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T B Parrish

Publications and source records attributed to T B Parrish.

At least 19 recordsLinked to original sources
PubMed2003

The posterior cingulate and medial prefrontal cortex mediate the anticipatory allocation of spatial attention.

The purpose of this study was to identify brain regions underlying internally generated anticipatory biases toward locations where significant events are expected to occur. Subjects fixated centrally and responded to peripheral targets preceded by a spatially valid (predictive), invalid (misleading), or neutral central cue while undergoing fMRI scanning. In some validly cued trials, reaction time was significantly shorter than in trials with neutral cues, indicating that the cue had successfully induced a spatial redistribution of motivational valence, manifested as expectancy. The largest cue benefits led to selectively greater activations within the posterior cingulate and medial prefrontal cortex. These two areas thus appear to establish a neural interface between attention and motivation. An inverse relationship to cue benefit was seen in the parietal cortex, suggesting that spatial expectancy may entail the inhibition of attention-related areas to reduce distractibility by events at irrelevant locations.

Adult↗
PubMed2002

Neural correlates of artificial grammar learning.

Artificial grammar learning (AGL) is a form of nondeclarative memory that involves the nonconscious acquisition of abstract rules. While data from amnesic patients indicate that AGL does not depend on the medial temporal lobe, the neural basis of this type of memory is unknown and was therefore examined using event-related fMRI. Prior to scanning, participants studied letter strings constructed according to an artificial grammar. Participants then made grammaticality judgments about novel grammatical and nongrammatical strings while fMRI data were collected. The participants successfully acquired knowledge of the grammar, as evidenced by correct identification of the grammatical letter strings (57.4% correct; SE 1.9). During grammaticality judgments, widespread increases in activity were observed throughout the occipital, posterior temporal, parietal, and prefrontal cortical areas, reflecting the cognitive demands of the task. More specific analyses contrasting grammatical and nongrammatical strings identified greater activity in left superior occipital cortex and the right fusiform gyrus for grammatical stimuli. Increased activity was also observed in the left superior occipital and left angular gyrus for correct responses compared to incorrect. Comparing activity during grammaticality judgments versus a matched recognition control task again identified greater activation in the left angular gyrus. The network of areas exhibiting increased activity for grammatical stimuli appears to have more in common with studies examining word-form processing or mental calculation than the fluency effects previously reported for nondeclarative memory tasks such as priming and visual categorization. These results suggest that a novel nondeclarative memory mechanism supporting AGL exists in the left superior occipital and inferior parietal cortex.

Adult↗
PubMed2002

Electromyography as a recording system for eyeblink conditioning with functional magnetic resonance imaging.

This study was designed to develop a suitable method of recording eyeblink responses while conducting functional magnetic resonance imaging (fMRI). Given the complexity of this behavioral setup outside of the magnet, this study sought to adapt and further optimize an approach to eyeblink conditioning that would be suitable for conducting event-related fMRI experiments. This method involved the acquisition of electromyographic (EMG) signals from the orbicularis oculi of the right eye, which were subsequently amplified and converted into an optical signal outside of the head coil. This optical signal was converted back into an electrical signal once outside the magnet room. Electromyography (EMG)-detected eyeblinks were used to measure responses in a delay eyeblink conditioning paradigm. Our results indicate that: (1) electromyography is a sensitive method for the detection of eyeblinks during fMRI; (2) minimal interactions or artifacts of the EMG signal were created from the magnetic resonance pulse sequence; and (3) no electromyography-related artifacts were detected in the magnetic resonance images. Furthermore, an analysis of the functional data showed areas of activation that have previously been shown in positron emission tomography studies of human eyeblink conditioning. Our results support the strength of this behavioral setup as a suitable method to be used in association with fMRI.

Acoustic Stimulation↗
PubMed2001

Impact of signal-to-noise on functional MRI of the human amygdala.

The impact of signal-to-noise (SNR) on fMRI of the amygdala was investigated during a picture encoding task. The SNR value required to observe reliable activation was determined by computer simulations. Blood oxygen level-dependent (BOLD) sensitivity maps were generated to indicate brain regions with sufficient SNR to test the statistical hypotheses. The results showed that the medial aspect of the amygdala had insufficient SNR to detect a 1% peak BOLD signal change for a t-test comparison in a majority of subjects. None of these subjects showed activation in regions with unacceptable SNR values, indicating a low false positive rate. Furthermore, hemispheric asymmetries in the BOLD sensitivity maps mirrored asymmetries in the activation patterns. Impoverished SNR was also found in the basal forebrain and orbitofrontal cortex. These findings emphasize the importance of considering SNR when interpreting fMRI results in the limbic forebrain.

Adult↗
PubMed2001

Heterogeneity of cingulate contributions to spatial attention.

Functional magnetic resonance imaging was used to investigate activation patterns within the cingulate region during tasks based on spatial attention. Subjects were asked to detect targets which appeared either at the site indicated by a cue or on the opposite side. A "cue effect" was identified by the presence of shorter reaction times to validly than invalidly cued targets, showing that an anticipatory bias had been generated in the direction of the cue. Target detection accuracy was consistently above 90% although cue effects and reaction times displayed substantial variations, from one task session to another. Activation within the anterior cingulate region was seen in 16 of the 26 sessions but showed no correlation with reaction time. Posterior cingulate activation was seen in only 6 of the 26 sessions. However, a random effects analysis showed that the task-related signal change in this region was strongly correlated with the speed of target detection. A post hoc analysis indicated that this correlation was significant only when cue effects were present. No other part of the cerebral cortex displayed significant correlations with reaction times or cue effects. These results suggest that the cingulate component of the attentional network has at least two functionally segregated sectors, an anterior one in BA 24/32 and a posterior cingulo-retrosplenial one in BA 23/29/30. The posterior sector appears to be associated with the speed of detecting spatial targets, especially when attention is under the influence of a cue-induced anticipatory bias. The anterior cingulate focus did not display such a relationship in our tasks and is likely to mediate other aspects of attentional deployment such as performance monitoring, response selection or target identification.

Adult↗
PubMed2001

Location- or feature-based targeting of peripheral attention.

Using event-related fMRI we determined the differential effects of feature- versus location-based cues for directing peripheral attention. Pairs of same-color targets appeared on the left and on the right. A predictive cue indicated whether the subsequent targeting of attention would be based on location (left versus right) or color (red versus blue). Subjects had to press a button when the relevant pair of targets (in the cued side of space or of the cued color) were identical in shape. The feature-based cue thus also led to a "global" expectancy of targets on either side of space whereas the location-based cue elicited a more "focal" expectancy limited to one side of space. The right inferior parietal lobule was more active when attention was targeted on the basis of location than of color. There was no difference between left-sided or right-sided attention in this region, indicating that it mediated the targeting to both sides of space. These results show that the right inferior parietal cortex plays a relatively selective role in mediating location-based and spatially focal modes of attentional deployment. Its relatively equal activation for leftward and rightward attentional shifts is also consistent with models of right hemispheric dominance of spatial attention.

Adult↗
PubMed2001

Functional specificity of superior parietal mediation of spatial shifting.

Using event-related functional magnetic resonance imaging (fMRI) we determined how brain activity changes when an attended target shifts its location. In the main experiment, a white square could appear at 10 possible eccentricities along the horizontal meridian. It remained on the screen for a variable period of time and then changed location. At any time the stimulus could dim briefly. Subjects had to press a button when the stimulus dimmed. In order to perform this task attention had to be locked onto the target and shift with it. Half of the runs were performed overtly and half covertly. The event of interest consisted of the shift in the location of the attentional target. The state of maintained attention occurring in between the shifts constituted the baseline. The superior parietal gyrus was activated bilaterally in response to attentional shifts. No other area showed a significant response to shifting. On the left side the amplitude of the superior parietal response correlated positively with the distance of the shift. On the right side a significant correlation was present only for overt shifts. In a separate experiment we compared the maintaining of attention at a single spatial location to passive fixation: the frontal eye fields, anterior cingulate, right dorsolateral prefrontal cortex, and inferior parietal lobule were significantly activated, indicating that the absence of a shift-related response in these areas in the main experiment was due to the fact that they were equally activated by maintaining and shifting attention. The response to spatial shifts and the correlation with the distance between the original and the new location points to a specific role of the superior parietal gyrus in shifting the locus of spatial attention.

Adult↗
PubMed2001

Hunger selectively modulates corticolimbic activation to food stimuli in humans.

Functional magnetic resonance imaging (fMRI) was used to determine whether visual responses to food in the human amygdala and related corticolimbic structures would be selectively altered by changes in states of hunger. Participants viewed images of motivationally relevant (food) and motivationally irrelevant (tool) objects while undergoing fMRI in alternately hungry and satiated conditions. Food-related visual stimuli elicited greater responses in the amygdala, parahippocampal gyrus. and anterior fusiform gyrus when participants were in a hungry state relative to a satiated state. The state-dependent activation of these brain structures did not generalize to the motivationally irrelevant objects. These results support the hypothesis that the amygdala and associated inferotemporal regions are involved in the integration of subjective interoceptive states with relevant sensory cues processed along the ventral visual stream.

Adult↗
PubMed2001

The development of specialized brain systems in reading and oral-language.

Functional magnetic resonance imaging (fMRI) was used to examine differences between children (9-12 years) and adults (21-31 years) in the distribution of brain activation during word processing. Orthographic, phonologic, semantic and syntactic tasks were used in both the auditory and visual modalities. Our two principal results were consistent with the hypothesis that development is characterized by increasing specialization. Our first analysis compared activation in children versus adults separately for each modality. Adults showed more activation than children in the unimodal visual areas of middle temporal gyrus and fusiform gyrus for processing written word forms and in the unimodal auditory areas of superior temporal gyrus for processing spoken word forms. Children showed more activation than adults for written word forms in posterior heteromodal regions (Wernicke's area), presumably for the integration of orthographic and phonologic word forms. Our second analysis compared activation in the visual versus auditory modality separately for children and adults. Children showed primarily overlap of activation in brain regions for the visual and auditory tasks. Adults showed selective activation in the unimodal auditory areas of superior temporal gyrus when processing spoken word forms and selective activation in the unimodal visual areas of middle temporal gyrus and fusiform gyrus when processing written word forms.

Adult↗
PubMed2000

Hybrid technique for dynamic imaging.

A number of data acquisition strategies have been introduced to speed up the image acquisition in dynamic settings. One such technique is the keyhole approach, which is based on reducing k-space coverage, and consequently the spatial resolution, of the dynamic information. Another is based on reducing the field of view of the dynamic information. These two techniques are complementary in that one reduces the field of view in k-space and the other does so in the spatial domain. A hybrid approach which combines the two is described in this study. In numerical simulations and experimental studies, this hybrid approach more accurately depicts the signal changes, outperforming the two techniques from which it is derived. Magn Reson Med 44:51-55, 2000.

Brain↗
PubMed2000

Impact of signal-to-noise on functional MRI.

Functional magnetic resonance imaging (fMRI) has recently been adopted as an investigational tool in the field of neuroscience. The signal changes induced by brain activations are small ( approximately 1-2%) at 1.5T. Therefore, the signal-to-noise ratio (SNR) of the time series used to calculate the functional maps is critical. In this study, the minimum SNR required to detect an expected MR signal change is determined using computer simulations for typical fMRI experimental designs. These SNR results are independent of manufacturer, site environment, field strength, coil type, or type of cognitive task used. Sensitivity maps depicting the minimum detectable signal change can be constructed. These sensitivity maps can be used as a mask of the activation map to help remove false positive activations as well as identify regions of the brain where it is not possible to confidently reject the null hypothesis due to a low SNR.

Artifacts↗
PubMed2000

Real-time monitoring of eye movements using infrared video-oculography during functional magnetic resonance imaging of the frontal eye fields.

Monitoring eye movements is a critical aspect of experimental design for studies of spatial attention and visual perception. However, obtaining online eye-movement recordings has been technologically difficult during functional magnetic resonance (MR) imaging studies. Previous approaches to monitoring eye movements either have distorted the MR images or have shown MR-related interference in the recordings. We report a technique using long-range infrared video-oculography to record eye movements without causing artifacts in the MR images. Analysis of the MR signal from a phantom obtained with the eye-movement equipment turned on or off confirmed the absence of significant additional noise in the MR time series. Eye movements of three subjects were monitored while they performed tasks of covert and overt shifts of spatial attention. Activation of the frontal eye fields during the covert task was seen even when the eye-movement recordings demonstrated no significant difference in saccadic eye movements between the baseline and the active conditions.

Adult↗
PubMed1999

Relationship of MRI delayed contrast enhancement to irreversible injury, infarct age, and contractile function.

BACKGROUND: Contrast MRI enhancement patterns in several pathophysiologies resulting from ischemic myocardial injury are controversial or have not been investigated. We compared contrast enhancement in acute infarction (AI), after severe but reversible ischemic injury (RII), and in chronic infarction. METHODS AND RESULTS: In dogs, a large coronary artery was occluded to study AI and/or chronic infarction (n = 18), and a second coronary artery was chronically instrumented with a reversible hydraulic occluder and Doppler flowmeter to study RII (n = 8). At 3 days after surgery, cine MRI revealed reduced wall thickening in AI (5+/-6% versus 33+/-6% in normal, P<0.001). In RII, wall thickening before, during, and after inflation of the occluder for 15 minutes was 35+/-5%, 1+/-8%, and 21+/-10% and Doppler flow was 19.8+/-5.3, 0.2+/-0.5, and 56.3+/-17.7 (peak hyperemia) cm/s, respectively, confirming occlusion, transient ischemia, and reperfusion. Gd-DTPA-enhanced MR images acquired 30 minutes after contrast revealed hyperenhancement of AI (294+/-96% of normal, P<0.001) but not of RII (98+/-6% of normal, P = NS). Eight weeks later, the chronically infarcted region again hyperenhanced (253+/-54% of normal, n = 8, P<0.001). High-resolution (0.5 x 0.5 x 0.5 mm) ex vivo MRI demonstrated that the spatial extent of hyperenhancement was the same as the spatial extent of myocyte necrosis with and without reperfusion at 1 day (R = 0.99, P<0.001) and 3 days (R = 0.99, P<0.001) and collagenous scar at 8 weeks (R = 0.97, P<0.001). CONCLUSIONS: In the pathophysiologies investigated, contrast MRI distinguishes between reversible and irreversible ischemic injury independent of wall motion and infarct age.

Animals↗
PubMed1999

Relationship of elevated 23Na magnetic resonance image intensity to infarct size after acute reperfused myocardial infarction.

BACKGROUND: Elevated 23Na MR image intensity after acute myocardial infarction has previously been shown to correspond to high tissue [Na+] and loss of myocardial viability. In this study, we explored the potential of in vivo 23Na MRI to assess infarct size and investigated possible mechanisms for elevated 23Na image intensity. METHODS AND RESULTS: Thirteen dogs and 8 rabbits underwent in situ coronary artery occlusion and reperfusion and were imaged by 23Na MRI. For anatomically matched left ventricular short-axis cross sections (n=46), infarct size measured by in vivo 23Na MRI correlated well with triphenyltetrazolium chloride staining (r=0.87, y=0.92x+3.37, P<0.001). Elevated 23Na image intensity was observed in infarcted myocardium (206+/-37% of remote in dogs, P<0.001; 215+/-58% in rabbits, P<0.002) but was not observed after severe but reversible ischemic injury (101+/-11% of baseline, P=NS). High-resolution ex vivo imaging revealed that regions of elevated 23Na image intensity appeared to be identical to those of infarcted regions (r=0.97, y=0.92x+1.52, P<0.001). In infarcted regions, total tissue [Na+] was elevated (89+/-12 versus 37+/-9 mmol/L in control tissue, 156+/-60% increase, P<0.001) and was associated with increased intracellular sodium (254+/-68% of control, P<0.005) and an increased intracellular sodium/potassium ratio (868+/-512% of control, P<0.002). Morphometric analysis demonstrated only a minor increase in extracellular volume (17+/-8% versus 14+/-5%, P<0.05) in the infarcted territory. CONCLUSIONS: Elevated 23Na MR image intensity in vivo measures infarct size after reperfused infarction in both a large and a small animal model. The mechanism of elevated 23Na image intensity is probably intracellular sodium accumulation secondary to loss of myocyte ionic homeostasis.

Animals↗
PubMed1999

MRI compatibility and visibility assessment of implantable medical devices.

We have developed a protocol to evaluate the magnetic resonance (MR) compatibility of implantable medical devices. The testing protocol consists of the evaluation of magnetic field-induced movement, electric current, heating, image distortion, and device operation. In addition, current induction is evaluated with a finite element analysis simulation technique that models the effect of radiofrequency fields on each device. The protocol has been applied to several implantable infusion pumps and neurostimulators with associated attachments. Experiments were performed using a 1.5-T whole-body MR system with parameters selected to approximate the intended clinical and worst case configuration. The devices exhibited moderate magnetic field-induced deflection and torque but had significant image artifacts. No heating was detected for any of the devices. Pump operation was halted in the magnetic field, but resumed after removed. Exposure to the magnetic field activated some of the neurostimulators.

Artifacts↗
PubMed1999

The large-scale neural network for spatial attention displays multifunctional overlap but differential asymmetry.

Functional magnetic resonance imaging (fMRI) was used to determine the brain regions activated by two types of covert visuospatial attentional shifts: one based on exogenous spatial priming and the other on foveally presented cues which endogenously regulated the direction of spatial expectancy. Activations were seen in the cortical and subcortical components of a previously characterized attentional network, namely, the frontal eye fields, posterior parietal cortex, the cingulate gyrus, the putamen, and the thalamus. Additional activations occurred in the anterior insula, dorsolateral prefrontal cortex, temporo-occipital cortex in the middle and inferior temporal gyri, the supplementary motor area, and the cerebellum. Direct comparisons showed a nearly complete overlap in the location of activations resulting from the two tasks. However, the spatial priming task displayed a more pronounced rightward asymmetry of parietal activation, and a conjunction analysis showed that the area of posterior parietal cortex jointly activated by both tasks was more extensive in the right hemisphere. Furthermore, the posterior parietal and temporo-occipital activations were more pronounced in the task of endogenous attentional shifts. The results show that both exogenous (based on spatial priming) and endogenous (based on expectancy cueing) shifts of attention are subserved by a common network of cortical and subcortical regions. However, the differences between the two tasks, especially in the degree of rightward asymmetry, suggests that the pattern of activation within this network may show variations that reflect the specific attributes of the attentional task.

Adult↗
PubMed1999

Neuroanatomic overlap of working memory and spatial attention networks: a functional MRI comparison within subjects.

Frontal and posterior parietal activations have been reported in numerous studies of working memory and visuospatial attention. To directly compare the brain regions engaged by these two cognitive functions, the same set of subjects consecutively participated in tasks of working memory and spatial attention while undergoing functional MRI (fMRI). The working memory task required the subject to maintain an on-line representation of foveally displayed letters against a background of distracters. The spatial attention task required the subject to shift visual attention covertly in response to a centrally presented directional cue. The spatial attention task had no working memory requirement, and the working memory task had no covert spatial attention requirement. Subjects' ability to maintain central fixation was confirmed outside the MRI scanner using infrared oculography. According to cognitive conjunction analysis, the set of activations common to both tasks included the intraparietal sulcus, ventral precentral sulcus, supplementary motor area, frontal eye fields, thalamus, cerebellum, left temporal neocortex, and right insula. Double-subtraction analyses yielded additional activations attributable to verbal working memory in premotor cortex, left inferior prefrontal cortex, right inferior parietal lobule, precuneus, and right cerebellum. Additional activations attributable to covert spatial attention included the occipitotemporal junction and extrastriate cortex. The use of two different tasks in the same set of subjects allowed us to provide an unequivocal demonstration that the neural networks subserving spatial attention and working memory intersect at several frontoparietal sites. These findings support the view that major cognitive domains are represented by partially overlapping large-scale neural networks. The presence of this overlap also suggests that spatial attention and working memory share common cognitive features related to the dynamic shifting of attentional resources.

Adult↗