in speech signals. This suggests that non-sinusoidal stimuli may be more effective when investigating these auditory mechanisms.
The Journal of the Acoustical Society of America 06/2008; 123(5):3716. · 1.55 Impact Factor
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ABSTRACT: Research has demonstrated that low frequency amplitude modulations in speech signals are crucially important to maintaining intelligibility. The current work demonstrates a flexible way of characterising the pulsatile bursts of energy found in the temporal envelopes of sub-band filtered speech. Speech was passed through a 128-filter Gammatone filterbank and the temporal envelope of each filter extracted using the Hilbert transform. Thirty-five raised cosine pulses were fitted to model the envelope of each filter and each pulse was defined by its amplitude, half-duration and centre position. The distribution of these pulses demonstrates that the most commonly found pulse half-duration in speech is around 10 ms and few pulses have half-durations longer than 25 ms. Highly intelligible vocoded speech is generated using the extracted pulses and these measures suggest that the auditory system may signal the position in time of the amplitude modulations rather than representing low-frequency information. This method creates a flexible framework within which to further probe the mechanisms involved and allows the ability to focus on cross-channel information in the time domain.
The Journal of the Acoustical Society of America 06/2008; 123(5):3734. · 1.55 Impact Factor
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ABSTRACT: Many experimental studies into human brain function now use magnetoencephalography (MEG) to non-invasively investigate human neuronal activity. A number of different analysis techniques use the observed magnetic fields outside of the head to estimate the location and strength of the underlying neural generators. One such technique, a spatial filtering method known as Beamforming, produces whole-head volumetric images of activation. Typically, a differential power map throughout the head is generated between a time window containing the response to a stimulus of interest and a window containing background brain activity. A statistical test is normally performed to reveal locations which show a significantly different response in the presence of the stimulus. Despite this being a widely used measure, for both phase-locked and non-phase-locked information, it requires a number of assumptions; namely that the baseline activity defined is stable and also that a change in total power is the most effective way of revealing the neuronal sources required for the task. This paper introduces a metric which evaluates the consistency of the response at each location within a cortical volume. Such a method of localisation negates the need for a baseline period of activity to be defined and also moves away from simply considering the energy content of brain activity. The paper presents both simulated and real data. It demonstrates that this new metric of stability is able to more accurately and, crucially, more reliably draw inferences about neuronal sources of interest.
NeuroImage.
