运动领域中的视觉-运动知觉的神经生理研究进展

运动领域中的视觉-运动知觉的神经生理研究现状主要包括功能磁共振成像(fMRI)、事件相关电位(ERPs)和视觉诱发电位(VEPs)三个方面。fMRI通过比较运动员和非运动员在运动观察和运动预测时大脑激活的差异,从而在神经生理层面对运动员的视觉-运动知觉优势进行解释。ERPs证明了长期的体育训练能够加强神经系统网络结构和可塑性。VEPs的研究结果表明运动员在对运动画面进行视觉认知加工时的神经活动都是通过视觉通路来调节的,并且其神经细胞的调节也可能与其从事的特定运动项目有关。     

Neuronal correlates of encoding and retrieval in episodic memory during a paired-word association learning task: a functional magnetic resonance imaging study

The investigation of memory function using functional magnetic resonance imaging (fMRI) is an expanding field of research. The aim of this study was to demonstrate brain-activity patterns related to a word-pair association task employing a whole-brain EPI sequence. Six right-handed, healthy male volunteers (mean age: 27.5 years) took part in the study. fMRI was performed at a field strength of 1.5 Tesla with 26–32 slices parallel to the AC-PC line, depending on individual brain size. Distributed brain regions were activated in episodic encoding and retrieval with similarities, but also (distinct) differences in activation patterns. Bilateral prefrontal cortical areas were involved when comparing encoding as well as retrieval to the reference condition (nonsense words). Furthermore, activation was observed in cerebellar areas during encoding, and activation in bilateral parietal areas (precuneus and inferior parietal cortex) was differentially more pronounced during retrieval. The activation of left dorsomedial thalamus during retrieval of high imagery-content word-pair associates may point to the role of this structure in episodic retrieval. The direct cognitive subtraction of encoding minus retrieval yielded a differentially larger left prefrontal activation. There was a differentially higher right prefrontal activation during retrieval than during encoding, underlining the proposed right/left asymmetry for episodic memory processes. Received: 10 June 1998 / Accepted: 23 April 1999     

Anatomy of verbal memory: a functional MRI study

The recent advent of new functional neuroimaging techniques such as functional magnetic resonance imaging (fMRI) makes it possible to examine cerebral activations in healthy individuals. In the present study, we attempted to examine the anatomical distribution of neocortical and mediotemporal activations in control subjects during episodic memory tasks over 24 h. BOLD (blood oxygenation-level-dependent) fMRI data were collected from 10 control subjects. Twenty-two contiguous images covering the whole brain were acquired using an EPI echoplanar sequence. Subjects were instructed to learn a list of 17 words, and to recall it immediately and after a 24-h interval. Individual and group analyses were performed using SPM96. The results demonstrated that a similar left occipito-temporofrontal network was activated during both immediate and 24-h-delayed retrieval conditions. In addition, the 24-h-delayed retrieval also activated a larger parietal region and the right hippocampus. These findings suggest that fMRI is a reliable method with which to perform anatomical studies.     

Accelerated parallel imaging for functional imaging of the human brain

Accelerated parallel imaging (PI) techniques have recently been applied to functional imaging experiments of the human brain in order to improve the performance of commonly used single-shot techniques like echo-planar imaging (EPI). Potential benefits of PI-fMRI include the reduction of geometrical distortions due to off-resonance signals, the reduction of signal-loss in areas with substantial signal inhomogeneity, increases of the spatial and temporal resolution of the fMRI experiment and reduction of gradient acoustic noise. Although PI generally leads to a substantial decrease in image signal-to-noise ratio (SNR), its effect on the temporal stability of the signal, which ultimately determines fMRI performance, is only partially determined by image SNR. Therefore, the penalty for using PI is generally not as severe as the SNR reduction. The majority of problems related to single-shot techniques become more severe at an increased magnetic field strength, making PI an important tool in achieving the full potential of fMRI at high field.     

Evoked potentials in motor cortical local field potentials reflect task timing and behavioral performance

Evoked potentials (EPs) are observed in motor cortical local field potentials (LFPs) during movement execution (movement-related potentials [MRPs]) and in response to relevant visual cues (visual evoked potentials [VEPs]). Motor cortical EPs may be directionally selective, but little is known concerning their relation to other aspects of motor behavior, such as task timing and performance. We recorded LFPs in motor cortex of two monkeys during performance of a precued arm-reaching task. A time cue at the start of each trial signaled delay duration and thereby the pace of the task and the available time for movement preparation. VEPs and MRPs were strongly modulated by the delay duration, VEPs being systematically larger in short-delay trials and MRPs larger in long-delay trials. Despite these systematic modulations related to the task timing, directional selectivity was similar in short and long trials. The behavioral reaction time was positively correlated with MRP size and negatively correlated with VEP size, within sessions. In addition, the behavioral performance improved across sessions, in parallel with a slow decrease in the size of VEPs and MRPs. Our results clearly show the strong influence of the behavioral context and performance on motor cortical population activity during movement preparation and execution.     

A comparison of signal instability in 2D and 3D EPI resting-state fMRI

Spatiotemporally structured noise, such as physiological noise, is a potential source of artifacts in functional magnetic resonance imaging (fMRI) and is the main limiting factor for the detection of small blood oxygen level-dependent (BOLD) signal variations. fMRI was employed to detect low-frequency BOLD signal fluctuations, which are thought to be related to spontaneous neuronal activity in the resting human brain. The sensitivity to noise, that is, signal variations of non-BOLD origin, was investigated for two- (2D) and three-dimensional (3D) imaging techniques. Incomplete relaxation between subsequent scans increases the level of temporally and spatially correlated signal variations originating from physiological and/or systemic noise. Although inflow effects are suspected to be reduced in 3D echo-planar imaging (EPI) compared with multi-slice 2D EPI, the noise level was higher in the 3D technique. The noise level in 3D fMRI experiments was significantly increased by instabilities of the transverse steady-state magnetization as the repetition time was of the order of T(2). By implementing radiofrequency spoiling, temporal signal fluctuations and erroneous inter-regional correlation in connectivity maps were diminished to a level present in data sets acquired with 2D EPI.     

Event related fMRI studies of voluntary and inhibited eye blinking using a time marker of EOG

Electrooculogram (EOG) measurements, along with infrared measurements, are commonly used to record eye blinking during functional magnetic resonance imaging (fMRI). We report herein, on the use of EOG in measuring voluntary and inhibited eye blinking during echo planar imaging (EPI) in an MR scanner. The inhibited eye blinking occurred during the period, in which subjects were requested not to blink their eyes. After the removal of gradient-field induced artifacts from the EOG signal, the waveform of the EOG clearly showed both voluntary and inhibited eye blinking. Using these data, each voluntary or inhibited eye-blinking event was used as the temporal cue for an event related fMRI. Activation of the bilateral parahippocampal, precentral gyrus and left supplementary motor area was observed for voluntary eye blinking, whereas the medial/superior frontal, precentral, cingulate, precuneus, and superior temporal gyrus appears to be involved in inhibited eye blinking. Based on these experimental results, we propose that the precentral gyrus is responsible for both voluntary and inhibited eye blinking. The parietal area (precuneus and superior temporal gyrus) appears to be exclusively related to inhibited eye blinking.     

Pushing functional MRI spatial and temporal resolution further: High‐density receive arrays combined with shot‐selective 2D CAIPIRINHA for 3D echo‐planar imaging at 7 T

To be able to examine dynamic and detailed brain functions, the spatial and temporal resolution of 7 T MRI needs to improve. In this study, it was investigated whether submillimeter multishot 3D EPI fMRI scans, acquired with high‐density receive arrays, can benefit from a 2D CAIPIRINHA sampling pattern, in terms of noise amplification (g‐factor), temporal SNR and fMRI sensitivity. High‐density receive arrays were combined with a shot‐selective 2D CAIPIRINHA implementation for multishot 3D EPI sequences at 7 T. In this implementation, in contrast to conventional inclusion of extra k_z gradient blips, specific EPI shots are left out to create a CAIPIRINHA shift and reduction of scan time. First, the implementation of the CAIPIRINHA sequence was evaluated with a standard receive setup by acquiring submillimeter whole brain T₂*‐weighted anatomy images. Second, the CAIPIRINHA sequence was combined with high‐density receive arrays to push the temporal resolution of submillimeter 3D EPI fMRI scans of the visual cortex. Results show that the shot‐selective 2D CAIPIRINHA sequence enables a reduction in scan time for 0.5 mm isotropic 3D EPI T₂*‐weighted anatomy scans by a factor of 4 compared with earlier reports. The use of the 2D CAIPIRINHA implementation in combination with high‐density receive arrays, enhances the image quality of submillimeter 3D EPI scans of the visual cortex at high acceleration as compared to conventional SENSE. Both the g‐factor and temporal SNR improved, resulting in a method that is more sensitive to the fMRI signal. Using this method, it is possible to acquire submillimeter single volume 3D EPI scans of the visual cortex in a subsecond timeframe. Overall, high‐density receive arrays in combination with shot‐selective 2D CAIPIRINHA for 3D EPI scans prove to be valuable for reducing the scan time of submillimeter MRI acquisitions.     

Phantom haptic device upgrade for use in fMRI

This paper presents an upgrade of a Phantom Premium 1.5 haptic device for use within a functional magnetic resonance imaging (fMRI) environment. A special mechanical extension that allows the haptic device to operate at a safe distance from the high-density magnetic field of an fMRI scanner has been developed. Extended haptic system was subjected to a series of tests to confirm electromagnetic compatibility with the fMRI scanner, for which key results are presented. With this fMRI compatible haptic platform a human brain activation during controlled upper limb movements can be studied. A simple virtual environment reaching task was programmed to study brain motor control functions. At the end preliminary results of an ongoing neurophysiological study are presented.     

Visual Evoked Potentials to Illusory Reversals of the Necker Cube

Sensory adaptation and cognitive evaluation have been proposed as explanations of illusory figure reversals. The effect of variations in the perceived orientation of a Necker cube on visual evoked potentials (VEPs) was studied to test these two hypotheses. VEPs associated with perceptual reversals and non-reversals of a Necker cube were compared with VEPs elicited by a sequence of physically varying cubical figures. Amplitude differences in the early VEP components consistent with adaptation effects were not detected. Both types of reversals were associated with a late positive component (400–700 ms), which was smaller in amplitude in the illusory condition. The late positivity to illusory reversals was also distinguished by a broad (200–700 ms) positive component over frontal and central recording sites which was absent over Oz. These findings suggest that illusory reversals are more difficult to discriminate than physical reversals, and require additional cognitive resources for evaluation.     

The hemispheric distribution of the transient rat VEP: a comparison of flash and pattern stimulation

Summary We investigated the hemispheric distribution of the rat visual evoked potential (VEP) to pattern reversal and flash stimuli, presented monocularly at a rate of 1 Hz. Pattern VEP components could be recorded only over an area bounded by the anatomical coordinates of area 17, while some flash VEP components were recordable outside the primary visual area. Monocular pattern stimulation, as expected, evoked dominant contralateral VEPs. Surprisingly, ipsilateral responses could be also recorded with the reference electrode near the nasal bone. These VEPs showed partial polarity inversion compared to the contralateral EPs. To assess the origin of the ipsilateral EPs, we also recorded EPs following surgical deafferentation of the ipsilateral cortex. Our data reveal that ipsilateral VEPs represent volume conducted potentials.     

Scanning strategies for simultaneous EEG-fMRI evoked potential studies at 3 T.

There are two basic strategies for applying simultaneous EEG-fMRI: either the fMRI data are acquired continuously, or the stimulus is presented during a brief gap in scanning when the EEG data is clear of gradient artefact. The former has the advantage that the protocol for the fMRI data acquisition is not affected by the presence of EEG. This study investigated the effect of these different strategies and the subsequent ballistocardiogram artefact removal methods (Average Artefact Subtraction (AAS) and Optimal Basis Set (OBS)) on EEG data quality recorded in response to a visual stimulus. Continuous scanning generally resulted in VEPs that were no worse, and in some cases were better, than those measured during a gap in scanning. The AAS and OBS methods lead to comparable results at the level of the grand average visual evoked potential (VEP), although when examined at the level of the single trial the OBS method was more effective. The spectral quality of the data was similar across scanning protocols, as demonstrated by the proportion of spectral power in each frequency band, although there was an effect of the artefact removal method on the overall spectral power. Some differences in the VEPs were also noted when a TR of 1.5 s was used relative to a TR of 3 s. The results indicate improved EEG quality when fMRI scanning is continuous and BCG artefacts are removed using the OBS method, confirming that EEG can be added to an fMRI experiment with minimal change to the experimental protocol.     

Functional imaging of the monkey brain.

Functional magnetic resonance imaging (fMRI) has become an essential tool for studying human brain function. Here we describe the application of this technique to anesthetized monkeys. We present spatially resolved functional images of the monkey cortex based on blood oxygenation level dependent (BOLD) contrast. Checkerboard patterns or pictures of primates were used to study stimulus-induced activation of the visual cortex, in a 4.7-Tesla magnetic field, using optimized multi-slice, gradient-recalled, echo-planar imaging (EPI) sequences to image the entire brain. Under our anesthesia protocol, visual stimulation yielded robust, reproducible, focal activation of the lateral geniculate nucleus (LGN), the primary visual area (V1) and a number of extrastriate visual areas, including areas in the superior temporal sulcus. Similar responses were obtained in alert, behaving monkeys performing a discrimination task.     

Does habituation depend on cortical inhibition? Results of a rTMS study in healthy subjects

Habituation, i.e. the decremental response to repeated sensorial stimulation, is studied in humans through evoked potential stimulation. Mechanisms underlying habituation are not yet cleared, even if inhibitory circuits are supposed to play an important role. Light deprivation (LD) increases visual cortical excitability likely through down-regulation of GABA circuits. We previously found that high-frequency repetitive transcranial magnetic stimulation (hf-rTMS) can revert these facilitatory effects likely restoring the activity of inhibitory circuits. Here, we studied the effects of LD and rTMS on habituation of visual evoked potentials (VEPs). The hypothesis was that if the inhibitory circuits have a role in habituation, then LD that downregulates GABA circuits, should impair habituation that in turn should be restored by hf-rTMS. Fifteen healthy subjects underwent VEPs recording in baseline (without LD), in LD alone (without rTMS), in LD and 1 Hz rTMS and in LD and 10 Hz rTMS. Habituation observed in baseline (without LD) was significantly impaired after LD; 10 Hz but not 1 Hz rTMS was able to restore normal habituation phenomena. VEPs habituation is impaired by LD but it could be restored if hf-rTMS is given during LD. As LD acts reducing GABA circuits activity and hf-rTMS likely upregulates such circuits, these data give support to the hypothesis that cortical inhibition can play a relevant role in mechanisms underlying habituation.     

Electrocortical signs of word categorization in saccade-related brain potentials and visual evoked potentials

Visual evoked potentials (VEPs) elicited by foveal presentation of words were compared to brain potentials evoked by the same words in a condition where subjects had to make a saccadic eye movement in order to perceive the words (saccade-related brain potentials, SRPs). Subjects had to categorize the words responding with a button press to stimuli belonging to the target (infrequent, P = 0.2) category. The VEP and SRP waveforms showed divergences in the early (up to 250 ms) components, but a marked similarity between the late components. Principal Component Analysis also revealed the same relationship between the two types of brain responses. Peak latency of the late SRP components measured from saccade offset showed an apparent processing advantage over the corresponding late components of VEPs. The N3 component, indexing semantic processing of visual patterns, peaked between 310 and 375 ms in the SRPs, while in the VEPs it appeared between 410 and 470 ms. The P4 component, associated with final stimulus evaluation, showed a similar latency benefit in favour of SRPs (420-500 ms vs 530-590 ms in VEPs). The mean reaction time was 74 ms shorter in the eye movement condition (measured from saccade offset) than in the VEP condition (703 vs 777 ms). The question of what kind of processes may contribute to the differences in mean RTs and to the latencies of the late components between the two conditions are discussed. We suggest that the late components (P3, N3 and P4) of the VEP and the SRP, respectively, index identical brain processes.     

Conflict-related activity in the caudal anterior cingulate cortex in the absence of awareness

The caudal anterior cingulate cortex (cACC) is thought to be involved in performance monitoring, as conflict and error-related activity frequently co-localize in this area. Recent results suggest that these effects may be differentially modulated by awareness. To clarify the role of awareness in performance monitoring by the cACC, we used rapid event-related fMRI to examine the cACC activity while subjects performed a dual task: a delayed recognition task and a serial response task (SRT) with an implicit probabilistic learning rule (i.e. the stimulus location followed a probabilistic sequence of which the subjects were unaware). Task performance confirmed that the location sequence was learned implicitly. Even though we found no evidence of awareness for the presence of the sequence, imaging data revealed increased cACC activity during correct trials which violated the sequence (high-conflict), relative to trials when stimuli followed the sequence (low conflict). Errors made with awareness also activated the same brain region. These results suggest that the performance monitoring function of the cACC extends beyond detection of errors made with or without awareness, and involves detection of multiple responses even when they are outside of awareness.     

Simultaneous multi‐slice (SMS) acquisition enhances the sensitivity of hemodynamic mapping using gas challenges

Hemodynamic mapping using gas inhalation has received increasing interest in recent years. Cerebrovascular reactivity (CVR), which reflects the ability of the brain vasculature to dilate in response to a vasoactive stimulus, can be measured by CO₂ inhalation with continuous acquisition of blood oxygen level‐dependent (BOLD) magnetic resonance images. Cerebral blood volume (CBV) can be measured by O₂ inhalation. These hemodynamic mapping methods are appealing because of their absence of gadolinium contrast agent, their ability to assess both baseline perfusion and vascular reserve, and their utility in calibrating the functional magnetic resonance imaging (fMRI) signal. However, like other functional and physiological indices, a major drawback of these measurements is their poor sensitivity and reliability. Simultaneous multi‐slice echo planar imaging (SMS EPI) is a fast imaging technology that allows the excitation and acquisition of multiple two‐dimensional slices simultaneously, and has been shown to enhance the sensitivity of several MRI applications. To our knowledge, the benefit of SMS in gas inhalation imaging has not been investigated. In this work, we compared the sensitivity of CO₂ and O₂ inhalation data collected using SMS factor 2 (SMS2) and SMS factor 3 (SMS3) with those collected using conventional EPI (SMS1). We showed that the sensitivity of SMS scans was significantly (p = 0.01) higher than that of conventional EPI, although no difference was found between SMS2 and SMS3 (p = 0.3). On a voxel‐wise level, approximately 20–30% of voxels in the brain showed a significant enhancement in sensitivity when using SMS compared with conventional EPI, with other voxels showing an increase, but not reaching statistical significance. When using SMS, the scan duration can be reduced by half, whilst maintaining the sensitivity of conventional EPI. The availability of a sensitive acquisition technique can further enhance the potential of gas inhalation MRI in clinical applications.     

The Effect of Slice Orientation on Auditory fMRI at the Level of the Brainstem

Although auditory information is processed in several subcortical nuclei, most fMRI studies focus solely on the auditory cortex and do not take brainstem responses into account. One common difficulty in obtaining clear functional brainstem recordings is due to heartbeat related motion, manifested in the rostro-caudal and in the ventro-dorsal directions in the contraction phase of the heart. The aim of this study was to investigate the effect of slice orientation on auditory functional magnetic resonance imagining (fMRI) measurements with respect to the pattern of brainstem oscillation. Fourteen healthy volunteers listened monaurally to modulated pink noise. Blood oxygenation level dependent (BOLD) contrast was performed with an echo-planar image (EPI) sequence using a 3T MRI system. Three different slice orientations were compared: approximately parallel, at 45°, and orthogonal to the brainstem. The standard deviation of the residuals, the effect size, the median t-values, and the number of activated voxels were calculated to quantify variability in activation between orientations. The data for the inferior colliculi indicated that a slice orientation with a 45° angle to the brainstem yielded the lowest sensitivity to motion (reflected in the standard deviation of the residuals). By contrast, the results did not suggest differences between the three imaging planes on the scanning of the auditory cortex. Findings indicate that the 45° slice orientation is the optimum orientation for accurate measurement at the upper brainstem level.     

Evidence for multiple generators in evoked responses using finite difference field mapping: Auditory evoked fields

Summary Electric potential maps and magnetic field maps have been used to study brain electrical activity. During the temporal course of an evoked cortical response, the electrical activity of specific neuronal subpopulations change in a sequential manner giving rise to measurable electrical potentials and magnetic fields. For these potentials and fields, both the amplitude and rate of amplitude change have characteristic, time-dependent waveforms. Presently, amplitude waveforms from multiple locations are used to generate magnetic field and electric potential maps which have been found to be useful in understanding the activity of the neurons which give rise to these maps (Romani 1990). This paper introduces a data transformation technique which results in a derived map that we have termed a "finite difference field map" (FDFM). This mapping technique provides information associated with the rate at which the amplitude of the neuronal electric activity changes. In this paper, some advantages of FDFM analysis are illustrated by application of this technique to the study of the auditory evoked cortical field (AECF) N1m waveform. Using data obtained from normal subjects it will be demonstrated that application of the FDFM technique allows the localization of the primary N1m source at an earlier latency than is possible using the conventional waveform data. The source location determined at an early latency by FDFM analysis was identical to that obtained at later latency from the conventional field data. These data suggest that the primary N1m source is stationary. In addition, analysis of the time sequence of FDFM field maps contains evidence of a second spatially separate source which is co-active with primary N1m source.