Emotion–attention network interactions during a visual oddball task

Emotional and attentional functions are known to be distributed along ventral and dorsal networks in the brain, respectively. However, the interactions between these systems remain to be specified. The present study used event-related functional magnetic resonance imaging (fMRI) to investigate how attentional focus can modulate the neural activity elicited by scenes that vary in emotional content. In a visual oddball task, aversive and neutral scenes were presented intermittently among circles and squares. The squares were frequent standard events, whereas the other novel stimulus categories occurred rarely. One experimental group [N=10] was instructed to count the circles, whereas another group [N=12] counted the emotional scenes. A main effect of emotion was found in the amygdala (AMG) and ventral frontotemporal cortices. In these regions, activation was significantly greater for emotional than neutral stimuli but was invariant to attentional focus. A main effect of attentional focus was found in dorsal frontoparietal cortices, whose activity signaled task-relevant target events irrespective of emotional content. The only brain region that was sensitive to both emotion and attentional focus was the anterior cingulate gyrus (ACG). When circles were task-relevant, the ACG responded equally to circle targets and distracting emotional scenes. The ACG response to emotional scenes increased when they were task-relevant, and the response to circles concomitantly decreased. These findings support and extend prominent network theories of emotion-attention interactions that highlight the integrative role played by the anterior cingulate.

Emotion-attention network interactions during a visual oddball task

Emotional and attentional functions are known to be distributed along ventral and dorsal networks in the brain, respectively. However, the interactions between these systems remain to be specified. The present study used event-related functional magnetic resonance imaging (fMRI) to investigate how attentional focus can modulate the neural activity elicited by scenes that vary in emotional content. In a visual oddball task, aversive and neutral scenes were presented intermittently among circles and squares. The squares were frequent standard events, whereas the other novel stimulus categories occurred rarely. One experimental group [N=10] was instructed to count the circles, whereas another group [N=12] counted the emotional scenes. A main effect of emotion was found in the amygdala (AMG) and ventral frontotemporal cortices. In these regions, activation was significantly greater for emotional than neutral stimuli but was invariant to attentional focus. A main effect of attentional focus was found in dorsal frontoparietal cortices, whose activity signaled task-relevant target events irrespective of emotional content. The only brain region that was sensitive to both emotion and attentional focus was the anterior cingulate gyrus (ACG). When circles were task-relevant, the ACG responded equally to circle targets and distracting emotional scenes. The ACG response to emotional scenes increased when they were task-relevant, and the response to circles concomitantly decreased. These findings support and extend prominent network theories of emotion-attention interactions that highlight the integrative role played by the anterior cingulate.

What is odd in the oddball task? Prefrontal cortex is activated by dynamic changes in response strategy

In the "oddball" target detection task, subjects respond to target stimuli that occur infrequently and irregularly within a series of standard stimuli. Although detection of these targets reliably evokes transient activity in prefrontal cortical regions, it has not been established whether this activity is due to selection of an infrequent response or to changes in response strategy. We investigated this issue using a novel variant of the oddball task that incorporated the Simon effect, while measuring hemodynamic brain activity in prefrontal cortex using functional magnetic resonance imaging (fMRI). Subjects viewed a series of circles and squares that required left and right button presses, respectively. On 90% of trials ("standard" trials), the stimuli were presented in the same visual hemifield as the hand of response, but on 10% of trials ("strategy-change" trials) they were presented in the opposite visual hemifield. Significant activation to the infrequent strategy-change trials was found in the anterior middle frontal gyrus (MFG), the posterior inferior frontal gyrus (IFG) and adjacent insular cortex, and in the anterior cingulate gyrus (ACG). These regions, which correspond to previous reports of oddball-related activation, were consistent across subjects. Behavioral results supported our interpretation that subjects potentiated a position-based response strategy, which was inhibited on the strategy-change trials. Activity within the MFG and ACG was much greater on error trials than on correct trials, while IFG activity was similar between error and correct trials. We conclude that the dorsolateral prefrontal cortex (dlPFC) is associated with dynamic changes in the mapping of stimuli to responses (e.g. response strategies), independently of any changes in behavior.

Linking Hemodynamic and Electrophysiological Measures of Brain Activity: Evidence from Functional MRI and Intracranial Field Potentials

We investigated the relation between electrophysiological and hemodynamic measures of brain activity through comparison of intracranially recorded event-related local field potentials (ERPs) and blood-oxygenation level dependent functional magnetic resonance imaging (BOLD fMRI). We manipulated the duration of visual checkerboard stimuli across trials and measured stimulus-duration-related changes in ERP and BOLD activity in three brain regions: peri-calcarine cortex, the fusiform gyrus and lateral temporal-occipital (LTO) cortex. ERPs were recorded from patients who had indwelling subdural electrodes as part of presurgical testing, while BOLD responses were measured in similar brain regions in a second set of subjects. Similar BOLD responses were measured in peri-calcarine and fusiform regions, with both showing monotonic but non-linear increases in hemodynamic amplitude with stimulus duration. In sharp contrast, very different ERP responses were observed in these same regions, such that calcarine electrodes exhibited onset potentials, sustained activity over the course of stimulus duration and prominent offset potentials, while fusiform electrodes only exhibited onset potentials that did not vary with stimulus duration. No duration-related ERP or BOLD changes were observed in LTO. Additional analyses revealed no consistent changes in the EEG spectrum across different brain sites that correlated with duration-related changes in the BOLD response. We conclude that the relation between ERPs and fMRI differs across brain regions.

A systematic genomewide linkage study in 353 sib pairs with schizophrenia

We undertook a genomewide linkage study in a total of 353 affected sib pairs (ASPs) with schizophrenia. Our sample consisted of 179 ASPs from the United Kingdom, 134 from Sweden, and 40 from the United States. We typed 372 microsatellite markers at approximately 10-cM intervals. Our strongest finding was a LOD score of 3.87 on chromosome 10q25.3-q26.3, with positive results being contributed by all three samples and a LOD-1 interval of 15 cM. This finding achieved genomewide significance (P<.05), on the basis of simulation studies. We also found two regions, 17p11.2-q25.1 (maximum LOD score [MLS] = 3.35) and 22q11 (MLS = 2.29), in which the evidence for linkage was highly suggestive. Linkage to all of these regions has been supported by other studies. Moreover, we found strong evidence for linkage (genomewide P<.02) to 17p11.2-q25.1 in a single pedigree with schizophrenia. In our view, the evidence is now sufficiently compelling to undertake detailed mapping studies of these three regions.

Polysensory interactions along lateral temporal regions evoked by audiovisual speech

Many socially significant biological stimuli are polymodal, and information processing is enhanced for polymodal over unimodal stimuli. The human superior temporal sulcus (STS) region has been implicated in processing socially relevant stimuli--particularly those derived from biological motion such as mouth movements. Single unit studies in monkeys have demonstrated that regions of STS are polysensory--responding to visual, auditory and somato-sensory stimuli, and human neuroimaging studies have shown that lip-reading activates auditory regions of the lateral temporal lobe. We evaluated whether concurrent speech sounds and mouth movements were more potent activators of STS than either speech sounds or mouth movements alone. In an event-related fMRI study, subjects observed an animated character that produced audiovisual speech and the audio and visual components of speech alone. Strong activation of the STS region was evoked in all three conditions, with greatest levels of activity elicited by audiovisual speech. Subsets of activated voxels within the STS region demonstrated overadditivity (audiovisual > audio + visual) and underadditivity (audiovisual < audio + visual). These results confirm the polysensory nature of STS region and demonstrate for the first time that polymodal interactions may both potentiate and inhibit activation.

Dynamic perception of facial affect and identity in the human brain

Functional magnetic resonance imaging (fMRI) was used to compare brain activation to static facial displays versus dynamic changes in facial identity or emotional expression. Static images depicted prototypical fearful, angry and neutral expressions. Identity morphs depicted identity changes from one person to another, always with neutral expressions. Emotion morphs depicted expression changes from neutral to fear or anger, creating the illusion that the actor was 'getting scared' or 'getting angry' in real-time. Brain regions implicated in processing facial affect, including the amygdala and fusiform gyrus, showed greater responses to dynamic versus static emotional expressions, especially for fear. Identity morphs activated a dorsal fronto-cingulo-parietal circuit and additional ventral areas, including the amygdala, that also responded to the emotion morphs. Activity in the superior temporal sulcus discriminated emotion morphs from identity morphs, extending its known role in processing biologically relevant motion. The results highlight the importance of temporal cues in the neural coding of facial displays.

Brain activity evoked by the perception of human walking: controlling for meaningful coherent motion

Many functional neuroimaging studies of biological motion have used as stimuli point-light displays of walking figures and compared the resulting activations with those evoked by the same display elements moving in a random or noncoherent manner. Although these studies have established that biological motion activates the superior temporal sulcus (STS), the use of random motion controls has left open the possibility that coordinated and meaningful nonbiological motion might activate these same brain regions and thus call into question their specificity for processing biological motion. Here we used functional magnetic resonance imaging and an anatomical region-of-interest approach to test a hierarchy of three questions regarding activity within the STS. First, by comparing responses in the STS with animations of human and robot walking figures, we determined (1) that the STS is sensitive to biological motion itself, not merely to the superficial characteristics of the stimulus. Then we determined that the STS responds more strongly to biological motion (as conveyed by the walking robot) than to (2) a nonmeaningful but complex nonbiological motion (a disjointed mechanical figure) and (3) a complex and meaningful nonbiological motion (the movements of a grandfather clock). In subsequent whole-brain voxel-based analyses, we confirmed robust STS activity that was strongly right lateralized. In addition, we observed significant deactivations in the STS that differentiated biological and nonbiological motion. These voxel-based analyses also revealed regions of motion-related positive activity in other brain regions, including MT or V5, fusiform gyri, right premotor cortex, and the intraparietal sulci.

Faces evoke spatially differentiated patterns of BOLD activation and deactivation

Using fMRI techniques sensitive to blood oxygen-level dependent (BOLD) contrast, we measured brain activity in participants (n=8) as they viewed images of faces presented periodically within a continuously changing montage of common objects. Consistent with prior studies, we identified regions of ventral extrastriate cortex, primarily in the fusiform and inferior temporal gyri and nearby cortex, that were activated by faces as measured by an increase in BOLD signal. In addition, we made the novel observation that faces deactivated other areas of ventral extrastriate cortex, primarily in the lingual and parahippocampal gyri and medial to activations. These deactivated regions, identified by a decrease in BOLD signal, may reflect populations of neurons that decrease their activity when faces appear, possibly as a consequence of category-specific inhibition.

Brain activation evoked by perception of gaze shifts: the influence of context

Prior studies from our laboratory [Journal of Neuroscience 18 (1998) 2188; Cognitive Neuropsychology 17 (2000) 221] have demonstrated that discrete regions of the superior temporal sulcus (STS) are activated when a subject views a face in which the eyes shift their gaze. Here we investigated the degree to which activity in the STS and other brain regions is modulated by the context of the perceived gaze shift; that is, when the shift correctly or incorrectly acquires a visual target. Fifteen subjects participated in an event-related functional magnetic resonance imaging experiment in which they viewed an animated face that remained present throughout each run. On each of 21 trials within each run, a small checkerboard appeared and flickered at one of six locations within the character's visual field. On "correct" trials, the character shifted its gaze towards the checkerboard after a delay of 1 or 3s. On "incorrect" trials, the character shifted its gaze towards empty space after the same delays. On "no shift" trials, the character's eyes did not move. Significantly larger hemodynamic responses (HDR) were evoked by gaze shifts compared to no gaze shifts in primarily right hemisphere STS. The gaze-evoked HDR was significantly delayed in peak amplitude for 3s compared to 1s shifts. For 1s shifts, a strong effect of context was observed in which errors evoked a HDR with extended duration. Although this study focused upon STS, similar effects were also observed in the intraparietal sulcus and fusiform gyrus.

Field potentials in the human hippocampus during the encoding and recognition of visual stimuli

Intracranial field potentials were recorded from electrodes implanted in the hippocampus in 12 epileptic patients. Potentials were elicited by stimuli presented during a delayed matching-to-sample test. Each trial began with a sample stimulus composed of a 3 x 3 grid of rectangular color patches. The sample was followed by a sequence of similar but task-irrelevant stimuli and the sequential presentation of two test stimuli, one of which was identical to the sample. Patients indicated their recognition of the test stimulus that matched the sample with a button press. High-amplitude negative potentials were consistently elicited by sample and test stimuli. Peak amplitudes occurred 300-500 ms after stimulus onset and were larger for the sample in all cases. The patterns of potential gradients observed between adjacent hippocampal contacts and the locations of maximal amplitudes, as verified by magnetic resonance imaging in seven patients, suggest that these potentials were produced by neuronal activity in posterior hippocampus. These field potentials appear to index a memory storage function engaged in response to events that will later be remembered. The hippocampal contribution to storing declarative memories can thus begin, in some circumstances, within the first half-second after the presentation of a to-be-remembered stimulus.

Gender differences in cognitive and neural correlates of remembrance of emotional words

Studies suggest that men and women have important differences in specific cognitive functions. Men show superior spatial memory and women demonstrate superior verbal memory, and women rely on emotional content to a greater degree in the processing of information. In spite of extensive research in neural correlates of human cognition, little is known about possible gender differences or the role of emotional content in the mediation of cognition. Two sets of lists of word pairs were developed, one with neutral (e.g., school-grocery) and the other with emotional (e.g., mutilate-beat) content. Male and female subjects were asked to rate emotions related to the words on several dimensions (e.g., nervous, fearful, happy). In a second experiment, men and women underwent positron emission tomographic (PET) measurement of brain blood flow during retrieval of word pairs. Words in the "emotional" category were rated more highly on the emotional dimensions, and women rated them as having more emotional impact than did the men. During retrieval of emotional words (but not neutral words) there was a different pattern of activation among the women compared with the men, with greater activation in bilateral posterior hippocampus and cerebellum, and decreased activity in medial prefrontal cortex, which are brain areas previously implicated in emotion. There were no significant differences in retrieval of emotional versus neutral words, or in differences in memory performance between men and women. The findings suggest differences in cognitive appraisal and involvement of a broader network of brain regions mediating emotion during remembrance of emotional words in women compared with men.

Category-sensitive excitatory and inhibitory processes in human extrastriate cortex

Single-cell recordings from the temporal lobe of monkeys viewing stimuli show that cells may be highly selective, responding for example to particular objects such as faces. However, stimulus-selective cells may be inhibited by nonpreferred stimuli. Can such inhibitory mechanisms be detected in human visual cortex? In previous recordings from the surface of human ventral extrastriate cortex, we found that specific categories of stimuli such as faces and words generate category-specific negative event-related potentials (ERPs) with a peak latency of about 200 ms (N200). Laminar recordings in animal cortex suggest that the human N200 reflects excitatory depolarizing potentials in apical dendrites of pyramidal cells. In this study we found that, at about half of word-specific N200 sites, faces generated a positive ERP (P200); conversely, at about half of face-specific sites, words generated P200s. The electrogenesis of N200 implies that P200 ERPs reflect hyperpolarizing inhibition of apical dendrites. These recordings, together with the prior animal recordings, provide strong circumstantial evidence that in human cortex populations of cells responsive to one stimulus category (such as faces) inhibit cells responsive to another category (such as words), probably by a type of lateral inhibition. Of the stimulus categories studied quantitatively, face-specific cells are maximally inhibited by words and vice versa, but other categories of stimuli may generate smaller P200s, suggesting that inhibition of category-specific cells by nonpreferred stimuli is a general feature of human extrastriate cortex involved in object recognition.

Enhanced spatial localization of neuronal activation using simultaneous apparent-diffusion-coefficient and blood-oxygenation functional magnetic resonance imaging

Functional MRI (fMRI) can detect blood oxygenation level dependent (BOLD) hemodynamic responses secondary to local neuronal activity. The most commonly used method for detecting fMRI signals is the gradient-echo echo-planar imaging (EPI) technique because of its sensitivity and speed. However, it is known that much of the signal obtained with this approach arises from large veins, with additional contribution from the capillaries and venules. Early experiments using diffusion-weighted gradient-echo EPI have suggested that intravoxel incoherent motion (IVIM) weighting can selectively attenuate contributions from large vessels based on the differences in the mobility of the blood within them, thereby revealing the contributions from hemodynamic changes in capillaries, which are in close spatial proximity to the activated neural tissue. Using this differential sensitivity of the various neurovascular compartments to IVIM weighting, we present a new approach for deriving functional maps of neural activity. This method is based on task-induced changes of the apparent diffusion coefficients (ADC), a signal that we demonstrate is generated in vascular compartments that only partially overlap with those generating the BOLD signal. The approach allows both the ADC-based maps and the more commonly used BOLD-based maps to be acquired simultaneously. The spatial overlap between these maps can be used to create composite maps that permit improved localization of the underlying neuronal activity patterns by identifying signals generated in those vascular components that are in closest proximity to the active neuronal populations of interest.

Deterministic and stochastic features of fMRI data: implications for analysis of event-related experiments

As the limits of stimuli presentation rates are explored in event-related fMRI design, there is a greater need to assess the implications of averaging raw fMRI data. Selective averaging assumes that the fMRI signal consists of task-dependent signal, random noise, and non-task dependent brain signal that can be modeled as random noise so that it tends to zero when averaged over a practical number of trials. We recorded a total of four fMRI data series from two normal subjects (subject 1, axially acquired; subject 2, coronally acquired) performing a simple visual event-related task and a water phantom with the same fMRI scanner imaging parameters. To determine which fraction of the fMRI data was deterministic as opposed to random, we created different data subsets by taking the odd or even time points of the full data sets. All data sets were first dimension-reduced with principal component analysis (PCA) and separated into 100 spatially independent components with independent component analysis (ICA). The mutual information between best-matching pairs of components selected from full data set-subset comparisons was plotted for each data set. Visual inspection suggested that 45-85 components were reproducible, and hence deterministic, accounting for 79-97% of the variance, respectively, in the raw data. The reproducible components exhibited much less trial-to-trial variability than the raw data from even the most activated voxel. Many (22-47) of reproducible components were significantly affected by stimulus presentation (P < 0.001). The most significantly-stimulus-correlated component was strongly time-locked to stimulus presentation and was directly stimulus correlated, corresponding to occipital brain regions. However, other spatially distinct task-related components demonstrated variable temporal relationships with the most significantly-stimulus-correlated component. Our results suggest that the majority of the variance in fMRI data is in fact deterministic, and support the notion that the data consist of differing components with differing temporal relationships to visual stimulation. They further suggest roles for restricting interpretations of the spatial extent of activation from event-related designs to a specific region of interest (ROI) and/or first separating the data into spatially independent components. Averaging the time courses of spatially independent components time-locked to stimulus presentation may prevent possible biases in the estimates of the spatial and temporal extent of stimulus-correlated activation and of trial-to-trial variability.

Dissociable prefrontal brain systems for attention and emotion

The prefrontal cortex has been implicated in a variety of attentional, executive, and mnemonic mental operations, yet its functional organization is still highly debated. The present study used functional MRI to determine whether attentional and emotional functions are segregated into dissociable prefrontal networks in the human brain. Subjects discriminated infrequent and irregularly presented attentional targets (circles) from frequent standards (squares) while novel distracting scenes, parametrically varied for emotional arousal, were intermittently presented. Targets differentially activated middle frontal gyrus, posterior parietal cortex, and posterior cingulate gyrus. Novel distracters activated inferior frontal gyrus, amygdala, and fusiform gyrus, with significantly stronger activation evoked by the emotional scenes. The anterior cingulate gyrus was the only brain region with equivalent responses to attentional and emotional stimuli. These results show that attentional and emotional functions are segregated into parallel dorsal and ventral streams that extend into prefrontal cortex and are integrated in the anterior cingulate. These findings may have implications for understanding the neural dynamics underlying emotional distractibility on attentional tasks in affective disorders.

Perceiving patterns in random series: dynamic processing of sequence in prefrontal cortex

We demonstrate that regions within human prefrontal cortex develop moment-to-moment models for patterns of events occurring in the sensory environment. Subjects viewed a random binary sequence of images, each presented singly and each requiring a different button press response. Patterns occurred by chance within the presented series of images. Using functional magnetic resonance imaging (fMRI), we identified activity evoked by viewing a stimulus that interrupted a pattern. Prefrontal activation was evoked by violations of both repeating and alternating patterns, and the amplitude of this activation increased with increasing pattern length. Violations of repeating patterns, but not of alternating patterns, activated the basal ganglia.

Myocarditis in sibling boxer puppies associated with Citrobacter koseri infection

Two sibling Boxer puppies presented with severe suppurative myocarditis in the absence of additional disseminated suppurative foci. The identification of gram-negative bacteria within areas of myocarditis in both puppies and the pure growth of large numbers of Citrobacter koseri from the myocardial lesions in one of the dogs were consistent with a bacterial etiology. The fact that C. koseri is an opportunist pathogen suggested intercurrent immunosuppression. The finding of a concomitant bacterial myocarditis in two canine siblings is novel. The case is also unusual in that syncope could be related to the myocardial injury.

Regional differences in the refractory period of the hemodynamic response: an event-related fMRI study

We investigated the characteristics of the hemodynamic response (HDR) to paired presentations of visual face stimuli using functional magnetic resonance imaging (fMRI). Photographs of faces were presented singly or in pairs with either a 1-s or 6-s intrapair interval (IPI). Each trial (single face or face pairs) was followed by an intertrial interval of 16-20 s. Faces were presented at fixation and passively viewed by the 10 subjects. Images were acquired at 1.5 Tesla using a gradient-echo echo-planar imaging sequence sensitive to blood-oxygenation-level-dependent (BOLD) contrast. To examine the refractory properties of the HDR, we subtracted the single-stimulus hemodynamic response from the composite response evoked by face pairs for all voxels significantly active on single face trials. The residual represents the contribution of the second stimulus to the fMRI signal. Event-related presentation of faces evoked activity in medial calcarine cortex and the fusiform gyrus bilaterally. In both calcarine and fusiform regions, the hemodynamic response to the second face in a pair was of lower amplitude and of increased latency at 1 s IPI, with significant recovery of both amplitude and latency toward single-stimulus values at 6 s IPI. At 1 s IPI, significantly greater recovery was found in posterior fusiform regions (50-60%) than in midfusiform regions (10-40%). These regional differences were not apparent at 6 s IPI. No differences were found across slices in calcarine cortex. There was a significant difference in mean latency to HDR peak between calcarine and fusiform cortex, with the HDR peaking about 400 ms earlier in calcarine cortex. We conclude that characteristics of the HDR, notably its amplitude, latency, and refractory properties, differ across visual cortical areas.

Dissociating the neural mechanisms of visual attention in change detection using functional MRI

We investigated using functional magnetic resonance imaging (fMRI) the neural processes associated with performance of a change-detection task. In this task, two versions of the same picture are presented in alternation, separated by a brief mask interval. Even when the two pictures greatly differ (e.g., as when a building is in different locations), subjects report that identification of the change is difficult and often take 30 or more seconds to identify the change. This phenomenon of "change blindness" provides a powerful and novel paradigm for segregating components of visual attention using fMRI that can otherwise be confounded in short-duration tasks. By using a response-contingent event-related analysis technique, we successfully dissociated brain regions associated with different processing components of a visual change-detection task. Activation in the calcarine cortex was associated with task onset, but did not vary with the duration of visual search. In contrast, the pattern of activation in dorsal and ventral visual areas was temporally associated with the duration of visual search. As such, our results support a distinction between brain regions whose activation is modulated by attentional demands of the visual task (extrastriate cortex) and those that are not affected by it (primary visual cortex). A second network of areas including central sulcus, insular, and inferior frontal cortical areas, along with the thalamus and basal ganglia, showed phasic activation tied to the execution of responses. Finally, parietal and frontal regions showed systematic deactivations during task performance, consistent with previous reports that these regions may be associated with nontask semantic processing. We conclude that detection of change, when transient visual cues are not present, requires activation of extrastriate visual regions and frontal regions responsible for eye movements. These results suggest that studies of change blindness can inform understanding of more general attentional processing.

The influence of memory load upon delay-interval activity in a working-memory task: an event-related functional MRI study

We conducted two fMRI studies to investigate the sensitivity of delay-period activity to changes in memory load during a delayed-recognition task for faces. In Experiment 1, each trial began with the presentation of a memory array consisting of one, two, or three faces that lasted for 3 sec. A 15-sec delay period followed during which no stimuli were present. The delay interval concluded with a one-face probe to which subjects made a button press response indicating whether this face was part of the memory array. Experiment 2 was similar in design except that the delay period was lengthened to 24 sec, and the memory array consisted of only one or three faces. We hypothesized that memory maintenance processes that spanned the delay interval would be revealed by their sensitivity to memory load. Long delay intervals were employed to temporally dissociate phasic activity engendered by the memory array from sustained activity reflecting maintenance. Regions of interest (ROIs) were defined anatomically for the superior frontal gyri (SFG), middle frontal gyri (MFG), and inferior frontal gyri (IFG), intraparietal sulci (IPS), and fusiform gyri (FFG) on a subject-by-subject basis. The mean time course of activity was determined for all voxels within these regions and for that subset of voxels within each ROI that correlated significantly with an empirically determined reference waveform. In both experiments, memory load significantly influenced activation 6--9 sec following the onset of the memory array with larger amplitude responses for higher load levels. Responses were greatest within MFG, IPS, and FFG. In both experiments, however, these load-sensitive differences declined over successive time intervals and were no longer significant at the end of the delay interval. Although insensitive to our load manipulation, sustained activation was present at the conclusion of the delay interval within MFG and other prefrontal regions. IPS delay activity returned to prestimulus baseline levels prior to the end of the delay period in Experiment 2, but not in Experiment 1. Within FFG, delay activity returned to prestimulus baseline levels prior to the conclusion of the delay interval in both experiments. Thus, while phasic processes engendered by the memory array were strongly affected by memory load, no evidence for load-sensitive delay-spanning maintenance processes was obtained.

Transverse instability of vector solitons and generation of dipole arrays

We develop a theory of modulational instability of multiparameter solitary waves and analyze the transverse instability of composite (or vector) optical solitons in a saturable nonlinear medium. We demonstrate theoretically and experimentally that a soliton stripe breaks up into an array of ( 2+1)-dimensional dipole-mode vector solitons, thus confirming the robust nature of those solitons as fundamental composite structures of incoherently coupled fields.

The effects of single-trial averaging upon the spatial extent of fMRI activation

We examined effects of trial averaging upon spatial extent, spatial topography, and temporal properties of fMRI activation. Two subjects participated in an event-related visual stimulation design. There was an exponential relation between number of trials and spatial extent, such that additional trials identified, on average, a constant proportion of the remaining voxels. At values typical of fMRI experimentation (e.g. 50 trials) only about 50% of eventually active voxels were significant; asymptotic values were approached by 150 trials. The variability of the estimated hemodynamic response decreased with signal averaging, becoming stable across samples of > or = 25 trials. Therefore, group or condition differences may result from differences in voxelwise noise exacerbated by averaging small numbers of trials.