Organic Neuromorphic Spiking Circuits—From Sensory Coding and Neurotransmitter-Mediated Plasticity to Retina-Inspired Computing Functions

The fundamental mechanisms of signal communication within the human body rely on the spiking frequency of action potentials.¹ Through biological receptors and afferent neuronal cells, stimuli from the external world are encoded into a spiking pattern and transmitted to the central nervous systems where they are processed via interneurons. Indeed, the direct dependence of spikes frequency on the magnitude of the stimulus is generally referred to as “sensory coding”.<br/>Neuromorphic electronics is currently experiencing a huge increase in research activity, aiming to mimic the architecture of the human brain to enable parallel computing with high energy efficiency² and local processing, thus advancing intelligent systems interfacing with the human body³.<br/>Replicating the interdependent functions of receptors, afferent neurons and interneurons with spiking circuits, sensors and biohybrid synapses is an essential first step towards merging neuromorphic circuits and biological systems, crucial for computing at the biological interface.<br/>Moreover, establishing an active interaction with biological tissues, especially with the central nervous system, requires adaptive computing systems that are not only able to receive biologically encoded inputs but also to process and communicate these.<br/>Recently, organic materials have been employed to build electronic circuits that mimic the spiking behaviour of neurons⁴^,⁵. Despite processing complex sensorial input, these systems still lack mechanisms to modulate the encoded signal⁶.<br/>However, a combination of bioelectronic devices may recreate a “neuronal pathway” that in nature relies on the cooperation of spiking (neurons and interneurons) and non-spiking elements such as mechano-chemical sensors (receptors), as well as neuromodulator junctions (chemical synapses).<br/>Here it is presented a novel adaptive spiking circuit that replicates afferent neurons “sensory coding” from external physical stimuli. The neuromodulatory activity of interneurons is emulated by associating the spiking circuit with biohybrid synapses demonstrating an interdependent chemical synaptic connection. To establish a full neuronal pathway, these key biological functions are combined, showing the signal transduction from light stimulus to spiking frequency and to dopamine-mediated plasticity: a retinal pathway primitive.<br/>This circuit constitutes a fundamental building block for programmable neural pathways that are able not only to perform “sensory coding”, transducing both physical as well as physiological environmental information, but also to locally execute bio-inspired pre-processing functions: an essential step towards realizing processors dynamically communicating with the nervous system.<br/><br/><b>1</b> Kandel, E. R. <i>et al.</i> 4, (McGraw-hill New York, 2000)<br/><b>2</b> Furber, S. <i>J. Neural Eng.</i> 13, 051001 (2016)<br/><b>3</b> Yoo, J. <i>et al.</i> <i>Current Opinion in Biotechnology</i> 72, 95–101 (2021)<br/><b>4</b> Mirshojaeian Hosseini, M. J. <i>et al.</i> <i>J. Phys. D: Appl. Phys.</i> 54, 104004 (2021)<br/><b>5</b> Harikesh, P. C. <i>et al.</i> <i>Nat Commun</i> 13, 901 (2022)<br/><b>6</b> Go, G. <i>et al.</i> <i>Advanced Materials</i> 2201864 (2022)