Backprop Alternative: Augmented Lagrangian Predictive Coding

Researchers introduced PC-ALM (Augmented Lagrangian Predictive Coding), a biologically plausible alternative to backpropagation that trains deep neural networks using only layer-local dynamics and feedback control systems rather than explicit backward passes. The method successfully trained residual MLPs with up to 1000 layers on standard benchmarks, nearly matching backpropagation's performance while addressing the biological implausibility of the brain implementing exact backprop. This work advances understanding of how distributed systems like the brain might solve credit assignment problems without the strict phase-locking requirements of traditional backpropagation.