Abstract

This protocol accompanies Methods: Extracellular Electrophysiology for our article: Resulaj, A., Ruediger, S., Olsen, S. R., and Scanziani, M. (2018). First spikes in visual cortex enable perceptual discrimination. eLife, 7, e34044. This protocol describes standard procedures in the field and was written following a reader request.

Materials and Reagents

Kwik-Cast (WPI)

Kwik-Sil (WPI)

1% agarose (Type IIIA; Sigma-Aldrich)

Electrodes (A1 × 32-Edge-5mm-20–177; Neuronexus)

Empower™ Dual Enzymatic Detergent (Metrex)

Krazy Glue (All Purpose)

Solutions

1. Artificial cerebrospinal fluid (ACSF; 142 mM NaCl, 5 mM KCl, 10 mM D-glucose, 10 mM Hepes, 3.1 mM CaCl2, 1.3 mM MgCl2).

2. 1% agarose (Type IIIA; Sigma-Aldrich; dissolve in ACSF). During the experiment, we kept it in a water bath at 40 degrees Celsius.

Procedures

1. We removed the QuickCast that covers the craniotomy with forceps. The craniotomy was made the day before.

2. We covered the edges of the dental cement in contact with the skull with a very thin layer of Kwik-Sil (optional step).

3. We added a drop of ACSF to cover the craniotomy.

4. We lowered the electrodes to the surface of the brain.

5. We checked signal quality. Acceptable level of noise: <50 uV peak-to-peak.

6. We lowered the recording electrodes into the brain at 4 um/sec. We continued lowering them 50 um past the desired depth of ~850–900 µm. We then returned to the desired depth.

7. We first presented black or white squares of ~10° to map the location of the receptive field across all channels of the probe.

8. To ensure that all receptive fields overlapped with the stimulus position during the behavioral task (initial 350 ms), we either moved the monitor slightly if the movement was approximately <15°, or reinserted the probe at a different location mediolaterally and repeated step #7.

9. We then filled a 1 ml syringe with 1% agarose. To dispense the agarose, we connected a pipette tip to the 1 ml syringe. We added a drop to our hand to check that it was not too hot.

10. We then dried out the ACSF covering the craniotomy with KimWipe.

11. We added a small drop of 1% agarose to cover the craniotomy and the tip of the electrodes.

12. We then added a drop of ACSF back again.

13. We lowered the electrodes into the brain and waited at least 30 min before we started recording the signals.

14. When the recording was done, we withdrew the electrodes.

15. We dried out the ACSF with a KimWipe.

16. We removed the agarose and Quick-Sil with forceps.

17. We added a fresh layer of Quick-Cast to protect the craniotomy. We added a small drop of Krazy Glue to the edges of the Quick-Cast to prevent the mouse from taking it off.

18. We put the tip of the electrode in detergent (EmPower) overnight to clean it. We rinsed with water the following day.

19. We did not attempt to record acutely on consecutive days from the same craniotomy. If one is not restricted to recording from the same retinotopic location on consecutive days like we were, one could record at different sites that are ~500 um apart on consecutive days for 3-5 days.