Fig. 1.

Schematic illustration of in vivo tetrode recordings in a freely moving rat during a magnetic field manipulation study. a Rat in a 1 m diameter circular arena placed in the centre of a 2 m × 2 m × 2 m triple-wrapped 3D Merritt coil system, which gives a 99% homogeneous field of 1 m diameter. Thus, the entire behavioural arena is within the homogeneous magnetic field. The 3D Merritt coil system allows a magnetic field of any azimuth or inclination to be produced. A tether cable connects the tetrode headstage on the rat to a commutator, with only the length of cable within the homogeneous area movable (~ 0.5 m). A cable from the commutator runs outside the electrically shielded room to a data acquisition box for amplification and filtering, before sending the signal to a computer for recording, processing and analysis. b Within the tether cable (red wire: voltage supply to the operational amplifiers in the headstage; black wire: the signal lines), the amplitude of the induced voltage in the largest conductive loop is a function of the enclosed area ($A$) of the loop, the magnetic field ($B$; yellow dashed arrows) in the homogeneous field (yellow shading), and the angular velocity $\left(\frac{\mathrm{\Delta }\theta }{\mathrm{\Delta }t}\right)$. c Example plot of induced voltage as a function of the enclosed area of the loop, demonstrated by three cable types – ribbon, straight bundle and twisted pairs. For all three examples, the length of the cable is set to 0.5 m, the rotation ($\theta$) is set to 90 degrees and the magnetic field intensity ($B$) to 50 µT. The rotation time ($\mathrm{\Delta }t)$ increases to 1 s. The distance between the voltage supply and signal wires is set to ribbon = 1 cm, straight bundle = 1 mm and twisted wire = 0 mm