Our visual system possesses a remarkable ability to extract summary statistical information from groups of similar objects, known as ensemble perception. The ability is particularly important when facing a complex visual scene, which can help extract summary statistics information from the scene at a glance. Previous studies have shown that ensemble perception may be characterized as an automatic compulsory process to some extent, and likely independent of awareness or attention resources. It leads to an alluring question that whether the statistical information automatically extracted from the surrounding similar stimuli can bias our perceptual decision making on the selected target stimuli. In the following two experiments, we explored the modulation effect of ensemble perception under different conditions on perceptual decision-making and the underlying cognitive and neural mechanism.
In Study 1, we adopted a new paradigm to investigate whether and how the readouts of ensemble statistics exert influences on our perceptual decision-making and what roles consciousness and attention play in this process. In a series of experiments, we designed two arrays of gratings arranged in concentric circles, and applied the Critical Flicker-Fusion Frequency (CFF) paradigm or Continuous flash suppression paradigm (CFS) to manipulate the awareness of one array of gratings (i.e., inducers). Then we investigated whether the participants' perceptual judgment on the orientation of another array of stimuli (targets) will be affected by the ensemble statistics of the inducers. The results showed that summary statistics extracted from the conscious and unconscious ensembles differentially affected the perceptual decision-making. Participants’ ensemble judgments of the targets tended to be attracted to the unconscious readout of the inducers’ summary statistics (attractive effect) but repelled from the conscious readout of the inducers’ summary statistics (repulsive effect). Both the conscious and the unconscious modulation effects disappeared under the high attentional load condition, indicating the necessity of sufficient attentional resources in triggering the interaction between multiple ensemble representations. The unconscious modulation effect was susceptible to the temporal separation and highly specific to the ensemble task, while the conscious modulation effect was immune to the asynchronous presentation of the inducers and the targets and could be generalized to the non- ensemble task. Taken together, these findings suggest that the conscious and the unconscious ensemble representations engage different mechanisms of the visual system, which in turn exert dissociable influences on perceptual decision-making and highlight the distinct roles of awareness and attention in ensemble perception.
In Study 2, with the help of the frequency-tagging approach in electroencephalogram (EEG), we measured the steady-state visual evoked potentials of two sets of stimuli when they were ensemble represented together or one of them was ensemble represented. The most important observation was the responses of intermodulation components (IM), or known as nonlinear SSVEPs, in different perceptual judgments. Experiment 1 found that the IM responses could be observed when the two sets of stimuli were ensemble represented together. Furthermore, the Difference IM component responses were significantly stronger in the trials in which the ensemble statistics were extracted correctly than incorrectly, even if the fundamental frequency responses were unchanged. The differences of the Difference IM responses between the two conditions were mainly distributed in the parieto-occipital and midfrontal regions of the brain. Experiment 2 found that when one set of the stimuli was ensemble represented, the Sum IM component response could only be observed in the trials in which ensemble perceptual judgment of the targets was repulsed from the ensemble statistic of the inducers. And in the unrepulsive trials, the Sum IM component response is more distributed in the occipital-parietal region electrodes, while in the repulsive trials, it is more distributed in the frontal region electrodes. These results suggest that ensemble processing involves the nonlinear integration of neural signals. The correct extraction of ensemble statistical information can not just rely on a simple sum of the responses to the members, but a high-level cognitive activity that occurs in the frontal area of the brain. Furthermore, the modulation effect of ensemble statistics on perceptual decision-making is related to the brain area where the nonlinear integration of neural signals occurs.
To sum up, the findings systematically describe the cognitive and neural mechanism of modulation effects of ensemble statistics on perceptual decision-making under different conditions.
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