Implementation of spin‐echo blood oxygen level‐dependent (BOLD) functional MRI in birds

The advent of high‐field MRI systems has allowed the implementation of blood oxygen level‐dependent functional MRI (BOLD fMRI) on small animals. An increased magnetic field improves the signal‐to‐noise ratio and thus allows an improvement in the spatial resolution. However, it also increases susceptibility artefacts in the commonly acquired gradient‐echo images. This problem is particularly prominent in songbird MRI because of the presence of numerous air cavities in the skull of birds. These T₂*‐related image artefacts can be circumvented using spin‐echo BOLD fMRI. In this article, we describe the implementation of spin‐echo BOLD fMRI in zebra finches, a small songbird of 15–25 g, extensively studied in the behavioural neurosciences of birdsong. Because the main topics in this research domain are song perception and song learning, the protocol implemented used auditory stimuli. Despite the auditory nature of the stimuli and the weak contrast‐to‐noise ratio of spin‐echo BOLD fMRI compared with gradient‐echo BOLD fMRI, we succeeded in detecting statistically significant differences in BOLD responses triggered by different stimuli. This study shows that spin‐echo BOLD fMRI is a viable approach for the investigation of auditory processing in the whole brain of small songbirds. It can also be applied to study auditory processing in other small animals, as well as other sensory modalities. Copyright © 2010 John Wiley & Sons, Ltd.     

Orienting and maintenance of spatial attention in audition and vision: multimodal and modality-specific brain activations

We studied orienting and maintenance of spatial attention in audition and vision. Functional magnetic resonance imaging (fMRI) in nine healthy subjects revealed activations in the same superior and inferior parietal, and posterior prefrontal areas in the auditory and visual orienting tasks when these tasks were compared with the corresponding maintenance tasks. Attention-related activations in the thalamus and cerebellum were observed during the auditory orienting and maintenance tasks and during the visual orienting task. In addition to the supratemporal auditory cortices, auditory orienting, and maintenance produced stronger activity than the respective visual tasks in the inferior parietal and prefrontal cortices, whereas only the occipital visual cortex and the superior parietal cortex showed stronger activity during the visual tasks than during the auditory tasks. Differences between the brain networks involved in auditory and visual spatial attention could be, for example, due to different encoding of auditory and visual spatial information or differences in stimulus-driven (bottom-up triggered) and voluntary (top-down controlled) attention between the auditory and visual modalities, or both.     

Multimodal Integration of fMRI and EEG Data for High Spatial and Temporal Resolution Analysis of Brain Networks

Two major non-invasive brain mapping techniques, electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), have complementary advantages with regard to their spatial and temporal resolution. We propose an approach based on the integration of EEG and fMRI, enabling the EEG temporal dynamics of information processing to be characterized within spatially well-defined fMRI large-scale networks. First, the fMRI data are decomposed into networks by means of spatial independent component analysis (sICA), and those associated with intrinsic activity and/or responding to task performance are selected using information from the related time-courses. Next, the EEG data over all sensors are averaged with respect to event timing, thus calculating event-related potentials (ERPs). The ERPs are subjected to temporal ICA (tICA), and the resulting components are localized with the weighted minimum norm (WMNLS) algorithm using the task-related fMRI networks as priors. Finally, the temporal contribution of each ERP component in the areas belonging to the fMRI large-scale networks is estimated. The proposed approach has been evaluated on visual target detection data. Our results confirm that two different components, commonly observed in EEG when presenting novel and salient stimuli, respectively, are related to the neuronal activation in large-scale networks, operating at different latencies and associated with different functional processes.     

An auditory oddball (P300) spelling system for brain-computer interfaces

This study was designed to develop and test an auditory event-related potential (ERP) based spelling system for a brain-computer interface (BCI) and to compare user's performance between the auditory and visual modality. The spelling system, where letters in a matrix were coded with acoustically presented numbers, was tested on a group of healthy volunteers. The results were compared with a visual spelling system. Nine of the 13 participants presented with the auditory ERP spelling system scored above a predefined criterion level control for communication. Compared to the visual spelling system, users' performance was lower and the peak latencies of the auditorily evoked ERPs were delayed. It was concluded that auditorily evoked ERPs from the majority of the users could be reliably classified. High accuracies were achieved in these users, rendering item selection with a BCI based on auditory stimulation feasible for communication.     

人脑对食物视觉刺激反应的功能磁共振成像的任务模式研究

虽然正电子发射断层成像 (positronemissiontomography ,PET)实验研究证明 ,杏仁核、眶额皮质与人脑在饥饿状态下对食物视觉刺激反应相关 ,但它对人体有微创、时间分辨率低、成本高的缺陷限制了人们在这方面的进一步研究。功能磁共振成像克服了这些缺陷 ,并且它的数据可以单例处理。但它的实验设计比PET的复杂 ,目前也没有较多任务模式设计经验可以借鉴。因此 ,要进行这方面的预实验。 5名健康志愿受试者随机分 3组 ,选择组块、事件相关和快速事件相关任务模式中的一种 ,禁食 12h后扫描。成像过程中 ,受试者接受随机播放的食物图片、非食物图片和空白模糊图片的视觉刺激。再采用SPM2软件处理功能像数据。对感兴趣的食品图片与中性物品图片的对比、食品图片与空白对照组图片对比的相关功能区域进行了t检验后发现 ,事件相关任务模式所得到的相关功能区 ,比其他两类任务模式的相应区域较多地包含了杏仁核、眶额皮质区 ,与既往PET的实验结果相符程度最大。可见事件相关任务模式是一种相对稳定的功能成像模式。     

Electrophysiological evidence for task effects on semantic priming in auditory word processing

Event-related brain potentials (ERPs) associated with semantic relatedness were recorded in two auditory word recognition tasks. In the Memorize task, subjects listened to a list of words in anticipation of a subsequent recognition test. In the Count Nonwords task, subjects silently counted the number of nonwords occurring within a list of words. Within each list, target words were either semantically related or unrelated to the immediately preceding word. As in comparable visual tasks, the amplitude of a negative ERP component (N400) was significantly attenuated when targets were preceded by semantically related primes. This attenuation was greater in the Memorize than in the Count Nonwords task. These data are consistent with the view that the sensitivity of N400 amplitude to semantic relatedness is modulated by task variables that manipulate depth of processing.     

Inter- versus intramodal integration in sensorimotor synchronization: a combined behavioral and magnetoencephalographic study

Although the temporal occurrence of the pacing signal is predictable in sensorimotor synchronization tasks, normal subjects perform on-the-beat-tapping to an isochronous auditory metronome with an anticipatory error. This error originates from an intermodal task, that is, subjects have to bring information from the auditory and tactile modality to coincide. The aim of the present study was to illuminate whether the synchronization error is a finding specific to an intermodal timing task and whether the underlying cortical mechanisms are modality-specific or supramodal. We collected behavioral data and cortical evoked responses by magneto-encephalography (MEG) during performance of cross- and unimodal tapping-tasks. As expected, subjects showed negative asynchrony in performing an auditorily paced tapping task. However, no asynchrony emerged during tactile pacing, neither during pacing at the opposite finger nor at the toe. Analysis of cortical signals resulted in a three dipole model best explaining tap-contingent activity in all three conditions. The temporal behavior of the sources was similar between the conditions and, thus, modality independent. The localization of the two earlier activated sources was modality-independent as well whereas location of the third source varied with modality. In the auditory pacing condition it was localized in contralateral primary somatosensory cortex, during tactile pacing it was localized in contralateral posterior parietal cortex. In previous studies with auditory pacing the functional role of this third source was contradictory: A special temporal coupling pattern argued for involvement of the source in evaluating the temporal distance between tap and click whereas subsequent data gave no evidence for such an interpretation. Present data shed new light on this question by demonstrating differences between modalities in the localization of the third source with similar temporal behavior.     

Memory for drawings in locations: spatial source memory and event-related potentials

Event-related potentials (ERPs) were recorded during recognition tasks for line drawings (items) or for both drawings and their spatial locations (sources). Recognized drawings elicited more positive ERPs than new drawings. Independent of accuracy in the spatial judgment, the old/new effect in the source recognition task was larger over the prefrontal scalp, and of longer temporal duration than in the item recognition task, suggesting that the source memory task engaged a qualitatively distinct memory process. More posterior scalp sites were sensitive to the accuracy of the source judgment, but this effect was delayed relative to the difference between studied and unstudied drawings, suggesting that source memory processes are completed after item recognition. Similarities and differences between spatial source memory and memory for conjunctions of other stimulus attributes are discussed, together with the role of prefrontal cortex in memory.     

Fundamental differences in change detection between vision and audition

We compared auditory change detection to visual change detection using closely matched stimuli and tasks in the two modalities. On each trial, participants were presented with a test stimulus consisting of ten elements: pure tones with various frequencies for audition, or dots with various spatial positions for vision. The test stimulus was preceded or followed by a probe stimulus consisting of a single element, and two change-detection tasks were performed. In the “present/absent” task, the probe either matched one randomly selected element of the test stimulus or none of them; participants reported present or absent. In the “direction-judgment” task, the probe was always slightly shifted relative to one randomly selected element of the test stimulus; participants reported the direction of the shift. Whereas visual performance was systematically better in the present/absent task than in the direction-judgment task, the opposite was true for auditory performance. Moreover, whereas visual performance was strongly dependent on selective attention and on the time interval separating the probe from the test stimulus, this was not the case for auditory performance. Our results show that small auditory changes can be detected automatically across relatively long temporal gaps, using an implicit memory system that seems to have no similar counterpart in the visual domain.     

Behavioral studies of auditory-visual spatial recognition and integration in rats

Rodents are useful animal models in the study of the molecular and cellular mechanisms underlying various neural functions. For studying behavioral properties associated with multisensory functions in rats, we measured the speed and accuracy of target detection by the reaction-time procedure. In the first experiment, we utilized simple two-alternative-choice tasks, in which spatial cues are visual or auditory modalities, and conducted a cross-modal transfer test in order to determine whether rats recognize amodal spatial information. Rats showed successful performance in the cross-modal transfer test and the speed to respond to sensory stimuli was constant under a rule-consistent condition despite the change in cue modality. In the second experiment, we developed audiovisual two-alternative-choice tasks, in which both auditory and visual stimuli were simultaneously presented but one of the two modalities was task-relevant, in order to determine whether the response to the sensory stimulation of one modality is enhanced by the stimulation of a different modality. If bimodal stimuli were spatially coincident, the speed for detecting the relevant stimulus was shortened and the extent of the effect was comparable to those in past studies of humans and other mammals. These results indicate the cross-modal spatial abilities of rats and our present paradigms may provide useful behavioral tasks for studying the neural bases of multisensory processing and integration in rats.     

Long-term physical exercise and somatosensory event-related potentials

We have compared the occurrence patterns of somatosensory event-related potentials (ERPs) in athletes (soccer players) and non-athletes. ERPs were elicited by two oddball tasks following separate somatosensory stimulation at the median nerve (upper-limb task) and at the tibial nerve (lower-limb task). In the athlete group the N140 amplitudes were larger during upper- and lower-limb tasks and the P300 amplitude and latency were larger and shorter, respectively, during the lower-limb task compared with non-athletes. On the other hand, no significant differences in the P300 amplitude and latency during the upper-limb task were observed between the athlete and non-athlete groups. These results indicate that plastic changes in somatosensory processing might be induced by performing physical exercises that require attention and skilled movements.     

Effective Connectivity of Cortical Sensorimotor Networks During Finger Movement Tasks: A Simultaneous fNIRS,fMRI, EEG Study

Recently, interest has been growing to understand the underlying dynamic directional relationship between simultaneously activated regions of the brain during motor task performance. Such directionality analysis (or effective connectivity analysis), based on non-invasive electrophysiological (electroencephalography—EEG) and hemodynamic (functional near infrared spectroscopy—fNIRS; and functional magnetic resonance imaging—fMRI) neuroimaging modalities can provide an estimate of the motor task-related information flow from one brain region to another. Since EEG, fNIRS and fMRI modalities achieve different spatial and temporal resolutions of motor-task related activation in the brain, the aim of this study was to determine the effective connectivity of cortico-cortical sensorimotor networks during finger movement tasks measured by each neuroimaging modality. Nine healthy subjects performed right hand finger movement tasks of different complexity (simple finger tapping-FT, simple finger sequence-SFS, and complex finger sequence-CFS). We focused our observations on three cortical regions of interest (ROIs), namely the contralateral sensorimotor cortex (SMC), the contralateral premotor cortex (PMC) and the contralateral dorsolateral prefrontal cortex (DLPFC). We estimated the effective connectivity between these ROIs using conditional Granger causality (GC) analysis determined from the time series signals measured by fMRI (blood oxygenation level-dependent-BOLD), fNIRS (oxygenated-O₂Hb and deoxygenated-HHb hemoglobin), and EEG (scalp and source level analysis) neuroimaging modalities. The effective connectivity analysis showed significant bi-directional information flow between the SMC, PMC, and DLPFC as determined by the EEG (scalp and source), fMRI (BOLD) and fNIRS (O₂Hb and HHb) modalities for all three motor tasks. However the source level EEG GC values were significantly greater than the other modalities. In addition, only the source level EEG showed a significantly greater forward than backward information flow between the ROIs. This simultaneous fMRI, fNIRS and EEG study has shown through independent GC analysis of the respective time series that a bi-directional effective connectivity occurs within a cortico-cortical sensorimotor network (SMC, PMC and DLPFC) during finger movement tasks.     

Effects of inter-stimulus interval (ISI) duration on the N1 and P2 components of the auditory event-related potential

The N1 and P2 components of the event-related potential are relevant markers in the processing of auditory information, indicating the presence of several acoustic phenomena, such as pure tones or speech sounds. In addition, the expression of these components seems to be sensitive to diverse experimental variations. The main purpose of the present investigation was to explore the role of inter-stimulus interval (ISI) on the N1 and P2 responses, considering two widely used experimental paradigms: a single tone task (1000 Hz sound repeated in a fixed rhythm) and an auditory oddball (80% of the stimuli were equal to the sound used in the single tone and the remaining were a 1500 Hz tone). Both tasks had four different conditions, and each one tested a fixed value of ISI (600, 1000, 3000, or 6000 ms). A sample of 22 participants performed these tasks, while an EEG was recorded, in order to examine the maximum amplitude of the N1 and P2 components. Analysis of the stimuli in the single tone task and the frequent tones in the oddball task revealed a similar outcome for both tasks and for both components: N1 and P2 amplitudes were enhanced in conditions with longer ISIs regardless of task. This response pattern emphasizes the dependence of both the N1 and P2 components on the ISI, especially in a scenario of repetitive and regular stimulation. The absence of task effects suggests that the ISI effect reported may depend on refractory mechanisms rather than being due to habituation effects.     

Theta rhythmicities following expected visual and auditory targets.

Evoked (EPs) as well as event-related potentials (ERPs) were recorded from two groups of 10 healthy, voluntary subjects in auditory and visual modalities. For ERP recordings 'the omitted stimulus paradigm' was employed, in which the subjects were expected to mark mentally the onset time (time prediction task) of the omitted stimulus (target). The standard auditory (AEP) and visual (VEP) evoked potentials and auditory and visual ERPs to the preceding stimuli of the omitted ones were analyzed in time and frequency domains. In the time domain the time prediction task induced increases of the amplitudes of waves existing in standard EPs; however, an additional wave or component could not be detected. Analysis of amplitude frequency characteristics (AFCs) revealed, however, selective, significant increases of the theta (3-6 Hz) frequency components of the responses concerned. These theta increases were especially evident in the frontal and parietal recording sites. Our findings suggest an association between the theta frequency components of transient evoked responses, the association areas of the brain and cognitive performance. The neurophysiological basis of scalp recorded ERPs are discussed in relation to the findings of animal studies with EEG and single unit recordings from cortical and subcortical structures.     

Source memory for the color of pictures: event-related brain potentials (ERPs) reveal sensory-specific retrieval-related activity

Remembering the context (i.e., source) in which an event occurred reveals episodic memory effects (EM) in the event-related brain potentials (ERP). In some verbal source memory experiments, a late prefrontal EM effect has been observed. In a different, pictorial source memory paradigm, a late, parieto-occipital EM effect was recorded. To assess whether these two EM effects stemmed from differences in task paradigms or from source-attribute differences, ERPs were recorded during source memory retrieval for object colors in two tasks. In the sequential task, old/new judgments were followed by source judgments (i.e., color). In the exclusion task, source memory judgments coincided with recognition judgments. For both tasks, late, parietao-occipital EM effects were observed. These findings suggest that it is not the nature of the task, but rather the perceptual characteristics of the source that lead to the presence of the parieto-occipital EM effect. The data further imply that memories for perceptual attributes such as color are stored in and retrieved from sensory-specific cortical areas.     

Transcranial magnetic stimulation (TMS) applied to left dorsolateral prefrontal cortex disrupts verbal working memory performance in humans

Working memory refers to the temporary maintenance and processing of information and involves executive processes that manipulate the contents of the working memory. The role of the executive function in the human left dorsolateral prefrontal cortex (LDLPFC) was explored using transcranial magnetic stimulation (TMS) after confirming the LDLPFC activation using fMRI. We applied double-pulse TMS having a 100-ms inter-pulse interval to LDLPFC immediately after the subjects finished reading the sentences of the reading span test (RST) task, an efficient measure of verbal working memory, in which dual tasks that include both sentence comprehension and word maintenance are required. Using eight normal participants, we found a significant deterioration of performance, i.e., decreased number of correctly reported words, in RST due to TMS stimulation of LDLPFC. Evidence suggests that transient functional disruption of the LDLPFC impairs performance in the maintenance processing of the RST task.     

Dependence of the cognitive set on the involvement of the ventral and dorsal visual systems in cognitive activity

Measures of the stability of a non-verbal visual set were compared in healthy human subjects in three series of experiments: 1) controls, in which a pair of set-forming stimuli (images of circles) were presented; 2) in the context of a test with a non-verbal set, subjects were presented with an additional task consisting of recognition of pseudowords (words); and 3) as before, but the additional task consisted of identifying the position of a target stimulus in a matrix of letters. There was a significant decrease in the stability (rigidity) of the non-verbal set on introduction of the additional task consisting of identifying the spatial position of a target stimulus; conversely, there was an increase in rigidity when the task consisted of recognizing the quality of a stimulus. Coherence analysis of cortical potentials in the alpha range showed that changes in the spatial organization of cortical electrical activity were significantly different, depending on the nature of the additional task: when the additional task involved recognition of a verbal stimulus, coherence connections were strengthened in the frontal-temporal-parietal areas of the right hemisphere; presentation in the context of a visuospatial task resulted in greater changes being observed in the anterior areas of the right hemisphere. It is suggested that the successful performance of mental functions requiring relatively rapid shifts in unconscious sets on changes in situation occurs in conditions of alternation of different types of cognitive tasks when cortical processing of visual information is mediated predominantly by one of the visual systems — either the ventral (“what?”) or the dorsal (“where?”) and, correspondingly, with the involvement of the anterior and posterior cortical selective attention systems.     

Auditory influences on visual temporal rate perception

Visual stimuli are known to influence the perception of auditory stimuli in spatial tasks, giving rise to the ventriloquism effect. These influences can persist in the absence of visual input following a period of exposure to spatially disparate auditory and visual stimuli, a phenomenon termed the ventriloquism aftereffect. It has been speculated that the visual dominance over audition in spatial tasks is due to the superior spatial acuity of vision compared with audition. If that is the case, then the auditory system should dominate visual perception in a manner analogous to the ventriloquism effect and aftereffect if one uses a task in which the auditory system has superior acuity. To test this prediction, the interactions of visual and auditory stimuli were measured in a temporally based task in normal human subjects. The results show that the auditory system has a pronounced influence on visual temporal rate perception. This influence was independent of the spatial location, spectral bandwidth, and intensity of the auditory stimulus. The influence was, however, strongly dependent on the disparity in temporal rate between the two stimulus modalities. Further, aftereffects were observed following approximately 20 min of exposure to temporally disparate auditory and visual stimuli. These results show that the auditory system can strongly influence visual perception and are consistent with the idea that bimodal sensory conflicts are dominated by the sensory system with the greater acuity for the stimulus parameter being discriminated.     

Spatial and cross-modal attention alter responses to unattended sensory information in early visual and auditory human cortex

Attending to a visual or auditory stimulus often requires irrelevant information to be filtered out, both within the modality attended and in other modalities. For example, attentively listening to a phone conversation can diminish our ability to detect visual events. We used functional magnetic resonance imaging (fMRI) to examine brain responses to visual and auditory stimuli while subjects attended visual or auditory information. Although early cortical areas are traditionally considered unimodal, we found that brain responses to the same ignored information depended on the modality attended. In early visual area V1, responses to ignored visual stimuli were weaker when attending to another visual stimulus, compared with attending to an auditory stimulus. The opposite was true in more central visual area MT+, where responses to ignored visual stimuli were weaker when attending to an auditory stimulus. Furthermore, fMRI responses to the same ignored visual information depended on the location of the auditory stimulus, with stronger responses when the attended auditory stimulus shared the same side of space as the ignored visual stimulus. In early auditory cortex, responses to ignored auditory stimuli were weaker when attending a visual stimulus. A simple parameterization of our data can describe the effects of redirecting attention across space within the same modality (spatial attention) or across modalities (cross-modal attention), and the influence of spatial attention across modalities (cross-modal spatial attention). Our results suggest that the representation of unattended information depends on whether attention is directed to another stimulus in the same modality or the same region of space.     

Auditory Evoked Potentials and Divided Attention

In a multi-channel divided attention task, 8 subjects listened to a sequence of tones delivered at one of two stimulation rates and at one of three spatial locations (channels): left ear, right ear, and an apparent position midway between left and right ears. Subjects were instructed to monitor one, two, or all three channels and detect slightly louder target tones in the monitored channel(s). Seven listening conditions were used, three in which subjects monitored one channel and ignored the others, another three in which two channels were simultaneously monitored and the third ignored, and a further condition in which all three channels were monitored. With a high stimulation rate, the N1 component of the vertex evoked potential (latency 70–130 msec) in both attended and unattended channels significantly decreased in amplitude with an increase in the number of monitored channels. At the same time, N1 was significantly larger when a channel was attended than when it was ignored. There were no significant effects under slow stimulation rate conditions. Like N1, target detectability (d') declined with increasing monitoring load only with a fast stimulation rate, but the correspondence between these two measures was not upheld in every condition. The results indicate that with a high “information load” in a multi-channel task, selective attention increases the vertex response in all attended channels, while divided attention decreases the response in both attended and unattended channels, thus suggesting that the N1 component of the auditory evoked potential reflects the apparent division of attentional capacity among competing auditory inputs.