The perceptual and phenomenal capacity of mental imagery.

Despite the brain's immense processing power, it has finite resources. Where do these resource limits come from? Little research has examined possible low-level sensory contributions to these limitations. Mental imagery is a fundamental part of human cognition that bridges cognition with sensory representations. Hence, imagery serves as a good candidate sensory process for probing how low capacity limitations might extend down the processing hierarchy. Here we introduce a novel technique to measure the sensory capacity of mental imagery, while removing the need for memory and any direct subjective reports. Contrary to our dynamic phenomenological experience, we demonstrate that visual imagery is severely limited by the perceptual and phenomenal bottleneck of visual representation. These capacity limits appear to be independent of generation time, depend on visual feature heterogeneity, are attenuated by concurrent retinal stimulation and are endowed with good metacognition. Additionally, the precision of visual representation declines rapidly with the number of stimuli, which is governed by a simple power law. We anticipate that this assay will be important for mapping the limits of human information processing.