Hello Tianyang,

I do not have experience in using the flexdrive but I have done tetrode recordings in different species including humans for many years. As far as I understand from your description you were successful with 5 tetrodes in one flexdrive and it was possible to record from some tetrodes in the other three drives but not with good S/N ratio. The recordings from the other tetrodes failed totally or was it possible to record signals? What kind of tetrodes did you use (I assume twisted wire?). Anyway, in principle the tissue damage with twisted wire tetrodes is higher as with tetrodes with smooth tip shapes. But as you mentioned in one drive you´ve got very good recordings on 5 tetrodes. Therefore I do not think that tissue damage is the main problem. As three of the drives did not work in the same way as the first drive I would assume that there could be a problem with the reference ground of these drives. Please check the contact of the reference ground contact to the animal. As you know it should be a low impedance contact. What kind of ground contact did you use? I also would check the connection between the tetrode connection board and the input of your preamplifier/data acquisition system. By the way, did you measure the impedance of the tetrodes before you loaded them to the flexdrive? Did you make a test recording from saline before you mounted the drive to the animal to see if the drive with the loaded tetrodes works? Is there a gold or platinum plating on the tetrodes tips? Maybe you can give me some more detailed information. That makes it easier for me to give you a recommendation. Best wishes, Dirk       

Hi Dirk,

Thanks for your reply. I do use twisted wire. And I do gold-plating and test the impedance before implantation, also I could test the impedance after implantation. In the rat brain, the impedance of the wires could range from 100 kΩ to 2 MΩ.

The baseline for all my recording is similar. Based on what I record, I could only get units with spike amplitude higher than 100 uv. To be honest, I am not happy with that. But I record in freely-moving rats, and it is not easy to get rid of the artifacts.

For the other three drives, I could get spikes, but just on one or two tetrodes after turning down the tetrodes. And the spikes are not so stable across two to three days. After longer time, the spikes disappeared. I assume the neurons died around the tetrodes and glial cells covered the tetrodes. I am thinking  for chronic recording with tetrodes, how to reduce the damage to the brain tissue? If the neurons die and glial cells grow more around my tetrodes, how could I get good spikes.

Best,

Tianyang

Dear Tianyang, thank you for your fast response and the additional information. You told me that the range of impedance of the tetrodes is between 100kOhm and 2MOhm. This is a relatively large range if we consider the fact that the wires have all the same diameter. I assume that the coating is stripped off the tips of some tetrode contacts when the tetrodes are introduced in the brain and this causes the impedance increase. As you know the tetrode basically is a multi-unit electrode and it should pick up the signals of several units at a time. The higher the impedance the smaller the recording sphere of a tetrode contact and the smaller also the number of units that the tetrode contact detects.    

Although many people use the twisted wire tetrodes and seem to get satisfactory results with these electrodes it is a matter of fact that the tips of these twisted wire tetrodes have sharp cutting edges that cause tissue damage when introduced in the brain. You can see a photo of a twisted wire stereo-electrode in the paper of McNaughton et al. (McNaughton B, Barnes CA, O´Keefe J. The stereotrode: a new technique for simultaneous isolation of several single units in the central nervous system from multiple unit records. J Neurosci Methods 1983; 8: 391-397.).

To see if you are able to get stable long term recordings with your flexdrive I would recommend to load your flexdrive with several single core electrodes with a conical tip shape (impedance 200-500kOhm, without plating at the tip) and try to make long term multi-unit recordings. If you will get stable multi-unit recordings over some days you can be sure that the tetrode itself causes the problem.

Basically it is difficult to hold cells over some days without readjusting the electrode position. Drift in the amplitudes and shapes of the spikes recorded at a site is often observed. This drift smears the clusters and increases the size of the clusters which can cause cluster overlap and thus makes it impossible to separate them. Details about this effect are published by Emondi et al. (see Journal of Neuroscience Methods 135 (2004) 95–105, Tracking neurons recorded from tetrodes across time, Emondi et al.). I think this effect is increased in the freely moving animal application that you have.

If the plating is removed from the tetrode tip, the impedance increases and the recording sphere of this tetrode contact decreases. Beside tissue movement this can be a reason that neurons disappear after some time of recording. I am not sure that the tissue damage caused by the tetrode would cause a gliosis around the tetrode tip after this short time. In long term recordings this is a problem, because then the electrode contact is insulated by glia cells. Twisted wire tetrodes have a slightly smaller recording radius as other tetrodes presently used in neuroscience. For example Mechler reported: “…Thomas tetrodes, the R50 recording radius was ≈80–85 μm for the smaller …tetrodes  and ≈95–100 μm for the larger tetrode. These ranges of tetrode sensitivity are generally consistent with the characteristic linear span (can be >200 μm) over which polytrodes reportedly register signals from cortical single units…but remain significantly larger than the ≈50- to 70-μm recording radii reported for twisted wire tetrodes…” (for details see: Three-dimensional localization of neurons in cortical tetrode recordings, Ferenc Mechler, Jonathan D. Victor, Ifije Ohiorhenuan, Anita M. Schmid, Qin Hu, Journal of Neurophysiology Published 1 August 2011 Vol. 106 no. 2, 828-848 DOI: 10.1152/jn.00515.2010).

I would recommend the following procedure: 1) first load the drive with low impedance not plated single electrodes (impedance between 100kOhm and 400kOhm, well suited for multi-unit recording) as mentioned before and make a long term test recording to make sure that the stability of the drive is good enough for this application. If not you should improve the stability, if yes, go to the next step 2) Consider using a different tetrode technology that causes less tissue damage and has a larger recording sphere. Speak with somebody who has made long term recordings with other tetrodes to get some user feedback (e.g. see: Saccades during visual exploration align hippocampal 3–8 Hz rhythms in human and non-human primates; Kari L. Hoffman, Michelle C. Dragan, Timothy K. Leonard, Cristiano Micheli, Rodrigo Montefusco-Siegmund and Taufik A. Valiante; Front. Syst. Neurosci., 30 August 2013 | http://dx.doi.org/10.3389/fnsys.2013.00043).

I hope this information was helpful. Let me know if you have any further question or if you need additional information. I would be pleased to help you. Best wishes, Dirk