Hesitations in manual tracking: a single-channel limit in response programming.

When subjects used a manual joystick to track the motion of a visual target (in either zero-order or first-order control), hesitations in tracking often occurred when the other hand responded to an auditory stimulus. These hesitations are related to postponement in the psychological refractory period effect. Because few hesitations occurred when the auditory stimulus was the no-go case of a go-no-go paradigm, hesitations must arise primarily during "late" processing associated with the concurrent response rather than during "early" perceptual or decision-making processes. Other findings suggest that the single-channel processing limit is in programming (as opposed to selecting or generating) concurrent responses. Blanking of the target also produced hesitations through a different mechanism.

Behavioral control of breathing in the cat.

Respiration depends upon brainstem neuronal circuits that produce the respiratory rhythm and relay it, via the ventrolateral columns, to motor neurons in the spinal cord. This brainstem system produces respiration automatically, i.e. without conscious effort, and is responsive to chemical and mechanical stimuli that signal imbalances in respiratory homeostasis. In addition to this automatic/metabolic respiratory system, there is a voluntary/behavioral system that controls the respiratory muscles during speaking, breath holding, and other voluntary respiratory acts. It has been proposed that this behavioral system involves corticofugal fibers that bypass the automatic system, course in the dorsolateral columns, and end at the level of the respiratory motor neurons. According to this scheme, the integration of behavioral control with automatic/metabolic control occurs at the level of the motor neurons and not within the automatic system. This proposed scheme has not been investigated experimentally. In the present study, we trained cats to control their respiration and recorded the activity of cells within the automatic system in the medulla during this behavioral control. We trained the animals to terminate inspiration and prolong expiration when a tone sounded. Microelectrode recordings from 40 medullary respiratory neurons showed that most cells, inspiratory and expiratory, became inactive during the behavioral apneic response. The exceptions were some expiratory cells that were activated during the task. These results suggest that the integration of behavioral influences occurs within the automatic system.

Ventilatory response to hypercapnia during sleep and wakefulness in cats.

Eucapnic breathing and ventilatory responses to hypercapnia were studied in seven cats during sleep and wakefulness. No significant differences were found in minute ventilation (VE), alveolar ventilation (VA), or alveolar PCO2 (PACO2) between wakefulness (W) and non-rapid-eye-movement (NREM) sleep, but VA and VE were less during rapid-eye-movement (REM) sleep than W, and PACO2 declined during REM compared with NREM. To test the hypercapnic response, cats were required to rebreathe from a bag containing 6% CO2 and 94% O2 (to eliminate the hypoxic response). The response curve was displaced to the right during NREM and REM; the slope was reduced only during REM to a value about 75% of W and NREM. Eye movements, quantifying phasic REM, were only slightly correlated (negatively) with the deviation of ventilation from the response curve. The hypercapnic response was diminished, not eliminated, during REM, even during phasic REM. The reduced slope arose principally from the failure of the expiratory time to shorten with hypercapnia as during W and NREM. The cat's hypercapnic response compared with the dog's, measured by others with the same methodology, suggests that differences between species may be more crucial than methodology in explaining earlier contradictory results.

Characteristics of midbrain respiratory neurons in sleep and wakefulness in the cat.

Midbrain neurons were recorded in sleep and wakefulness in chronic cats. In the first phase of this study, we attempted to detect respiratory neurons by observing neuronal activity on an oscilloscope and listening to it after audioamplification. We studied 780 neurons in this non-quantitative way and failed to detect any respiratory activity. In the second phase, 203 neurons were analyzed statistically: 15% of these had activity patterns significantly related to the respiratory cycle. In the third and final phase, we studied the details of the activity of midbrain respiratory neurons. Sixteen of 281 single neurons had discharge patterns significantly related to the respiratory cycle, but only 3 of the 16 were related to breathing with a consistency that essentially excluded the possibility of a false positive error. All 3 of these cells were only intermittently respiratory, and the intermittency varied from cell to cell and tended to depend upon the state of consciousness. These results differ substantially from reports that more than 30% of midbrain and diencephalic cells are respiratory neurons. The differences may be explained by the previous authors' use of a statistic, the respiratory nodulation index, that, as shown here and in a previous stimulation study, produces a large number of false positive errors.

Sleep-related apneic and apneustic breathing following pneumotaxic lesion and vagotomy.

Sleep-wakefulness state was found to be a crucial determinant of respiratory pattern in chronic cats with bilateral lesions of the rostral pontine pneumotaxic complex (PC). Lesions resulted in increased TE, TI, and VT in all sleep and waking states. Several state-specific respiratory effects were also observed: (1) comparatively eupneic breathing during alert wakefulness (WI); (2) greatly increased TE in slow wave sleep (SWS); (3) decreased TE during rapid eye movement sleep (REM), relative to SWS; (4) increased tendency for prolonged TI (brief apneusis) during REM. Bilateral vagotomy at 2-5 weeks after PC lesion exaggerated these effects and caused distinct apneusis during REM. The results confirm that the PC is not essential for the occurrence of either rhythmic breathing or for expression of state changes in respiration, although the effects of the PC on breathing in the intact cat may vary as a function of sleep-wakefulness state. It is suggested that other regulatory systems that influence the central respiratory rhythm generator (RRG) are similarly modulated by state, and that variations in respiratory pattern observed following PC lesion and vagotomy are the result of state-dependent changes in the balance between multiple inputs to the RRG.

Erroneous classification of neuronal activity by the respiratory modulation index.

Failure to record respiratory activity in the mesencephalon of the chronic cat led us to analyze the formula (the respiratory modulation index, RMI) used by Hugelin and his colleagues to discriminate respiratory neurons. Using computer simulations, we compared RMI with the analysis of variance (F) and the non-parametric Friedman's test (chi 2). Samples were drawn repeatedly from simulated distributions of neuronal activity and were allocated to successive bins representing the respiratory cycle. Allocations of bins were made randomly so that only a chance relationship existed between the simulated activity and respiratory cycle. These simulations revealed that the RMI erroneously yields values indicative of a respiratory relationship and does so as a function of sample size and the variability and shape of the distribution of non-respiratory activity. Although some of the simulated conditions violated assumptions of the F test and, to a lesser degree, the chi 2, these statistics erred at rates close to the chosen 5% level. When respiratory activity was stimulated, chi 2 and F were more sensitive than RMI in detecting the relationship. We conclude that the high incidence of respiratory activity reported by the Hugelin group is based upon a faulty statistic and is highly questionable.

Respiratory activity and sleep--wakefulness in the deafferented, paralyzed cat.

The purpose of this study was to assess the role, if any, that peripheral feedback plays in the distinct respiratory patterns which are characteristic of the various states of consciousness. Those states include wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep. Eight adult cats, implanted with electrodes and skull bolts for sleep recordings and head restraint, sustained extensive deafferentations (bilateral vagotomy and pneumothorax, spinal transection at T-1 level, bilateral section of the phrenic nerves), were paralyzed with gallamine, artificially ventilated (ensuring stable blood gases), and held at a constant temperature. Central respiratory activity was determined by phrenic nerve recordings. During NREM sleep, respiratory activity slowed as in intact cats. During REM sleep without phasic events, phrenic activity did not differ from that in NREM sleep. During REM sleep with phasic phenomena, fast and irregular "breathing" was observed. It is concluded that states of consciousness have a direct effect on central respiratory activity. Possible mechanisms for this effect are discussed.

Sleep state effects on breathing after spinal cord transection and vagotomy in the cat.

The role of chest wall and vagal afferents on breathing during wakefulness (W), nonrapid eye movement sleep (NREM), and REM sleep was assessed in 16 adult cats implanted with electrodes and skull bolts for sleep recordings with head restraint. Breathing was monitored with a pneumotachograph. Following control recordings establishing characteristic respiratory patterns during each state, cats sustained spinal cord transections at T-1, vagotomies, or both. The transections decreased variability of breathing rate, while vagotomies decreased rate but increased variability and tidal volume. These deafferentations alone or in combination failed to eliminate the major effects of state upon breathing pattern. Different states of consciousness were associated with significant changes on every measured breathing parameter, but the interactions of these effects with the deafferentations were small or nonsignificant. The vagus, however, appears to play its largest role during NREM. We hypothesize that while vagal afference functions during all states to terminate inspiration, during W and REM separate but functionally equivalent mechanisms of central origin supplement the vagus in facilitating the termination of inspiration. The absence of these mechanisms during NREM accounts for the increased vagal influence during this state.

Increased upper airway resistance to breathing during sleep in the cat.

At the onset of sleep, upper airway resistance shifted to higher levels which were maintained throughout sleep. With each inspiration, there was a decrease in upper airway resistance. These respiratory changes in resistance were smaller in wakefulness (on low baseline resistances) than those in nonrapid eye movement sleep (on high baseline resistances). In rapid eye movement sleep, modulations with inspiration were diminished and were intermittently absent, and baseline resistance was high.

Neuronal activity specific to REM sleep and its relationship to breathing.

The search for the neural substrate of a state of consciousness has led to the expectation that there may be neurons which discharge tonically and rhythmically during that state alone. We have now recorded in the cat the first evidence of neurons whose rhythmic discharge is consistent with the hypothesis of a tonically active neural substrate for rapid eye movement (REM) sleep. These neurons, located in the area of gigantocellular and lateral medullary tegmental fields, begin rhythmically discharging simultaneously with the cortical desynchronization at REM sleep onset and cease firing at arousal from REM; they are essentially silent at all other times. Modulations of the discharge rate correlate with the phasic events of REM sleep, such as ataxic breathing and eye movement bursts. In addition, there is a high correlation between their discharge rate and respiratory frequency analyzed on a breath-by-breath basis. The significant rhythmicity of their discharge coupled with high REM selectivity contrasts them with putative REM generators reported by others in the pons and suggests their crucial role in REM generation.