Biomedical subjects

Rhodri Cusack

Publications and source records attributed to Rhodri Cusack.

14 recordsLinked to original sources

PubMed2006

Branch cut surface placement for unwrapping of undersampled three-dimensional phase data: application to magnetic resonance imaging arterial flow mapping.

We demonstrate in both simulated and real cases the effect that undersampling of a three-dimensional (3D) wrapped phase distribution has on the geometry of phase singularity loops and their branch cut surfaces. The more intuitive two-dimensional (2D) problem of setting branch cuts between dipole pairs is taken as a starting point, and then branch cut surfaces in flat and ambiguous 3D loops are discussed. It is shown that the correct 2D branch cuts and 3D branch cut surfaces should be placed where the gradient of the original phase distribution exceeded pi rad voxel(-1). This information, however, is lost owing to undersampling and cannot be recovered from the sampled wrapped phase distribution alone. As a consequence, empirical rules such as finding the surface of minimal area or methods based on the wrapped phase gradient will fail to find the correct branch cut surfaces. We conclude that additional information about the problem under study is therefore needed to produce correct branch cut surfaces that lead to an unwrapped phase distribution with minimum local errors. An example with real data is provided in which downsampled phase contrast magnetic resonance imaging data are successfully unwrapped when the position of the vessel walls and the physical properties of the flowing blood are taken into account.

Algorithms↗

PubMed2006

Dissociable contributions of the mid-ventrolateral frontal cortex and the medial temporal lobe system to human memory.

Although the prefrontal cortex and regions of the medial temporal lobe are commonly co-activated in neuroimaging studies, their precise respective contributions to human memory remain unclear. In this event-related fMRI study, conditions requiring volunteers to simply look at pictures of abstract art were compared with conditions in which they were explicitly instructed to remember similar stimuli for later recognition. Looking, with no explicit instruction to remember, was associated with significant increases in signal intensity in the medial temporal lobe in 19 of the 20 volunteers scanned, but not in a region of the mid-ventrolateral prefrontal cortex that has previously been implicated in memory encoding and retrieval. Behavioral data collected outside the scanner on the same task revealed that recognition of these stimuli was, however, above chance. When the task instructions were changed to encourage the volunteers to remember the stimuli, significant increases in signal intensity were observed bilaterally, in the mid-ventrolateral frontal cortex, but there was no concomitant increase within the medial temporal lobe region. Moreover, behavioral data collected outside the scanner confirmed that recognition of these stimuli was significantly improved relative to the 'just look' trials. These results suggest that the mid-ventrolateral frontal cortex and the medial temporal lobe region make dissociable contributions to human memory that correspond closely to 'top-down' and 'bottom-up' notions of cognitive control, respectively.

Adolescent↗

PubMed2006

Modulation of spatial bias in the dual task paradigm: evidence from patients with unilateral parietal lesions and controls.

Lateral attentional bias is common after unilateral brain damage. It has sometimes been proposed that lateral bias is increased by concurrent cognitive demands, perhaps because of lost top-down compensation. However, an important limitation of previous studies is the sole use of right hemisphere patients. Here we employed a dual task paradigm to measure spatial bias on a visual task while manipulating demands of a concurrent auditory task. Bias was examined in patients with left or right parietal lesions and controls. In Experiment 1 the addition of either a non-spatial or spatial auditory task led to a rightward shift in visual bias. This same rightward shift occurred in controls, left parietals and right parietals. Experiment 2 examined whether the participant's response hand affected their bias. In addition, it attempted to distinguish between the hypothesis that modulatory effects are strongly dependent on lateralization of the concurrent task, and the hypothesis that dual tasks cause a general rightward shift. Response hand was found to have no effect on spatial bias. In addition, bias did not differ between left hemisphere (verbal) and right hemisphere (pitch) concurrent tasks, though the trend was for a smaller rightward shift with the verbal task. Our results show that dual tasks do not exacerbate patients' underlying deficits; instead they cause a global shift in attention to the right. This shift may resemble general rightward shifts that have previously been linked to reduced arousal.

Acoustic Stimulation↗

PubMed2005

Is susceptibility to perceptual migration and fusion modality-specific or multimodal?

A previous paper reported high susceptibility to spatial migration (allochiria) of tactile stimuli in about 25% of healthy individuals (High Error subjects). When synchronous stimuli touched the two hands, if the unattended stimulus was temporally modulated when the attended one was not (and was thus more salient than the latter), it "migrated" to and fused with or replaced the stimulus on the attended hand. When subjects rated similarity of the attended stimulus accompanied by a distractor to each stimulus alone, scaling distributions tested against a sampling model showed most High Error subjects experienced fused stimuli, others experienced replacement and Low Error subjects experienced neither. We argued that these migrations are equivalent to allochiria and that this underlies neglect and extinction. This study assessed whether the individual difference is modality-specific or not. In auditory and visual equivalents of the tactile rating experiment, the difference between High and Low Error subjects was replicated in audition, but no migration occurred in vision. However, when two words were briefly presented visually before a mask with cued report of one, letter migrations to equivalent locations did occur and the individual difference was reproduced. This constitutes the first report of individual differences in auditory fusion and visual letter migration. Migration occurred in egocentric coordinates but apparently preserved structural homology. Different migration rates between the modalities paralleled relative salience of the unattended to the attended stimulus. The multimodality of the individual difference suggests that its source is supramodal, in deficient binding of perceptual content to location.

Adult↗

PubMed2005

Attentional functions of parietal and frontal cortex.

A model of normal attentional function, based on the concept of competitive parallel processing, is used to compare attentional deficits following parietal and frontal lobe lesions. Measurements are obtained for visual processing speed, capacity of visual short-term memory (VSTM), spatial bias (bias to left or right hemifield) and top-down control (selective attention based on task relevance). The results show important differences, but also surprising similarities, in parietal and frontal lobe patients. For processing speed and VSTM, deficits are selectively associated with parietal lesions, in particular lesions of the temporoparietal junction. We discuss explanations based on either grey matter or white matter lesions. In striking contrast, measures of attentional weighting (spatial bias and top-down control) are predicted by simple lesion volume. We suggest that attentional weights reflect competition between broadly distributed object representations. Parietal and frontal mechanisms work together, both in weighting by location and weighting by task context.

Acoustic Stimulation↗

PubMed2005

An evaluation of the use of passive shimming to improve frontal sensitivity in fMRI.

The presence of the head in an MRI scanner leads to inhomogeneities in the magnetic field. These cause the 'susceptibility artifacts' of image distortion and signal dropout. In this paper, we evaluate a technique called passive shimming, which has the potential to reduce field inhomogeneities and the resultant artifacts. A piece of a magnetically active material (pyrolytic graphite) is held on the roof of the participant's mouth by a plastic mouth mould. We evaluate the effects in several different ways. We show that the presence of a shim reduces field inhomogeneity across much of the brain. From field maps, we generate simulations of EPI image intensity and BOLD sensitivity. Both of these are mainly improved by the presence of shim, although there were small reductions in some regions. Measured EPI image intensity also mostly increased. Finally, we ran a reward-punishment task in our subjects, and found that the presence of a shim increased functional sensitivity in the orbitofrontal cortex. Using the BOLD sensitivity measure, we provide estimates of the improvement to be expected in functional studies for a range of neural structures. Passive shims are quick to make and reasonably comfortable to wear, and have substantial potential for researchers investigating inferior frontal brain regions using MRI.

Artifacts↗

PubMed2005

Automated post-hoc noise cancellation tool for audio recordings acquired in an MRI scanner.

There are several types of experiment in which it is useful to have subjects speak overtly in a magnetic resonance imaging (MRI) scanner, including those studying the articulatory apparatus and the neural basis of speech production, and fMRI experiments in which speech is used as a response modality. Although it is relatively easy to record sound from the bore, it can be difficult to hear the speech over the very loud acoustic noise from the scanner. This is particularly a problem during echo-planar imaging, which is usually used for fMRI. We present a post-hoc sound cancellation algorithm, and describe a Windows-based tool that implements it. The tool is fast and operates with minimal user intervention. We evaluate cancellation performance in terms of the improvement in signal-to-noise ratio, and investigate the effect of the recording medium. A substantial improvement in audibility was obtained.

Algorithms↗

PubMed2005

The intraparietal sulcus and perceptual organization.

The structuring of the sensory scene (perceptual organization) profoundly affects what we perceive, and is of increasing clinical interest. In both vision and audition, many cues have been identified that influence perceptual organization, but only a little is known about its neural basis. Previous studies have suggested that auditory cortex may play a role in auditory perceptual organization (also called auditory stream segregation). However, these studies were limited in that they just examined auditory cortex and that the stimuli they used to generate different organizations had different physical characteristics, which per se may have led to the differences in neural response. In the current study, functional magnetic resonance imaging was used to test for an effect of perceptual organization across the whole brain. To avoid confounding physical changes to the stimuli with differences in perceptual organization, we exploited an ambiguous auditory figure that is sometimes perceived as a single auditory stream and sometimes as two streams. We found that regions in the intraparietal sulcus (IPS ) showed greater activity when 2 streams were perceived rather than 1. The specific involvement of this region in perceptual organization is exciting, as there is a growing literature that suggests a role for the IPS in binding in vision, touch, and cross-modally. This evidence is discussed, and a general role proposed for regions of the IPS in structuring sensory input.

Acoustic Stimulation↗

PubMed2004

Effects of differences in the pattern of amplitude envelopes across harmonics on auditory stream segregation.

When a mixture of sound from many sources arrives at the ear, the auditory system attempts to segregate it into different perceptual streams. For sequences of sounds, the effects of basic acoustic properties (e.g., frequency separation, rate) on streaming are well understood, but much less is known about the effects of more complex acoustic attributes. Dynamic variations in frequency spectrum are known to have an important effect on the timbre of sounds. We investigated whether dynamic variations also affect stream segregation. Periodic tones were used, comprising harmonics 1-6, and presented in long sequences (ABA-ABA- ...). Tones A and B always differed in fundamental frequency, a factor known to influence streaming, and had either the same or different patterns of spectral variation. The total amount of variation (spectral flux) was held constant. Listeners judged moment-by-moment the grouping of these sequences, and the measure used was the proportion of time a sequence was heard as segregated. We found that sequences of complex tones with different patterns of spectral variation are more likely to segregate when this results in different patterns of change in the frequency centroid over time (Experiments 1 and 2), but not when it does not (Experiment 3).

Acoustic Stimulation↗

PubMed2004

Categorical and dimensional reports of experienced affect to emotion-inducing pictures in depression.

Self-reported affect to positive and negative emotional pictures was contrasted in people with major depressive disorder (MDD) and never-depressed control participants (n = 25 in each group). The results revealed significant differences in response to positive images (reduced arousal, less pleasant valence, decreased happiness, increased sadness) in MDD but no clear group differences in response to negative stimuli. Extending earlier findings of reduced responsiveness to positive, but not negative, stimuli in MDD (D. M. Sloan, M. E. Strauss, S. W. Quirk, & M. Satajovik, 1997; D. M. Sloan, M. E. Strauss, & K. L. Wisner, 2001), the data indicate that blunted response to positive stimuli is found when both categorical and dimensional ratings are elicited. Further, the data replicate earlier findings of elevated sadness reports to positive stimuli (J. Rottenberg, K. L. Kasch, J. J. Gross, & I. H. Gotlib, 2002), which may reflect broader difficulties in regulating emotions in MDD.

Affect↗

PubMed2004

Effects of location, frequency region, and time course of selective attention on auditory scene analysis.

Often, the sound arriving at the ears is a mixture from many different sources, but only 1 is of interest. To assist with selection, the auditory system structures the incoming input into streams, each of which ideally corresponds to a single source. Some authors have argued that this process of streaming is automatic and invariant, but recent evidence suggests it is affected by attention. In Experiments 1 and 2, it is shown that the effect of attention is not a general suppression of streaming on an unattended side of the ascending auditory pathway or in unattended frequency regions. Experiments 3 and 4 investigate the effect on streaming of physical gaps in the sequence and of brief switches in attention away from a sequence. The results demonstrate that after even short gaps or brief switches in attention, streaming is reset. The implications are discussed, and a hierarchical decomposition model is proposed.

Attention↗

PubMed2003

An evaluation of the use of magnetic field maps to undistort echo-planar images.

When a head is placed in an MRI scanner, differences between the magnetic susceptibility of tissue, bone, and air distort the magnetic field. While some of the resulting inhomogeneity can be corrected by the shimming process, much of it cannot, and this causes distortion (sometimes referred to as geometric distortion) of echo-planar images (EPIs). One strategy for the correction of distortion is to acquire a map of the magnetic field achieved in each subject and then to use this to undistort their EPIs after reconstruction (). Here, we present five experiments to evaluate the application of such a strategy on data from a 3-T scanner. We show that after undistortion, the shape of EPIs is more similar to the true shape of the brain, and we investigate the effect of head movement on the efficacy of undistortion. If undistortion was applied first, it was found that less nonlinear warping was required to transform EPIs into a standard space, particularly in the phase-encode direction. We show that if SPM 99 normalization is used to perform a nonlinear warp to standard space, the prior application of undistortion increases the statistical power of group studies with motor and auditory tasks. We show that this increase in power is due to an increase in the overlap of activation of different subjects. Finally, we evaluate where in the brain undistorting EPIs might be expected to have the greatesteffect, in terms both of mislocalization of activationand of a reduction in power. Overall, undistorting EPIs using field maps has proved extremely successful, improving the anatomical localization of activation and increasing statistical power.

Algorithms↗

PubMed2003

Perceptual asymmetries in audition.

Visual feature extraction has been investigated using search experiments. Targets that contain a feature not present in the distractors are easier to detect than if they do not, leading to search asymmetries. If sounds are decomposed into features in the auditory system, there might be asymmetries in analogous tasks. Six experiments investigating this are described. Strong asymmetries were identified, with frequency-modulated targets easier to detect among pure-tone distractors than vice versa and longer sounds easier to select from short distractors than the reverse. It is demonstrated that this asymmetry is not a result of peripheral limitations. In contrast, no asymmetries were observed between high- and low-frequency tones or between short 3-tone sequences differing only in their temporal structure. The results are discussed with reference to models of perceptual grouping and attention, the applicability of analogies between vision and audition, and possible physiological correlates. The paradigm provides a new way in which to investigate auditory feature extraction.

Auditory Perception↗

PubMed2003

Cross-modal and non-sensory influences on auditory streaming.

Carlyon et al (2001 Journal of Experimental Psychology: Human Perception and Performance 27 115-127) have reported that the buildup of auditory streaming is reduced when attention is diverted to a competing auditory stimulus. Here, we demonstrate that a reduction in streaming can also be obtained by attention to a visual task or by the requirement to count backwards in threes. In all conditions participants heard a 13 s sequence of tones, and, during the first 10 s saw a sequence of visual stimuli containing three, four, or five targets. The tone sequence consisted of twenty repeating triplets in an ABA - ABA ... order, where A and B represent tones of two different frequencies. In each sequence, three, four, or five tones were amplitude modulated. During the first 10 s of the sequence, participants either counted the number of visual targets, counted the number of (modulated) auditory targets, or counted backwards in threes from a specified number. They then made an auditory-streaming judgment about the last 3 s of the tone sequence: whether one or two streams were heard. The results showed more streaming when participants counted the auditory targets (and hence were attending to the tones throughout) than in either the 'visual' or 'counting-backwards' conditions.

Acoustic Stimulation↗