An adaptable CMOS depressing synapse with detection of changes in input spike rate

Sensory pathways in the brain attain large dynamic ranges and novelty detection through adaptive mechanisms. We present a CMOS neuromorphic circuit emulating the adaptation dynamics of short-term depressing synapses for both transient and steady state. The circuit detects abrupt changes in the input firing rate following the Weber-Fechner relation, where the transient response is proportional to the fractional change of the input firing rate. In the steady state, the input-output relationship follows the one over frequency law in the excitatory postsynaptic potential (EPSP) amplitude. The circuit also detects novel change after a long interval of inactivity. The design comprises a small number of transistors, while capturing the desired input-output relationship. The amplitude of both transient and steady state EPSP are tunable. To our knowledge this is the first CMOS design to approach the Weber-Fechner relation.