Transient brain activity used in magnetic resonance imaging to detect functional areas

Multimodal deep neural decoding reveals highly resolved spatiotemporal profile of visual object representation in humans

Perception and categorization of objects in a visual scene are essential to grasp the surrounding situation. Recently, neural decoding schemes, such as machine learning in functional magnetic resonance imaging (fMRI), has been employed to elucidate the underlying neural mechanisms. However, it remains unclear as to how spatially distributed brain regions temporally represent visual object categories and sub-categories. One promising strategy to address this issue is neural decoding with concurrently obtained neural response data of high spatial and temporal resolution. In this study, we explored the spatial and temporal organization of visual object representations using concurrent fMRI and electroencephalography (EEG), combined with neural decoding using deep neural networks (DNNs). We hypothesized that neural decoding by multimodal neural data with DNN would show high classification performance in visual object categorization (faces or non-face objects) and sub-categorization within faces and objects. Visualization of the fMRI DNN was more sensitive than that in the univariate approach and revealed that visual categorization occurred in brain-wide regions. Interestingly, the EEG DNN valued the earlier phase of neural responses for categorization and the later phase of neural responses for sub-categorization. Combination of the two DNNs improved the classification performance for both categorization and sub-categorization compared with fMRI DNN or EEG DNN alone. These deep learning-based results demonstrate a categorization principle in which visual objects are represented in a spatially organized and coarse-to-fine manner, and provide strong evidence of the ability of multimodal deep learning to uncover spatiotemporal neural machinery in sensory processing.

Voxel-Wise Linearity Analysis of Increments and Decrements in BOLD Responses in Human Visual Cortex Using a Contrast Adaptation Paradigm

The linearity of BOLD responses is a fundamental presumption in most analysis procedures for BOLD fMRI studies. Previous studies have examined the linearity of BOLD signal increments, but less is known about the linearity of BOLD signal decrements. The present study assessed the linearity of both BOLD signal increments and decrements in the human primary visual cortex using a contrast adaptation paradigm. Results showed that both BOLD signal increments and decrements kept linearity to long stimuli (e.g., 3 s, 6 s), yet, deviated from linearity to transient stimuli (e.g., 1 s). Furthermore, a voxel-wise analysis showed that the deviation patterns were different for BOLD signal increments and decrements: while the BOLD signal increments demonstrated a consistent overestimation pattern, the patterns for BOLD signal decrements varied from overestimation to underestimation. Our results suggested that corrections to deviations from linearity of transient responses should consider the different effects of BOLD signal increments and decrements.

Novel Technique for Noninvasive Detection of Localized Dynamic Brain Signals by Using Transcranial Static Magnetic Fields

The techniques for noninvasive measurement of brain function such as electroencephalography (EEG), magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), and functional near-infrared spectroscopy (fNIRS) have been used in diagnosing brain conditions. However, the conventional techniques have critical limitations of spatial or temporal resolution. Here, we developed a novel technique which enables the precise measurement of dynamic brain signals and localized identification of active brain regions. In this technique, termed as magnetically biased field (MBF), human brain signal is measured as the fluctuation of a transcranial static magnetic field emitted by a coil placed on the scalp. The validity of MBF was confirmed by the measurement of somatosensory evoked signals. Fast somatosensory evoked signals were successfully observed. Localized maximum positive and negative deflections appeared at the region which represents the right primary somatosensory area contralateral to the stimulated hand. The ability of MBF to detect dynamic brain activity precisely can have numerous applications such as diagnosing brain diseases and brain-machine interfaces.

Comparison of [(15)O] H2O Positron Emission Tomography and Functional Magnetic Resonance Imaging in Activation Studies

[(15)O] H2O positron emission tomography (PET) has long been out of use in activation studies on the brain. Indeed, it is true that blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) has better spatial resolution and temporal resolution than PET, as well as no radiation exposure. However, PET and fMRI differ in their scope. Compared to fMRI, [(15)O] H2O PET offers advantages such as being quantifiable, less deteriorated by movement, and allowing for longitudinal studies. This article aimed to reassess the merits of PET in this context.

Distinct information representation and processing for goal-directed behavior in the dorsolateral and ventrolateral prefrontal cortex and the dorsal premotor cortex

Although the lateral prefrontal cortex (lPFC) and dorsal premotor cortex (PMd) are thought to be involved in goal-directed behavior, the specific roles of each area still remain elusive. To characterize and compare neuronal activity in two sectors of the lPFC [dorsal (dlPFC) and ventral (vlPFC)] and the PMd, we designed a behavioral task for monkeys to explore the differences in their participation in four aspects of information processing: encoding of visual signals, behavioral goal retrieval, action specification, and maintenance of relevant information. We initially presented a visual object (an instruction cue) to instruct a behavioral goal (reaching to the right or left of potential targets). After a subsequent delay, a choice cue appeared at various locations on a screen, and the animals could specify an action to achieve the behavioral goal. We found that vlPFC neurons amply encoded object features of the instruction cues for behavioral goal retrieval and, subsequently, spatial locations of the choice cues for specifying the actions. By contrast, dlPFC and PMd neurons rarely encoded the object features, although they reflected the behavioral goals throughout the delay period. After the appearance of the choice cues, the PMd held information for action throughout the specification and preparation of reaching movements. Remarkably, lPFC neurons represented information for the behavioral goal continuously, even after the action specification as well as during its execution. These results indicate that area-specific representation and information processing at progressive stages of the perception-action transformation in these areas underlie goal-directed behavior.

Selective averaging of rapidly presented individual trials using fMRI

A major limitation in conducting functional neuroimaging studies, particularly for cognitive experiments, has been the use of blocked task paradigms. Here we explored whether selective averaging techniques similar to those applied in event-related potential (ERP) experiments could be used to demonstrate functional magnetic resonance imaging (fMRI) responses to rapidly intermixed trials. In the first two experiments, we observed that for 1-sec trials of full-field visual checkerboard stimulation, the fMRI blood oxygenation level-dependent (BOLD) signal summated in a roughly linear fashion across successive trials even at very short (2 sec and 5 sec) intertrial intervals, although subtle departures from linearity were observed. In experiments 3 and 4, we observed that it is possible to obtain robust activation maps for rapidly presented randomly mixed trial types (left- and right-hemifield visual checkerboard stimulation) spaced as little as 2 sec apart. Taken collectively, these results suggest that selective averaging may enable fMRI experimental designs identical to those used in typical behavioral and ERP studies. The ability to analyze closely spaced single-trial, or event-related, signals provides for a class of experiments which cannot be conducted using blocked designs. Trial types can be randomly intermixed, and selective averaging based upon trial type and/or subject performance is possible.

Functional optical signal analysis: a software tool for near-infrared spectroscopy data processing incorporating statistical parametric mapping

Optical topography (OT) relies on the near infrared spectroscopy (NIRS) technique to provide noninvasively a spatial map of functional brain activity. OT has advantages over conventional fMRI in terms of its simple approach to measuring the hemodynamic response, its ability to distinguish between changes in oxy- and deoxy-hemoglobin and the range of human participants that can be readily investigated. We offer a new software tool, functional optical signal analysis (fOSA), for analyzing the spatially resolved optical signals that provides statistical inference capabilities about the distribution of brain activity in space and time and by experimental condition. It does this by mapping the signal into a standard functional neuroimaging analysis software, statistical parametric mapping (SPM), and forms, in effect, a new SPM toolbox specifically designed for NIRS in an OT configuration. The validity of the program has been tested using synthetic data, and its applicability is demonstrated with experimental data.

The concept of 'developing the brain': a new natural science for learning and education

From the viewpoint of biology, learning and education can be defined as the processes of forming neuronal connections in response to external environmental stimuli, and of controlling or adding appropriate stimuli, respectively. Learning and education can thus be studied as a new field of natural sciences with the entire human life span as its subject, thus including various problems such as fetal environment, childcare, language acquisition, general/special education, and rehabilitation. Non-invasive imaging of higher-order brain functions in humans will clarify the brain's developmental processes, and will provide various evidence for learning sciences. This new approach is called 'developing the brain' or 'brain science and education'. The origin of the concept and its present state are described and its future prospects are briefly analyzed.

The application of functional MRI of memory in temporal lobe epilepsy: a clinical review

Functional MRI (fMRI) is a useful tool for noninvasively localizing areas in the brain involved in specific cognitive functions. Since its introduction, there has been considerable speculation regarding the role it may play in the presurgical assessment of temporal lobe epilepsy (TLE). This review considers the progress made to date in using fMRI to investigate memory processing in the medial temporal lobe in normal subjects and in those with TLE. Results so far suggest that fMRI will be incorporated into the presurgical assessment of TLE in the coming years to improve definition of eloquent cerebral areas, with the objective of minimizing the adverse cognitive sequelae of anterior temporal lobe resection.

Brain imaging in awake infants by near-infrared optical topography

Studies of young infants are critical to understand perceptual, motor, and cognitive processing in humans. However, brain mechanisms involved are poorly understood, because the use of brain-imaging methods such as functional magnetic resonance imaging in awake infants is difficult. In the present study we show functional brain imaging of awake infants viewing visual stimuli by means of multichannel near-infrared spectroscopy, a technique that permits a measurement of cerebral hemoglobin oxygenation in response to brain activation through the intact skull without subject constraint. We found that event-related increases in oxyhemoglobin were evident in localized areas of the occipital cortex of infants aged 2-4 months in response to a brief presentation of a checkerboard pattern reversal while they maintained fixation to attention-grabbing stimuli. The dynamic change in cerebral blood oxygenation was qualitatively similar to that observed in the adult brain. This result introduces near-infrared optical topography as a method for investigating the functional development of the brain in early infancy.

Rapid self-paced event-related functional MRI: feasibility and implications of stimulus- versus response-locked timing

Many cognitive paradigms require self-paced responses or examine events that occur at unpredictable times. To explore whether functional MRI (fMRI) can accommodate such paradigms, a method allowing rapid, unpredictable trial pacing was developed and tested on 17 subjects using activation of the motor network as a model. Trial onset was determined solely by the subjects' self-paced responses and trials occurred, on average, less than 2 s apart. The hemodynamic response was estimated both in relation to stimulus onset (stimulus-locked) and in relation to behavioral response time (response-locked). Results yielded robust activation maps and hemodynamic response estimates. Specifically, significant activation in motor cortex, supplementary motor area (SMA), and cerebellum was observed both at the group and at the individual-subject level, confirming predicted patterns of brain activity. Moreover, the self-paced design resulted in even temporal sampling of the hemodynamic response across the image acquisition, allowing estimation of response parameters. Stimulus-locked analysis demonstrated strong correlation between hemodynamic- and behavioral-response timing both within and across subjects. Conversely, response-locked analysis showed minimal correlation with behavioral timing, suggesting effective resynchronization of the timing parameters. These results demonstrate fMRI procedures that can accommodate rapid, arbitrarily timed events and, in doing so, provide precise temporal estimates of the hemodynamic response.

An event-related fMRI study of overt and covert word stem completion

In fMRI studies of language processing, it would be extremely useful to obtain high-quality images during tasks requiring spoken output. Recent studies have suggested that this may be possible, particularly if event-related fMRI methods are used. This study assesses the feasibility of acquiring interpretable images during speech by applying event-related methods to visual word stem completion, a task that has been studied extensively. On each trial, a different three-letter word stem (e.g., COU) was presented visually and subjects were required to generate a word beginning with that stem (e.g., COUSIN). In covert runs, subjects were instructed to say the word once to themselves, without moving their lips. In overt runs, subjects were instructed to say the word once aloud. Ten subjects were scanned during six overt runs and six covert runs at three presentation rates. Data were analyzed using an implementation of the general linear model making no assumptions about response shape. Images were relatively free of artifacts, and regions demonstrating task-related activation were similar to those reported in previous imaging studies. Regions active during overt task performance were similar to those active during covert task performance, with the addition of several regions commonly associated with motor aspects of speech production. Consistent with other studies, magnitude of activation was greater in the overt condition than in the covert condition, and there was a modest decrease in magnitude at the fastest presentation rate. Together, these results help to validate the use of event-related fMRI during tasks that require spoken output. Press

Cerebral cortical representation of automatic and volitional swallowing in humans

Although the cerebral cortex has been implicated in the control of swallowing, the functional organization of the human cortical swallowing representation has not been fully documented. Therefore, the present study determined the cortical representation of swallowing in fourteen healthy right-handed female subjects using single-event-related functional magnetic resonance imaging (fMRI). Subjects were scanned during three swallowing activation tasks: a naïve saliva swallow, a voluntary saliva swallow, and a water bolus swallow. Swallow-related laryngeal movement was recorded simultaneously from the output of a bellows positioned over the thyroid cartilage. Statistical maps were generated by computing the difference between the magnitude of the voxel time course during 1) a single swallowing trial and 2) the corresponding control period. Automatic and volitional swallowing produced activation within several common cortical regions, the most prominent and consistent being located within the lateral precentral gyrus, lateral postcentral gyrus, and right insula. Activation foci within the superior temporal gyrus, middle and inferior frontal gyri, and frontal operculum also were identified for all swallowing tasks. In contrast, activation of the caudal anterior cingulate cortex was significantly more likely in association with the voluntary saliva swallow and water bolus swallow than the naïve swallow. These findings support the view that, in addition to known brain stem areas, human swallowing is represented within a number of spatially and functionally distinct cortical loci which may participate differentially in the regulation of swallowing. Activation of the insula was significantly lateralized to the right hemisphere for the voluntary saliva swallow, suggesting a functional hemispheric dominance of the insula for the processing of swallowing.

Separating processes within a trial in event-related functional MRI I. The Method

Many behavioral paradigms involve temporally overlapping sensory, cognitive, and motor components within a single trial. The complex interplay among these factors makes it desirable to separate the components of the total response without assumptions about shape of the underlying hemodynamic response. We present a method that does this. Four conditions were studied in four subjects to validate the method. Two conditions involved rapid event-related studies, one with a low-contrast (5%) flickering checkerboard and another with a high-contrast (95%) checkerboard. In the third condition, the same high-contrast checkerboard was presented with widely spaced trials. Finally, multicomponent trials were formed from temporally adjacent low-contrast and high-contrast stimuli. These trials were presented as a rapid event-related study. Low-contrast stimuli presented in isolation (partial trials) made it possible to uniquely estimate both the low-contrast and high-contrast responses. These estimated responses matched those measured in the first three conditions, thereby validating the method. Nonlinear interactions between adjacent low-contrast and high-contrast responses were shown to be significant but weak in two of the four subjects.

Cognitive neuroscience of episodic memory encoding

This paper presents a cognitive neuroscientific perspective on how human episodic memories are formed. Convergent evidence from multiple brain imaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) suggests a role for frontal cortex in episodic memory encoding. Activity levels within frontal cortex can predict episodic memory encoding across a wide range of behavioral manipulations known to influence memory performance, such as those present during levels of processing and divided attention manipulations. Activity levels within specific frontal and medial temporal regions can even predict, on an item by item basis, whether an episodic memory is likely to form. Furthermore, separate frontal regions appear to participate in supplying code-specific information, including distinct regions which process semantic attributes of verbal information as well as right-lateralized regions which process nonverbal information. We hypothesize that activity within these multiple frontal regions provides a functional influence (input) to medical temporal regions that bind the information together into a lasting episodic memory trace.

A new approach to measure single-event related brain activity using real-time fMRI: feasibility of sensory, motor, and higher cognitive tasks

Real-time fMRI is a rapidly emerging methodology that enables monitoring changes in brain activity during an ongoing experiment. In this article we demonstrate the feasibility of performing single-event sensory, motor, and higher cognitive tasks in real-time on a clinical whole-body scanner. This approach requires sensitivity optimized fMRI methods: Using statistical parametric mapping we quantified the spatial extent of BOLD contrast signal changes as a function of voxel size and demonstrate that sacrificing spatial resolution and readout bandwidth improves the detection of signal changes in real time. Further increases in BOLD contrast sensitivity were obtained by using real-time multi-echo EPI. Real-time image analysis was performed using our previously described Functional Imaging in REal time (FIRE) software package, which features real-time motion compensation, sliding window correlation analysis, and automatic reference vector optimization. This new fMRI methodology was validated using single-block design paradigms of standard visual, motor, and auditory tasks. Further, we demonstrate the sensitivity of this method for online detection of higher cognitive functions during a language task using single-block design paradigms. Finally, we used single-event fMRI to characterize the variability of the hemodynamic impulse response in primary and supplementary motor cortex in consecutive trials using single movements. Real-time fMRI can improve reliability of clinical and research studies and offers new opportunities for studying higher cognitive functions.

An approach to probe some neural systems interaction by functional MRI at neural time scale down to milliseconds

In this paper, we demonstrate an approach by which some evoked neuronal events can be probed by functional MRI (fMRI) signal with temporal resolution at the time scale of tens of milliseconds. The approach is based on the close relationship between neuronal electrical events and fMRI signal that is experimentally demonstrated in concurrent fMRI and electroencephalographic (EEG) studies conducted in a rat model with forepaw electrical stimulation. We observed a refractory period of neuronal origin in a two-stimuli paradigm: the first stimulation pulse suppressed the evoked activity in both EEG and fMRI signal responding to the subsequent stimulus for a period of several hundred milliseconds. When there was an apparent site-site interaction detected in the evoked EEG signal induced by two stimuli that were primarily targeted to activate two different sites in the brain, fMRI also displayed signal amplitude modulation because of the interactive event. With visual stimulation using two short pulses in the human brain, a similar refractory phenomenon was observed in activated fMRI signals in the primary visual cortex. In addition, for interstimulus intervals shorter than the known latency time of the evoked potential induced by the first stimulus ( approximately 100 ms) in the primary visual cortex of the human brain, the suppression was not present. Thus, by controlling the temporal relation of input tasks, it is possible to study temporal evolution of certain neural events at the time scale of their evoked electrical activity by noninvasive fMRI methodology.

Neural correlates of episodic retrieval success

Episodic memory retrieval involves multiple component processes, including those that occur when information is correctly remembered (retrieval success). The present study employed rapid-presentation event-related functional MRI that allowed different trial types with short intertrial intervals to be sorted such that the hemodynamic response associated with retrieval success could be extracted. Specifically, in an old/new episodic recognition task, hit trials (correctly recognized old items) and correct rejection trials (correctly rejected new items) were directly compared. The comparison revealed a mostly left-lateralized set of brain regions. Differential activation was most robust in left lateral parietal cortex and medial parietal cortex. Additional regions of differential activation included left anterior prefrontal cortex at or near Brodmann area 10, anterior insula, thalamus, anterior cingulate cortex, frontal cortex along inferior frontal gyrus, premotor cortex, and presupplementary motor area. These results suggest that left frontal and parietal regions modulate activity based on the successful retrieval of information from episodic memory. We discuss these findings in the context of several recent investigations that provide converging results as well as prior studies that have failed to detect these changes.

Characterizing the hemodynamic response: effects of presentation rate, sampling procedure, and the possibility of ordering brain activity based on relative timing

Rapid-presentation event-related functional MRI (ER-fMRI) allows neuroimaging methods based on hemodynamics to employ behavioral task paradigms typical of cognitive settings. However, the sluggishness of the hemodynamic response and its variance provide constraints on how ER-fMRI can be applied. In a series of two studies, estimates of the hemodynamic response in or near the primary visual and motor cortices were compared across various paradigms and sampling procedures to determine the limits of ER-fMRI procedures and, more generally, to describe the behavior of the hemodynamic response. The temporal profile of the hemodynamic response was estimated across overlapping events by solving a set of linear equations within the general linear model. No assumptions about the shape were made in solving the equations. Following estimation of the temporal profile, the amplitude and timing were modeled using a gamma function. Results indicated that (1) within a region, for a given subject, estimation of the hemodynamic response is extremely stable for both amplitude (r(2) = 0.98) and time to peak (r(2) = 0.95), from one series of measurements to the next, and slightly less stable for estimation of time to onset (r(2) = 0.60). (2) As the trial presentation rate changed (from those spaced 20 s apart to temporally overlapping trials), the hemodynamic response amplitude showed a small, but significant, decrease. Trial onsets spaced (on average) 5 s apart showed a 17-25% reduction in amplitude compared to those spaced 20 s apart. Power analysis indicated that the increased number of trials at fast rates outweighs this decrease in amplitude if statistically reliable response detection is the goal. (3) Knowledge of the amplitude and timing of the hemodynamic response in one region failed to predict those properties in another region, even for within-subject comparisons. (4) Across subjects, the amplitude of the response showed no significant correlation with timing of the response, for either time-to-onset or time-to-peak estimates. (5) The within-region stability of the response was sufficient to allow offsets in the timing of the response to be detected that were under a second, placing event-related fMRI methods in a position to answer questions about the change in relative timing between regions.

Higher-order brain function analysis by trans-cranial dynamic near-infrared spectroscopy imaging

Near-infrared spectroscopy is discussed from the viewpoint of human higher-order brain function analysis. Pioneering work in this field is reviewed; then we describe our concept of noninvasive trans-cranial dynamic optical topography and its instrumentation. Also, the validity of its functional images is assessed from both physical and physiological viewpoints. After confirming the validity of this method, we have applied it to a wide variety of fields such as clinical medicine, cognitive science, and linguistics in collaboration with researchers at several other institutes. Further application possibilities and the future of trans-cranial dynamic optical topography are also discussed. © 1999 Society of Photo-Optical Instrumentation Engineers.

Rapid imaging of olfaction by functional MRI (fMRI): identification of presence and type of hyposmia

PURPOSE

Our goal was to develop a rapid, simple, near-real-time method of functional MRI (fMRI) to measure brain activation in response to olfactory stimuli, to use it to identify patients with smell loss (hyposmia), and to differentiate their types of hyposmia.

METHOD

fMRI was obtained in 16 patients with Type I hyposmia (who could detect but not recognize odors), 5 patients with Type II hyposmia (who could both detect and recognize odors, albeit with less than normal acuity), and 2 volunteers with normal olfactory acuity by use of a rapid echo planar imaging technique in which one coronal brain section from the anterior cortical region was studied and a single olfactory stimulus was used. Actual scanning time performed by a variation of methods previously published required 26 s. Three patients with Type I hyposmia were treated with theophylline 250-500 mg for 4-6 months and were studied before and after treatment.

RESULTS

Brain activation in response to olfactory stimuli was demonstrated using a new, rapid, and simple fMRI technique. Patients with Type I hyposmia had less activation than patients with Type II hyposmia. Both patient groups had less activation than normal volunteers. Activation in patients with Type I hyposmia was essentially absent from regions of the middle frontal, orbitofrontal, and temporal cortex and was totally absent in regions of inferior frontal, insular, and cingulate cortex. Activation in patients with Type II hyposmia was greatest in the middle frontal cortex and the orbitofrontal cortex bilaterally and was present in regions of inferior frontal, temporal, and cingulate cortex. Each patient with Type I hyposmia treated with theophylline had improved smell function to Type II hyposmia and after treatment demonstrated activation in inferior frontal and cingulate cortex bilaterally, whereas before treatment, no activation in these regions was apparent.

CONCLUSION

We describe a simple, rapid technique that can be used in a practical clinical setting to identify patients with hyposmia and to differentiate patients with different types of olfactory loss. These studies confirm the presence and classification of patients with Type I and Type II hyposmia. Results of this study suggest that regions of the frontal cortex may act to guide or direct olfactory signals to other brain areas such as temporal and cingulate regions. Although these latter regions are involved with olfactory recognition, their role in olfactory memory, olfactory meaning, and attention needs to be considered.

When encoding yields remembering: insights from event-related neuroimaging

To understand human memory, it is important to determine why some experiences are remembered whereas others are forgotten. Until recently, insights into the neural bases of human memory encoding, the processes by which information is transformed into an enduring memory trace, have primarily been derived from neuropsychological studies of humans with select brain lesions. The advent of functional neuroimaging methods, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), has provided a new opportunity to gain additional understanding of how the brain supports memory formation. Importantly, the recent development of event-related fMRI methods now allows for examination of trial-by-trial differences in neural activity during encoding and of the consequences of these differences for later remembering. In this review, we consider the contributions of PET and fMRI studies to the understanding of memory encoding, placing a particular emphasis on recent event-related fMRI studies of the Dm effect: that is, differences in neural activity during encoding that are related to differences in subsequent memory. We then turn our attention to the rich literature on the Dm effect that has emerged from studies using event-related potentials (ERPs). It is hoped that the integration of findings from ERP studies, which offer higher temporal resolution, with those from event-related fMRI studies, which offer higher spatial resolution, will shed new light on when and why encoding yields subsequent remembering.

Common inhibitory mechanism in human inferior prefrontal cortex revealed by event-related functional MRI

Inhibition of an ongoing reaction tendency for adaptation to changing environments is a major function of the human prefrontal cortex. This function has been investigated frequently using the go/no-go task and set-shifting tasks such as the Wisconsin Card Sorting Test (WCST). Studies in humans and monkeys suggest the involvement of the dorsolateral prefrontal cortex in the two task paradigms. However, it remains unknown where in the dorsolateral prefrontal cortex this function is localized, whether a common inhibitory mechanism is used in these task paradigms and how this inhibitory function acts on two different targets, i.e. the go response in the go/no-go task and the cognitive set in the WCST. In the go/no-go task of this study, subjects were instructed to either respond (go trial) or not respond (no-go trial), depending on the cue stimulus presented. The signals of functional MRI (fMRI) related to the inhibitory function should be transient by nature. Thus, we used the temporal resolution of fMRI (event-related fMRI) by which transient signals in go and no-go trials can be analysed separately and compared with each other. We found a focus that showed transient no-go dominant activity in the posterior part of the inferior frontal sulcus in the right hemisphere. This was true irrespective of whether the subjects used their right or left hands. These results suggest that the transient activation in the right inferior prefrontal area is related to the neural mechanism underlying the response inhibition function. Furthermore, this area was found to be overlapped spatially with the area that was activated transiently during cognitive set shifting in the WCST. The transient signals in the go/no-go task peaked 5 s after the transient expression of the inhibitory function, and the transient signals in the WCST peaked 7s after the transient expression, reflecting different durations of neuronal activity in the two inhibitory task paradigms. These results imply that the right inferior prefrontal area is commonly involved in the inhibition of different targets, i.e. the go response during performance of the go/no-go task and the cognitive set during performance of the WCST.

Activation of lateral extrastriate areas during orthographic processing of Japanese characters studied with fMRI

We investigated the early orthographic processing in the occipital cortex using Japanese morphogram by functional magnetic resonance imaging. Kanji (Japanese morphogram) is one system of character used in the Japanese language, each character of which has a specific meaning and pronunciations. To ensure that the effects of the general visual properties of Kanji were excluded from Kanji-related activation, we created strict control stimuli, the "scrambled Kanji" that had the same luminance, contrast, and retinotopical size as those of the original Kanji. In the Kanji vs scrambled Kanji task, we found significant activation in the left inferior occipital gyrus. However, we found no activation in earlier visual cortices, including the primary visual cortex, indicating that the scrambled Kanji served as an effective control stimulus for this task. In the Kanji vs blank screen task, much more areas, including earlier visual cortices, were activated. The activation that we found in the Kanji vs scrambled Kanji task was compatible with the results of previous studies of English letter strings by other groups, suggesting that the left inferior occipital gyrus plays an essential role in orthographic processing common to these two different writing systems.

Temporal and spatial MRI responses to subsecond visual activation

The temporal and spatial characteristics of oxygenation-sensitive MRI responses to very brief visual stimuli (five Hz reversing black and white checkerboard pattern versus darkness) were investigated (nine subjects) by means of serial single-shot gradient-echo echo-planar imaging (2.0 T, TR=400 ms, mean TE=54 ms, flip angle 30 degrees). The use of a 0.2-s stimulus and a 90-s control phase resulted in an initial latency phase (about 2 s, no signal change), a positive MRI response (2.5% signal increase peaking at 5 s after stimulus onset), and a post-stimulus undershoot (1% signal decrease peaking at 15 s after stimulus onset) lasting for about 50-60 s. The finding that a subsecond visual stimulus elicits both a strong positive MRI response and a long-lasting undershoot provides further evidence for the neuronal origin of slow signal fluctuations seen in the absence of functional challenge and their utility for mapping functional connectivity. The additional observation that a reduction of the inter-stimulus control phase from 90 s to 9.8 s does not seem to affect the spatial extent of cortical activation in pertinent maps is of major relevance for the design and analysis of "event-related" MRI studies.

Transient activation of inferior prefrontal cortex during cognitive set shifting

The Wisconsin Card Sorting Test, which probes the ability to shift attention from one category of stimulus attributes to another (shifting cognitive sets), is the most common paradigm used to detect human frontal lobe pathology. However, the exact relationship of this card test to prefrontal function and the precise anatomical localization of the cognitive shifts involved are controversial. By isolating shift-related signals using the temporal resolution of functional magnetic resonance imaging, we reproducibly found transient activation of the posterior part of the bilateral inferior frontal sulci. This activation was larger as the number of dimensions (relevant stimulus attributes that had to be recognized) were increased. These results suggest that the inferior frontal areas play an essential role in the flexible shifting of cognitive sets.

Functional-anatomic study of episodic retrieval. II. Selective averaging of event-related fMRI trials to test the retrieval success hypothesis

In a companion paper (R. L. Buckner et al., 1998, NeuroImage 7, 151-162) we used fMRI to identify brain areas activated by episodic memory retrieval. Prefrontal areas were shown to differentiate component processes related to retrieval success and retrieval effort in block-designed paradigms. Importantly, a right anterior prefrontal area was most active during task blocks involving greatest retrieval success, consistent with an earlier PET study by M. D. Rugg et al. (1996, Brain 119, 2073-2083). However, manipulation of these variables within the context of blocked trials confounds differences related to varying levels of retrieval success with potential shifts in subjects' strategies due to changes in the probability of target events across blocks. To test more rigorously the hypothesis that certain areas are directly related to retrieval success, we adopted recently developed procedures for event-related fMRI. Fourteen subjects studied words under deep encoding and were then tested in a mixed trial paradigm where old and new words were randomly presented. This recognition testing procedure activated similar areas to the blocked trial paradigm, with all areas showing similar levels of activation across old and new items. Of critical importance, significant activation was detected in right anterior prefrontal cortex for new items when subjects correctly indicated they were new (correct rejections). These findings go against the retrieval success hypothesis as formally proposed and provide an important constraint for interpretation of this region's role in episodic retrieval. Furthermore, anterior prefrontal activation was found to occur late, relative to other brain areas, suggesting that it may be involved in retrieval verification or monitoring processes or perhaps even in anticipation of subsequent trial events (although an alternative possibility, that the late onset is mediated by a late vascular response, cannot be ruled out). These findings and their relation to the results obtained in the companion blocked-trial paradigm are discussed.

No-go dominant brain activity in human inferior prefrontal cortex revealed by functional magnetic resonance imaging

We investigated the response inhibition function of the prefrontal cortex associated with the go/no-go task using functional magnetic resonance imaging in five human subjects. The go/no-go task consisted of go and no-go trials given randomly with roughly equal probability. In go trials a green square was presented and the subjects had to respond by promptly pushing a button using their right or left thumbs, but in no-go trials a red square was presented and subjects were instructed not to respond. When brain activity in no-go trials is dominant over that in go trials in areas in the prefrontal cortex, this no-go dominant brain activity would reflect the neural processes for inhibiting inherent response tendency. We used a new strategy of image data analysis by which transient brain activity in go or no-go trials can be analysed separately, and looked for the prefrontal areas in which the brain activity in no-go trials is dominant over that in go trials. We found the no-go dominant foci in the posterior part of the right inferior frontal sulcus reproducibly among the subjects. This was true whether the right or left hand was used. These results suggest that this region in the prefrontal cortex is related to the neural mechanisms underlying the response inhibition function.

Event-related functional MRI: past, present, and future

The past two decades have seen an enormous growth in the field of human brain mapping. Investigators have extensively exploited techniques such as positron emission tomography and MRI to map patterns of brain activity based on changes in cerebral hemodynamics. However, until recently, most studies have investigated equilibrium changes in blood flow measured over time periods upward of 1 min. The advent of high-speed MRI methods, capable of imaging the entire brain with a temporal resolution of a few seconds, allows for brain mapping based on more transient aspects of the hemodynamic response. Today it is now possible to map changes in cerebrovascular parameters essentially in real time, conferring the ability to observe changes in brain state that occur over time periods of seconds. Furthermore, because robust hemodynamic alterations are detectable after neuronal stimuli lasting only a few tens of milliseconds, a new class of task paradigms designed to measure regional responses to single sensory or cognitive events can now be studied. Such "event related" functional MRI should provide for fundamentally new ways to interrogate brain function, and allow for the direct comparison and ultimately integration of data acquired by using more traditional behavioral and electrophysiological methods.

Functional neuroimaging studies of encoding, priming, and explicit memory retrieval

Human functional neuroimaging techniques provide a powerful means of linking neural level descriptions of brain function and cognition. The exploration of the functional anatomy underlying human memory comprises a prime example. Three highly reliable findings linking memory-related cognitive processes to brain activity are discussed. First, priming is accompanied by reductions in the amount of neural activation relative to naive or unprimed task performance. These reductions can be shown to be both anatomically and functionally specific and are found for both perceptual and conceptual task components. Second, verbal encoding, allowing subsequent conscious retrieval, is associated with activation of higher order brain regions including areas within the left inferior and dorsal prefrontal cortex. These areas also are activated by working memory and effortful word generation tasks, suggesting that these tasks, often discussed as separable, might rely on interdependent processes. Finally, explicit (intentional) retrieval shares much of the same functional anatomy as the encoding and word generation tasks but is associated with the recruitment of additional brain areas, including the anterior prefrontal cortex (right > left). These findings illustrate how neuroimaging techniques can be used to study memory processes and can both complement and extend data derived through other means. More recently developed methods, such as event-related functional MRI, will continue this progress and may provide additional new directions for research.

Functional-anatomic correlates of object priming in humans revealed by rapid presentation event-related fMRI

Human functional-anatomic correlates of object repetition were explored in a cohort of 20 subjects using fMRI. Subjects performed an object classification task where the target objects were either novel or repeated. Objects appeared rapidly, one every 2 s, in a randomly intermixed task design similar to traditional behavioral, event-related potential (ERP), and single-unit physiological studies. Recently developed event-related fMRI methods were used to analyze the data. Clear effects of repetition were observed. Brain areas in midlevels of the processing hierarchy, including extrastriate visual cortex extending into inferotemporal cortex and left dorsal prefrontal cortex, showed reductions in the amount of activation after repetition. By contrast, early visual areas and output motor areas were activated equally by both novel and repeated objects and did not show effects of repetition, suggesting that the observed correlates of repetition were anatomically selective. We discuss these findings in relation to previous positron emission tomography (PET) and fMRI studies of item repetition and single-unit physiological studies; we also address the broad impact that rapid event-related fMRI is likely to have on functional neuroimaging.

Late onset of anterior prefrontal activity during true and false recognition: an event-related fMRI study

Previous studies using PET and fMRI to examine memory retrieval have been limited by the requirement to test different types of items in separate blocks and to average data across items and response types within blocks. We used recently developed procedures for analyzing event-related mixed trial data from fMRI experiments to compare brain activity during true recognition of previously studied words and false recognition of semantic associates. A previous PET study using blocked testing procedures reported similarities and differences in rCBF patterns associated with true and false recognition (Schacter et al., 1996a). We examined brain activity during blocked testing of studied words and nonstudied semantic associates (similar to PET), and also during event-related mixed trials, where studied words and nonstudied semantic associates are intermixed. Six subjects initially heard lists of semantically related words and were later tested for old/new recognition with studied words and nonstudied semantic associates, either in separate blocks or intermixed randomly for the event-related analysis. Compared to a fixation control condition, a variety of regions previously reported in the PET study showed significant activation for both true and false recognition, including anterior prefrontal, frontal opercular, medial parietal, and visual cortex extending into hippocampal/parahippocampal regions. Differences across trial types were not clearly present. Event-related analyses of time course data show a relatively late onset and sustained duration for anterior prefrontal signal changes compared to signal changes in other activated regions. Further study is needed to resolve whether this late onset originates from variance in hemodynamic response properties or is attributable to delayed neural activity. The delayed onset is consistent with the idea that anterior prefrontal regions participate in postretrieval monitoring processes.