Optopatch Update May 2001
We've been selling the Optopatch for several years now, and although the basic design hasn't changed too much during this time, we've steadily been making a series of more detailed refinements. While we may continue to make improvements in the future, we're actually beginning to wonder what these might be, so we now feel it is time to promote this product rather more actively. In particular, the capacitance measurement facilities have turned out to perform extremely well - more about that below.
But first, here's a brief summary of how the design has developed. In retrospect, the first examples really weren't too bad, and the performance improvements we've obtained since then have been more down to simple things like improving the screening of the headstage cable than to the refinements we've made to the internal circuitry. However, refinements there most certainly have been, generally to make the amplifier easier to use. In particular, we revamped the RC compensation system to operate in units of resistance rather than conductance. We also added a very comprehensive metering system, so that many parameters, including resistance and capacitance values, could be displayed directly. This is particularly helpful, because the switchable range and offset facilities actually made it quite difficult to work out the values just from looking at the control settings. The filtering facilities have also been enhanced; as well as now being able to provide the eight-pole Bessel filter option (which works beautifully, and we strongly recommend it), we also realised it would be useful to incorporate a variable two-pole prefilter, to remove some high-frequency noise before amplifying the signal, so that went in too. On the capacitance measurement side, we incorporated as standard the option to switch off the lock-in amplifier system by a logic-level control pulse (previously this had been available only as a custom modification). Current clamping was great right from the start, but even here we managed to make an improvement, as we added an "oscillation catcher" circuit to protect the cell if the amplifier is made unstable by overcompensation of the electrode capacitance (yet another Cairn first, we think). The same principle also protects against instability due to overcompensation of the series resistance in voltage clamp mode. And finally, as to the optical headstage, the patent was granted a few months ago, and we now hope to push ahead with developing the lower-noise "gold" version - at last!
Many of these improvements have been suggested by our users, and we'd particularly like to thank the following people for their support. Boris Barbour (Ecole Normale Supérieure, Paris) provided many helpful comments and suggestions on most aspects of the design, especially the idea of incorporating the oscillation catcher circuit. However, Boris does not make membrane capacitance measurements, and for checking out this part of the amplifier we are very grateful to Corne Kros and Stuart Johnson at the University of Sussex here in the UK. We'd always had high hopes for our "track-in" system, and they were the first to really use it to full effect. We've just finished a joint paper, which clearly demonstrates the excellent discrimination that the technique gives between resistance and capacitance changes on both model and real (mouse inner hair) cells. We'll give further details once it has been accepted for publication. The Neural Systems and Behavior course at Wood's Hole has requested an Optopatch again this year, and Martin (who, needless to say, is the author of this latest propaganda) hopes to spend a few days there keeping it company. Hopefully more about that at the next update!
