Optimum stimulus size for the human brain to respond to motion: a magnetoencephalographic study.

OBJECTIVE: To determine if there is an optimum spatial extent for the detection of moving objects in humans. METHODS: We investigated human brain responses to motion at various speeds (2.9-23.5 deg/s) and stimulus sizes (2.2 × 2.9 deg to 44.8 × 57.4 deg) using magnetoencephalography. The results were compared with those for the flickers with the same stimulus sizes and temporal frequencies. RESULTS: For every size, response latency was inversely related to speed. Further, the latency was lowest at a stimulus size of 16.8 × 22.4 deg for every speed. Although response latency was inversely related to the temporal frequency of the flicker stimulation for all stimulus sizes, it was not affected by stimulus size as much as motion stimulus. CONCLUSIONS: For visual motion detection, the most efficient stimulus size is around 16.8 × 22.4 deg. SIGNIFICANCE: The results suggest that spatial summation mechanism is important for the detection of visual motion but a loss of reference frame information affects the detection of larger motion stimuli, supporting the view that illusory self-motion (vection) is caused by poor reference frame information for motion detection.

Changes in regional blood flow induced by unilateral subthalamic nucleus stimulation in patients with Parkinson's disease.

Changes in regional cerebral blood flow (rCBF) induced by unilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) were investigated in 7 consecutive patients with Parkinson's disease, 4 men and 3 women (mean age 62.3 +/- 8.1 years), who underwent rCBF measurement by N-isopropyl-p-(iodine-123)-iodoamphetamine single photon emission computed tomography at rest before and after unilateral STN DBS preoperatively in the on-drug condition, and postoperatively in the on-drug and on-stimulation condition. Statistical parametric mapping was used to identify significant changes in rCBF from the preoperative to the postoperative conditions. rCBF was increased in the bilateral cingulate cortices and bilateral cerebellar hemispheres. rCBF was decreased in the bilateral medial frontal cortices and left superior temporal cortex. Unilateral STN DBS produced rCBF changes in the bilateral cingulate cortices, cerebellar hemispheres, and medial frontal cortices. These findings indicate that unilateral STN DBS affects rCBF in both hemispheres.

Temporal dynamics of adaptation to natural sounds in the human auditory cortex.

We aimed at testing the cortical representation of complex natural sounds within auditory cortex by conducting 2 human magnetoencephalography experiments. To this end, we employed an adaptation paradigm and presented subjects with pairs of complex stimuli, namely, animal vocalizations and spectrally matched noise. In Experiment 1, we presented stimulus pairs of same or different animal vocalizations and same or different noise. Our results suggest a 2-step process of adaptation effects: first, we observed a general item-unspecific reduction of the N1m peak amplitude at 100 ms, followed by an item-specific amplitude reduction of the P2m component at 200 ms after stimulus onset for both animal vocalizations and noise. Multiple dipole source modeling revealed the right lateral Heschl's gyrus and the bilateral superior temporal gyrus as sites of adaptation. In Experiment 2, we tested for cross-adaptation between animal vocalizations and spectrally matched noise sounds, by presenting pairs of an animal vocalization and its corresponding or a different noise sound. We observed cross-adaptation effects for the P2m component within bilateral superior temporal gyrus. Thus, our results suggest selectivity of the evoked magnetic field at 200 ms after stimulus onset in nonprimary auditory cortex for the spectral fine structure of complex sounds rather than their temporal dynamics.

Human brain activation in response to olfactory stimulation by intravenous administration of odorants.

To identify the BOLD effects related to olfaction in humans, we recorded functional magnetic resonance imaging (fMRI) scans in response intravenously instilled thiamine propyl disulfide (TPD) and thiamine tetrahydrofurfuryl disulfide monohydrochloride (TTFD). TPD and TTFD evoked a strong and weak odor sensation, respectively. Since we did not spray the odor stimuli directly, this method is expected to reduce the effect caused by direct stimulation of the trigeminal nerve. For the analysis of fMRI data, statistical parametric mapping (SPM2) was employed and the areas significantly activated during olfactory processing were located. Both strong and weak odorants induced brain activities mainly in the orbitofrontal gyrus (Brodmann's area: BA 11) in the left hemisphere. TPD (a strong odorant) induced activity in the subthalamic nucleus in the left hemisphere and the precentral gyrus (BA 6) and insula in the right hemisphere. TTFD (a weak odorant) induced activity in the superior frontal gyrus (BA 11) in the right hemisphere. In both circumstances, there was an increase in blood flow at the secondary olfactory cortex (SOC) but not the primary olfactory cortex (POC), probably due to a habituation effect in the POC. From the present results, we found brain activity in not only odor-specific regions but also regions whose levels of activity were changed by an intensity difference of odor stimuli.

Long-term stimulation of the subthalamic nucleus in hemiparkinsonian rats: neuroprotection of dopaminergic neurons.

OBJECT: The goal of this study was to evaluate the neuroprotective effects conferred by long-term electrical stimulation of the subthalamic nucleus (STN) against degeneration of dopaminergic neurons by assessing motor functional and immunohistological findings in hemiparkinsonian rats. METHODS: In 13 of 25 rats, a concentric microelectrode was stereotactically implanted into the right STN under the guidance of extracellular microelectrode recording. After this had been done the animals were given an injection of 6-hydroxydopamine (6-OHDA) into the right striatum. Seven of the rats received continuous stimulation (frequency 130 Hz, intensity 80-100 microA) for 2 weeks (Group A); the other six did not receive any stimulation during this period (Group B). Twelve rats did not receive electrode implantation and underwent 6-OHDA injection only; these animals served as a control group (Group C). After 2 weeks, motor function in the rats was evaluated by conducting an amphetamine-induced rotation test. Finally, tyrosine hydroxylase-immunoreactive neurons in the pars compacta of the substantia nigra (SNc) were counted to evaluate the extent of degeneration of dopaminergic neurons. Ipsilateral rotation was significantly decreased in Group A, regardless of the effects of stimulation delivered during the test (p < 0.05). Rats in Group B demonstrated typical circling as did those in Group C, except that on stimulation Group B rats immediately stopped circling or changed direction. Tyrosine hydroxylase-immunoreactive neurons in the SNc were significantly preserved in the animals in Group A, whereas neurons in animals in Groups B and C were moderately depleted (p < 0.01). CONCLUSIONS: Acutely, STN stimulation improved rotation symmetry in rats with moderate SNc degeneration. When STN stimulation had been applied for the preceding 2 weeks, motor function was better and SNc neural degeneration was significantly milder. Subthalamic nucleus stimulation thus appears to protect dopaminergic neurons in this hemiparkinsonian model, in addition to improving motor function in these animals.