Imagine walking into a party hall and looking at the complex scene, what information could you get? It seems impossible to perceive everything accurately, but you might be able to quickly access some summary statistics about the scene, such as the overall hue and brightness, the location and orientation of the crowd. This ability to extract ensemble statistical information from groups of similar objects is called ensemble perception.

Previous studies have shown that ensemble perception can be characterized to some extent as an automatic, compulsive process, suggesting that the summary statistics can be rapidly extracted even without intention, helping to enrich our visual perceptual experience. The intriguing question here is whether the processing of non-target ensemble statistics exerts influences on the perceptual judgments of targets. More critically, what is the role of awareness and attention in this process?

To address these questions, a research team led by Prof. JIANG Yi from the Institute of Psychology of the Chinese Academy of Sciences manipulated the visibility of the stimuli using a critical flicker-fusion frequency method and investigated how conscious and unconscious ensemble statistics modulate the perceptual judgments and their cognitive mechanisms in a series of experiments.

Specifically, in Experiments 1 and 2, two arrays of gratings were presented and arranged in concentric circles, with one array of the gratings (i.e., inducers) made visible or invisible, to examine how the average orientation of visible or invisible inducers influenced the average orientation judgments of another set of stimuli (i.e., targets).

The results showed that regardless of whether the inducers were positioned on the inner circle or the outer circle, the average orientation of the conscious and unconscious inducers oppositely modulated the perceptual judgments of the vertical targets. The ensemble judgments of the targets tended to be repelled by the average orientation of the visible inducers, but attracted to the average orientation of the invisible inducers.

Figure 1. Procedure, stimuli, and results of Experiments 1 and 2. (Image by LIU Dingrui)

In Experiment 3, the researchers manipulated the attentional load of the task by introducing rapid serial visual presentation (RSVP) to investigate whether and how attentional resources affected the conscious and unconscious modulation effects. The results showed that in the single-task condition where participants only had to judge the average orientation of the targets, the conscious and unconscious modulation effects were consistent with the findings of Experiments 1 and 2.

However, in the dual-task condition where participants had to perform the average orientation judgment and the RSVP task simultaneously, both conscious and unconscious modulation effects were both severely impaired. This suggests that available attentional resources are indispensable for ensemble perception and/or the modulation effects.

In addition, from Experiments 4 and 5, the researchers found that when introducing a short temporal gap between the presentation of the inducers and the targets, or increasing the discrimination between the inducers and the targets (by replacing the group of target gratings with a single target grating), abolished the attractive modulation effect induced by the unconscious summary statistics, but preserved the repulsive modulation effect induced by the conscious summary statistics. These results suggest that only the unconscious attraction effect depend on the simultaneity and similarity between the inducers and the targets.

Figure 2. Procedure, stimuli, and results of Experiments 3–5. (Image by LIU Dingrui)

Taken together, this study demonstrates that conscious and unconscious processing of ensemble statistics have different and even opposite effects on perceptual decision-making, with unconscious processing of ensemble statistics inducing an attractive modulation effect, whereas conscious processing of ensemble statistics induces a repulsive modulation effect.

They both rely on the available attentional resources, but only the unconscious modulation effect is highly sensitive to the temporal separation and discrimination between the target and the inducer. This indicates that perceptual decisions about summary statistics may involve two different mechanisms in the visual system, a bottom-up mechanism underlying the unconscious attractive modulation effect, and both bottom-up and top-down mechanisms underlying the conscious repulsive modulation effect.

In addition, awareness and attention play distinct roles in ensemble perception: awareness acts as a 'guide' that determines how to cognitively use the extracted summary statistics, while attention acts as a 'switch' that determines whether to extract the summary statistics from ensemble stimuli.

The study, entitled "Conscious and unconscious processing of ensemble statistics oppositely modulate perceptual decision-making," has been published in American Psychologist on Feb. 28. 

                                  Figure 3. A schematic illustration of the proposed mechanism for the unconscious and conscious modulation effects. (Image by LIU Dingrui)