12Brains are able to integrate memory from the recent past into their current computations, 13 seemingly without effort. This ability is critical for cognitive tasks such as speech understand-14 ing or working with sequences of symbols according to dynamically changing rules. But it has 15 remained unknown how networks of spiking neurons in the brain can achieve that. We show 16 that the presence of neurons with spike frequency adaptation makes a significant difference: 17 Their inclusion in a network moves its performance for such computing tasks from a very low 18 level close to the level of human performance. While artificial neural networks with special 19 long short-term memory (LSTM) units had already reached such high performance levels, 20 they lack biological plausibility. We find that neurons with spike-frequency adaptation, which 21 occur especially frequently in higher cortical areas of the human brain, provide to brains a 22 functional equivalent to LSTM units. 23
Introduction 24Our brains are able to constantly process new information in the light of recent experiences and 25 dynamically changing rules, seemingly without any effort. But we do not know how networks of 26 spiking neurons (SNNs) in the brain accomplish that. The performance of both spike-based and 27 rate-based models for recurrent neural networks in the brain have stayed on a rather low perfor-28 mance level for such tasks, far below the performance level of the human brain and artificial neural 29 network models. Artificial neural network models that perform well on such tasks use, instead of 30 neuron models, a special type of unit called Long Short-Term Memory (LSTM) unit. LSTM units 31 store information in registers -like a digital computer -where it remains without perturbance 32 by network activity for an indefinite amount of time, until is is actively updated or recalled. Hence 33 these LSTM units are not biologically plausible, and it has remained an open problem how neural 34 networks in the brain achieve so high performance on cognitively demanding tasks that require 35 integration of information from the recent past into current computational processing. We pro-36 pose that the brain achieves this -at least for some tasks -without separating computation and 37 short-term memory in different network modules: Rather it intertwines computing and memory 38 with the help of inherent slow dynamic processes in neurons and synapses. 39 Arguably the most prominent internal dynamics of neurons on the time scale of seconds -40 which is particularly relevant for many cognitive tasks -is spike-frequency adaptation (SFA). It 41 1 is expressed by a substantial fraction of neocortical neurons (Allen Institute, 2018). SFA reduces 42 the excitability of a neuron in response to its firing, see Fig. 1. Neurons with SFA have often 43 been included in SNN models that aim at modelling the dynamics of brain networks (Gutkin and 44 Zeldenrust, 2014), but not in computational studies. We show that neurons with SFA do in fact 45 si...