The most direct approach to bridge the gap between brain activity and behavior is to record electrical activity of single cells in awake, behaving animals. Physical restraint of the animal subjects is generally inherent to technically demanding electrophysiological studies. For some experiments with highly mobile animals such as birds, however, it would be extremely desirable to have the animal free from the encumbrance and restriction of connecting wires. In these cases, the use of telemetry could be very advantageous and might enable a new perspective for combined behavioral/electrophysiological approaches.

Compared to the transmission of 'slow' bioelectric potentials like electrocardiograms, electromyograms or electroencephalograms, telemetry systems broadcasting activity of single nerve cells have to meet specific demands. Action potentials are fast (ca. 1 ms) and exhibit small amplitudes in the range of 100µV when recorded extracellularly. Continuous improvements of electronic components and the application of highly-integrated circuits nowadays offers the possibility for easy construction of high-quality, miniature systems with more than one transmission channel.

A new miniature FM-stereo radio transmitter (Figure 1) was designed that permits multiple single-unit recordings from two electrodes simultaneously [1]. Input from two independent channels is multiplexed to form a stereo composite signal that modulates a radio frequency carrier. The high quality of broadcasted extracellular signals enabled separation of single units based on differences in spike waveforms. This device has been used successfully to record from behaving barn owls [2-4].

Figure 1. Miniature stereo radio-transmitter.

Recording several single cells from different electrodes offers three distinct advantages. First, data collection is faster since two channels instead of one were transmitted at a time. The probability of maintaining a stable recording at least at one of the two electrodes increases. Second, so-called stereotrodes [5] - two closely spaced electrodes whose signals are compared for spike separation - may be used to enable reliable spike separation in certain brain regions. And third, it offers the possibility to investigate correlations between small, distributed neuronal ensembles. Multi-channel radiotelemetry that meets the demands of modern electrophysiology might open a new perspective for combined behavioral/neurophysiological approaches in freely-behaving animals.

References

1. Nieder, A. (2000) Miniature stereo radio transmitter for simultaneous recording of multiple single-neuron signals from behaving owls. J. Neurosc. Methods, 101,157-164.
2. Nieder, A. & Wagner, H. (1999) Perception and neuronal coding of subjective contours in the owl. Nature Neurosci., 2, 660-663.
3. Nieder, A. & Wagner, H. (2000) Horizontal-disparity tuning of neurons in the visual forebrain of the behaving barn owl. J. Neurophysiol., 83, 2967-2979.
4. Nieder, A. & Wagner, H. (2001) Hierarchical processing of horizontal-disparity information in the visual forebrain of behaving owls. J. Neurosci., 21, 4514-4522.
5. McNaughton, B.L., O'Keefe, J. & Barnes C.A. (1983) The stereotrode: a new technique for simultaneous isolation of several single units in the central nervous system from multiple unit records. J. Neurosci. Methods, 8, 391-397.

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