Our lab is building an affordable supercom … Our lab is building an affordable supercomputer, Neurogrid, whose specialty is simulations large enough to include interactions between cortical areas yet detailed enough to account for distinct cellular properties. Inspired by GRAPE-6, a $60K supercomputer that has revolutionized astrophysics, Neurogrid provides an affordable option for brain simulations. It uses analog computation to emulate ion-channel activity and uses digital communication to softwire structured connectivity patterns.
Because their operation is parallel or serial, respectively, these technologies impose different constraints. Analog computation constrains the number of distinct ion-channel populations that can be simulated—unlike digital computation, which simply takes longer to run bigger simulations. Digital communication constrains the number of synaptic connections that can be activated per second—unlike analog communication, which simply sums additional inputs onto the same wire. Working within these constraints, Neurogrid achieves its goal of simulating multiple cortical areas in real-time by making the following judicious choices.
Neurogrid simulates one million neurons by using two subcellular compartments (per neuron), a choice motivated by cortical studies. Nonlinear interactions between projections that terminate in distinct cortical layers have been replicated in a pyramidal-cell model with just two compartments. Furthermore, varying their electrical coupling replicates the firing patterns of various pyramidal-cell types. Using the smallest number of compartments that captures these behaviors lets us minimize the number of distinct ion-channel populations that need to be simulated.
Neurogrid simulates six billion synaptic connections by using local analog communication, another choice motivated by cortical studies. Cortical axons synapse profusely in a local area, course along for a while, then do it again. Thus, nearby neurons receive inputs from largely the same axons, as expected from the map-like organization of cortical areas. Local wires running between neighboring silicon neurons emulate these patches, invoking postsynaptic potentials within a programmable radius. Using a patch radius of 6 lets us increase the number of synaptic connections a hundredfold—from 600 million to six billion—without increasing digital communication. —without increasing digital communication.