Size discrimination and V1 surface area.

<p>A. Individual discrimination slopes plotted against the surface area of contralateral V1 (expressed as percentage of the cortical hemisphere). Blue: small inducers. Red: large inducers. B. Individual discrimination slopes for the control stimulus (without any inducers) plotted against contralateral V1 surface area. In both plots, left and right pointing triangles denote measurements from left and right visual hemifields in individual participants, respectively. Solid lines are linear regression fits.</p

Illusion strength and V1 surface area.

<p>A. Individual illusion strengths plotted against the surface area of contralateral V1 (expressed as percentage of the cortical hemisphere). Blue: small inducers. Red: large inducers. B. Bias in size perception for the control stimulus (without any inducers) plotted against contralateral V1 surface area. C. “Classical illusion index” calculated from the illusion strengths for small and large inducers plotted against contralateral V1 surface area. In all plots, left and right pointing triangles denote measurements from left and right visual hemifields in individual participants, respectively. Solid lines are linear regression fits.</p

The Ebbinghaus illusion.

<p>A. In the classical form of the illusion two identical circles are surrounded by smaller (left) or larger (right) inducers. This causes a perceived difference in the size of the two central circles. B–D. Example stimuli (all without any physical size difference between test and reference) for the three stimulus conditions. Participants fixated the small white dot while two circles were shown to the left or right of fixation. One circle (the left in all these examples) was the reference and always remained constant. The other circle was the test stimulus and could either be surrounded by small inducers (B), large inducers (C), or no inducers (D). The hemifield where the test stimulus appeared was pseudo-randomized and counterbalanced for each participant. E. The size of the test stimulus varied between 9 different test/reference size ratios on a logarithmic scale (shown here schematically).</p

Anatomical definition of V1.

<p>A. Left: Retinotopic polar angle maps from a typical participant shown on an inflated reconstruction of the grey-white matter boundary. Shades of gray indicate gyri and sulci. The colour indicate polar angle coordinates of visual field positions mapped onto the cortex. Right: The probability (p>0.7) that occipital vertices fall within aV1 as determined by anatomical criteria <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060550#pone.0060550-Hinds1" target="_blank">[19]</a>. The overlaid black line denotes the boundaries of V1 delineated functionally through retinotopic mapping. B. Individual illusion strengths plotted against the surface area of contralateral aV1 (expressed as percentage of the cortical hemisphere). Blue: small inducers. Red: large inducers. C. Anatomical aV1 surface areas plotted against retinotopic V1 surface areas. In both plots, left and right pointing triangles denote measurements from left and right visual hemifields/cortical hemispheres in individual participants, respectively. Solid lines are linear regression fits.</p

Utrocular discrimination outside the scanner and inside the scanner.

<p>Behavioral accuracy for judging which eye saw the grating stimulus is plotted for the low and high spatial frequency. Grey lines and open circles denote the performance for individual participants. Black line and solid circles denote the mean across participants (error bars depict standard error of the mean).</p

Illustration of the stimuli used in the experiment.

<p>Participants used free fusion to view the two rings comprising random line patterns on each side of the screen. On separate trials, we either stimulated the left or right eye, with a low or high spatial frequency grating presented inside the ring. The gratings flickered by reversing contrast polarity. In each fMRI run each stimulus condition was shown only once (as depicted here, presented in a pseudo-randomized order) and each trial lasted 19.2 s. In the behavioral experiments outside the scanner trials lasted 350 ms and 624 trials were presented per run. Stimulation trials were interleaved with fixation periods in which only the fusion rings and fixation crosses were presented.</p

Decoding stimulus eye-of-origin for the low spatial frequency stimulus (A) and the high spatial frequency stimulus (B).

<p>Decoding accuracy (proportion correct) obtained for the 100 most discriminative voxels is plotted for the three ROIs averaged across participants. Error bars depict standard error of the mean. The asterisks indicate that accuracy was significantly above chance (permutation test, p<0.01, Bonferroni corrected).</p

Mass-univariate response per condition.

<p>A–B. The signal change (z-score) averaged across the group of participants plotted for each ROI and stimulus condition. Data are grouped by anatomical eye (A) or by eye dominance (B). Error bars depict standard error of the mean. C–D. Univariate decoding stimulus eye-of-origin for the low spatial frequency stimulus (C) and the high spatial frequency stimulus (D). Decoding accuracy (proportion correct) obtained for the mean of the 100 most active voxels is plotted for the three ROIs averaged across participants. Error bars depict standard error of the mean. The asterisks indicate that accuracy was significantly above chance (permutation test, p<0.01, Bonferroni corrected).</p

Learning effects in experiment 2.

<p>A. The effect training had on orientation discrimination in experiment 2 was quantified as the change in orientation discrimination sensitivity between test sessions 1 and 5 (the training effect). This was calculated by subtracting the slopes for the pre-test (session 1) from the slopes for the post-test (session 5) for each participant and then calculating the average across all participants. The training effect was significantly greater around the trained than the untrained reference orientations. B. Tilt illusion magnitude (with surround) and perceptual biases (without surround) were estimated by extrapolating the PSE from the psychometric curve fitted to the test sessions. We calculated the difference in PSE between session 1 and session 5 by subtracting the PSE for the pre-test (session 1) from the PSE for the post-test (session 5) and averaging these differences across all participants. Critically, illusion magnitude (which was bias corrected by subtracting the PSE without surround for each participant) did not change significantly between session 1 and session 5 at the trained or untrained conditions. In both panels error bars indicate ±1 standard error of the mean across participants. The insets illustrate the stimulus conditions but note that which orientation was trained was counterbalanced across participants.</p

Learning effects in experiment 1.

<p>A. The effect training had on orientation discrimination in experiment 1 was quantified as the change in orientation discrimination sensitivity between test sessions 1 and 5 (the training effect). This was calculated by subtracting the slopes for the pre-test (session 1) from the slopes for the post-test (session 5) for each participant and then calculating the average across all participants. The training effect was significantly greater around the trained than the untrained reference orientations. B. Tilt illusion magnitude (with surround) and perceptual biases (without surround) were estimated by extrapolating the PSE from the psychometric curve fitted to the test sessions. We calculated the difference in PSE between session 1 and session 5 by subtracting the PSE for the pre-test (session 1) from the PSE for the post-test (session 5) and averaging these differences across all participants. Critically, illusion magnitude (which was bias corrected by subtracting the PSE without surround for each participant) did not change significantly between session 1 and session 5 at the trained or untrained conditions. In both panels error bars indicate ±1 standard error of the mean across participants. The insets illustrate the stimulus conditions but note that which orientation was trained was counterbalanced across participants.</p