Signal-, set- and movement-related activity in the human brain: an event-related fMRI study.
Electrophysiological studies on monkeys have been able to distinguish sensory and motor signals close in time by pseudorandomly delaying the cue that instructs the movement from the stimulus that triggers the movement. We have used a similar experimental design in functional magnetic resonance imaging (fMRI), scanning subjects while they performed a visuomotor conditional task with instructed delays. One of four shapes was presented briefly. Two shapes instructed the subjects to flex the index finger; the other two shapes coded the flexion of the middle finger. The subjects were told to perform the movement after a tone. We have exploited a novel use of event-related fMRI. By systematically varying the interval between the visual and acoustic stimuli, it has been possible to estimate the significance of the evoked haemodynamic response (EHR) to each of the stimuli, despite their temporal proximity in relation to the time constant of the EHR. Furthermore, by varying the phase between events and image acquisition, we have been able to achieve high temporal resolution while scanning the whole brain. We dissociated sensory and motor components of the sensorimotor transformations elicited by the task, and assessed sustained activity during the instructed delays. In calcarine and occipitotemporal cortex, the responses were exclusively associated with the visual instruction cues. In temporal auditory cortex and in primary motor cortex, they were exclusively associated with the auditory trigger stimulus. In ventral prefrontal cortex there were movement-related responses preceded by preparatory activity and by signal-related activity. Finally, responses associated with the instruction cue and with sustained activity during the delay period were observed in the dorsal premotor cortex and in the dorsal posterior parietal cortex. Where the association between a visual cue and the appropriate movement is arbitrary, the underlying visuomotor transformations are not achieved exclusively through frontoparietal interactions. Rather, these processes seem to rely on the ventral visual stream, the ventral prefrontal cortex and the anterior part of the dorsal premotor cortex. (+info)
Cerebellar Purkinje cell simple spike discharge encodes movement velocity in primates during visuomotor arm tracking.
Pathophysiological, lesion, and electrophysiological studies suggest that the cerebellar cortex is important for controlling the direction and speed of movement. The relationship of cerebellar Purkinje cell discharge to the control of arm movement parameters, however, remains unclear. The goal of this study was to examine how movement direction and speed and their interaction-velocity-modulate Purkinje cell simple spike discharge in an arm movement task in which direction and speed were independently controlled. The simple spike discharge of 154 Purkinje cells was recorded in two monkeys during the performance of two visuomotor tasks that required the animals to track targets that moved in one of eight directions and at one of four speeds. Single-parameter regression analyses revealed that a large proportion of cells had discharge modulation related to movement direction and speed. Most cells with significant directional tuning, however, were modulated at one speed, and most cells with speed-related discharge were modulated along one direction; this suggested that the patterns of simple spike discharge were not adequately described by single-parameter models. Therefore, a regression surface was fitted to the data, which showed that the discharge could be tuned to specific direction-speed combinations (preferred velocities). The overall variability in simple spike discharge was well described by the surface model, and the velocities corresponding to maximal and minimal discharge rates were distributed uniformly throughout the workspace. Simple spike discharge therefore appears to integrate information about both the direction and speed of arm movements, thereby encoding movement velocity. (+info)
Complete compensation in skilled reaching success with associated impairments in limb synergies, after dorsal column lesion in the rat.
Each of the dorsal columns of the rat spinal cord conveys primary sensory information, by way of the medullary dorsal column nucleus, to the ventrobasal thalamus on the contralateral side; thus the dorsal columns are an important source of neural input to the sensorimotor cortex. Damage to the dorsal columns causes impairments in synergistic proximal or whole-body movements in cats and distal limb impairments in primates, particularly in multiarticulated finger movements and tactile foviation while handling objects, but the behavioral effects of afferent fiber lesions in the dorsal columns of rodents have not been described. Female Long-Evans rats were trained to reach with a forelimb for food pellets and subsequently received lesions of the dorsomedial spinal cord at the C2 level, ipsilateral to their preferred limb. Reaching success completely recovered within a few days of dorsal column lesion. Nevertheless, a detailed analysis of high-speed video recordings revealed that rotatory limb movements (aiming, pronation, supination, etc.) were irreversibly impaired. Compensation was achieved with whole-body and alternate limb movements. These results indicate the following: (1) in the absence of the dorsal columns, other sensorimotor pathways support endpoint success in reaching; (2) sensory input conveyed by the dorsal columns is important for both proximal and distal limb movements used for skilled reaching; and (3) detailed behavioral analyses in addition to endpoint measures are necessary to completely describe the effects of dorsal column lesions. (+info)
Mechanical stimulation of starfish sperm flagella.
1. The responses of starfish sperm flagella to mechanical stimulation with a microneedle were analysed. Flagellar movement was recorded by high-speed microcinematography and by stroboscopic observation. 2. The amplitude of the bending wave of a flagellum was restricted over its entire length when the microneedle was brought near to the flagellum at its proximal region. Beyond the restricted part, the amplitude of the wave, and the bend angle, became smaller than those of a normally beating flagellum, while the curvature was practically unchanged. 3. When the tip of the microneedle was in contact with the flagellum, propagation of the bending wave beyond the microneedle was inhibited. The part of the flagellum between the base and the microneedle continued beating in some cases and stopped beating in other cases. The flagellum beyond the arrested part stopped beating and remained straight. When the microneedle was removed, the bending wave which existed in the part of the flagellum proximal to the microneedle, or the wave which was passively formed de novo at the time of the removal of the microneedle, propagated over the arrested part towards the tip. 4. A flagellum amputated by a microneedle in a medium containing ATP continued beating with a small amplitude, small curvature, small bend angle and low frequency. When the amputated flagellum was passively bent by a microneedle at the region near the point of amputation, this bend propagated towards the tip with a constant bend angle. 5. The beating frequency of the flagellum could be modulated by the application of a rhythmic external force generated by vibrating a microneedle near the flagellum. The beating was completely synchronized with vibration of the microneedle in the frequency range from 23 Hz to 43 Hz. (+info)
Interactions of membrane potential and cations in regulation of ciliary activity in Paramecium.
Ciliary activity in Paramecium was investigated in different external solutions using techniques of voltage clamp and high frequency cinematography. An increase in the external concentration of K, Ca or Mg ions decreased the resting potential. It had no effect on ciliary activity. When the membrane potential was fixed, an increase in external Ca or Mg and, to a lesser extent, an increase in K concentration, raised the frequency of normal beating or decreased the frequency of reversed beating of the cilia. Similar effects resulted from membrane hyperpolarization with constant ionic conditions. Increase in concentration of Ca, but not of Mg or K, enhanced hyperpolarization-induced augmentation of ciliary frequency. Increase in Ca concentration also specifically augmented the delayed increase in inward current during rapid hyperpolarizing clamp. The results support the view that [Ca]i regulates the frequency and direction of ciliary beating. It is suggested that the insensitivity of the ciliary motor system to elevations of the external concentrations of ions results from compensation of their effects on [Ca]i. Depolarization itself appears to increase [Ca]i while elevation of the external ion concentrations at a fixed membrane potential appears to decrease [Ca]i. (+info)
Myosin II-independent F-actin flow contributes to cell locomotion in dictyostelium.
While the treadmilling and retrograde flow of F-actin are believed to be responsible for the protrusion of leading edges, little is known about the mechanism that brings the posterior cell body forward. To elucidate the mechanism for global cell locomotion, we examined the organizational changes of filamentous (F-) actin in live Dictyostelium discoideum. We labeled F-actin with a trace amount of fluorescent phalloidin and analyzed its dynamics in nearly two-dimensional cells by using a sensitive, high-resolution charge-coupled device. We optically resolved a cyclic mode of tightening and loosening of fibrous cortical F-actin and quantitated its flow by measuring temporal and spatial intensity changes. The rate of F-actin flow was evaluated with respect to migration velocity and morphometric changes. In migrating monopodial cells, the cortical F-actin encircling the posterior cell body gradually accumulated into the tail end at a speed of 0.35 microm/minute. We show qualitatively and quantitatively that the F-actin flow is closely associated with cell migration. Similarly, in dividing cells, the cortical F-actin accumulated into the cleavage furrow. Although five times slower than the wild type, the F-actin also flows rearward in migrating mhcA- cells demonstrating that myosin II ('conventional' myosin) is not absolutely required for the observed dynamics of F-actin. Yet consistent with the reported transportation of ConA-beads, the direction of observed F-actin flow in Dictyostelium is conceptually opposite from a barbed-end binding to the plasma membrane. This study suggests that the posterior end of the cell has a unique motif that tugs the cortical actin layer rearward by means of a mechanism independent from myosin II; this mechanism may be also involved in cleavage furrow formation. (+info)
MST neuronal responses to heading direction during pursuit eye movements.
As you move through the environment, you see a radial pattern of visual motion with a focus of expansion (FOE) that indicates your heading direction. When self-movement is combined with smooth pursuit eye movements, the turning of the eye distorts the retinal image of the FOE but somehow you still can perceive heading. We studied neurons in the medial superior temporal area (MST) of monkey visual cortex, recording responses to FOE stimuli presented during fixation and smooth pursuit eye movements. Almost all neurons showed significant changes in their FOE selective responses during pursuit eye movements. However, the vector average of all the neuronal responses indicated the direction of the FOE during both fixation and pursuit. Furthermore, the amplitude of the net vector increased with increasing FOE eccentricity. We conclude that neuronal population encoding in MST might contribute to pursuit-tolerant heading perception. (+info)
Microtubule-dependent plus- and minus end-directed motilities are competing processes for nuclear targeting of adenovirus.
Adenovirus (Ad) enters target cells by receptor-mediated endocytosis, escapes to the cytosol, and then delivers its DNA genome into the nucleus. Here we analyzed the trafficking of fluorophore-tagged viruses in HeLa and TC7 cells by time-lapse microscopy. Our results show that native or taxol-stabilized microtubules (MTs) support alternating minus- and plus end-directed movements of cytosolic virus with elementary speeds up to 2.6 micrometer/s. No directed movement was observed in nocodazole-treated cells. Switching between plus- and minus end-directed elementary speeds at frequencies up to 1 Hz was observed in the periphery and near the MT organizing center (MTOC) after recovery from nocodazole treatment. MT-dependent motilities allowed virus accumulation near the MTOC at population speeds of 1-10 micrometer/min, depending on the cell type. Overexpression of p50/dynamitin, which is known to affect dynein-dependent minus end-directed vesicular transport, significantly reduced the extent and the frequency of minus end-directed migration of cytosolic virus, and increased the frequency, but not the extent of plus end-directed motility. The data imply that a single cytosolic Ad particle engages with two types of MT-dependent motor activities, the minus end- directed cytoplasmic dynein and an unknown plus end- directed activity. (+info)