J. p. SEGUNDO, C. GALEANO, |. A. SOMMER-SMITH AND J. A. ROIG 277 



provide a more susceptible background, upon which T effects if any 

 would become more notorious (Figs. 6 — left column — hrst, fourth line; 

 8 — II, III, IV; 9; 10 — I). Figs. 6 — first line, 8 — III and 9 — I illustrate 

 specially well the advantages of double stimuli showing instances in which 

 T markedly modified potentials evoked by second shock. 



Efficacy of T was critically related with two issues, (i) Voltage of SS 

 background stimulation: responses (behavioural and EEG) to values 

 producing reduced or intermediate conduct effects were susceptible; 

 lability ciccreased and disappeared if stronger stimuli were used, (ii) Control 

 responses (evoked by SS prior to T) : if T was presented against background 

 EP exhibiting important slow components, electrographic T effects 

 were clear-cut; as control potentials became smaller, EEG effects became 

 less obvious in a parallel fashion (behavioural effects occurred no matter 

 the type of background potentials). 



When SS was not discontinued after T, cats eventually (10-30 seconds) 

 readoptcd behavioural and electrographic patterns exhibited prior to 

 sound application. If applied during SS, intense intercurrent stimuli 

 (e.g. loud crash) provoked effects described below: if T were presented a 

 few seconds later, animals usually did not respond to it ('external inhibi- 

 tion'). 



Under Flaxedil, T provoked modifications of EP similar to those seen, 

 in the same trained preparation, when freely mobile (Fig. 9) : hence, 

 electrographic alterations were not a consequence of concomitant 

 performance. Pupillary dilatation occurred in flaxedilized animals upon 

 application of T.^ 



Electrographically T effects resembled results of reducing SS voltage. 

 In Fig. 6 — upper line, T (left column) a passage from 8 volts excitation to 

 5 volts (right column) reduced waves N2 and N3 (after second shock) and 

 P3 (after both shocks) (Pi was larger with 8 volts during T than with 5 

 volts). Observed with ink-writer, T application induced a shift similar to 

 that occurring in SS interruption (specially when spike Pi was not 

 prominent) (Fig. 2). With oscilloscope (showing potentials more clearly), 

 resemblance was not complete unless voltages used were threshold for 

 EP: in these circumstances, responses were effaced by T and st^ obscured 

 by 'spontaneous' rhythms that patterns became comparable to those 

 obtained without peripheral excitation. 



Changes in configuration of cortical evoked potentials are known to be 



' Apart from this one, other learned EEG responses were found to subsist in preparations 

 immobihzed with Flaxedil (Galeano, Roig, Segundo and Somnier-Sniith, 1959; Segundo, 

 Sommer-Smith, Galeano and Roig, 1959). 



