What is the name for the minimum amount of stimulation a person can detect on a given sensory channel?

Sensory Thalamic Stimulation

The first trials of stimulation in the STh nuclei were, as briefly mentioned in theIntroduction, performed byMazars and colleagues in the early 1960s (1960, 1973). Somewhat later but probably independent of the European experience, the first trials of STh stimulation were carried out in the United States (Hosobuchi et al 1973,Adams et al 1974).

Only a few experimental studies have sought to elucidate the mechanisms by which pain can be relieved by STh stimulation. In view of the similarities between this form of stimulation and SCS, it was natural to postulate the presence of supraspinal gating mechanisms. In experiments in monkeys it was shown that stimulation in the ventrobasal complex of the thalamus can inhibit spinothalamic tract WDR neurons in the DH activated both by innocuous and noxious peripheral stimuli (Gerhart et al 1983).

In an electrophysiological study designed to mimic the condition of neuropathic pain, cats were subjected to trigeminal deafferentation that resulted in increased spontaneous neuronal discharge in the spinal trigeminal nucleus (Namba and Nishimoto 1988). Stimulation, both of the sensory thalamus and the internal capsule, inhibited this deafferentation hyperactivity in almost half of the neurons, and there were also long-lasting post-stimulatory effects. A behavioral study byKupers and Gybels (1993) performed on a rat model of mononeuropathy is also of particular interest. Following partial sciatic nerve injury, these animals displayed signs of neuropathy in the form of tactile hypersensitivity in the hindpaw of the nerve-ligated leg. Stimulation applied to the sensory thalamus resulted in marked suppression of this hypersensitivity.

There is much evidence that conditions of neuropathic pain, central pain in particular, lead to profound functional changes in the sensory thalamus. A series of crucial studies using microstimulation and recording in patients during stereotactic interventions have demonstrated that in patients with such pain, the thalamic somatotopy is reorganized and there are marked signs of neural hyperexcitability and changes in response properties (e.g.,Lenz et al 1998).

Summary

Sensory input and subsequent processing are definitely present in sleep, but show different characteristics than during wakefulness. The interaction between sleep and sensory physiology is an important factor because any sufficiently intense sensory stimulation always produces an awakening, from any stage of sleep.

Interestingly enough, each sensory system has an efferent pathway, with centrifugal projections ending in virtually all core afferents and on the receiver itself. Therefore, incoming sensory information can alter the physiology of sleep and wakefulness, and these states modulate incoming information.

Normal sleep depends on many aspects of sensory input. Neural networks that command sleep and wakefulness are modulated by many sensory inputs, a proportion of the ‘passive’ effects must be associated with active mechanisms of sleep. Gains or losses sensory inputs produce imbalances in neuronal networks involved in the sleep–wake cycle, changing their relative proportions of active and not being mere passive processes. For example, the almost complete deafferentation in cats caused a state of drowsiness in these animals (Vital-Durand and Michel, 1971).

Technical Considerations: Minimizing Stimulation of Airway Sensory Receptors

As a general rule, cardiovascular responses to airway maneuvers can be minimized by limiting airway sensory receptor stimulation, starting with manipulation of the larynx itself. In a double-blind study, cricoid pressure with 4.5 kg of force resulted in a significantly greater HR and BP response to endotracheal intubation than occurred in patients whose cricoid area was gently palpated.29 This underrecognized effect of cricoid pressure should be considered when estimating the risk-benefit ratio of this procedure in individual patients.

Laryngoscopy is a moderately stimulating procedure, and the use of a straight blade (e.g., a Miller blade) with elevation of the vagally innervated posterior aspect of the epiglottis results in significantly higher arterial BP than does the use of a curved blade (e.g., a Macintosh blade).30 Video-assisted laryngoscopy (VAL), which does not require alignment of the anatomical axes for adequate visualization of the glottis and subsequent intubation, has the potential to minimize the pressor response to airway manipulation. This is evidenced by manikin studies demonstrating that VAL requires less force to displace oropharyngeal tissues than direct laryngoscopy (DL) with a Macintosh blade.31,32

Use of the Pentax-AWS video laryngoscope (Pentax, Tokyo, Japan) has been reported to attenuate the hemodynamic response to endotracheal intubation after fentanyl/propofol induction when compared with either the GlideScope (Verathon, Bothell, WA) or the Macintosh laryngoscope (Fig. 7.2).33 This finding is not universal. An earlier study comparing the Pentax-AWS to Macintosh laryngoscopy reported no significant differences in systolic BP, diastolic BP, or HR after intubation, and two separate studies comparing the GlideScope and Macintosh laryngoscope also failed to find significant differences in the hemodynamic response to intubation with these devices.34,35 One possible explanation for the lack of hemodynamic advantage to less stimulating intubation devices is that the act of endotracheal intubation is far more hemodynamically stimulating than laryngoscopy itself. For example, the use of a lighted intubation stylet fails to prevent hemodynamic stimulation once an ETT is advanced past the vocal cords.36

Insertion of a laryngeal mask airway (LMA) after induction of general anesthesia with thiopental or propofol and fentanyl has been shown to cause a lower cardiovascular and endocrine response than laryngoscopy or endotracheal intubation.37–40 The LMA has the advantage of avoiding the vagally-mediated infraglottic stimulation entailed by the use of a laryngoscope, thus requiring lighter levels of general anesthesia. Furthermore, because neuromuscular blockade is not required for airway control, spontaneous ventilation and avoidance of the adverse hemodynamic consequences of PPV are possible. In contrast, endotracheal intubation via an intubating LMA results in a hemodynamic and endocrine response similar to that of DL and intubation after propofol induction.41 Therefore if endotracheal intubation is necessary, there may not be a hemodynamic advantage to instrumenting the airway with the intubating LMA or other less stimulating devices. Notably, placement of a Combitube (Kendall-Sheridan Catheter Corp., Argyle, NY) was found to cause significantly greater elevations in BP and catecholamine release when compared with endotracheal intubation or LMA placement.40

Sensory Stimulation.

Sensory stimulation is the application of environmental stimuli by an external agent for the purpose of promoting arousal and behavioral responsiveness.84 Formalized sensory stimulation programs as a treatment for patients in a coma or vegetative state became popular in the 1980s despite a lack of scientific evidence proving or disproving their effectiveness. The programs varied in intensity and frequency of intervention, as well as targeted senses. At a minimum, most programs included stimulation of visual, auditory, olfactory, kinesthetic, and tactile senses.

Proponents of such programs contended that patients with disorders of consciousness suffered from environmental deprivation and that this “deprivation could lead to widespread impairments of intellectual and perceptual processes accompanied by changes in cerebral electrical activity.”85 Controversy concerning the benefits of such programs continued throughout the 1990s, prompting the Cochrane Collaboration in 1999 to formally assess the effectiveness of sensory stimulation programs through a systematic review of the literature. Including only randomized controlled trials that compared sensory stimulation with standard rehabilitation programs, the Cochrane group found “no reliable evidence to support or rule out the effectiveness of multi-sensory programs in patients in coma or vegetative state.”86 There is no literature on the effectiveness of sensory stimulation programs in patients in minimally conscious state. Clinicians need to understand the high degree of uncertainty of outcomes with these programs and take into account other prognostic factors for outcome before initiating or recommending sensory stimulation intervention.

Information Processing in ELL

Sensory stimulation

Sensory stimuli, experiences, and emotions continuously influence brain structure. The rationale for using sensory stimulation with the aim of promoting plasticity and recovering consciousness in patients with DOCs comes from protocols of environmental enrichment that, as previously described, are associated with positive biologic and behavioral effects in experimental conditions. The assumptions of this approach lie in the concept that environmental changes after a severe brain injury manifest as virtual isolation of the patient (for example, during an intensive care unit stay) and have potentially detrimental effects on his/her recovery. However, there are significant differences between the environmental enrichment adopted in animal models and sensory stimulation, essentially depending on the fact that the first creates the conditions for a high level of behavioral interactions, whereas the second requires that specific stimuli be actively administered. There is no consensus on what stimuli should be used, but it is generally accepted that stimuli with emotional salience and autobiographic content may be more effective in promoting consciousness recovery (Abbate et al., 2014). Studies with different techniques suggest that seeing the patient's own face or familiar faces (Sharon et al., 2013; Bagnato et al., 2015) or listening to the patient's own name (Kempny et al., 2018) may lead to more effective activation of specific brain areas rather than stimuli without autobiographic content, especially in patients who are in an MCS. Similarly, a patient's preferred flavor or smell is generally used for stimulation rather than generic flavors or smells. A recent study showed that sensory stimulation based mainly on autobiographic content (a picture of the patient's closest family member; his/her favorite music, flavor and smell; and nonspecific tactile stimulation of the arms) may lead to some improvements in responsiveness in patients with MCS but not in patients with UWS (Cheng et al., 2018). Because the awareness of one's own experiences and personality (self-awareness) and the awareness of the external world (perceptual awareness) involve different cortical areas (Tacikowski et al., 2017), sensory stimulation programs should include stimuli with both autobiographic and non-autobiographic content to maximize the chances of achieving cortical activation and plastic changes. Despite the encouraging theoretical premises, the current evidence for the use of sensory stimulation in patients with DOCs is still low, and no standardized procedures are widely accepted. The increasing diffusion of virtual reality systems for cognitive rehabilitation will likely facilitate the adoption of new and more standardized programs of sensory stimulation in patients with DOCs over the coming years.

Summary

Sensory input plays an important role in the learning and refinement of movement. The integration of multiple sensory input and the association-coordination of sensory and motor information form the basis for cognition and perception. Sensory input from the external environment initiates the activity that shapes synaptic connections. The autonomic nervous system and the reticular system act as gatekeepers, determining which sensory information reaches consciousness and which is dampened. The thalamus is a central relay station that directs the flow of sensory information to association cortices. All sensory systems have similar characteristics: transduction and coding, representation, and integration at all levels of the nervous system. Sensory systems develop early in utero to be ready to function at birth. Some, such as vision and hearing, must have additional input to completely develop the neural pathway. Sensory abilities change with age; deficits in any sensory system, whether from congenital absence, trauma, or decline with age, can result in functional impairment of movement. Presbystasis, presbyopia, and presbycusis are all seen in older adults but not to the same degree in all individuals. Age-related changes in the sensory systems, therefore, are neither uniform nor universal.

Abstract

Sensory input induces gamma band activity in the reticular activating system that participates in preconscious awareness, the process necessary to support a state capable of reliably assessing the world around us on a continuous basis. This process is manifested rapidly upon waking, following increased blood flow in the thalamus and brain stem that occurs ahead of increases in cortical blood flow. This mechanism is also involved in the preconscious activity necessary for the preparation for voluntary movements. This occurs in advance of the subjective sensation of will or intention to move, that is, preconsciously.

Sensory Training (Braille)

Sensory stimulation can functionally reorganize the brain.259 This occurs not only after injury267 but with heavy hand use like in musicians250,251 and braille readers.525 However, cortical de-differentiation is reported in patients with FHd294 and animals trained intensively until the development of involuntary, uncontrolled hand movements when performing the target task.75,296,299,300 Improved cortical differentiation is reported in multifinger braille readers.525 Thus, teaching patients to read braille523,524 and perform other sensory discrimination activities522,535,536 has been recommended as a method of driving the reorganization needed to restore motor control in patients with FHd.