Unless we fixate directly on it, it is hard to see an object among other objects. This breakdown in object recognition, called crowding, severely limits peripheral vision. The effect is more severe when objects are more similar. When observers mistake the identity of a target among flanker objects, they often report a flanker. Many have taken these flanker reports as evidence of internal substitution of the target by a flanker. Here, we ask observers to identify a target letter presented in between one similar and one dissimilar flanker letter. Simple substitution takes in only one letter, which is often the target but, by unwitting mistake, is sometimes a flanker. The opposite of substitution is pooling, which takes in more than one letter. Having taken only one letter, the substitution process knows only its identity, not its similarity to the target. Thus, it must report similar and dissimilar flankers equally often. Contrary to this prediction, the similar flanker is reported much more often than the dissimilar flanker, showing that rampant flanker substitution cannot account for most flanker reports. Mixture modeling shows that simple substitution can account for, at most, about half the trials. Pooling and nonpooling (simple substitution) together include all possible models of crowding. When observers are asked to identify a crowded object, at least half of their reports are pooled, based on a combination of information from target and flankers, rather than being based on a single letter.Electronic supplementary materialThe online version of this article (doi:10.3758/s13414-011-0229-0) contains supplementary material.

As the visual world changes, its representation in our consciousness must be constantly updated. Given that the external changes are continuous, it appears plausible that conscious updating is continuous as well. Alternatively, this updating could be periodic, if, for example, its implementation at the neural level relies on oscillatory activity. The flash-lag illusion, where a briefly presented flash in the vicinity of a moving object is misperceived to lag behind the moving object, is a useful tool for studying the dynamics of conscious updating. Here, we show that the trial-by-trial variability in updating, measured by the flash-lag effect (FLE), is highly correlated with the phase of spontaneous EEG oscillations in occipital (5-10 Hz) and frontocentral (12-20 Hz) cortices just around the reference event (flash onset). Further, the periodicity in each region independently influences the updating process, suggesting a two-stage periodic mechanism. We conclude that conscious updating is not continuous; rather, it follows a rhythmic pattern.T he external world is in a state of continuous flux. The brain needs to track these changes and update its conscious representations to keep up with reality. This updating takes time and can be prone to systematic errors, as demonstrated, for example, by the flash-lag effect (1-4) (FLE). In FLE, a briefly presented stationary object (flash) near a moving object is misperceived to lag behind (1-4) or the moving stimulus is misperceived to be ahead (5, 6). This spatiotemporal illusion is classically taken to reflect the efficiency of updating and has been used to study the dynamics of conscious updating (3,7,8). The mechanisms underlying the FLE remain unclear, with various processes, such as motion extrapolation (1, 2), postdiction (3, 4), differential neuronal latencies (9-11), empirical experience (12), and attention (13-16) proposed to explain or modulate the misperception. Our study is not intended to address this debate, but we exploit the uncontroversial relation between FLE and conscious updating (3,7,8,(13)(14)(15)(16) to determine whether the updating process is continuous or periodic.Our perceptual experience of continuous external changes in the world appears seamless. Hence, it is intuitive to assume that object representations in awareness are updated continuously. However, a seamless appearance is not a guarantor of a continuous process: for example, a movie appears continuous even though it is based on discrete images displayed periodically. Discrete processing in perception is an idea that has been espoused and discarded several times (17-25). However, recent studies indicate that at least some aspects of perception, such as the detection of a perithreshold stimulus, rely on discrete or periodic sampling (26-28). Here, we ask whether one of the highest levels of perception, namely conscious updating, is periodic as well. ResultsTo test our hypothesis, we measured the flash-lag effect as an index of the efficiency of conscious updating on each trial: we asked o...