Early Detection of Covert Consciousness With Functional Near-Infrared Spectroscopy

Advances in the evaluation and management of patients with disorders of consciousness have revealed a pressing need to disentangle the cognitive capacities of the injured brain from overt behavior, particularly when ordinary avenues of self-expression are lost, limited, or obscured.¹ Although the understanding that consciousness and related qualities of subjective experience cannot be adequately assessed through behavior alone has been long contemplated by philosophers,^2–4 the clinical realization that a patient’s vitiated motoric repertoire does not imply unconsciousness was only more recently empirically established through functional neuroimaging and electro-encephalography (EEG)-based studies, wherein behaviorally unresponsive patients, when asked to imagine or attempt actions, have demonstrated corresponding cortical activation. In such paradigms, when a time-locked, task-appropriate pattern appears, it suggests residual comprehension and cognitive control sufficient to volitionally follow a verbal instruction, and is accordingly interpreted as evidence of covert consciousness (also known as cognitive motor dissociation).^5–9 Although the worldwide incidence and prevalence of covert consciousness has yet to be systematically studied, limited estimates have been as high as 25%,¹⁰ signaling a phenomenon of substantial public concern that clinicians and society are beginning to contend with.¹¹

Kazazian and colleagues extend this lesson in the intensive care unit (ICU) setting by demonstrating that functional near-infrared spectroscopy (fNIRS), a portable bedside imaging technology that indirectly measures cortical function by detecting changes in cerebral blood oxygenation through near-infrared light absorption, may detect engagement in cognitive tasks among behaviorally unresponsive, acutely brain injured patients.¹² Of 32 critically ill patients studied in a prospective, single arm, single site study, 8 (25%) displayed evidence of volitional modulation of brain activity in response to motor imagery commands, a finding seen not only among patients considered to be in the vegetative state / unresponsive wakefulness syndrome (VS/UWS, N = 5) or low-level minimally conscious state (MCS–, N = 1), but also in coma (N = 2),¹² underscoring the potential for profound severance of cognitive and motor function during early care windows when clinical decisions are most consequential. The findings and their implications raise important clinical, epistemological, and ethical questions (see the Table), many still unsettled as this field advances apace.

TABLE.

Detection of Covert Consciousness With fNIRS: Clinical, Conceptual, and Ethical Implications [Color table can be viewed at www.annalsofneurology.org]

	Key Challenges	Corresponding Opportunities
Clinical detection of consciousness	How to accurately evaluate capacities of the mind when ordinary avenues of expression are lost, limited, or obscured? A vitiated motoric repertoire can masquerade as unconsciousness.	fNIRS and other neurotechnologies carry promise to assess cognition among overtly unresponsive patients in settings of critical illness or injury
DoC neurorecovery and prognostication	Early, static neuroprognostication risks self-fulfilling prophecy and premature limitation of LST; while evidence links covert consciousness to improved recovery, effect sizes vary by modality and patient factors, and whether fNIRS carries comparable prognostic weight is unclear	Investigate impact of covert consciousness testing and status on patient-centered outcomes and LST; pair early testing with longitudinal follow-up of overt responsiveness, communication, QoL, and therapy participation to enhance neuroprognostication frameworks and future study
Access and implementation	Only a handful of centers worldwide have demonstrated capabilities to perform guideline-directed covert consciousness assessment, leaving most patients and clinicians without access; serial testing is pragmatically challenging, with narrow windows to detect fluctuating awareness	Portable technologies that can be applied serially at the bedside such as fNIRS may foster broadened access and help overcome pragmatic obstacles; implementation science approaches to democratize access to testing and resource sharing are needed; community hospital involvement may help clarify barriers
Benchmarking and calibration	Consciousness is only directly knowable from the first-person perspective, with no definite external criterion standard, complicating appraisal of test specificity and sensitivity. Testing may be affected by factors outside of consciousness simpliciter, and fNIRS signal quality may be influenced by hair and skin variables.	Future work might include demonstrably conscious and unconscious control participants to benchmark and calibrate test sensitivity and specificity; track variables that may influence signal quality; examine cross modal convergence, and create central repositories of covert consciousness testing data to facilitate pooled analyses across sites
Communicating results	Results remain investigational and require careful interpretation and communication; for some, recognizing that a patient is “still there” despite loss of all ordinary means of self-expression can bring renewed hope, for others, it may intensify anguish by evoking images of an intact mind trapped in a state of total privation	Opportunities exist to enhance pre- and post-test counseling; embed clinical ethics to guide value-sensitive communication; understand perils of disability bias and self-fulfilling prophecy, and align decision making with individual preferences
Treatment implications	Optimal care pathways following detection of covert consciousness remain unclear and understudied; no therapeutic clinical trials to date specifically target this population	Clinical trials of interventions to support recovery and of neural interfaces (eg, BCIs) to restore autonomy are needed; ensure explicit recognition of an experiencing person; assess and manage potential discomfort and reduce isolation via multimodal stimulation

Salient challenges are presented alongside corresponding areas of opportunity and need.

BCIs = brain-computer interfaces; DoC = disorders of consciousness; fNIRS = functional near-infrared spectroscopy; LST = life-sustaining therapy (eg, ventilator and feeding tube); QoL = quality of life.

Implementation of Covert Consciousness Testing

In the years since the release of American^13,14 and European¹⁵ professional society guidelines recommending evaluation for covert consciousness in settings of diagnostic ambiguity, only a handful of medical centers worldwide have demonstrated capabilities to perform such assessments.¹⁶ Consequently, most patients and clinicians who might benefit from covert consciousness testing lack access. This implementation gap may be distressing not only for family members eager to understand their loved one’s level of awareness and capacity for recovery following critical illness or injury, but also for clinicians aspiring to provide guideline-concordant care and counseling. The reasons for this gap are numerous and generally underexamined. They span issues of inconsistent access to the requisite neurotechnologies, analytical expertise, clinical validation, workflow management, and regulatory hurdles.¹⁷ Functional magnetic resonance imaging (fMRI), for example, typically demands transport out of the ICU, MRI-compatible monitoring, patient tolerance of recumbency in the scanner, and robust analytic infrastructure. Advanced EEG is more portable, yet provides comparatively limited spatial resolution, requires sophisticated data processing pipelines, and analytical expertise that most medical centers lack. Unlike neurobehavioral assessment,¹⁸ the practical challenge of serially administering these tests further limits the window in which fluctuating awareness can be identified. The fNIRS occupies a potential middle ground: portable, relatively safe, and able to provide useful spatial and temporal information that fits cognitive task designs.¹⁹ These traits point to scalability, and Kazazian and colleagues have paved the way for further study and validation necessary toward responsible clinical implementation and dissemination. Deepening understanding of the neural correlates of consciousness,^20,21 alongside innovations in machine learning, resting state,^22–24 stimulusbased,^25,26 automated examination techniques,^27–29 and teleneurology further promise to aid in democratizing access to state-of-the-art assessments of consciousness and neurorecovery potential. Parallel educational initiatives and regulatory analyses might clarify pathways to resource sharing, training, and regulatory clearance so that access does not remain confined to few specialized centers.³⁰

Validation and Calibration

Since early application of language-based testing in 1997³¹ and the subsequent introduction of command-following paradigms in 2006,⁷ verbal stimuli have been the back-bone of covert consciousness testing, but have notable limitations. Hearing impairment, aphasia, inattention, apathy, abulia, or apraxia, all potential sequalae of brain injury, can confound responsiveness even when consciousness is preserved, triggering false negative results. False positive findings have been less well characterized, in part because no definitive gold standard exists for determining consciousness simpliciter in other minds.³² Unlike most other capacities of clinical interest, consciousness is only directly experienced from the first-person perspective,³³ and cannot itself be plainly measured. Lacking the terra firma of an external reference standard complicates discrimination between true and spurious positive responses in those who cannot otherwise indicate that they are conscious, although rigorous task design and stringent statistical thresholds are intended to mitigate misclassification. Future investigations can strengthen validity by including demonstrably conscious and unconscious controls to benchmark and calibrate test sensitivity and specificity.

fNIRS signals are prone to a range of other variables, including hair density, texture, and skin pigmentation.³⁴ Routine reporting of these factors, planned subgroup analyses, along with open sharing of data, code, and processing pipelines can promote generalizability, render multi-site syntheses feasible, and inform development of next-generation neurotechnologies.³⁵ External validity also depends on including diverse care settings. Involving community hospitals in early feasibility phases may help to clarify pragmatic barriers that academic centers might miss.³⁶ Cross-modal comparisons (ie, triangulating findings from different test modalities) can support development of composite measures³⁷ addressing the absence of a single gold standard test of consciousness, as envisioned by European guidelines¹⁵ and International Federation of Clinical Neurophysiology (IFCN) consensus.³⁸

Communicating Findings

In the ICU, where sedation, delirium, intercurrent illness, and fluctuating arousal often converge to obscure lucid awareness, the discovery of covert consciousness can be especially striking for families and clinicians alike. For some, recognizing that a loved one is “still there” despite the loss of all ordinary means of self-expression can bring comfort and renewed hope; for others, it may intensify anguish by evoking images of an intact mind trapped in a state of total privation. Both reactions are understandable and ethically significant. Such findings therefore require meticulous counseling and sensitive communication. Families typically want more information, yet results can be difficult to interpret, especially while the science remains investigational. Pre- and post-test counseling, teach-back techniques, and careful attention to family informational needs and approaches to meaning-making³⁹ can safeguard responsible disclosure.⁴⁰ Clinical ethics engagement often supports these conversations, especially when diagnostic or prognostic uncertainty intersects with deeply held goals and values.⁴¹ Avoiding disability bias,⁴² exploring the patient’s previously expressed preferences when they are known, and distinguishing substituted judgment from best-interest reasoning when they are not, are pillars of optimal care for patients with disorders of consciousness and may be supported through ethics consultation.⁴³

Clinical Ramifications and Implications for Neurorecovery

Covert consciousness has been associated in other contexts with improved outcomes and time to recovery, although effect sizes vary by timing, etiology, and paradigm.^44–46 The extent to which fNIRS confers comparable prognostic insight remains unknown, and merits evaluation in future research. An integrated approach might pair early, multimodal consciousness screening with longitudinal follow-up of changes in behavioral diagnosis, communication capacity, overt responsiveness, and therapy participation, and relating those changes to early findings. The impact of covert consciousness on life-sustaining treatment decisions warrants additional study, particularly considering potential for early prognostic counseling to introduce bias or foster self-fulfilling prophecy effects.^47–50

Following the detection of covert consciousness, a central clinical question becomes what to do next. This remains particularly challenging because no clinical trials to date have specifically targeted this population. American Academy of Neurology/American Congress of Rehabilitation Medicine/National Institute on Disability, Independent Living, and Rehabilitation Research (AAN/ACRM/NIDLRR) guidelines recommend that “where there is no behavioral evidence of consciousness on clinical examination but functional neuroimaging or electrophysiologic testing suggests the possibility of preserved conscious awareness, frequent neurobehavioral reevaluations may be conducted to identify emerging signs of conscious awareness and decisions to reduce the intensity of rehabilitation treatment may be delayed for those individuals receiving active rehabilitation management, with the length of time over which these are done determined by an agreement between the treating clinician and the health care proxy” (2F, level C).¹³ When discovered, care practices might also shift toward explicit recognition of an experiencing person, which can include reducing isolation through multimodal stimulation, deliberately managing potential discomfort, and fostering opportunities for family connection. Detection of covert consciousness can also help orient neurorehabilitation priorities, motivate arousal optimization, and early mobilization when safe. Efforts to establish communication might naturally follow. Adaptive binary paradigms, even if limited, could facilitate participation in care decisions, but typically require some reliable volitional output, even if minimal, often supported through the expertise of skilled speech–language therapists. Brain-computer interfaces carry potential to expand communication options,⁵¹ however, impairments common after severe brain injury complicate approaches designed for cognitively intact users.⁵² If reliable communication is established, it can enable preference-sensitive choices about symptom management, daily routines, and participation in therapy. With potential return of fuller interaction, structured reflection on the phenomenology of emerging consciousness can refine clinical practice and patient-centered care models, supporting or challenging a priori beliefs about the interiority of such conditions. These considerations dovetail with an urgent need for therapeutic research to move the field beyond the detection of covert consciousness toward tailored interventions to promote recovery and improve long-term outcomes.⁵³

Bedside fNIRS offers a promising path to expand access to covert consciousness testing. Kazazian and colleagues provide compelling early evidence that it can function even in the acute setting and among the most behaviorally impaired.¹² Yet these results remain investigational, and invite inquiry into how such findings might be validated across sites and translated into care. Implementation science, embedded ethics, and philosophical analysis may yield valuable insights for clinicians and investigators navigating implications for prognosis, communication, and the responsibilities owed to patients whose hidden inner life and awareness, obscured by injury or illness, are now revealed, even if only in part. The potential ramifications extend beyond individual care decisions, shaping design of future studies and how recovery is imagined for those once thought unreachable.