Neural basis of stable perception of an ambiguous apparent motion stimulus.

Although it has been shown that an alternative dominant percept induced by an ambiguous visual scene has neural correlates in various cortical areas, it is not known how such a dominant percept is maintained until it switches to another. We measured the primary visual response to the two-frame bistable apparent motion stimulus (stroboscopic alternative motion) when observers continuously perceived one motion and compared this with the response for another motion using magnetoencephalography. We observed a response component at around 160 ms after the frame change, the amplitude of which depended on the perceived motion. In contrast, brain responses to less ambiguous and physically unambiguous motions in both the horizontal and vertical directions did not evoke such a component. The differential response evoked by the bistable apparent motion is therefore distinct from directionally-selective visual responses. The results indicate the existence of neural activity related to establish and maintain one dominant percept, the magnitude of which is related to the ambiguity of the stimulus. This is in the line with the currently proposed idea that dominant percept is established in the distributed cortical areas including the early visual areas. Further, the existence of the neural activity induced only by the ambiguous image suggests that the competitive neural activities for the two possible percepts exist even when one dominant image is continuously perceived.

Neural processes of attentional inhibition of return traced with magnetoencephalography.

Inhibition of return (IOR) is a phenomenon that involves reaction times (RTs) to a spatially cued target that are longer than RTs to an uncued target when the interval between the cue and target is prolonged. Although numerous studies have examined IOR, no consensus has yet been reached regarding the neural mechanisms responsible for it. We used magnetoencephalography (MEG) and measured the human neural responses underlying the time course of IOR, applying a typical spatial cueing paradigm. The cue-target interval was 600+/-200 ms. Three experimental conditions were employed. Cued; the cue and target were presented at the same location. Uncued; the two stimuli were presented at opposite locations. Neutral; the cue stimulus was presented bilaterally. We found differences in the amplitudes of signals in the postero-temporal and bilateral temporal areas, and peak latencies in a central area between the cued and uncued conditions. These signals were localized to the extrastriate cortex, bilateral temporal-parietal junction (TPJ), and primary motor cortex, respectively. Bilateral TPJ activities are related to the identification of salient events in the sensory environment both within and independent of the current behavioral context and may play an important role in IOR in addition to extrastriate and the primary motor cortex.

Reaction times of movement preparation in patients with Parkinson's disease.

We studied the onset of preparatory postural responses and subsequent voluntary movements by measuring soleus muscle activities in the standing position in 20 patients with Parkinson's disease. We measured the postural response in preparing to rise on tiptoe as the onset of the premotion silent period (PMSP). Our patients showed no delay when compared with age-matched healthy controls, but did show a significant delay in the onset of voluntary movement. The elongated PMSP (increased duration of the preparatory postural adjustments) was related to the severity of bradykinesia. Results indicate that the conventional reaction time is increased in patients with Parkinson's disease, even though there is no delay of central processing for the preparation of voluntary movements, and that there is bradykinesia of involuntary postural movements.

Magnetic response of human extrastriate cortex in the detection of coherent and incoherent motion.

Although direction selectivity is a cardinal property of neurons in the visual motion detection system, movement of numerous elements without global direction (incoherent motion) has been shown to activate human and monkey visual systems, as does coherent motion which has global direction. We used magnetoencephalography to investigate the neural process underlying responses to these types of motions in the human extrastriate cortex. Both motions were created using a random dot kinematogram and four speeds (0, 0.6, 9.6 and 25 degrees /s). The visual stimuli were composed of two successive motions at different speeds; a coherent motion at a certain speed that changed to incoherent motion at another speed or vice versa. Magnetic responses to the change in motion consisted of a few components, the first of which was always largest. The peak latency of the first component was inversely related to the speed of the preceding motion, but for both motions it was not affected by the speed of the subsequent motion. For each subject, the estimated origin of the first component was always in the extrastriate cortex, and this changed with the speed of the preceding motion. For both motions, the location for the slower preceding motion was lateral to that for the faster preceding motion. Although the latency changes of the two motions differed, their overall response properties were markedly similar. These findings show that the speed of incoherent motion is represented in the human extrastriate cortex neurons to the same degree as coherent motion. We consider that the human visual system has a distinct neural mechanism to perceive random dots' motion even though they do not move in a specific direction as a whole.

Somatosensory evoked magnetic fields following passive finger movement.

The somatosensory evoked magnetic field (SEF) following passive finger movement and electrical stimulation of finger was studied in 10 normal subjects. Four main components were identified in SEFs recorded at the hemisphere contralateral to the moved finger: 1M(P), 2M(P), 3M(P) and 4M(P). The 1M(P) was clearly identified only in three subjects and was smaller than other components. The equivalent current dipoles (ECDs) of 1M(P) were located around the finger area of the primary sensorimotor cortex and oriented either posteriorly or anteriorly. We speculate that it was generated in areas 3a or 2 of the primary sensory cortex. The 2M(P) and 3M(P) were usually combined as one large deflection with two peaks. Because the ECDs of 2M(P) and 3M(P) were located around the finger area of the sensorimotor cortex and both oriented posteriorly, they were considered to be generated in area 4 and/or 3b, and their activities have temporal overlapping. The 4M(P) has large inter-individual difference in terms of amplitude and latency. The ECD of 4M(P) was also located around the finger area of the primary sensorimotor cortex, and oriented anteriorly. The 4M(PI), the main component recorded from the hemisphere ipsilateral to the moved finger, was located in the upper bank of the sylvian fissure, probably the second sensory cortex (SII). Five components, 1M(E), 2M(E), 3M(E), 4M(E) and 4M(EI), corresponding to 1M(P), 2M(P), 3M(P), 4M(P) and 4M(PI), were identified following electrical stimulation of the same finger. However, SEFs following passive movement were clearly different from SEFs following electrical stimulation, in terms of waveforms and source locations, probably due to differences of ascending fibers and receptive fields.

Random dots blinking: a new approach to elucidate the activities of the extrastriate cortex in humans.

To investigate cortical activities related to the visual recognition of characters, we recorded the magnetoencephalography (MEG) in six normal subjects who were encouraged to discriminate capital English letters displayed for a brief period. To reduce the primary responses evoked by the luminance change in the striate cortex (V1), we used a novel stimulus method, random dots blinking (RDB), by means of the temporal changes of patterns using a large number of small random dots. Along with the MEG recording, we also measured the discrimination accuracy rate (%) to know how well the subjects recognized the letters. One clear component, about 300 ms in peak latency, was identified in all six subjects. Its peak amplitude and the discrimination accuracy rate increased similarly as the character display duration became longer. Its signal source was estimated in the extrastriate cortex, around the fusiform gyrus, in the right hemisphere. We suspect that the activity in these cortical areas has strong relation to the conscious perception of characters.

Human cortical area responding to stimuli in apparent motion.

Apparent motion is the perception of the realistic smooth motion of an object which flashes first at one place then at another. To investigate human cortical responses to stimuli in apparent motion, we used a multichannel biomagnetometer to record the magnetic fields evoked by these stimuli in four normal subjects. The results showed the presence of a localized cortical area exclusively sensitive to apparent motion stimuli that is identical to that for smooth motion. In three subjects this area corresponded to the human homologue of MT/V5. Moreover, the same region in the extrastriate cortex was involved in the short range (0.1 degree) apparent motion process as well as the long range (1.0 degree) process.

Very slow oscillatory activities in lateral geniculate neurons of freely moving and anesthetized rats.

In urethane anesthetized rats many lateral geniculate neurons display a strong very slow oscillatory behavior in the range of 0.025-0.01 Hz. One of the aims of the present study was to determine whether very slow oscillatory activity in this range can also be obtained in barbiturate anesthetized and in awake animals, respectively. Although very slow oscillations were found in geniculate neurons both during awakeness and during anesthesia, significant differences in peak frequencies of oscillations under the three experimental conditions (barbiturate, urethane, awake) were demonstrated. In addition, we have tested the influence of glutamate antagonists and GABA agonists as well as antagonists on the very slow oscillatory activity in urethane anesthetized rats. Very slow oscillatory activity which could be blocked by the continuous illumination of the eyes was re-induced by iontophoresis of NMDA and non-NMDA glutamate antagonists. GABA(A) as well as GABA(B) agonists also caused a significant re-induction of very slow oscillatory activity under light conditions. In the dark, muscimol, a GABA(A) agonist, significantly enhanced the very slow oscillatory activity, i.e. muscimol either induced it or reduced the frequency of very slow oscillations. For the whole sample, GABA antagonists did not have a significant influence on the very slow oscillatory activity. Autocorrelation analysis based on the spike interval histograms and determination of the spectrum of autocorrelograms revealed the significance of periodicity.

Effect of optokinetic stimulation on human balance recovery in unexpected forward fall.

We examined whether optokinetic stimulation (OKS) affects balance recovery in an unexpected forward fall. Ten healthy male subjects participated in the study. Each was held in an initial, leaning-forward position supported by a cable connected to a strong magnet which enable unpredicted release. Instruction was given to move against forward fall by taking steps. To evaluate the relation between OKS velocities and balance recovery, ten stages of OKS velocities ranging from -100 (upward) degree/sec to +100 (downward) degree/sec were presented randomly. Balance recovery against forward fall was characterized by the reaction time, heel off, maximum vertical push, and heel contact. The latencies of these events decreased as the downward velocity of OKS increased; the latencies increased with the increment in the upward velocity of OKS. Changes in the latencies of the parameters took place in the early phase, in which the parameters of balance recovery depended little on the proprioceptive afferents from the lower limbs. This suggests that vection induced by the OKS affected the condition of the motor program which controls the initiation of the motor response. In other words, information from the visual system may modulate the condition of the motor program before the onset of movement.

Odorant evoked magnetic fields in humans.

We investigated the olfactory evoked magnetic fields (OEFs) in 14 normal subjects. Pulses of odorant air containing amyl acetate or phenethyl alcohol, and odorless air were administered to the subject through a nasal tube. A clear and consistent OEF component, 1M, was identified in all subjects, and a second component, 2M, was detected in seven subjects, but no consistent component was identified in response to the odorless air. The peak latencies of the 1M and 2M components were approximately 320 and 630 ms, respectively. The waveforms produced by the odorless air were subtracted from the waveforms produced by the odorant air to obtain the 'subtraction' waveform, which indicated the 1M and 2M component more clearly. Their equivalent current dipoles (ECDs) were estimated in the regions around the Sylvian fissure symmetrically in both hemispheres. Therefore, these areas are proposed to be involved in olfactory perception in humans.

Burst and oscillation as disparate neuronal properties.

We have developed methods to detect and discern burst and oscillatory patterns of neuronal activity. In them, a burst period is defined as an interval in which there are a significantly higher number of spikes as compared to other intervals in the spike train. Oscillation is defined as a spike train in which significant periodicity is detected in its autocorrelogram. The main feature of our burst detection method is that discharge density (i.e., the number of spikes in a short interval) is used instead of the interspike interval. This enables one to assess the likelihood of having burst periods in a spike train. We use the Lomb periodogram to detect periodicity in an autocorrelogram. This method gives one significance of periodicity detected and enables the detection of multiple frequencies in an autocorrelogram. The advantage of these methods is discussed in comparison with the other methods used to detect bursting and oscillatory activity.

Timing of motion representation in the human visual system.

Visual stimulus in apparent motion evokes a magnetic field from the extrastriate cortex in humans. To investigate what this magnetic field represents, we measured the latencies of the responses in three subjects to the stimuli in apparent motion at various spatial separations. These different latencies were inversely related to the spatial separations of the stimuli (range of 74 to 182 ms) and correlated with each subject's reaction time. The direction of motion affected neither the latency of the magnetic response nor the reaction times. Estimations of the origins of the evoked magnetic fields showed they were always in the same area. In two subjects, the sites were around the meeting point of the ascending limb of the inferior temporal sulcus and the lateral occipital sulcus. In the third subject, the site was in the vicinity of the angular gyrus. The difference between the magnetic response and reaction time was fairly constant (about 64 ms) among the subjects. We consider the magnetic response to be related to the generation of a motion image: First, the response clearly corresponded to human reaction times to the same stimuli: Second, the fact that the magnetic response was related to the spatial separations but independent of the direction of motion is not explained if the response is evoked simply by both the onset and offset of the object in the stimulus. Furthermore, individual reaction times were mainly delayed by the speed of the process that generated the motion image.

Asymmetry of the human visual field in magnetic response to apparent motion.

Predominance of the lower visual field has been shown in various visual tasks, but whether the upper visual field is involved in a specific neural process is unknown. We used magnetoencephalography to study the effect of orientation and direction on the responses of five subjects to apparent motion from the human extrastriate cortex. The first magnetic response always was the largest, and the peak latency of about 200 ms did not change with the stimulus conditions. Amplitudes of the first responses were highest when motions were oriented at the horizontal meridian, decreasing with the degree of the angle between motion orientation and the horizontal meridian. There was no difference in amplitude between the two directions in the lower visual field, whereas the value of the response to downward motion in the upper visual field was significantly larger than that to upward motion. These amplitude changes are not due to differences in the anatomical distribution of neural activities because the estimated origins for the first responses always were in the same cortical area (around the occipito-parieto-temporal region) and the directions of the current vectors did not change with the stimulus conditions, and the estimated current strength changed with the stimulus conditions as did the response amplitude. These findings suggest that the human extrastriate cortex has a directional preference for downward versus upward motion in the upper visual field.

Hypokinesia of associated movement in Parkinson's disease: a symptom in early stages of the disease.

We compared the degrees of impairment between intended voluntary movement and its simultaneous automatic associated movement in Parkinson's disease (PD). We studied wrist dorsiflexion as a movement associated with grip in 20 patients with PD and in 20 normal controls. The patients showed a significantly smaller dorsiflexion as compared with the controls. The decrease in associated movement was related to the severity of clinical stage of the disease, while gripping was performed well in each stage. The temporal relationship between grip and associated movement was the same for both groups of subjects. The patients showed no disturbance of amplitude or velocity for a single motor act of wrist dorsiflexion. Persons with PD have a greater reduction of automatic associated movement than intended voluntary movement. This may be one of the bases of clinical symptoms of PD patients in early stages of the disease.

Effects of visual and auditory stimulation on somatosensory evoked magnetic fields.

DESIGN AND METHODS: We investigated the effects of continuous visual (cartoon and random dot motion) and auditory (music) stimulation on somatosensory evoked magnetic fields (SEFs) following electrical stimulation of the median nerve on 12 normal subjects using paired t test and two way ANOVA for the statistics. RESULTS: In the hemisphere contralateral to the stimulated nerve, the middle-latency components (35-60 ms in latency) were significantly enhanced by visual, but not by auditory stimulation. The dipoles of all components within 60-70 ms following stimulation were estimated to be very close each other, around the hand area of the primary sensory cortex (SI). In the ipsilateral hemisphere, the middle-latency components (70-100 ms in latency), the dipoles of which were estimated to be in the second sensory cortex (SII), were markedly decreased in amplitude by both the visual and auditory stimulation. CONCLUSIONS: These changes in waveform by visual and auditory stimulation are thought to be due to the effects of the activation of polymodal neurons, which receive not only somatosensory but also visual and/or auditory inputs, in areas 5 and/or 7 as well as in the medial superior temporal region (MST) and superior temporal sulcus (STS), although a change of attention might also be a factor causing such findings.

Perception of apparent motion is related to the neural activity in the human extrastriate cortex as measured by magnetoencephalography.

To determine the neural correlate of apparent motion perception, we measured magnetic responses to visual stimuli in apparent motion and compared the results with subjective rating of the quality of perceived motion with varied stimulus timing. The latency of the magnetic response was about 150 ms, and its origin was estimated to be in the occipito-parieto-temporal junction. The strength of the first component in the response varied with the stimulus timing, the maximum value being at the interval 0. The change could not be explained by the simple summation of onset and offset responses and this value was related to the subjective rating of quality (smoothness) of motion measured of the stimulus. Results indicate there is a localized cortical region of neural activity which is closely related to the subjective assessment of quality of perceived motion.

A first comparison of the human multifocal visual evoked magnetic field and visual evoked potential.

Our objectives were to determine the feasibility of recording reliable multifocal visual evoked magnetic fields (mfVEFs), to investigate the maximum stimulus eccentricity for which the mfVEF responses can be obtained, and to study how this changes with checksize (spatial frequency tuning). Using a checksize of 30', we recorded 8-channel pattern-onset mfVEFs three times to obtain responses from 19 channels located around the inion. Multifocal visual evoked potentials (mfVEPs) were recorded under the same conditions. Eccentricity changes with spatial frequency were studied using checksizes from 7.5' to 60'. We obtained, for the first time, reliable mfVEFs, and found they could be elicited from more peripheral stimulus elements than could mfVEPs. The larger the checksize, the greater the eccentricity reached.

Transfer index of MR relaxation enhancer: a quantitative evaluation of MR contrast enhancement.

To clarify the effects of Gd-DTPA on biological water, we examined the effects of the compound on spin-lattice relaxation rate with various concentrations of gelatin solutions. The results indicate that the effects of relaxation rate of Gd-DTPA in biological water fundamentally correspond to those in aqueous solution. To evaluate the distribution of Gd-DTPA in tissues, we introduced a transfer index that represents the product of tissue-blood ratio of Gd-DTPA and the ratio of extracellular volume of a tissue based on the above findings. The index depends neither on dose of the compound nor on Larmor frequency. The clinical significance of the index was studied in patients with brain tumors. The indexes varied from 0.038 to 0.51, depending on the biological characteristics of the tumors. The transfer index may be used in the quantitative evaluation of MR relaxation enhancement, which may be applied to monitoring therapeutic efficacy and to estimating tissue perfusion.

Microelectrode-guided pallidotomy: technical approach and its application in medically intractable Parkinson's disease.

OBJECT: The authors describe the microelectrode recording and stimulation techniques used for localizing the caudal sensorimotor portion of the globus pallidus internus (GPi) and nearby structures (internal capsule and optic tract) in patients undergoing GPi pallidotomy. METHODS: Localization is achieved by developing a topographic map of the abovementioned structures based on the physiological characteristics of neurons in the basal ganglia and the microexcitable properties of the internal capsule and optic tract. The location of the caudal GPi can be determined by "form fitting" the physiological map on relevant planes of a stereotactic atlas. A sensorimotor map can be developed by assessing neuronal responses to passive manipulation or active movement of the limbs and orofacial structures. The internal capsule and optic tract, respectively, can be identified by the presence of stimulation-evoked movement or the patient's report of flashes or speckles of light that occur coincident with stimulation. The optic tract may also be located by identifying the neural response to flashes of light. The anatomical/physiological map is used to guide lesion placement within the sensorimotor portion of the pallidum while sparing nearby structures, for example, the external globus pallidus, nucleus basalis, optic tract, and internal capsule. The lesion location and size predicted by using physiological recording together with thin-slice high-resolution magnetic resonance imaging reconstructions of the lesion were confirmed in one patient on histological studies. CONCLUSIONS: These data provide important information concerning target identification for ablative or deep brain stimulation procedures in idiopathic Parkinson's disease and other movement disorders.

Spin-lattice relaxation times of bound water--its determination and implications for tissue discrimination.

Measurements were made of T1 of bound water (T1b) and bound water fraction (alpha) for gelatin solutions and human tissues (sera, brain tumor, cerebral white matter). Bound water fraction in each sample was measured by means of thermal analysis (differential scanning calorimetry: DSC). T1 values were measured by FONAR QED 80-alpha. T1b values were calculated by an equation derived from the fast-exchange two-state model. In the study of gelatin solutions, the relationship between T1 and water content differed depending on the sort of solutions. This was considered to be due to differences in T1b values. In each biological tissue the values of T1b and alpha had different distributions. These results indicate that values of T1b and alpha for biological tissues may be altered in correspondence to the changes in pathophysiological states in those tissues.