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Yan Song, Bin Yang, Fang Wang, Xiaoli Ma; Neural Mechanism of Inverse Oblique Effect on Broad-band Noise Stimuli: An ERP Study. Journal of Vision 2010;10(7):929. doi: 10.1167/10.7.929.
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When resolution acuity or contrast sensitivity is evaluated using simple stimuli such as lines or gratings, visual performance is often best for horizontal and vertical orientations and worst for oblique orientations. It's the well known ‘oblique effect’. However, using more complex stimuli of broad-band spatial frequencies and orientations, Essock et al. found an inverse oblique effect, which visual performance is worst for horizontal orientation and best for oblique orientations. The anisotropy in the number of cortical neurons tuned to different orientations in visual cortex has accounted for the oblique effect by previous fMRI and physiological studies. But the neural mechanism of this inverse oblique effect remains largely unknown. In the present study, seventeen subjects were first tested for orientation salience threshold before they were recorded EEGs. Thresholds were highest for horizontal orientation and lowest for oblique orientations, which was consistent with Essock et al.'s work. Then, we recorded high resolution electroencephalography from the whole-scalp sensor array while subjects took part in an orientation identification task, in which orientation salience was 1.2~2.5 times of the threshold of horizontal orientation. We found the response accuracies were lower and the response times were longer for cardinal orientations than for oblique orientations. The event-related potential results revealed that the difference between cardinal orientations and oblique orientations occurred around 200 ms post stimulus onset, which was much later than the traditional oblique effect. Besides, the P300 latency was much earlier for oblique orientations than for cardinal orientations. These findings indicated that this inverse oblique effect of broad-band noise stimuli might occur at relatively higher levels of visual information processing and might involve more complex neural mechanism than oblique effect.
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