Elsevier

Neuroscience Research

Volume 46, Issue 4, August 2003, Pages 423-433
Neuroscience Research

Human V5 demonstrated by magnetoencephalography using random dot kinematograms of different coherence levels

https://doi.org/10.1016/S0168-0102(03)00119-6Get rights and content

Abstract

To investigate the cortical mechanisms for motion perception in human V5, we measured visual evoked magnetic fields in response to random dot kinematograms (RDKs) of three different coherence levels (50, 70 and 100%) using a 122-channel whole-head magnetometer. As the coherence level increased, the peak amplitude measured by the root mean square (RMS) of the local response increased significantly (7.4±1.0, 9.5±1.5 and 15.5±3.2 fT/cm on the right, 6.4±0.3, 7.8±0.7 and 12.5±0.9 fT/cm on the left; for the coherence level of 50, 70 and 100%, respectively). There was no significant difference between the hemispheres. As for the peak latency, there was no significant difference in terms of coherence levels or hemispheres. The response was localized posterior to the junction of the ascending limb of the inferior temporal and lateral occipital sulci (human V5). These findings indicate that processing of global motion in terms of the synchronized portion correlates well with the response amplitude but not with its latency. Thus, we could estimate the magnetic responses of human V5 non-invasively by presenting different coherence levels of the visual motion stimuli. Hemispheric laterality was recognized, although the dominant side varied among subjects.

Introduction

To understand the cortical mechanisms of human motion perception, it is important to detect response variables specific to the characteristics of visual stimuli. It has been widely accepted that human V5 is activated by moving stimuli (Zeki et al., 1991, Sunaert et al., 1999, Ahlfors et al., 1999). Human V5 is located posterior to the junction of the ascending limb of the inferior temporal and lateral occipital sulci (Zeki et al., 1991) and contains a high concentration of neurons that are sensitive to the speed and orientation of motion stimulus (Zeki, 1978, Baker et al., 1981, Maunsell and Van Essen, 1983, Albright, 1984). Human V5 can detect the direction of movement of many dots in which only a small portion moves coherently, while the remaining portion moves incoherently (Baker et al., 1991, Newsome and Pare, 1988). In this case, the motion of each dot must first be detected in the local area then integrated in a large spatial scale (Williams and Sekuler, 1984). Thus, random dot kinematograms (RDKs) can be appropriate stimuli for eliciting this kind of perception. As for the orientation of stimulus motion, there are a few studies that investigated the relationship between brain responses and coherence levels of the motion using functional magnetic resonance imaging (fMRI) or visual evoked potential (VEP) (Heeger et al., 2000, Rees et al., 2000, Niedeggen and Wist, 1999). However, these data might not have provided sufficient information due to the limitation in terms of temporal or spatial resolution. However, magnetoencephalography (MEG) has a high spatial and temporal resolution on the order of mm and ms, respectively. It also allows the tracking of weak evoked responses from more concentrated brain regions compared with the EEG technique, since the skull is transparent to magnetic fields but not to electric fields (Malmivuo et al., 1997). Despite these advantages, there are few MEG studies that investigated the correlation between brain response and coherence levels. Therefore, our present experiment used changing coherence levels to more systematically investigate the effects of coherent motion on transient evoked cortical responses in humans.

Section snippets

Subjects

Eight healthy right-handed subjects (six men and two women) with no ophthalmologic or neurological abnormality participated in this study. Handedness was evaluated by the Edinburgh Handedness Inventory (mean±S.D., LQ=0.96±0.077). Their ages ranged from 22 to 35 (mean±S.D., 30.4±5.7) years. All subjects had normal or corrected-to-normal visual acuity at the time of this study. Informed consent was obtained from each subject after a full explanation of the experiment.

Visual stimulation

We used RDKs for visual

Waveforms of magnetic fields evoked by coherent dot motion

Fig. 2A shows magnetic responses of one subject (S2) induced by horizontal coherent motion with 100% coherence level. A mono phasic peak was recognized over the bilateral temporo-parietal area and the middle occipital area. Fig. 2B shows the largest channel from both hemispheres and represents the magnetic responses at three coherence levels (50, 70, 100%). The peak was 20.8 fT/cm in amplitude at 244 ms for 50% coherence level, 21.6 fT/cm at 229 ms for 70% and 42.9 fT/cm at 211 ms for 100% in

Discussion

We investigated visual human motion perception with MEG by presenting dot motion at three different coherence levels. Since the first ECD locations were estimated in a consistent area within the extrastriate region of each hemisphere irrespective of coherence levels and no additional ECDs were detected in this area, the use of RMS over the temporal area was appropriate to represent local neuronal activity for intra-subject comparisons. Therefore, we could compare cortical activities even when

Acknowledgements

This study was partly supported by Grants-in-Aids for Scientific Research on Priority Area (C) (Advanced Brain Science) 12210012 from the Japan Ministry of Education, Culture, Sports, Science and Technology.

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