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Long Luu, Mingsha Zhang, Misha Tsodyks, Ning Qian; Cross-fixation interactions of orientations suggest that orientation decoding occurs in a high-level area of visual working memory. Journal of Vision 2020;20(11):216. doi: https://doi.org/10.1167/jov.20.11.216.
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Visual encoding (how stimuli evoke neuronal responses) is known to progress from low to high levels. Decoding (how responses lead to perception), in contrast, is less understood but is widely assumed to follow the same, low-to-high-level hierarchy. According to this assumption, orientation decoding must occur in low-level areas such as V1, and consequently, two orientations on opposite sides of the fixation should not interact with each other perceptually. However, Ding et al (PNAS, 2017) provided evidence against the assumption and proposed that visual decoding may follow the opposite, high-to-low-level hierarchy in working memory. If two orientations on opposite sides of the fixation are both task relevant and enter a high-level working-memory area in a delay period, then they should interact with each other. We tested this prediction.
Subjects maintained central fixation when two lines with an orientation difference of 5° were flashed on opposite sides of the fixation, with a center-to-center distance of 16° of visual angle. Their eye positions were monitored with an infrared eye tracker and trials with broken fixation were aborted. After a delay, subjects reported the two orientations by drawing and adjusting an indicator line at the fixation. In one condition, the indicator line disappeared after the first report, and was redrawn for the second report, to minimize potential interference. We found that the two lines showed a large and significant repulsion between them, demonstrating the predicted cross-fixation interactions in working memory. The pattern was consistent across 14 subjects. Control conditions and analyses ruled out alternative explanations such as interactions across trials on the same side of the fixation. Moreover, we quantitatively accounted for the repulsion with the retrospective Bayesian decoding model in Ding et al. We conclude that our results support the theory that visual perception may be viewed as high-to-low-level decoding in working memory.
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