Christian Tetzlaff

Carlo Michaelis, Andrew B. Lehr, Winfried Oed et al.

Developing intelligent neuromorphic solutions remains a challenging endeavour. It requires a solid conceptual understanding of the hardware's fundamental building blocks. Beyond this, accessible and user-friendly prototyping is crucial to speed up the design pipeline. We developed an open source Loihi emulator based on the neural network simulator Brian that can easily be incorporated into existing simulation workflows. We demonstrate errorless Loihi emulation in software for a single neuron and for a recurrently connected spiking neural network. On-chip learning is also reviewed and implemented, with reasonable discrepancy due to stochastic rounding. This work provides a coherent presentation of Loihi's computational unit and introduces a new, easy-to-use Loihi prototyping package with the aim to help streamline conceptualisation and deployment of new algorithms.

Arash Golmohammadi, Christian Tetzlaff

Intrinsic within-type neuronal heterogeneity is a ubiquitous feature of biological systems, with well-documented computational advantages. Recent works in machine learning have incorporated such diversities by optimizing neuronal parameters alongside synaptic connections and demonstrated state-of-the-art performance across common benchmarks. However, this performance gain comes at the cost of significantly higher computational costs, imposed by a larger parameter space. Furthermore, it is unclear how the neuronal parameters, constrained by the biophysics of their surroundings, are globally orchestrated to minimize top-down errors. To address these challenges, we postulate that neurons are intrinsically diverse, and investigate the computational capabilities of such heterogeneous neuronal parameters. Our results show that intrinsic heterogeneity, viewed as a fixed quenched disorder, often substantially improves performance across hundreds of temporal tasks. Notably, smaller but heterogeneous networks outperform larger homogeneous networks, despite consuming less data. We elucidate the underlying mechanisms driving this performance boost and illustrate its applicability to both rate and spiking dynamics. Moreover, our findings demonstrate that heterogeneous networks are highly resilient to severe alterations in their recurrent synaptic hyperparameters, and even recurrent connections removal does not compromise performance. The remarkable effectiveness of heterogeneous networks with small sizes and relaxed connectivity is particularly relevant for the neuromorphic community, which faces challenges due to device-to-device variability. Furthermore, understanding the mechanism of robust computation with heterogeneity also benefits neuroscientists and machine learners.

Carlo Michaelis, Andrew B. Lehr, Christian Tetzlaff

Neuromorphic hardware has several promising advantages compared to von Neumann architectures and is highly interesting for robot control. However, despite the high speed and energy efficiency of neuromorphic computing, algorithms utilizing this hardware in control scenarios are still rare. One problem is the transition from fast spiking activity on the hardware, which acts on a timescale of a few milliseconds, to a control-relevant timescale on the order of hundreds of milliseconds. Another problem is the execution of complex trajectories, which requires spiking activity to contain sufficient variability, while at the same time, for reliable performance, network dynamics must be adequately robust against noise. In this study we exploit a recently developed biologically-inspired spiking neural network model, the so-called anisotropic network. We identified and transferred the core principles of the anisotropic network to neuromorphic hardware using Intel's neuromorphic research chip Loihi and validated the system on trajectories from a motor-control task performed by a robot arm. We developed a network architecture including the anisotropic network and a pooling layer which allows fast spike read-out from the chip and performs an inherent regularization. With this, we show that the anisotropic network on Loihi reliably encodes sequential patterns of neural activity, each representing a robotic action, and that the patterns allow the generation of multidimensional trajectories on control-relevant timescales. Taken together, our study presents a new algorithm that allows the generation of complex robotic movements as a building block for robotic control using state of the art neuromorphic hardware.

Sebastian Herzog, Christian Tetzlaff, Florentin Wörgötter

The structure of the majority of modern deep neural networks is characterized by uni- directional feed-forward connectivity across a very large number of layers. By contrast, the architecture of the cortex of vertebrates contains fewer hierarchical levels but many recurrent and feedback connections. Here we show that a small, few-layer artificial neural network that employs feedback will reach top level performance on a standard benchmark task, otherwise only obtained by large feed-forward structures. To achieve this we use feed-forward transfer entropy between neurons to structure feedback connectivity. Transfer entropy can here intuitively be understood as a measure for the relevance of certain pathways in the network, which are then amplified by feedback. Feedback may therefore be key for high network performance in small brain-like architectures.