The Perceived Size of a Part Changes When it is Separated from the Whole

This study investigates the visual phenomenon of increasing stimulus size [Bertulis & Bielevicius. Expansion of perceived size. J. Perception, 2025]. In psychophysical experiments, observers adjusted the test distance to match the length of the entire stimulus— the pentagon—and its components: the apex and the rectangular segment, which can be called the pedestal, both when separated and combined. The shape of the pentagon changed during presentations due to three simultaneous variables: a) the length of the apex gradually increased from 30 to 180 arc minutes, b) the horizontal edges of the pentagon decreased from 150 to 0 arc minutes; and c) the apex angle narrowed from 100 to 23 degrees, while the pentagon’s length and height remained constant at 180 and 72 arc minutes, respectively. Based on the averages of the experimental results, the pentagonal-shaped stimulus demonstrated an expansion effect, which, due to shape changes, ranged from 2 to 19 arc minutes. For the isolated apex with increased length and narrowed opening angle, expansion values were higher and, ranging from 10 to 33 arc minutes, showed a consistent upward trend without reaching a peak. In the pentagon’s structure, the apex with similar shape and size dynamics displayed a different expansion profile. The expansion value gradually increased from about 9 to 35 arc minutes (its maximum), then decreased to 30 and 19 arc minutes. Similarly, the pedestal displayed different patterns when viewed as a separate object and as part of the pentagon. The isolated pedestals showed expansion (7 to 30 arc min) throughout. Inside the pentagon, narrow pedestals (30, 60 arc min) did not even exhibit expansion and instead showed a negative error sign. Wider pedestals produced relatively weak expansion, peaking at 8 arc minutes before decreasing to 1.5 arc minutes. The combined curves of the isolated apex and pedestal did not match the experimental curve of the whole pentagon. Based on the data, the perceived shape seems to be a key factor in determining perceived size and, therefore, the expansion effect. During the visual size-matching process, the spatial proportions within the perceived shape are likely to change when combined with another shape, even though the geometry remains unchanged. The representation of a shape (such as a pentagon) in higher neural networks is not simply a sum of its parts’ representations (apex and pedestal).