Temporal and spatial patterns of cortical activation during assisted lower limb movement

Human gait is a complex process in the central nervous system that results from the integrity of various mechanisms, including different cortical and subcortical structures. In the present study, we investigated cortical activity during lower limb movement using EEG. Assisted by a dynamic tilt table, all subjects performed standardized stepping movements in an upright position. Source localization of the movement-related potential in relation to spontaneous EEG showed activity in brain regions classically associated with human gait such as the primary motor cortex, the premotor cortex, the supplementary motor cortex, the cingulate cortex, the primary somatosensory cortex and the somatosensory association cortex. Further, we observed a task-related power decrease in the alpha and beta frequency band at electrodes overlying the leg motor area. A temporal activation and deactivation of the involved brain regions as well as the chronological sequence of the movement-related potential could be mapped to specific phases of the gait-like leg movement. We showed that most cortical capacity is needed for changing the direction between the flexion and extension phase. An enhanced understanding of the human gait will provide a basis to improve applications in the field of neurorehabilitation and brain–computer interfaces.     

Superadditive BOLD activation in superior temporal sulcus with threshold non-speech objects

Evidence from neurophysiological studies has shown the superior temporal sulcus (STS) to be a site of audio-visual integration, with neuronal response to audio-visual stimuli exceeding the sum of independent responses to unisensory audio and visual stimuli. However, experimenters have yet to elicit superadditive (AV > A+V) blood oxygen-level dependent (BOLD) activation from STS in humans using non-speech objects. Other studies have found integration in the BOLD signal with objects, but only using less stringent criteria to define integration. Using video clips and sounds of hand held tools presented at psychophysical threshold, we were able to elicit BOLD activation to audio-visual objects that surpassed the sum of the BOLD activations to audio and visual stimuli presented independently. Our findings suggest that the properties of the BOLD signal do not limit our ability to detect and define sites of integration using stringent criteria.     

A temporal feature of memory for plans

How do we remember future plans? In this study, three experiments were conducted to examine this issue. Thirty to forty different undergraduate or graduate students participated as subjects in each experiment. In Experiment 1, subjects were asked to memorize plans for a day, assuming that they were of tomorrow in the 'future' condition and that they were of yesterday in the 'past' condition. The result showed that plans for the morning and the evening were recalled better than plans for the daytime (the U-shape effect), only in the future condition. In Experiment 2, plans were presented without time, so that subjects could not use any specific schema associated with time. In this condition, the U-shape effect disappeared. In Experiment 3, subjects were required to memorize plans for two days, tomorrow and the day after tomorrow. The result showed the U-shape effect for each day, not for two consecutive days. These results lead to the conclusion that some 'temporal information' about a day may affect the memory for future plans.     

Neural responses to visual texture patterns in middle temporal area of the macaque monkey.

1. We studied how neurons in the middle temporal visual area (MT) of anesthetized macaque monkeys responded to textured and nontextured visual stimuli. Stimuli contained a central rectangular "figure" that was either uniform in luminance or consisted of an array of oriented line segments. The figure moved at constant velocity in one of four orthogonal directions. The region surrounding the figure was either uniform in luminance or contained a texture array (whose elements were identical or orthogonal in orientation to those of the figure), and it either was stationary or moved along with the figure. 2. A textured figure moving across a stationary textured background ("texture bar" stimulus) often elicited vigorous neural responses, but, on average, the responses to texture bars were significantly smaller than to solid (uniform luminance) bars. 3. Many cells showed direction selectivity that was similar for both texture bars and solid bars. However, on average, the direction selectivity measured when texture bars were used was significantly smaller than that for solid bars, and many cells lost significant direction selectivity altogether. The reduction in direction selectivity for texture bars generally reflected a combination of decreased responsiveness in the preferred direction and increased responsiveness in the null (opposite to preferred) direction. 4. Responses to a texture bar in the absence of a texture background ("texture bar alone") were very similar to the responses to solid bars both in the magnitude of response and in the degree of direction selectivity. Conversely, adding a static texture surround to a moving solid bar reduced direction selectivity on average without a reduction in response magnitude. These results indicate that the static surround is largely responsible for the differences in direction selectivity for texture bars versus solid bars. 5. In the majority of MT cells studied, responses to a moving texture bar were largely independent of whether the elements in the bar were of the same orientation as the background elements or of the orthogonal orientation. Thus, for the class of stimuli we used, orientation contrast does not markedly affect the responses of MT neurons to moving texture patterns. 6. The optimum figure length and the shapes of the length tuning curves determined with the use of solid bars and texture bars differed significantly in most of the cells examined. Thus neurons in MT are not simply selective for a particular figure shape independent of whatever cues are used to delineate the figure.     

Temporal encoding of two-dimensional patterns by single units in primate inferior temporal cortex. I. Response characteristics

We seek a general approach to determine what stimulus features visual neurons are sensitive to and how those features are represented by the neuron's responses. Because lesions of inferior temporal (IT) cortex interfere with a monkey's ability to perform pattern discrimination tasks we studied IT neurons. Previous single-unit studies have shown that IT neurons sometimes respond more strongly to complex stimuli (brushes, hands, faces) than to simple stimuli (bars, slits, edges). However, it is not known how specific stimulus parameters are represented by responses. We studied the responses of IT neurons in alert behaving monkeys to a large set of two-dimensional black and white patterns. The stimulus set was based on 64 Walsh functions that can be used to represent any picture with a resolution of one part in eight along each of two dimensions. The responses to these stimuli spanned a continuum from inhibition to strong excitation. A statistical test showed that the spike count was determined by which Walsh stimulus was presented. Hence, these stimuli form an adequate set for testing IT neurons. The responses showed temporal modulation of the spike train that could not be represented by a change in the spike count alone. Examples of this modulation were changes in latency, changes in the duration of the response, and alternating periods of excitation and inhibition. This temporal modulation may be important in representing stimulus parameters. The next paper in this series develops a method for quantifying this temporal modulation and shows that it is dependent on the stimulus. The third paper in this series shows that this temporal modulation contains more information about stimulus parameters than is contained in the spike count alone.     

Complex temporal response patterns with a simple retinal circuit

The retina can respond to a wide array of features in the visual input. It was recently reported that the retina can even recognize complicated temporal input patterns and signal violations in the patterns. When a sequence of flashes was presented, ganglion cells exhibited a variety of firing profiles and many cells showed an "omitted stimulus response" (OSR), in which they fired strongly if a flash in the sequence was omitted. We examined the synaptic origins of the OSR by recording excitatory synaptic currents from ganglion cells in the salamander retina in response to periodic flash sequences. Consistent with previous spike recordings, ganglion cells exhibited an OSR in their current response and the OSR shifted in time with a change in flash frequency such that it could predict when the next flash should have occurred. Although the behavior may seem sophisticated, we show that a simple linear-nonlinear model with a spike threshold can account for the OSR in on ganglion cells and that the variety of complex firing profiles seen in other ganglion cells can be explained by adding contributions from the off pathway. We discuss the physiological and simulation results and their implications for understanding retinal mechanisms of visual information processing.     

Learning complex temporal patterns with resource-dependent spike timing-dependent plasticity

Studies of spike timing-dependent plasticity (STDP) have revealed that long-term changes in the strength of a synapse may be modulated substantially by temporal relationships between multiple presynaptic and postsynaptic spikes. Whereas long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength have been modeled as distinct or separate functional mechanisms, here, we propose a new shared resource model. A functional consequence of our model is fast, stable, and diverse unsupervised learning of temporal multispike patterns with a biologically consistent spiking neural network. Due to interdependencies between LTP and LTD, dendritic delays, and proactive homeostatic aspects of the model, neurons are equipped to learn to decode temporally coded information within spike bursts. Moreover, neurons learn spike timing with few exposures in substantial noise and jitter. Surprisingly, despite having only one parameter, the model also accurately predicts in vitro observations of STDP in more complex multispike trains, as well as rate-dependent effects. We discuss candidate commonalities in natural long-term plasticity mechanisms.     

Identification of cerebral networks by classification of the shape of BOLD responses

Changes in regional blood oxygen level dependent (BOLD) signals in response to brief visual stimuli can exhibit a variety of time-courses. To demonstrate the anatomical distribution of BOLD response shapes during a match to sample task, a formal analysis of their time-courses is presented. An event-related design was used to estimate regional BOLD responses evoked by a cue word, which instructed the subject to attend to the motion or color of an upcoming target, and those evoked by a briefly presented moving target consisting of colored dots. Regional BOLD time-courses were adequately represented by the linear combination of three orthogonal waveforms. BOLD response shapes were then classified using a fuzzy clustering scheme. Three classes (sustained, phasic, and negative) best characterized cue responses. Four classes (sustained, sustained-phasic, phasic, and bi-phasic) best characterized target responses. In certain regions, the shape of the BOLD responses was modulated by the instruction to attend to the target's motion or color. A left frontal and a posterior parietal region showed sustained activity when motion was cued and transient activity when color was cued. A right thalamic and a left lateral occipital region showed sustained activity when color was cued and transient activity when motion was cued. Following the target several regions showed more sustained activity during motion than color trials. In summary, the effect of the task variable was focal following the cue and widespread following the target. We conclude that the temporal patterns of neural activity affected the shape of the BOLD signal.     

Temporal processing of active and passive head movement

The brain can know about an active head movement even in advance of its execution by means of an efference copy signal. In fact, sensory correlates of active movements appear to be suppressed. Passive disturbances of the head, however, can be detected only by sensory feedback. Might the perceived timing of an active head movement be speeded relative to the perception of a passive movement due to the efferent copy (anticipation hypothesis) or delayed because of sensory suppression (suppression hypothesis)? We compared the perceived timing of active and passive head movement using other sensory events as temporal reference points. Participants made unspeeded temporal order and synchronicity judgments comparing the perceived onset of active and passive head movement with the onset of tactile, auditory and visual stimuli. The comparison stimuli had to be delayed by about 45 ms to appear coincident with passive head movement or by about 80 ms to appear aligned with an active head movement. The slow perceptual reaction to vestibular activation is compatible with our earlier study using galvanic stimulation (Barnett-Cowan and Harris 2009). The unexpected additional delay in processing the timing of an active head movement is compatible with the suppression hypothesis and is discussed in relation to suppression of vestibular signals during self-generated head movement.     

Temporal encoding of two-dimensional patterns by single units in primate inferior temporal cortex. III. Information theoretic analysis

Ablation and single-unit studies in primates have shown that inferior temporal (IT) cortex is important for pattern discrimination. The first paper in this series suggested that single units in IT cortex of alert monkeys respond to a set of two-dimensional patterns with complex temporal modulation of their spike trains. The second paper quantified the waveform of the modulated responses of IT neurons with principal components and demonstrated that the coefficients of two to four of the principal components were stimulus dependent. Although the coefficients of the principal components are uncorrelated, it is possible that they are not statistically independent. That is, several coefficients could be determined by the same feature of the stimulus, and thus could be conveying the same information. The final part of this study examined this issue by comparing the amount of information about the stimulus that can be conveyed by two codes: a temporal waveform code derived from the coefficients of the first three principal components and a mean rate code derived from the spike count. We considered the neuron to be an information channel conveying messages about stimulus parameters. Previous applications of information theory to neurophysiology have dealt either with the theoretical capacity of neuronal channels or the temporal distribution of information within the spike train. This previous work usually used a general binary code to represent the spike train of a neuron's response. Such a general approach yields no indication of the nature of the neuron's intrinsic coding scheme because it depends only on the timing of spikes in the response. In particular, it is independent of any statistical properties of the responses. Our approach uses the principal components of the response waveform to derive a code for representing information about the stimuli. We regard this code as an indication of the neuron's intrinsic coding scheme, because it is based on the statistical properties of the neuronal responses. We measured how much information about the stimulus was present in the neuron's responses. This transmitted information was calculated for codes based on either the spike count or on the first three principal components of the response waveform. The information transmitted by each of the first three principal components was largely independent of that transmitted by the others. It was found that the average amount of information transmitted by the principal components was about twice as large as that transmitted by the spike count.(ABSTRACT TRUNCATED AT 400 WORDS)     

Simultaneous recording of EEG and BOLD responses: a historical perspective.

Electromagnetic fields as measured with electroencephalogram (EEG) are a direct consequence of neuronal activity and feature the same timescale as the underlying cognitive processes, while hemodynamic signals as measured with functional magnetic resonance imaging (fMRI) are related to the energy consumption of neuronal populations. It is obvious that a combination of both techniques is a very attractive aim in neuroscience, in order to achieve both high temporal and spatial resolution for the non-invasive study of cognitive brain function. During the last decade a number of research groups have taken up this challenge. Here, we review the development of the combined EEG-fMRI approach. We summarize the main data integration approaches developed to achieve such a combination, discuss the current state-of-the-art in this field and outline challenges for the future success of this promising approach.     

Spatiotopic selectivity of BOLD responses to visual motion in human area MT

Many neurons in the monkey visual extrastriate cortex have receptive fields that are affected by gaze direction. In humans, psychophysical studies suggest that motion signals may be encoded in a spatiotopic fashion. Here we use functional magnetic resonance imaging to study spatial selectivity in the human middle temporal cortex (area MT or V5), an area that is clearly implicated in motion perception. The results show that the response of MT is modulated by gaze direction, generating a spatial selectivity based on screen rather than retinal coordinates. This area could be the neurophysiological substrate of the spatiotopic representation of motion signals.     

Effects of nonspatial selective and divided visual attention on fMRI BOLD responses

Using an uncertainty paradigm and functional magnetic resonance imaging (fMRI) we studied the effect of nonspatial selective and divided visual attention on the activity of specific areas of human extrastriate visual cortex. The stimuli were single ovals that differed from an implicit standard oval in either colour or width. The subjects’ task was to classify the current stimulus as one of two possible alternatives per stimulus dimension. Three different experimental conditions were conducted: “colour-certainty”, “shape-certainty” and “uncertainty”. In all experimental conditions, the stimulus differed in only one stimulus dimension per trial. In the two certainty conditions, the subjects knew in advance which dimension this would be. During the uncertainty condition they had no such previous knowledge and had to monitor both dimensions simultaneously. Statistical analysis of the fMRI data (with SPM2) revealed a modest effect of the attended stimulus dimension on the neural activity in colour sensitive area V4 (more activity during attention to colour) and in shape sensitive area LOC (more activity during attention to shape). Furthermore, cortical areas known to be related to attention and working memory processes (e.g., lateral prefrontal and posterior parietal cortex) exhibit higher activity during the condition of divided attention (“uncertainty”) than during that of selective attention (“certainty”).     

Neuronal activity in primate parietal cortex area 5 varies with intended movement direction during an instructed-delay period

Summary A monkey was trained to make arm movements to visual targets immediately after presentation of a GO signal, either in a visual reaction-time paradigm (CONTROL task), or after an instructed-delay period of variable duration, during which a CUE stimulus signalled the direction of the impending movement (DELAY task). The activity of 98 area 5 cells recorded in 2 hemispheres varied with movement direction in the CONTROL task. This included 60 early cells which showed directional activity changes prior to movement onset. In the DELAY task, 54/98 cells (55%) showed activity changes during the instructed-delay period which varied with the direction of the impending movement. Most of these (45/54, 83%) were early cells. Forty proximal arm-related cells were recorded in adjacent area 2. In contrast to area 5, only 2/40 area 2 cells showed any evidence of changes in activity varying with intended movement direction during the instructed-delay period. The origin of area 5 activity changes during an instructed-delay period which are related to intended direction of a delayed movement is uncertain, but its presence is consistent with a number of proposed roles for area 5.     

Hormone and recovery responses to resistance exercise with slow movement

This study examined acute hormone and recovery responses to resistance exercise with slow movements. Six men performed three types of exercise regimens (five sets of knee extension exercise): (1) high-intensity resistance exercise with normal movement (HN; 1 s for lifting action, 1 s for lowering action), (2) low-intensity resistance exercise with slow movement (LS; 3 s for lifting action, 3 s for lowering action), and (3) low-intensity resistance exercise with normal movement (LN; 1 s for lifting action, 1 s for lowering action). The intensity in the first set was set at approximately 80% of 1RM for HN and 40% of 1RM for LS and LN. In the HN and LS, the subjects performed each exercise set until exhaustion. In the LN, both intensity and number of repetitions were matched with those for LS. The total work volume in the HN showed approximately double the value of LS and LN (P < 0.05). Electromyography (EMG) data indicated that LS showed sustained EMG signals throughout the exercise. During the exercise, the HN and LS showed lower muscle oxygenation levels. After the exercise, LS caused significantly greater norepinephrine and free testosterone responses (delta value) than in the HN and LN (P < 0.05). However, no significant difference was observed in the recovery of maximal isometric strength, isokinetic strength, and jump performance between the HN and LS. These results indicate that slow movements during the resistance exercise are important for the enhancement of hormonal responses, especially catecholamine and free testosterone, but they do not affect muscle strength recovery.     

Spatial and temporal constraints on performance in children with movement co-ordination problems

Eight 10-year-old children manifesting movement co-ordination problems (MCP), as assessed by the Movement Assessment Battery for Children (MABC), and a matched control group of eight children of a similar age without such problems, were required to carry out a laboratory ball-catching task. The task was constrained in such a way as to allow separate kinematic analyses of reaching (Experiment 1) and grasping (Experiment 2) subactions. Significant differences between the groups, in favour of the control group, were found with respect to both spatial and temporal performance in intercepting the moving ball. The MCP children were shown to initiate reaching movements later and to initiate grasping movement of the fingers earlier in time than the controls. MCP children also made more spatial errors. These findings are discussed in the context of the distinction made in the neuropsychological literature between proximal and distal motor control systems and the visual perceptual system.