Ataxiagraph
The human ability to maintain an upright stance against the continuous pull of gravity represents one of the most sophisticated sensorimotor achievements of the central nervous system. When neurological illness disrupts this delicate equilibrium, the body’s involuntary oscillations expand, yielding a clinical manifestation termed static ataxia. Historically, quantifying this physical instability transitioned from mere visual inspection to rigorous scientific instrumentation through the invention of the ataxiagraph, an apparatus designed to capture, record, and evaluate the micro-movements of postural sway.
1. Concise Definition
An ataxiagraph is a medical and physiological instrument designed to graphically record and measure involuntary bodily oscillations—commonly designated as postural sway—in an individual standing in an upright position. Operating primarily to evaluate static motor coordination and postural equilibrium, the apparatus documents variations in movement when sensory inputs, particularly vision, are systematically varied or eliminated.
In its classic mechanical configuration, an ataxiagraph translates the multidirectional displacement of the patient’s head, shoulders, or center of mass onto a recording medium, historically producing a permanent graphic tracing known as an ataxiagram. In modern neurophysiology and biomechanics, the term also encompasses computerized, sensor-driven posturographic systems that digitally capture kinematic oscillations to quantify the degree of sensory or cerebellar ataxia.
2. Etymology & Linguistic Origin
The term ataxiagraph represents a classical neoclassical compound derived from two distinct Ancient Greek roots. The initial element originates from ataxia (ἀταξία), composed of the privative prefix a- (ἀ-, signifying “without” or “lack of”) and taxis (τάξις, denoting “arrangement,” “order,” or “coordination”). Historically, ancient Hellenic medicine used the word to describe general disorder, irregularity of symptoms, or bodily unrest, before 19th-century clinical neurology restricted its meaning specifically to the loss of coordinated muscular movement.
The terminal combining element is -graph, derived from the Greek substantive graphē (γραφή, “writing” or “drawing”) and the verb graphein (γράφειν, “to scratch, write, or record”). When united, the resulting compound literally signifies an “instrument for drawing or recording incoordination.” The term gained professional currency in the English medical lexicon during the late 1880s, popularized by American and European neurologists seeking objective, graphic methods to capture phenomena previously observable only via qualitative clinical examination.
3. Pronunciation & Grammatical Form
In contemporary English phonology, ataxiagraph is pronounced with primary stress on the third syllable: /əˈtæk.si.əˌɡræf/ or /ˌeɪ.tækˈsiː.əˌɡræf/. Its derived noun designating the tracing produced by the device is ataxiagram (/əˈtæk.si.əˌɡræm/), while the method or science of conducting such graphic recordings is termed ataxiagraphy (/əˌtæk.siˈɒɡ.rə.fi/). The associated adjective is ataxiagraphic (/əˌtæk.si.əˈɡræf.ɪk/).
Grammatically, the word operates strictly as a countable noun in singular (ataxiagraph) and plural (ataxiagraphs) forms. It enters into standard biomedical syntax as both a subject and an object (“the ataxiagraph revealed marked anteroposterior drift”) and frequently functions as a noun adjunct in technical literature, as in ataxiagraph tracing, ataxiagraph assessment, or ataxiagraph calibration.
4. Detailed Conceptual Explanation
Postural stability in bipedal human stance is not a static condition but an inherently dynamic process of continuous equilibrium maintenance. Even the healthiest human body never stands entirely motionless; rather, it undergoes miniature, physiological adjustments known as postural sway. This sway reflects continuous corrective torques generated by the ankle, knee, and hip musculature in response to subtle deviations of the body’s center of mass beyond its base of support. The central nervous system regulates this balance through a dynamic feedback loop that synthesizes input from three primary sensory modalities: proprioception (sensory signals from muscles, tendons, and joints), the vestibular system (gravitational and angular acceleration sensors of the inner ear), and vision.
The ataxiagraph was conceptualized to interrogate this exact sensorimotor feedback loop. When a person stands quietly with their feet placed side by side, postural control mechanisms maintain the center of pressure within narrow physiological boundaries. If the afferent pathways transmitting proprioceptive signals through the dorsal columns of the spinal cord become diseased, the subject relies almost exclusively on visual feedback to preserve equilibrium. When the examiner requests that the patient close their eyes, removing visual compensation, the uncoordinated corrective impulses cause the body to sway wildly or fall. This fundamental clinical observation, formalised as Romberg’s test, was historically qualitative; the examiner observed whether the patient wavered or required physical support. The ataxiagraph converted this subjective observation into a reproducible geometric coordinate system.
By capturing the physical perimeter, total trajectory length, and directional orientation of sway, the ataxiagraph establishes both physiological baselines and pathological deviations. The scope of the device ranges from distinguishing subtle subclinical neuropathies to documenting severe, macroscopic cerebellar or tabetic disequilibrium. Its diagnostic boundaries, however, are explicitly confined to static ataxia—incoordination manifested during maintained stationary postures—as opposed to dynamic or kinetic ataxia, which emerges during voluntary goal-directed limb movements such as the finger-to-nose or heel-to-shin maneuvers.
Moreover, the conceptual mechanics of ataxiagraphy necessitate distinguishing between high-frequency, low-amplitude physiological tremors and true ataxic disequilibrium. Pathological sway captured by the instrument typically exhibits low-frequency, high-amplitude excursions with irregular, multivectoral drift. By isolating these parameters across eyes-open and eyes-closed trials, the ataxiagraph clarifies the sensory weighting profile of the individual, demarcating whether equilibrium failure stems from peripheral sensory deprivation, dorsal column interruption, or higher-level cerebellar processing deficits.
5. Historical Development
The inception of the ataxiagraph unfolded in the vibrant late-19th-century period of objective instrumentation in neurology, an era marked by the clinical characterization of neurological syphilis, neurodegenerative cord diseases, and cerebellar syndromes. In 1887, the American physician and neurologist Guy Hinsdale (1858–1948), working in Philadelphia under the intellectual influence of S. Weir Mitchell and William Osler, conducted groundbreaking investigations into postural equilibrium. Hinsdale sought an objective means to document the bodily sway of patients suffering from tabes dorsalis (locomotor ataxia), as well as healthy controls, institutionalized blind individuals, and children.
Hinsdale engineered a mechanical apparatus suspended above the patient’s head. The subject wore a lightweight cap equipped with an upright, spring-mounted stylus or pen. Positioned directly above was a stationary horizontal frame securing a sheet of smoked paper or glazed cardboard. As the patient attempted to stand motionless for a standardized interval (typically one minute), the stylus gently contacted the soot-covered surface, scratching a continuous, intricate line that mirrored every micro-movement of the cranium. These records yielded striking visual contrasts: healthy adults produced tight, circumscribed geometric webs averaging less than an inch in total diameter, whereas tabetic subjects traced sprawling, erratic loops that frequently veered abruptly off the paper before the test concluded.
Shortly thereafter, prominent New York neurologist Charles Loomis Dana (1852–1935) refined and standardized this configuration, popularizing the moniker ataxiagraph in his landmark 1890s textbooks and clinical manuals. Dana’s ataxiagraph employed a rigid upright gallows frame adjustable to the patient’s height, minimizing mechanical resistance and friction so that the recorded line reflected genuine bodily trajectory rather than inertial resistance of the pen. Dana utilized the ataxiagraph systematically across diverse populations, establishing standard normal parameters for physiological sway in both the anteroposterior and lateral planes, and demonstrating that sway increased predictably with eye closure even in healthy individuals.
Throughout the early to mid-20th century, the mechanical ataxiagraph evolved in tandem with experimental psychology and psychomotor research. Investigators such as Clark L. Hull in the 1920s and 1930s modified ataxiagraphic devices using string-and-pulley linkages connected to smoked drums (kymographs) or mechanical counters to assess suggestibility, hypnosis, and fatigue. Concurrently, Walter R. Miles introduced the “ataxiameter,” a related instrument that converted multidirectional sway into cumulative linear inches using mechanical escapements. In the latter half of the 20th century, the advent of electronic strain gauges, piezoelectric crystals, and analog-to-digital converters transformed mechanical ataxiagraphy into modern computerized dynamic posturography, yet the core clinical principle pioneered by Hinsdale and Dana remains fully intact.
6. Theoretical Foundations
The ataxiagraph operates upon theoretical foundations derived from biomechanical physics, sensory integration theory, and cybernetic motor control. Biomechanically, the erect human body is modeled as an inverted pendulum pivoting around the talocrural (ankle) joint. Because the body’s center of gravity resides relatively high—approximately at the level of the second sacral vertebra—and rests upon a comparatively narrow base of support bounded by the lateral borders of the feet, upright stance is intrinsically unstable. Maintaining equilibrium requires continuous negative-feedback stabilization, wherein tiny perturbations away from vertical alignment elicit rapid, proportional reflex corrections.
Under sensory integration theory, equilibrium depends upon the central nervous system’s capacity to synthesize redundant afferent streams. The visual system establishes an external inertial frame of reference by monitoring optic flow; the vestibular apparatus detects angular and linear acceleration of the cranium; and the somatosensory-proprioceptive system gauges mechanical pressure beneath the plantar surface of the feet and stretch receptors within the calf muscles and Achilles tendons. The ataxiagraph operationalizes this multi-sensory model by systematically disrupting individual sensory channels. By eliminating visual input, the instrument tests the integrity of the somatosensory and vestibular circuits. By introducing compliant foam surfaces or narrow tandem stances in modernized paradigms, it systematically challenges the remaining pathways.
Cybernetic models of motor control interpret the ataxiagram as an output signal representing error-correction algorithms within the spinocerebellar pathways. In a neurologically intact system, postural corrections occur with rapid latency and minimal amplitude, appearing on the ataxiagram as a dense, high-frequency, tightly bound cluster. In cerebellar pathology, the internal models of limb and trunk dynamics fail to predict the biomechanical consequences of movement, yielding delayed corrections, overshooting (hypermetria), and continuous corrective oscillations. Consequently, the tracing transforms from an organized stochastic pattern into a widely dispersed, chaotic trajectory reflecting defective closed-loop feedback control.
7. Key Components, Types & Dimensions
The classification and physical architecture of ataxiagraphic instrumentation span historical mechanical systems to cutting-edge electronic configurations:
- Cephalic Tracing Stylus: The traditional mechanical element comprising an adjustable, low-friction writing point (such as a brass stylus, ink reservoir pen, or fine pencil) secured to a lightweight headpiece worn by the subject. Modern counterparts replace this stylus with optical markers, electromagnetic motion trackers, or reflective spherical nodes.
- Recording Substrate: The medium upon which movement vectors are inscribed. Historically, this consisted of a horizontal glass or cardboard plate coated with a fine film of camphor soot (smoked paper), subsequently preserved in shellac. Modern computerized devices substitute this with high-resolution digital touchscreens, two-dimensional digitizer tablets, or coordinate-tracking software.
- Support Gantry / Adjustable Frame: A stable, non-resonant vertical column with a horizontal cantilevered arm that can be raised or lowered to match the subject’s exact standing height, ensuring the recording surface remains perfectly parallel to the horizontal plane.
- Mechanical Pulley and Escapement Systems (Ataxiameter Variants): Historical modifications that translated directional movements via non-elastic silk threads running to ratchets and cyclometers, recording total cumulative movement across distinct axes (anteroposterior versus mediolateral) rather than continuous geometric paths.
- Center of Pressure (COP) Force Plate Systems: The modern electronic evolution of the ataxiagraph, wherein the patient stands upon an instrumented platform containing multiple strain-gauge or piezoelectric force transducers. Rather than tracking head movement, these systems measure the ground reaction forces and moments, continuously calculating the trajectory of the body’s center of pressure.
- Inertial Sensor-Based Ataxiagraphs: Modern lightweight wearable instruments incorporating tri-axial micro-electro-mechanical systems (MEMS) accelerometers and gyroscopes secured over the lumbar spine (L3–L5) or sternum, interfacing with diagnostic software to recreate digital ataxiagraphic sway plots in real time.
8. Examples & Illustrative Cases
To understand how an ataxiagraph clarifies clinical presentation, consider the illustrative comparison between sensory disequilibrium and cerebellar disease:
Case Illustration 1: Tabetic Sensory Ataxia. A 52-year-old patient presents with progressive gait instability, paresthesias in the lower limbs, and recurrent stumbles in darkened environments. When evaluated on a classical ataxiagraph with feet placed together and eyes open, the tracing records a modest, roughly circular trajectory measuring 18 millimeters in maximum excursion, remaining well within normative limits. However, the instant the patient is instructed to close their eyes, the tracing dramatically changes: the stylus moves in expansive, jagged swings exceeding 85 millimeters, displaying severe lateral and retro-pulsional oscillations before the patient loses balance and requires physical support. This drastic divergence between eyes-open and eyes-closed tracings provides graphic confirmation of dorsal column sensory loss characteristic of tabes dorsalis or severe peripheral sensory neuropathy.
Case Illustration 2: Midline Cerebellar Degeneration. A 61-year-old chronic alcohol user presents with broad-based gait ataxia and truncal instability. When placed under ataxiagraphic examination, the patient’s eyes-open recording reveals a marked, rhythmic anteroposterior oscillation centered around 3 Hertz—a phenomenon known as titubation. The maximum sway displacement spans 60 millimeters. Upon eye closure, the geometric pattern and overall amplitude of the ataxiagram remain practically unchanged, displaying approximately 64 millimeters of sway. The lack of significant exacerbation with eye closure demonstrates that the disequilibrium is not secondary to somatosensory deafferentation, but represents an intrinsic failure of cerebellar motor coordination that vision cannot meaningfully compensate.
9. Measurement & Assessment
The quantification of postural sway via ataxiagraphic records involves several rigorous mathematical, spatial, and temporal parameters:
- Total Sway Path Length: The cumulative linear distance traversed by the tracking stylus or center of pressure over a standardized recording epoch (typically 30 to 60 seconds), expressed in millimeters or centimeters. Elevated path length denotes diminished postural stability.
- Sway Area (95% Confidence Ellipse): The spatial surface area covering 95% of the coordinate trajectory points recorded on the ataxiagram. This parameter reflects the total spatial dispersion of postural corrections and serves as an index of overall stability.
- Anteroposterior (AP) vs. Mediolateral (ML) Excursion: The maximal linear displacement achieved along the sagittal versus coronal axes. Disproportionate mediolateral sway is clinically critical, as elevated lateral sway correlates strongly with fall risk in elderly individuals and patients with cerebellar disorders.
- Mean Velocity of Sway: Calculated by dividing total sway path length by the trial duration (mm/s). Increased velocity signifies excessive neuromuscular effort and rapid, reactive postural adjustments.
- The Romberg Ratio (Quotient): The mathematical ratio of sway parameter values (such as sway area or path length) obtained with eyes closed divided by those obtained with eyes open:
Romberg Ratio = Sway (Eyes Closed) / Sway (Eyes Open). A ratio approximating 1.0 indicates no visual dependence, while ratios exceeding 2.5 to 3.0 denote clinically significant sensory ataxia. - Frequency Spectrum Analysis (Power Spectral Density): In computerized posturographic systems, Fast Fourier Transform (FFT) algorithms decompose the sway pattern into distinct frequency bands: low-frequency sway (<0.1 Hz) corresponds to visual-vestibular adaptation, intermediate frequencies (0.1–0.5 Hz) reflect cerebellar and spinal postural reactions, and high frequencies (>1.0 Hz) indicate muscle stiffness, tremor, or anxiety.
10. Applications & Practical Significance
The practical utility of the ataxiagraph extends across multiple domains within medicine, neurorehabilitation, and industrial ergonomics. In clinical neurology, its historical and modern equivalents serve as essential diagnostic adjuncts for detecting early, subclinical stages of sensorimotor conditions, such as diabetic peripheral neuropathy, multiple sclerosis, spinocerebellar degenerations, and normal pressure hydrocephalus, often revealing balance impairments before gross gait abnormalities become visible to the unaided eye.
In geriatric medicine and fall-prevention clinics, ataxiagraphic analysis identifies frail older adults at imminent risk of devastating fall injuries. Longitudinal posturographic evaluations help clinicians gauge whether targeted physical therapy regimens, strength training, or assistive mobility devices successfully restore stable postural dynamics.
Within pharmacology and toxicology, ataxiagraphy provides a quantitative bioassay for tracking the central nervous system effects of therapeutic drugs and toxic substances. Historical researchers used ataxiagraphs to document the motor-depressant actions of anesthetics, alcohol, and sedatives. Today, digital posturography remains an industry standard in clinical trials evaluating the sedative side effects of novel neuropsychiatric medications, as well as assessing vestibular toxicity induced by aminoglycoside antibiotics or industrial solvents.
Furthermore, in sports medicine, instrumented postural sway tracking has become an integral component of comprehensive concussion protocols. Athletes suspected of mild traumatic brain injury undergo posturographic balance assessment (such as the Balance Error Scoring System and computerized posturography) to identify subtle balance deficits that persist even after subjective cognitive symptoms have resolved, guiding safe return-to-play decisions.
11. Research & Empirical Evidence
Over a century of empirical inquiry has validated the fundamental principles initially demonstrated by early ataxiagraphic studies. In his foundational late-19th-century papers, Guy Hinsdale compiled the first large-scale empirical normative database of human sway. Examining hundreds of subjects across diverse age groups, Hinsdale discovered that postural sway is non-zero across all healthy human beings, that sway increases predictably when visual fixation is removed, and that children exhibit significantly greater sway than adults, reflecting the developmental maturation of neuromuscular control.
In subsequent decades, psychological researchers expanded these findings into the cognitive domain. Clark L. Hull (1929) utilized customized ataxiagraphic setups to measure postural sway as an index of behavioral suggestibility. In Hull’s experimental paradigm, subjects were exposed to verbal suggestions of falling forward or backward; those demonstrating pronounced involuntary ataxiagraphic deviations along the suggested axis were identified as exhibiting high hypnotic suggestibility, establishing a bridge between somatic motor control and psychological processes.
Modern clinical neuroscience has replicated and expanded these findings using digital force platforms and wearable inertial sensors. Landmark studies in movement disorders have demonstrated that distinct neurodegenerative syndromes exhibit specific sway signatures. For instance, spinocerebellar ataxia types 1, 2, and 3 produce distinct alterations in sway path length and jerkiness that correlate tightly with genetic repeat expansions and functional disability scales. Similarly, empirical trials in multiple sclerosis demonstrate that computerized sway assessments can detect sensorimotor deterioration up to two years before changes become apparent on the Expanded Disability Status Scale (EDSS), validating the ataxiagraphic approach as an objective digital biomarker in contemporary medicine.
12. Cultural & Cross-Cultural Considerations
While the physiological reflexes maintaining bipedal balance are universal across human populations, the clinical interpretation and operational execution of ataxiagraphic testing are influenced by cultural, lifestyle, and ergonomic variables. Cross-cultural biomechanical studies have revealed that habitual postural practices significantly shape baseline postural control. In cultures where floor-sitting, cross-legged postures, or deep squatting are practiced daily from childhood, individuals routinely develop superior hip joint mobility, altered proprioceptive acuity, and enhanced ankle stability compared to individuals raised in Westernized societies dominated by chair-sitting ergonomics.
Footwear traditions represent another critical cultural variable in ataxiagraphic assessment. Populations that predominantly walk barefoot or utilize thin, non-restrictive footwear demonstrate distinct plantar sensory thresholds and broader base-of-support mechanics, yielding naturally smaller sway areas during bare-foot posturographic testing compared to individuals accustomed to rigid, cushioned athletic or high-heeled shoes. Standardization of testing protocols—stipulating whether subjects are tested unshod, in standardized socks, or wearing specific footwear—is therefore essential to avoid confounding diagnostic results across diverse geographical regions.
Moreover, linguistic and cultural nuances surrounding clinical instructions can substantially alter ataxiagraphic outcomes. Because postural sway is sensitive to voluntary cognitive effort, anxiety, and task comprehension, ambiguous testing directives (for instance, the distinction between being told to “stand naturally” versus “stand as still as a statue”) produce noticeable shifts in muscle stiffness and sway frequency. Ensuring cross-cultural equivalence in test administration requires standardized, translated, and culturally validated instructional protocols.
13. Criticisms, Debates & Limitations
Despite its significant contributions to diagnostic neurology, ataxiagraphic instrumentation has historically faced substantive methodological debates and technical limitations. A primary critique raised against early mechanical ataxiagraphs concerned the mechanical friction and inertia of the apparatus itself. In devices utilizing head-mounted styluses rubbing against smoked surfaces, the friction generated between the writing tip and the paper could dampen tiny physiological oscillations or introduce artificial mechanical artifacts, distorting the subtle nuances of human sway.
A second persistent clinical challenge involves the non-specificity of sway measurements. While an ataxiagraph excels at detecting that a balance disturbance exists, an abnormal tracing alone rarely pinpoints the exact anatomical site of pathology. A wide, chaotic ataxiagram can stem from peripheral neuropathy, vestibular neuritis, spinocerebellar degeneration, musculoskeletal pain, medication toxicity, or severe fatigue. Consequently, ataxiagraphic data cannot serve as a standalone diagnostic modality; it must always be interpreted alongside a meticulous neurological history, physical examination, and complementary neuroimaging or electrophysiological studies.
A third ongoing debate concerns the phenomenon of test-retest learning effects and psychological confounding. Repeated ataxiagraphic trials frequently show an apparent reduction in sway as subjects adapt to the novel testing environment, overcome initial anxiety, and adopt conscious postural stabilization strategies. Conversely, dual-task cognitive interference—such as counting backward while standing—can either increase sway due to divided attentional resources or paradoxically reduce sway by shifting the subject’s focus away from conscious postural control toward automated spinal reflexes. These confounding psychological factors have historically complicated attempts to establish rigid universal cutoffs for pathological static ataxia.
14. Related Terms & Distinctions
To ensure precision in clinical and scientific terminology, the ataxiagraph must be distinguished from several related diagnostic concepts, instruments, and physical records:
- Ataxiameter: A closely related mechanical or electronic instrument designed primarily to calculate the cumulative linear distance of postural sway rather than generating a continuous two-dimensional graphic path. While an ataxiagraph produces a geometric drawing, an ataxiameter typically outputs a numerical scalar value via cyclometers or digital integrators.
- Ataxiagram: The physical graphic tracing, soot record, or digital coordinate plot produced by an ataxiagraph. The ataxiagraph is the recording instrument, whereas the ataxiagram is the resultant graphical document.
- Stabilograph (Stabilometer): A posturographic platform designed specifically to measure the displacements of the subject’s center of pressure beneath the feet. While classical ataxiagraphs tracked cephalic (head) or upper-body displacement, stabilometers measure ground-reaction forces at the plantar interface.
- Kinetograph: A general physiological recording device used to register active bodily movements, muscular contractions, or mechanical work, in contrast to the ataxiagraph’s specific focus on involuntary oscillations during static quiet stance.
- Romberg Sign: The clinical observation of disproportionately increased postural unsteadiness upon eye closure. Romberg’s test is the clinical maneuver, whereas ataxiagraphy provides the objective, quantitative instrumentation used to measure and record the severity of that unsteadiness.
- Tremor: Rhythmic, involuntary oscillatory movements of a body part characterized by high frequency (typically 4–12 Hz) and low amplitude, whereas static ataxia involves lower-frequency, multivectoral, non-rhythmic wandering of the entire body’s center of gravity.
15. Summary / Key Takeaways
The ataxiagraph stands as a foundational milestone in the history of clinical neurology and biomechanical assessment, bridging the divide between subjective bed-side observation and objective measurement. By converting invisible or fleeting postural sway into permanent graphic tracings, the apparatus unveiled the fundamental dynamics of human equilibrium and illuminated the critical roles played by sensory integration, proprioception, and cerebellar motor coordination.
From its 19th-century origins under the pioneering work of Guy Hinsdale and Charles Loomis Dana using smoked paper and head-mounted styluses, the core concept of ataxiagraphy has evolved directly into modern computerized dynamic posturography and digital wearable sensor networks. Across clinical diagnosis, fall-risk prevention, and pharmacological research, the ataxiagraphic paradigm continues to provide invaluable empirical insights into how the nervous system navigates the perpetual challenge of vertical balance.
References
- Dana, C. L. (1901). Text-Book of Nervous Diseases: Being a Compendium for the Use of Students and Practitioners of Medicine (5th ed.). William Wood & Company.
- Hinsdale, G. (1887). The station of man, considered physiologically and clinically; with a description of the ataxiagraph. The American Journal of the Medical Sciences, 94(4), 814–828.
- Hull, C. L. (1929). Quantitative methods of investigating hypnotic suggestion: Part I. The Journal of Abnormal and Social Psychology, 24(2), 153–169.
- Miles, W. R. (1922). Static equilibrium as a useful test of motor control. The Journal of Industrial Hygiene, 3(10), 316–331.
- Visser, J. E., Carpenter, M. G., van Kordelaar, M., & Bloem, B. R. (2008). The clinical utility of posturography. Clinical Neurophysiology, 119(11), 2424–2436.