Strength of the nervous system is defined as the capacity of cortical cells to endure or protracted stimulation without showing transmarginal inhibition (Gray 1964).
Transmarginal inhibition (TMI) operationalized in terms of the stimulus intensity at which the relationship between stimulus intensisty and responsiveness shifts from positive to negative.
It appeared that the intensity at which TMI takes place is low for a weak nervous system and high for a strong system.
Nebylitsyn (1960) found a close relationship between strength，in terms of TMI，and simple reaction time; the weak nervous system yields faster RTs than the stronger nervous system. This relationship particularly holds for low intensities. At higher intensities the difference in RT between the weak and the strong nervous system tends to disappear. Eysenck (1966)，in attempting to equate Pavlovian typological properties with his personality
dimensions，contends that "the Pavlovian notion of ‘strong' and ‘weak' nervous systems bears a striking similarity to the notions of extraverted and introverted personality types. The ‘weak' personality type appears to resemble the introvert，the ‘strong' personality type the extravert."