A. Illustration of the hypothesized e↵ect of SF adaptation on pRF size. Low and high SF adaptation decreases the responsiveness (indicated by paler green circles) of neurons having respectively large and small receptive fields, and the consequent pRF size estimations (orange circles) reflect the aggregate of the neurons that are not a↵ected by the adapter (green circles). B. Stimulus sequence of a single trial in the behavioural experiment. The adaptation phase included an adapter stimulus with high or low SF, either in the right or left visual hemifield (only present in adaptation conditions). The test phase consisted of a mid-SF reference stimulus on the adapted side, and a test stimulus on the non-adapted side, which varied in SF, subject to two interleaved adaptive staircases. An empty display was shown until the participant responded. Participants were required to maintain their fixation on the dot at the center at all times and judge the relative SF of the two stimuli shown in the test phase. SF: spatial frequency.

A. Psychometric functions for a representative participant. The x-axis represents the log-transformed SF ratio (test/reference SF), where negative values indicate a decrease and positive values indicate an increase in perceived SF. The y-axis represents the proportion of trials participants perceived the test stimuli as having a higher SF. B. Log transformed SF ratios (perceived/actual) of all participants across three adaptation conditions. Coloured circles represent the average of left and right adaptation conditions for each participant, and the gray diamonds show the group average. Positive values on the y-axis denote perceptual overestimation of SF, while negatives denote the underestimation of SF. Un-shaded circles indicate the participants who were not included in the fMRI arm of the study. Circle sizes are proportional to the number of sessions completed. SF: spatial frequency.

Stimulus sequence of a pRF mapping run.

Scans started with a baseline period, showing an empty gray display. This was followed by the initial adaptation period. Adaptation consisted of two noise stimuli on both visual hemifields, having di↵erent SFs (in this case, high SF was on the left and low SF was on the right, e.g. HL configuration). During the mapping sequence, mid-SF bars swept the screen step by step, in eight directions. Two blank phases were presented in the middle of eight bar directions and also at the end of the mapping sequence. Each mapping component (bars and blanks) was preceded by a short top-up adaptation stimulus, consistently in all volumes throughout the mapping phase. Finally, the runs were concluded with another baseline. All noise stimuli were refreshed at 20 Hz, and a central fixation point persisted throughout. Participants pressed a key upon a color change in the fixation.

Polar angle (A), eccentricity (B), and pRF size (C) maps for the left hemisphere of representative participant, on a spherical model of the gray-white matter boundary (see online data repository for other participants).

The maps were plotted after the data were denoised and nR2 thresholded, so that only the vertices used in the analyses would be shown (see text for thresholding details). The first row shows the maps from high-SF adapted condition, and the second row shows those from low-SF adapted condition; annotated as High and Low, respectively. Color charts of each map type are shown on the bottom row. Polar angle maps in column A show the angle from the fixation point. e.g. orange represents the vertical center of the upper visual hemifield. The eccentricity maps in column B show the distance from the fixation point, from 0○ to 8○ of visual angle, represented by the range of colors from black to white (inner to outer parts of the eccentricity color wheel). pRF size maps in column C show the standard deviation (sigma) values of the Gaussian pRF profile. Blue indicates the pRF sizes that are closer to zero, and yellow shows pRF sizes greater than or equal to 2○ of visual angle. White lines on the maps represent the borders of visual ROIs, delineated using the average of the two conditions. The ROI labels are shown only in the first row of A for simplicity.

A. Median pRF sizes (y-axes) were plotted against low and high SF adaptation conditions (x-axes) for all ROIs (separate plots). Colored circles represent each participant. Filled circles indicate a significant participant-wise median comparison, with FDR corrected p < 0.05. Gray diamonds show the group means. B. Median of eccentricity binned pRF size di↵erences, averaged across participants. pRF size di↵erences were calculated by subtracting the pRF sizes of the low SF adapted condition from those of high SF (H-L). Positive and negative values on the y-axis are related to respectively larger and smaller pRF sizes in the high SF adapted condition. The average of the two conditions was used for eccentricity binning. The shaded area represents the standard error of the mean between participants. The data illustrated in both panels were first denoised and thresholded as described in the text. Note di↵erent scales between regions.

A. Stimulus sequence of a single trial in the control experiment. The adaptation phase is exactly the same as that in the main experiment. In the test phase, the adapted visual hemifield always included a mid-SF, maximum-contrast (100%) stimulus. The stimulus on the non-adapted visual hemifield had a mid-SF; but varied in contrast, subject to two interleaved adaptive staircases. In the response phase, participants were required to indicate the stimulus with relatively high contrast. This sequence was repeated 60 times for each measurement. B. Perceived contrast as a function of SF adaptation condition. Coloured circles represent the average log-transformed PSE values of each participant; gray diamonds indicate the participant average. SF: spatial frequency.

pRF size comparison between the two adaptation conditions for all ROIs (columns) and each participant (rows).

Dots represent the pRF size, σ, of each vertex as observed in the high SF adapter condition (y axes) and the low SF adapter condition (x axes). Dots were colored based on 2D kernel density estimates. Note that the data represented here is only from the voxels that survived the denoising and nR2 thresholding in both conditions. Note di↵erent scales in plots.)

Log-transformed pRF size ratio between high and low SF adapted conditions (log of H/L) for left and right hemispheres of Participant 002 (same participant as Figure 4).

Hot colors indicate positive numbers, which represent larger pRF sizes in high SF adapted condition as compared to the low SF adapted condition; cold colors indicate the opposite. The brightness of the colors represents the magnitude of the ratios. The brightest pink (or blue) indicates pRF sizes that were at least four times as large in the high (or low) SF condition (loge4 ⇡ 1.4). White lines show the borders of the ROIs. Maps only show the vertices where the average of two conditions’ normalized goodness of fit was above 0.2, (nR2 + nR2)/2 > 0.2. Transparency changes with respect to the proportion of the average nR2. Vertices shown with opaque colors have the highest average nR2 values. H: High; L:Low

A. Time series of percent signal change for two adaptation conditions in each ROI, averaged across eight participants. Regions were annotated on the right side of each panel. Red and blue lines respectively represent high and low SF adapted conditions. The shaded regions indicate ±1 standard error between the participants. The vertical dashed lines, respectively from left to right, indicate the end of the initial baseline period (volume 25), the end of the initial adaptation period (volume 55), and the start of the final baseline period (volume 305). The volumes between the second and third dashed lines correspond to the mapping sequence. B. Median of eccentricity binned noise ceiling values, averaged across participants. The average of the two conditions was used for eccentricity binning. The shaded area represents the standard error of the mean between participants.

A. Median of eccentricity binned pRF size di↵erences, averaged across participants, using the same set of vertices in two conditions. B. Median of eccentricity binned sigma values for both high and low SF adaptation conditions, averaged across participants. (Note di↵erent scales between visual regions in panels A and B.) C. Median of eccentricity binned eccentricity values, averaged across participants. For all panels, the average of the two adaptation conditions was used for eccentricity selection (x-axes). The shaded regions represent the standard error of the mean between participants.

Log-transformed and bias-corrected SF ratios (perceived/actual SF) were plotted against the adaptation conditions, separately for left and right visual fields.

Positive (negative) values of y-axis indicate an increase (decrease) in perceived SF and zero means no perceptual deviation from the actual SF

Distance between the eye position and the fixation point (left) and pupil size (right) plotted against the two adaptation conditions.

Colored circles represent each participant and gray diamonds show their mean. The exclamation marks on the legend items denote the participants who were excluded from the statistical analyses.