Biomedical subjects

Nicola Hewson-Stoate

Publications and source records attributed to Nicola Hewson-Stoate.

5 recordsLinked to original sources

PubMed2006

Cortical response to auditory motion suggests an asymmetry in the reliance on inter-hemispheric connections between the left and right auditory cortices.

The aim of the current study was to measure the brain's response to auditory motion using electroencephalography (EEG) to gain insight into the mechanisms by which hemispheric lateralization for auditory spatial processing is established in the human brain. The onset of left- or rightward motion in an otherwise continuous sound was found to elicit a large response, which appeared to arise from higher-level nonprimary auditory areas. This motion onset response was strongly lateralized to the hemisphere contralateral to the direction of motion. The response latencies suggest that the ipsilateral response to the leftward motion was produced by indirect callosal projections from the opposite hemisphere, whereas the ipsilateral response to the rightward motion seemed to receive contributions from direct thalamocortical projections. These results suggest an asymmetry in the reliance on inter-hemispheric projections between the left and right auditory cortices for auditory spatial processing.

Acoustic Stimulation↗

PubMed2006

Vowel processing evokes a large sustained response anterior to primary auditory cortex.

The present study uses electroencephalography (EEG) and a new stimulation paradigm, the 'continuous stimulation paradigm', to investigate the neural correlate of phonological processing in human auditory cortex. Evoked responses were recorded to stimuli consisting of a control sound (1000 ms) immediately followed by a test sound (150 ms). On half of the trials, the control sound was a noise and the test sound a vowel; to control for unavoidable effects of spectral change at the transition, the roles of the stimuli were reversed on the other half of the trials. The acoustical properties of the vowel and noise sounds were carefully matched to isolate the response specific to phonological processing. As the unspecific response to sound energy onset has subsided by the transition to the test sound, we hypothesized that the transition response from a noise to a vowel would reveal vowel-specific processing. Contrary to this expectation, however, the most striking difference between vowel and noise processing was a large, vertex-negative sustained response to the vowel control sound, which had a fast onset (30-50 ms) and remained constant throughout presentation of the vowel. The vowel-specific response was isolated using a subtraction technique analogous to that commonly applied in neuroimaging studies. This similarity in analysis methodology enabled close comparison of the EEG data collected in the present study with relevant functional magnetic resonance (fMRI) literature. Dipole source analysis revealed the vowel-specific component to be located anterior and inferior to primary auditory cortex, consistent with previous data investigating speech processing with fMRI.

Adult↗

PubMed2005

Further nonlinearities in neurovascular coupling in rodent barrel cortex.

An essential prerequisite for the accurate interpretation of noninvasive functional brain imaging techniques, such as blood oxygen level dependent (BOLD) fMRI, is a thorough understanding of the coupling relationship between neural activity and the haemodynamic response. The current study investigates this relationship using rat barrel cortex as a model. Neural input was measured by applying current source density (CSD) analysis to multi-laminar field potentials to remove ambiguities regarding the origin of the signal inherent in single electrode recordings. Changes in cerebral blood flow (CBF) were recorded with a laser Doppler flowmetry probe. The magnitude of neural and CBF responses were modulated over a large range by altering both the intensity and frequency of electrical whisker pad stimulation. Consistent with previous findings [Devor, A., et al., 2003. Neuron 39, 353-359; Sheth, S.A., et al., 2004. Neuron 42, 347-355] a power law function well described the relationship between neural activity and haemodynamics. Despite the nonlinearity of the coupling over the whole data set, the relationship was very well approximated by a linear function over mid-range stimuli. Altering the frequency of stimulation at 1.2 mA shifted the neural activity and corresponding haemodynamic response along this linear region, reconciling recent reports of a nonlinear relationship [Devor, A., et al., 2003. Neuron 39, 353-359; Jones, M., et al., 2004. NeuroImage 22, 956-965; Sheth, S.A., et al., 2004. Neuron 42, 347-355] with previous work that found a linear coupling relationship when altering stimulation frequency [Martindale, J., et al., 2003. J. Cereb. Blood Flow Metab. 23, 546-555; Ngai, A.C., et al., 1999. Brain Res. 837, 221-228; Sheth, S., et al., 2003. NeuroImage 19, 884-894]. Using stimuli within this linear range in imaging studies would simplify the interpretation of findings.

Animals↗

PubMed2005

The effect of hypercapnia on the neural and hemodynamic responses to somatosensory stimulation.

Modern non-invasive imaging techniques utilize the coupling between neural activity and changes in blood flow, volume and oxygenation to map the functional architecture of the human brain. An understanding of how the hemodynamic response is influenced by pre-stimulus baseline perfusion is important for the interpretation of imaging data. To address this issue, the present study measured hemodynamics with optical imaging spectroscopy and laser Doppler flowmetry, while multi-channel electrophysiology was used to record local field potentials (LFP) and multi-unit activity (MUA). The response to whisker stimulation in rodent barrel cortex was recorded during baseline (normocapnia) and elevated perfusion rates produced by two levels of hypercapnia (5 and 10%). With the exception of the 'washout' of deoxyhemoglobin, which was attenuated, all aspects of the neural and hemodynamic response to whisker stimulation were similar during 5% hypercapnia to those evoked during normocapnia. In contrast, 10% hypercapnia produced cortical arousal and a reduction in both the current sink and MUA elicited by stimulation. Blood flow and volume responses were reduced by a similar magnitude to that observed in the current sink. The deoxyhemoglobin 'washout', however, was attenuated to a greater degree than could be expected from the neural activity. These data suggest that imaging techniques based on perfusion or blood volume changes may be more robust to shifts in baseline than those based on the dilution of deoxyhemoglobin, such as conventional BOLD fMRI.

Anesthesia↗

PubMed2004

Nonlinear coupling of neural activity and CBF in rodent barrel cortex.

The relationship between neural activity and accompanying changes in cerebral blood flow (CBF) and oxygenation must be fully understood before data from brain imaging techniques can be correctly interpreted. Whether signals in fMRI reflect the neural input or output of an activated region is still unclear. Similarly, quantitative relationships between neural activity and changes in CBF are not well understood. The present study addresses these issues by using simultaneous laser Doppler flowmetry (LDF) to measure CBF and multichannel electrophysiology to record neural activity in the form of field potentials and multiunit spiking. We demonstrate that CBF-activation coupling is a nonlinear inverse sigmoid function. Comparing the data with previous work suggests that within a cortical model, CBF shows greatest spatial correlation with a current sink 500 microm below the surface corresponding to sensory input. These results show that care must be exercised when interpreting imaging data elicited by particularly strong or weak stimuli and that hemodynamic changes may better reflect the input to a region rather than its spiking output.

Animals↗