J. p. SEGUNDO, C. GALEANO, J. A. SOMMER-SMITH AND J. A. ROIG 267 



through a 0.02 (jfd. condenser (to reduce movement artefacts) and then in 

 parallel to both beams of a Dumont 322-A cathode ray oscilloscope. 

 Triggering by stimulators enabled display (and photography) of cortical 

 evoked potentials: in upper beam, control potentials during silence 

 immediately preceding tone initiation; in lower beam, responses during 

 tone immediately following tone initiation. Grass S 4 models (with SIU 

 4A isolation units) served as stimulators. Tones were generated by a 

 Hewlitt Packard 200A audio-oscillator connected with a loudspeaker. 



Training proceeded in two stages. 



Stage I. A tone T and a subcutaneous stimulus SS were applied ni- 

 dependently and exhibited no constant temporal relationship. T was kept 

 constant for each preparation (frequency, 200-2000 c.p.s.; duration, 4-10 

 seconds). SS lasted from 30 seconds to 30 minutes and consisted in rectan- 

 gular pulses (applied singly or in pairs separated by 10-100 msec.) with the 

 following characteristics: voltage, 1.5-15 volts; pulse duration, o.i msec; 

 frequency, 2.5-8 c.p.s. Voltages were adjusted so as to obtain visible 

 manifestations described below; choices between single or paired shocks 

 and between different frequencies were decided in each animal depending 

 on what parameters elicited larger late, slow and labile cortical pheno- 

 mena (see below). 



The tu-st presentation of T occurred either alone (three cats) or during 

 SS (four cats) : behavioural and EEG effects were noted. Thereafter, T was 

 applied independently of SS and repeatedly until no response occurred: 

 when habituation had thus ensued, it was again tested against a back- 

 ground SS. 



Stage II. The basic feature of the training schedule was that prolonged 

 SS were interrupted systematically 2-5 seconds after initiation of T 

 (Fig. iC): hence, T initiation consistently preceded SS cessation. Sessions 

 lasted up to 90 minutes and involved up to twenty-five such combina- 

 tions. 



Acquired responses to T were eventually established (see below). 

 Since they involved simultaneous behavioural and EEG effects, the pos- 

 sibility arose that the latter were a result of the former: T clectrographic 

 effects were tested in trained animals immobilized by intravenous Flaxedil 

 (artificial respiration; rectal temperature over 36 C). 



'Learned' effects produced by T application (during SS) were compared 

 with effects of agents known to affect cortical sensory responses, as: 

 (i) voltage reduction (or cessation) of SS (seven cats); (ii) MRF stimulation 

 (Grass S 4 model and SIU 4A: voltage 0.25-2 volts; pulse duration, 

 0.1 msec; frequency, 200 c.p.s.; total duration, 1-2 seconds) (two cats); 



