COUPLING THE PREPARATION TO THE STIMULATOR 



by their very nature cannot be screened — are measuring signals perhaps of 

 the order of microvolts. On stimulating the preparation the record may be 

 completely obscured by stimulus artefact, i.e. direct conductive and capacitive 

 coupling between stimulating and recording electrodes. Furthermore, one 



Figure 38.10 



may make the distressing discovery that the preparation is stimulated with 

 only one stimulating electrode in place, or even that the site of stimulation is 

 nowhere near the stimulating electrode at all ! 



The situation is represented rather artificially in Figure 38.10. The irregular 

 lump is the preparation, A and B are stimulating electrodes, C is a recording 

 electrode feeding a single ended — for argument's sake — recording amplifier, 



Figure 38.11 



and the preparation is earthed at D. The stimulator is represented in the 

 traditional manner as the secondary of an induction coil. 



Now let the stimulator pass a steady current. A system of equipotential 

 surfaces will be set up in the body of the preparation in some such manner 

 as Figure 38.11a, and it is clear that C and D will in general assume diff'erent 

 potentials, both intermediate between A and B. We can represent this state 



609 



