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13 of 17 cats which received unilateral olfactory tubercle lesions exhibited contralateral sensory inattention and ipsilateral circling and hyperresponsiveness. The possible neural substrates involved in these asymmetries are discussed, including ascending amine systems and ascending and descending connections with the orbital cortex.

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Much of the classical work on spinal reflex activity dealt with the facilitating and inhibiting effects of stimuli upon the responses to subsequent stimuli. Such effects were seen following both the repetition of a single stimulus (temporal summation and reflex fatigue) and the interaction of two different stimuli (facilitation and inhibition).1 These studies were made easier by two characteristics of the spinal preparation : The parts innervated by the isolated spinal cord were no longer functionally connected with the senses of sight, sound, and smell; and these parts showed very little spontaneous activity, thus permitting any movement that occurred to be causally related to specific stimuli.On a behavioral level a similar concern for the interaction between stimuli can be found in Pavlov's studies of conditioned reflexes.2 The terms internal inhibition and external inhibition are closely analogous to reflex fatigue and inhibition. In order to relate quantitatively the magnitude of the response to the effects of specific stimuli, it was necessary to take elaborate precautions to control the distracting sights, sounds, and odors inevitably associated with the pro¬ cedure and to select a response, salivary secretion, that had a low and predictable level of spontaneous activity. The transition from the alert state to sleep involves a marked decrease in responsiveness to cer¬ tain stimuli, and many of Pavlov's observa-tions had a direct bearing on the relation of such stimuli to the production of sleep,2There has been a renewal of interest in the effect of stimuli upon the responsiveness to subsequent stimuli and in the general problem of sleep-wake fulness and specific, as well as generalized, alertness, as a result of the recent studies on the reticular forma¬ tion and related parts of the brain. The first of these experiments dealt with the descending influences of these regions upon spinal reflexes; electrical stimulation of the reticular formation facilitated or inhibited reflex activity.3 Subsequent use of electrical recording techniques defined its ascending function of maintaining and regulating wakefulness and generalized responsiveness. Lesions of the reticular formation produced a permanent condition of sleep or coma; the animals were both very unresponsive to stimuli, as judged by the lack of motor response, and had an EEG pattern charac¬ teristic of sleep.4 Stimulation of this region, either directly by electrical stimulation 5 or indirectly by peripheral stimuli,6 caused a change in the EEG in the direction of lowamplitude, high-frequency activity. Since this desynchronization of the EEG was accompanied by behavioral signs of arousal and alertness in the unanesthetized animal,6 this pattern was considered to represent an activation of cortical function. More recent studies have demonstrated electrophysiologic correlates of facilitation7 and internal7,8 and external 810 inhibition.The work on generalized arousal dealt with gross changes in responsiveness to stimuli, such as those accompanying the tran...

We have observed that midsagittal reticular formation lesions in cats produce bilateral deficits in attention to and localization of various sensory modalities. To correlate these changes with previously reported changes following lesions in other areas of the brain, we propose the existence of two interdependent inhibitory pathways which originate in the frontal lobes.