Neurobiological mechanism for credit assignment across time and depth

Determine the mechanisms by which the brain performs credit assignment and learning across both recurrent temporal dynamics and hierarchical cortical depth (cortical layers and regions).

Background

The paper argues that backpropagation through time (BPTT) is implausible for biological implementation and reviews forward-mode alternatives like RTRL and E-prop. Despite proposing a mathematical extension of E-prop to deep networks, the authors emphasize that the broader neurobiological question of how the brain achieves credit assignment simultaneously over time and hierarchical depth remains unresolved.

This open problem frames the motivation for biologically plausible algorithms that can handle both temporal recurrence and multi-layer cortical organization.