Vectorized instructive signals in cortical dendrites - Nature

"Learning is the product of changes in the strength of synaptic connections between neurons. Synaptic modifications can have difficult-to-predict effects on network output, particularly in complex hierarchical networks such as the brain. The challenge of determining how individual synapses should be altered to improve task performance is known as the credit assignment problem."

"Whereas this problem is effectively solved in artificial neural networks (ANNs) by the backpropagation-of-error algorithm, how credit assignment is solved in the brain remains unknown. Central to both artificial and biologically inspired solutions to credit assignment is the vectorization of instructive signals, as opposed to the broadcasting of a single scalar teaching signal."

"One hypothesis is that in cortex, credit-related information is spatially, rather than temporally, segregated in the apical dendrites of pyramidal neurons. This aligns with anatomical and circuit evidence that feedforward inputs are received perisomatically and feedback inputs are received in the distal dendrites. However, direct evidence regarding the subcellular mechanisms of credit assignment is lacking."

Learning occurs through modifications in synaptic connection strength between neurons. The credit assignment problem involves determining how individual synapses should be altered to improve task performance in complex networks. Artificial neural networks solve this through backpropagation-of-error algorithms, but the brain's mechanism remains unclear. Recent theories propose biological circuits could use target learning or backpropagation-like algorithms. Both solutions require vectorized instructive signals rather than single scalar teaching signals, and these signals must be separable from feedforward inputs. Evidence suggests cortical credit information may be spatially segregated in apical dendrites of pyramidal neurons, with feedforward inputs received near the soma and feedback inputs in distal dendrites. Direct evidence of subcellular credit assignment mechanisms is currently lacking.

Read at Nature