LGN cells were intracellularly recorded with glass micropipettes. Electrical stimuli of different amplitude and frequency were applied to the optic tract close to the optic chiasm. The cells were classified according to stimulus response latencies of action potentials as belonging to class I (1.0-16 msec) Or class II (1.7-3.0 MSEC). Class I EPSPs had shorter latencies (1.0-1.5 msec), durations (4-12 msec), rise times to peak (0.5-1.4 msec), and decay times (3.0-8.5 msec); the synaptic transmission time was on the average 0.41 msec. Class II EPSPs (1.6-2.6 msec latency) had longer durations (10-30 msec), rise times (1.6-3.7 msec), and decay times (9.0-25 msec); the synaptic transmission time was on the average 0.67 msec. With repetitive stimulation the EPSPs of latency class I revealed almost no stimulus frequency dependence between 1 and 120 HZ, while class 22 EPSPs decrease in amplitude between 30 and 70% with increasing frequency. Comparable temporal summation of excitation occurred in cells of both latency classes. Negative serial correlation coefficients of first order were found for consecutive EPSP amplitudes of all cells recorded for sufficient periods of time. The IPSPs were subdivided into two groups according to their optic tract response latency. Group 1 IPSPs had shorter latencies (2.0-2.6 msec), durations (15-50 msec), and times from the onset to maximal hyperpolarization (2.4-4.2 msec) than group 2 IPSPs (3.0-4.8 msec latency, 40-100 msec duration, 2.7-7.5 msec time from onset to extremum). The group 2 IPSPs decreased in amplitude by about 90% when the stimulus frequency was increased form 1 to 50 HZ, while the group 1 IPSPs displayed a comparable decrease in the frequency range between 50 and 120 HZ. Effective temporal summation was found in group 2 IPSPs in the frequency range below 70 HZ, and in group 1 IPSPs at stimulus frequencies between 70 and 120 HZ. The EPSP peak latencies and the latencies to the minimum of IPSPs proved to be invariant with respect to PSP amplitude and stimulus fre quency in individual cells. The latencies to the extrema of EPSPs and IPSPs as well as the amplitude values were symmetrically distributed.
