3.3.4. EAG recording
After the insect antenna or sensory organ has been successfully mounted, start recording the signal on the computer monitor. A stimulus should only be applied when a relatively stable baseline signal is obtained.
Depending on the nature of the application of the stimulus, EAG recordings can either be described as continuous or discontinuous. A continuous recording involves the continuous application of the stimulus over the antennal preparation throughout the recording session while a discontinuous recording refers to the application of stimulus at intervals. Coupled gas chromatography electroantennographic detection (GC-EAD) is the most widely used technique for continuous recording.
Coupled GC-EAD recordings can be used to record antennal responses to both natural and synthetic mixtures of chemicals. The GC chemically separates the mixture into individual components which are then detected by the antenna, followed by identification of the antennally-active components by coupled gas chromatography-mass spectrometry (GC-MS). However, a coupled GC-EAD system is expensive to setup and run, bulky and requires lengthy recording times. Compared to continuous GC-EAD recording, discontinuous EAG recording is a simple, less bulky and fast way of testing individual chemicals at different doses for antennal activity. Discontinuous recordings are mostly used in dose response studies to compare antennal responses to identified chemicals. This usually involves quantification of responses to the various chemicals, averaging these responses and comparing them using various conventional statistical tests (see the BEEBOOK paper on statistics (Pirk et al., 2013)). The drawback of this method is that it is not suitable for testing mixtures of chemicals especially natural unidentified mixtures, and it cannot be used to identify unknown compounds.
Both GC-EAD and EAG recording systems complement each other and it is advantageous to have both systems in place in any insect electrophysiology laboratory. A coupled GC-EAD system can easily be converted into stimulus puff EAG recording system by switching-off the GC and using the opening at the side of the metallic stimulus delivery tube to puff odours over mounted antenna.