Yet he can understand the meaning of a spoken message. This achievementis based upon associations which were developed between sensory stimulation and central patterns of nerve excitation at an early stage in the learning process. Whenever similar patterns of verbal stimuli are presented, the old memories and the corresponding contents of consciousness become reactivated and comprehended. Thus, there is no trace of a causal evolution of understanding of verbal stimuli by way of an uninterrupted chain of conscious correlates of the sensory mechanisms. Instead, central patterns of excitation are elicited as though by resonance when specific sensory messages arrive. However logical this may sound it does not explain the process of transformation itself, which seems to be a separate biological faculty. No road to its understanding seems open at present. It has been known for a long time surface elements in the neighborhood of a dark field appear lighter than elements that lie away from the light-dark border. This difference in brightness is felt explicitly although objective checking shows that all the light fields are identical in tone. Therefore, until a short while ago the so-called simultaneous contrast had been interpreted as a subjective phenomenon. Recently this contrast effect was shown to be already manifest in the neural plane of the visual system (12). This was something of a sur- prise. The evidence discloses that contrast phenomena arebasically produced by collateral inhibition involving neigh- boring elements at retinal as well as central levels. The physiological effect of this mechanism is the sharpening of the border between light and dark areas in the visual field. Thus the correspondence between subjective impressions and patterns of neural activity in the visual system is documented. A second example of the close interrelationship between function of brain systems and mental processes is the well-known fact that consciousness is lost when critical areas of the brainstem are damaged (23). In contrast, consciousness remains unaffected when only parts of the cerebral cortex are damaged. At most, a limited defect may result, such as a scotoma of per8 DECEMBER 1967 and time are undisturbed. With this, proof exists that specific psychic functions are bound to specific nervous structures. Still, the intervening proc- ess between stimulation of nervous elements and the formation of a conscious perception remains beyond our grasp. A third example concerns the release of definite sensations through artificially induced stimulation of the brain. One recalls the activation of a characteristic behavior pattern in ex- perimental animals (goats) after intra- diencephalic injection of hypertonic saline. Their response was a massive intake of water (/4). This behavioral reaction is identical to that induced by long-term deprivation of water. The is increased, and the thirsty animal is impelled, as the human is under simi- and Neural Events light the situation and is capable of answering questions intelligently. His selfawareness and his orientation in space electrolyte concentration in the tissue Subjective Experience that ceptive integration. In spite of this defect, the patient remains conscious of lar conditions, to quench its thirst by drinking. The observation implies that the stimulated area of the brain contains receptors which control osmotic pressure by regulating the water balance. The immediate impulse is normally given by the specific sensation of thirst in association with the positively conditioned satisfaction of removing the thirst sensation. Another example of drive behavior elicited by stimulation of the diencephalon concerns food intake (/5). Here, extreme voracity may develop under the influence of central excitation, resembling that seen after prolonged fasting or as a consequence of insulin- induced fall of the blood sugar level. Such increased intake of food (bulimia) is also seen in some psychically dis- turbed human subjects. Thus, physio- logical hunger, experimentally induced hyperphagia, and pathological bulimia to the direct stimulation of efferent pathways? This is obviously not the explanation. For, if the experimenter reaches toward the cat at the climax of excitment, it strikes at him angrily in a well-directed attack. At the onset of stimulation the animal may inspect the environment and, in its search for a refuge, may suddenly jump off the laboratory table and flee to a hiding place. These observations indicate strongly that the stimulation, induced attack, and flight reactions are not purely motor effects; rather, they represent interactions between highly integrated central patterns of motivational behavior and conscious visual perceptions of the environment. The experiment with goats mentioned above is even more convincing (16). In the standard experimental procedure the animals are first acquainted with a well-defined source of water. After being deprived of water for a period they begin to seek water at this source. To reach it, they must surmount an obstruction and climb a ladder. After many trials the second part of the experiment begins—-namely, electrical stimulation of specific structures of the diencephalon when the animal is hydrated. The trained animal uses the ladder promptly, goes directly to the familiar water vessel, and drains it. The short time between the beginning of stimulation and the action described leads to one conclusion: the effect of electrical stimulation of specific diencephalic structures manifests itself in the subjective sphere as a drive, thirst. This drive gives inducement and direc- tion to the behavior, through which the tension of drive is relieved. Thus, it seems appropriate to conclude that, under the influence of brain stimulation, experiencesstored in the memory are actively integrated with instant perceptions and released as behavioral responses. appear interrelated inasmuch as they may be subserved by identical cerebral systems. Another set of observations refers to manifestation of rage and fear elicited by stimulation of the hypothala- mus in cats (J5). With the onset of artificial stimulation the cat begins to snarl, hiss, and spit. It arches its back or crouches, it bristles, and it lashes its tail. Thus, the typical defense reaction develops, as in an animal threatened by an enemy—for example, a cat threatened by an attacking dog. The question is raised, Are these effects due Effects of Psychotropic Drugs Electrical brain stimulation is not the only means whereby subjective experiences may beelicited or modified; they may be influenced considerably by the action of chemical agents. One of the best-known examples is the effect of ethyl alcohol. With moderate doses, the individual’s mood is usually improved; he experiences an in- creased desire for adventure, including an urge for verbal communication. His euphoria is accompanied. by a’ suppres1281