Subjective experience—conscious awareness—requires a specific neural architecture, not merely activity in higher cortical regions. The authors propose that any system capable of subjective experience must implement stacked forward models that predict the output of neural processing from inputs, enabling prediction, error detection, and feedback control. They call this the hierarchical forward models algorithm. This framework defines a minimal but not sufficient neural architecture necessary for subjective experience. It implies that animals lacking this architecture cannot have subjective experience, regardless of behavior or brain similarities to humans. The approach shifts focus from which brain regions are active to what computations are performed.
Subjective experience of sensory stimuli depends on the brain's awareness of its own internal neural processing, which in turn relies on predictions from hierarchically organized forward models of internal sensory processing. This specialized neural architecture is required for conscious awareness; without it, a nervous system cannot subjectively experience stimuli. Using nociception as a model, the authors show that the Drosophila brain lacks the internal neural connectivity needed for hierarchical forward models. They conclude that Drosophila and insects with similar neuroanatomy do not subjectively experience noxious stimuli and therefore cannot feel pain.