Arousal, Attention, and Other Fundamental Functions

AROUSAL

Arousal is a psychophysiological readiness to react or respond to stimuli. Most patients requiring mental status examination do not need to be examined for arousal; however, some patients fluctuate in level of alertness and awareness. Basic arousal is present if there is nonspecific, nonreflex responsiveness to any verbal or physical stimuli. Arousal results from activation of the cerebral cortex and is centered in the ascending reticular activating system (ARAS) in the upper brain stem (primarily midbrain) with projections to the intralaminar and centromedian thalamic nuclei, the autonomic and endocrine systems in the dorsal hypothalamus, and, ultimately, the neocortex. Neurologists often divide disorders of arousal into those that affect the brain stem, with associated brain stem neurological findings, and those that more diffusely affect the neocortex. Primary states of arousal include alertness or wakefulness, lethargy or somnolence, sleep and hypnosis, obtundation, stupor or semicoma, and coma ( Table 7.1 ). In addition, there are related coma-like states that include the vegetative state, the minimally conscious state, akinetic mutism, coma vigil, and locked-in syndrome.

The primary arousal states vary in responsiveness to environmental stimuli. Patients with lethargy or somnolence tend to drift off if not stimulated, and those with obtundation require vigorous or greater than normal verbal or physical stimulation to arouse. Patients with stupor or semicoma are limited to responses to noxious or painful stimuli by groaning or withdrawal to pain, and those with coma are totally unresponsive except for reflex action, including decorticate or decerebrate posturing, depending on whether the dysfunction is above or below the red nucleus in the midbrain, respectively. The definition of coma additionally specifies the absence of normal sleep-wake cycles documented on electroencephalography. These categories of disturbed arousal are not discrete, and there is a continuum of arousal from alertness to deep coma. In contrast, coma-like states, such as vegetative state and minimally conscious state, have some apparent responsivity, with eye openings and sleep-wake cycles.

ATTENTION

“Everyone knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence. It implies withdrawal from some things to deal effectively with others and is a condition, which has a real opposite in the confused, dazed, scatterbrained state, which in French is called distraction, and Zerstreutheit in German.” William James, The Principles of Psychology, Vol. 1, Chapter XI. Attention. 1890, pg 404.

Basic attention is selective attention. As explained by William James, attention is the ability to focus and concentrate arousal and mental activity on a targeted external or internal stimulus to the exclusion of others, that is, to avoid being distracted by extraneous stimuli. Like arousal, attention works by enhancing neocortical processing and is a prerequisite for effective instrumental cognitive abilities. Attention is more of a distributed system than arousal, and many areas of the brain can disrupt attention. Attention depends on the brain stem, both arousal from the ARAS and the mechanism for actual movement of attention; the reticular nucleus of the thalamus for modulating and “gating” the sensory input so that selectivity emerges; the parietal cortex for ipsilateral disengaging and contralateral shifting attention and its spatial distribution (particularly right parietal); and the prefrontal cortex for the mental control or complex aspects of shifting, maintaining, and dividing attention ( Fig. 7.1 ). Given the distributed system mediating attention, this cognitive process is quite vulnerable to metabolic disruptions. Patients with abnormal attention have difficulty concentrating and experience impersistence, ease of distraction, and increased vulnerability to interference. Abnormal attention is the hallmark of delirium (see Chapter 3), the most common cause of behavioral disturbance among hospitalized elderly patients.

MENTAL CONTROL

Mental control is the management of attention. It is a heterogeneous concept that includes complex attention and short-term or working memory. Complex attention involves the ability to maintain attention, divide attention between two or more stimuli, shift attention between stimuli, vigilance or readiness to shift attention, and spatial attention. These complex attentional activities interact with working memory, an aspect of executive functions (see Chapter 13). Maintaining or sustaining attention is altered in most clinic or bedside tests of attention. Divided attention, however, can be quite difficult to test independent of executive functions. The ability to shift attention involves mechanisms for engaging, disengaging, and actual movement of the attentional spotlight. Vigilance has an optimal level; for example, hypervigilance leads to easy distraction to irrelevant stimuli, as is the case in delirium tremens. Spatial disorders of attention, such as hemispatial neglect and Balint syndrome, are discussed in Chapter 10 .

PSYCHOMOTOR SPEED

Speed of processing and latency of response onset are cognitive abilities that can be very sensitive to aging, white matter disease, frontal lobe disorders, and other conditions. “Psychomotor” speed combines or conflates, two general neurological processes, basic cognitive speed and basic motor speed. The former is a mental decision-making process, and the latter is basic motor reaction time or speed of movement. Psychomotor speed is particularly a function of the basal ganglia (striatum, globus pallidus, subthalamic nucleus, substantia nigra) and their white matter connections from frontal lobe regions (dorsolateral, orbitofrontal, anterior cingulate, supplementary motor area). These frontal cortical areas promote action through input to the striatum (caudate nuclei and putamen), which, in turn, directly inhibits the globus pallidus (and, less directly, the subthalamic nucleus), thus releasing the thalamus and other structures for action selection and motor execution. Some neurological and neuropsychiatric disorders result in increased speed, but most slow basic mental decision times, both response choice and response selection, even beyond basic motor reaction time. Slowed psychomotor speed and delayed latency of response initiation are common in delirium, frontal conditions, advanced dementia, and, of course, parkinsonian disorders; whereas increase speed may occur in hyperkinetic delirium and agitated, anxious, or manic patients.