392 BRAIN MECHANISMS AND LEARNING 



after isolation is more prolonged in the isolated cortex than in the cortex before 

 isolation. On the other hand, if it is tested within the first half-hour, it is very hard 

 to get. So the question is — what do you mean by threshold and when were these 

 tested in relation to the isolation procedure? Also did the epilcptical activity in the 

 mirror cortex in some cases increase after isolation? Have you tested the sensitivitv 

 to mctrazol given intravenously of the cortex from the other side ? 



MoRRELL. Thresholds were determined by electrical stimulation measured in 

 terms of: the amount of current required to establish self-sustained afterdischarges. 

 Excitability is usually depressed immediately after isolation, may be augmented a 

 few hours later and is usually depressed again days or weeks later. We often did 

 find that spontaneous electrical abnormality increased for a short period after 

 isolation in some cases. In most cases it did not persist and gradually disappeared. 

 It is interesting that the electrical abnormality of the mirror region frequently 

 increased after isolation of the primary cortical lesion. We have administered 

 systemic metrazol to most of these animals and found that the paroxysmal dis- 

 charges usually did begin in the isolated epileptic regions. This is not surprising 

 because I think it is a common observation that seizures induced by systemic 

 metrazol start in any region where trauma to nerve cells has been produced. 



Gerard. I am delighted with these findings, and with the demonstration of 

 transmission of influence across a cut — one, incidentally, considerably more than 

 200 micra. This is very satisfactory. I do not know of another clear-cut case since 

 our frog brain work. On the question of spontaneity of the isolated part, I suspect 

 that we have been too easily assuming that all parts will be alike, I remind you 

 that even with the frog's brain, where a traction of a milligramme bit will give 

 regular electrical beats, not any part of the brain will do. The olfactory bulb is 

 particularly vigorous. A second point: h;;ive you to your own satisfaction excluded 

 possible secondary effects through the pia, either neural or humoral — like the 

 findings of Burn and the automatic activity described by Bach, where effects on 

 the slab were induced indirectly and not electrically? Finally, on the R.N. A. story I 

 would like to mention work done by Edith Maynard in our laboratory. She has 

 followed spontaneous or evoked discharges in the isolated lobster cardiac ganglion. 

 A ganglion soaked in the R.N.A.ase for 8 hours, and shown then to have no 

 R.N. A. bv histological tests, continues to show normal physiological activity. It is, 

 however, possible that the R.N.A.ase is not destroyed by the fixation process and 

 only enters the cell at the time of fixation ; so that this observation rec]uires further 

 check. It is, however, most interesting, especially considering the evidence of 

 Tobias that peripheral fibres soaked in proteoK'tic enzymes could practically 

 dissolve without significant change in the action potentials, whereas with lipases 

 conduction was quickly abolished with no evidence of histological change. 



MoRRELL. It is very difficult to exclude the effects which might be produced by 

 substances carried through the pial vessels. One has to maintain the pial circulation 

 intact in order to have a viable slab. 1 do not know any way of getting around this 

 difficulty. 





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