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Rats were trained to discriminate between two bursts of random noise that differed in intensity. In a two-lever, discrete-trial procedure, correct responses were reinforced with brain stimulation, and incorrect responses produced timeout. Responding was studied as a function of the decibel difference between the stimuli, the probabilities of presenting the stimuli, the relative duration of timeout consequent upon the two types of incorrect responses, and the absolute duration of timeout consequent upon incorrect responses. The results showed that the distribution of responses between the two levers depended upon the stimulus probabilities, but were independent of either the absolute or relative durations of timeout. When the stimulus probabilities were varied, the response probabilities did not match the stimulus probabilities; when the relative durations of timeout were varied, the animals did not obtain the maximum rate of reinforcement per unit time. Instead, the animals distributed their responses so as to obtain the maximum number of reinforcements at each level of discrimination. In addition, the level of discrimination increased as a function of the decibel difference between the stimuli.

In 94 patients with head injuries, conduction between the lower brainstem and the cerebral cortex was studied by recording evoked potentials from the scalp and neck following stimulation of the median nerves. A conduction time 3 SD or more above the normal mean (5.74 +/- 0.46 msec) or the absence of an evoked potential over one or both hemispheres was considered abnormal. During successive periods in the first 35 days after injury, the evoked potentials correlated with the outcome (classified as good or not good) in 75 to 84% of patients. Within 3 1/2 days the outcome was correctly predicted in 38 of 49 patients (78%). Six of the 7 surviving patients with persistent asymmetries of the cerebral evoked potentials remained hemiplegic. Serial studies over one year showed that both conduction time and amplitude recovered exponentially over many months, but differences persisted between the patients who made a good recovery and those who remained disabled.

Somatosensory conduction time between the dorsal column nuclei and the cerebral cortex may be measured following median nerve stimulation by recording evoked potentials from both scalp and neck. Central conduction times were significantly increased relative to normal (5.6 +/ 0.5 msec) in 11 of 24 comatose patients. Results within 10 and 35 days of onset of coma were correlated with the final clinical outcome. Conduction times were independent of serum phenobarbital (0 to 630 mumol per liter) and of central body temperature (35.0 to 38.5 degrees C). Serial studies in coma demonstrated (1) short-term increases during temporary metabolic disorders, and (2) sustained increases with gradual recovery over many months, particularly after head injury.