Qualitative clinical assessments of the recovery of awareness after severe brain injury require an assessor to differentiate purposeful behavior from spontaneous behavior. As many such behaviors are minimal and inconsistent, behavioral assessments are susceptible to diagnostic errors. Advanced neuroimaging tools can bypass behavioral responsiveness and reveal evidence of covert awareness and cognition within the brains of some patients, thus providing a means for more accurate diagnoses, more accurate prognoses, and, in some instances, facilitated communication. The majority of reports to date have employed the neuroimaging methods of functional magnetic resonance imaging, positron emission tomography, and electroencephalography (EEG). However, each neuroimaging method has its own advantages and disadvantages (e.g., signal resolution, accessibility, etc.). Here, we describe a burgeoning technique of non-invasive optical neuroimaging—functional near-infrared spectroscopy (fNIRS)—and review its potential to address the clinical challenges of prolonged disorders of consciousness. We also outline the potential for simultaneous EEG to complement the fNIRS signal and suggest the future directions of research that are required in order to realize its clinical potential.
Topographic cortical maps are essential for spatial localisation of sensory stimulation and generation of appropriate task-related motor responses. Somatosensation and nociception are finely mapped and aligned in the adult somatosensory (S1) cortex, but in infancy, when pain behaviour is disorganised and poorly directed, nociceptive maps may be less refined. We compared the topographic pattern of S1 activation following noxious (clinically required heel lance) and innocuous (touch) mechanical stimulation of the same skin region in newborn infants (n=32) using multi-optode functional near-infrared spectroscopy (fNIRS). Within S1 cortex, touch and lance of the heel elicit localised, partially overlapping increases in oxygenated haemoglobin concentration (D[HbO]), but while touch activation was restricted to the heel area, lance activation extended into cortical hand regions. The data reveals a widespread cortical nociceptive map in infant S1, consistent with their poorly directed pain behaviour.
Habituation to recurrent non-threatening or unavoidable noxious stimuli is an important aspect of adaptation to pain and indicates the ability of the brain to encode expectation of imminent nociception. However, it is not known whether the newborn brain can predict and habituate to recurrent noxious inputs. We used electroencephalography to investigate changes in cortical microstates, which represent the complex sequential processing of noxious inputs, following repeated clinically-required heel lances in term and preterm infants. Noxious stimulus repetition decreased the engagement of early sensory-related microstates and associated behavioural and physiological responses in term infants, while preterm infants did not show signs of adaptation. Nevertheless, both groups displayed a switch between different microstates at longer latencies. These data suggests that the preterm brain is capable of encoding high-level contextual differences in pain, but cannot update its prediction, which allows for adaptation, emphasising the vulnerability of this population to recurrent pain.
In neonates, a noxious stimulus elicits pain-related facial expression changes and distinct brain activity as measured by electroencephalography (EEG), but past research has revealed an inconsistent relationship between these responses. Facial activity is the most commonly used index of neonatal pain in clinical settings, with clinical thresholds determining if analgesia should be provided, however, we do not know of these thresholds are associated with differences in how the neonatal brain processes a noxious stimulus. The objective of the and without clinically-significant pain behaviour. Results revealed a sequence of nociceptive cortical network activation that was independent of pain-related behavior, however, a separate but interleaved sequence of early activity was related to the magnitude of the immediate behavioural response. Importantly, the degree of pain-related behavior is related to how the brain processes a stimulus and not simply the degree of cortical activation. This suggests that neonates who exhibit clinically-significant pain behaviours process the stimulus differently and that neonatal pain-related behaviours reflect just a portion of the overall cortical pain response.
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