Mini review: Multielectrode recordings in insect brains

Abstract

Currently, more and more laboratories are acquiring the capability of simultaneously detecting the extracellular activity of neurons in anaesthetized and awake animals by multielectrode recordings. In insects, multielectrode recordings are challenging due to the small size of the nervous system. Nevertheless, multielectrode recordings have been successfully established in brains of cockroaches, honeybees, fruit flies and grasshoppers to study sensory processing related to mechanosensation, olfaction, vision and audition. The number of neurons which can be recorded using such multielectrodes did not exceed 5 and likely depends on factors like recorded compartment of the neuron, impedance of the multielectrode, number of wires included in the multielectrode and threshold for spike detection. Signal-to-noise ratio (SNR) of the recordings obviously depends on the material and method used for the production of multielectrodes. To mark the location of the recording, different methods like current-driven copper deposition, labelling with fluorescent dye and electrocoagulation of nervous tissue are used. As expected, multielectrode recordings are more difficult in freely moving compared to restrained insects due to movement artifacts and requirement for fixed placement of the multielectrode at a particular recording site in the central nervous system (CNS). Specific characteristics of different preparations and sensory systems that include, disentangling spike collisions connected to auditory stimulation, increase in SNR after extended recording periods in olfactory systems and photoelectrical effects from compound eyes associated with visual stimulation, may require special attention and particular adaptations.