| Summary: | 碩士 === 國立陽明大學 === 醫學工程研究所 === 95 === Psychophysical and perceptual studies with human subjects have shown that visual acuity in terms of sensitivity and discriminability is better when the details of stimuli are oriented horizontally or vertically than when they are placed obliquely. This phenomenon is called the “oblique effect” .
In this study, we aimed to see differential activation of visual cortex in response to grating images with varying orientations and spatial frequencies. Ten healthy volunteers (5 men, 5 women; age 24-37 years) participated in the study after giving informed consent. We recorded their magnetoencephalographic (MEG) responses to grating stimuli that were horizontal, vertical, 45 oblique, and 135 oblique in orientation. For each orientation condition, the spatial frequencies of achromatic square-wave gratings varied between 0.5 – 12 cpd. The activation sources were analyzed with equivalent current dipole (ECD) modeling and then were coregistered with magnetic resonance images (MRI) of the subject’s own brain. In separate sessions, we also evaluated their subjective contrast sensitivity to the 4 different orientation conditions.
The result showed the latency of the magnetic response increased with increasing spatial frequency and longer latencies were found for oblique grating stimuli than horizontal and vertical ones (p<0.05). The magnetic filed strength tuning functions of the spatial frequency for four different orientation gratings had inverse-U shaped bandpass characteristics, peaking at 3-6 cpd. The CSF(contrast sensitivity function) showed low and high spatial frequency attenuation with peaking at 4-6 cpd which were similar with magnetic field strength tuning function (0.71< r <0.992). The ECD of P100m signal coregistered with MRI data revealed the visual area which the principle evoked cortical activity originates from were all near the calcarine sulcus which is in the primary visual cortex. The alteration of the visual evoked magnetic fields characteristics caused by different orientations and spatial frequencies gratings was in good agreement with the oblique effect observed in psychophysical studies.
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