Damian G Kelty-Stephen

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• Ph.D. Experimental Psychology
• Assistant Professor at SUNY New Paltz

Research suggests that aging involves a "loss of complexity" in postural control, reflected in reduced fractal temporal correlations of sway fluctuations. However, this view may be simplistic, overlooking the potential complexity of reorganization that compensates for age-related decline. To explore this, we studied CoM–CoP coupling in young and ol...

The question of how to measure movement and then model has had a history of seeming simple at first and growing more complex. Richardson found that wind velocity challenged conventional Euclidean intuitions that supported linear modeling with whole-number dimensions. Biological movement coordination is no less complex than wind, and it has proven t...

Natural and behavioral sciences increasingly model measurement time series as random multiplicative cascades---nonlinear processes that divide, branch, or aggregate structures across generations, creating multiplicative interactions that break ergodicity. Multifractal formalisms provide a framework for studying cascades, where multifractal spectrum...

Hip arthroplasty or hip replacement surgery for total osteoarthritis aims to restore mobility, not just in the replaced joint but also in reintegration into whole-body movement. While traditional biomechanical approaches provide insights into joint-specific outcomes, they often overlook broader neuromuscular adaptations emerging before and after su...

Ecological psychology emphasizes the nesting of organisms within contexts to explain perception and action, contrasting with non-ecological psychological focus on internal states. This ecological approach via nesting relies on higher-order structures across many scales, manifesting in key Gibsonian concepts like affordances and optic flow. These dy...

Michael T. Turvey viewed perceptual facility with words as a nonlinear prestress of whole-body synergies, forming a multifractal tensegrity rather than a computational cognitive mechanism. This reanalysis of an upright-postural Stroop-like conjunction search links three aspects of multifractal synergies to the cognitive processes involved in the vi...

The question of how to measure movement and then model has had a history of seeming simple at first and growing more complex. Richardson found that wind velocity challenged conventional Euclidean intuitions that supported linear modeling with whole-number dimensions. Biological movement coordination is no less complex than wind, and it has proven t...

Contemporary dynamical models of human postural control propose an intermittent controller regulating the postural centre of pressure (CoP) about a stable saddle-shaped manifold along anatomical anteroposterior (AP) and mediolateral (ML) axes, releasing CoP in an outwards spiral when inactive. Experimental manipulations can evoke this saddle-type t...

Conventional human movement science often considers body parts and processes separately as belonging to complementary but distinct domains (motor control, cognition). This study breaks from tradition by exploring how mechanical and cognitive demands jointly shape whole-body functional coordination in healthy young adults. Forty-eight participants p...

How do concurrent cognitive and mechanical perturbations affect whole-body postural coordination? Stability, resilience, and robustness are fundamental properties of complex biological systems that enable the maintenance of coordination patterns, functional adaptation, and resistance to disruption. We investigated how cognitive-motor integration (T...

Long-latency reflexes (LLRs) are critical precursors to intricate postural coordination of muscular adaptations that sustain equilibrium following abrupt disturbances. Both disturbances and adaptive responses reflect excursions of postural control from quiescent Gaussian stability under a narrow bell curve, excursions beyond Gaussianity unfolding a...

Rehabilitating a stable upright posture requires tailoring therapeutic training to the multiscale dynamics of postural control. wobble board training offers a unique approach to postural rehabilitation by fostering safe, shorter-scale variability scaffolding an individual's exploration of longer-scale multijoint coordination between the postural ce...

The interaction-dominant approach to perception and action, originally formulated in the mid-1990s, has matured and gained remarkable momentum as an entailment of the dynamical hypotheses proposed at that time. This framework seeks to explain the fluid and intricate interplay of causality spanning the entire organism by integrating high-dimensional...

This investigation extends prior research on postural control on wobble board by analyzing the impact of instability and cognitive load on postural control across the entire angular spectrum of the support surface, transcending the conventional focus on anteroposterior (AP) and mediolateral (ML) axes. Young adult subjects engaged in a Trail Making...

Evidence of multifractal structures has spread to a wider set of physiological time series supporting the intricate interplay of biological and psychological functioning. These dynamics manifest as random multiplicative cascades, embodying nonlinear relationships characterized by recurring division, branching, and aggregation processes implicating...

The task for an embodied cognitive understanding of humans’ actions with tools is to elucidate how the human body, as a whole, supports the perception of affordances and dexterous action with objects in relation to other objects. Here, we focus on the relationship between humans’ actions with handheld tools and bipedal posture. Posture plays a pivo...

This study investigated the interplay of bodily degrees of freedom (DoFs) governing the collective variable comprising the center of pressure (CoP) and center of mass (CoM) in postural control through the analytical lens of multiplicative interactions across scales. We employed a task combination involving the trail-making task (TMT), which imposes...

Contemporary dynamical models of human postural control propose an intermittent controller regulating the postural center of pressure (CoP) about a stable saddle-shaped manifold along anatomical anteroposterior (AP) and mediolateral (ML) axes, releasing CoP in an outwards spiral when inactive. Experimental manipulations can evoke this saddle-type t...

Long-latency reflexes (LLRs) are critical precursors to the intricate postural coordination of muscular adaptations that sustain equilibrium following abrupt disturbances. Both disturbances and adaptive responses reflect excursions of postural control from quiescent Gaussian stability under a narrow bell curve, excursions beyond Gaussianity unfoldi...

Biological and psychological processes have been conceptualized as emerging from intricate multiplicative interactions among component processes across various spatial and temporal scales. Among the statistical models employed to approximate these intricate nonlinear interactions across scales, one prominent framework is that of cascades. Despite d...

Evidence of multifractal structures has spread to a wider set of physiological time series supporting the intricate interplay of biological and psychological functioning. These dynamics manifest as random multiplicative cascades, embodying nonlinear relationships characterized by recurring division, branching, and aggregation processes implicating...

Research suggests that aging involves a "loss of complexity" in postural control, reflected in reduced fractal temporal correlations of sway fluctuations. However, this view may be simplistic, overlooking the potential complexity of reorganization that compensates for age-related decline. To explore this, we studied CoM–CoP coupling in young and ol...

Dexterous postural control subtly complements movement variability with sensory correlations at many scales. The expressive poise of gymnasts exemplifies this lyrical punctuation of release with constraint, from coarse grain to fine scales. Dexterous postural control upon a 2D support surface might collapse the variation of center of pressure (CoP)...

Anomalous diffusion processes, characterized by their nonstandard scaling of the mean squared displacement, pose a unique challenge in classification and characterization. In a previous study (Mangalam \textit{et al.}, 2023, \textit{Physical Review Research} \textbf{5}, 023144), we established a comprehensive framework for understanding anomalous d...

Individuals with Parkinson's disease exhibit tremors, rigidity, and bradykinesia, disrupting normal movement variability and resulting in postural instability. This comprehensive study aimed to investigate the link between the temporal structure of postural sway variability and Parkinsonism by analyzing multiple datasets from young and older adults...

Dexterous postural control subtly complements movement variability with sensory correlations at many scales. The expressive poise of gymnasts exemplifies this lyrical punctuation of release with constraint, from coarse grain to fine scales. Dexterous postural control upon a 2D support surface might collapse the variation of center of pressure (CoP)...

A rich and complex temporal structure of variability in postural sway characterizes healthy and adaptable postural control. However, neurodegenerative disorders such as Parkinson's disease, which often manifest as tremors, rigidity, and bradykinesia, disrupt this healthy variability. This study examined postural sway in young and older adults, incl...

Affordance has been a centerpiece of ecological approaches to psychology. Gibson developed this novel concept through the 1960s and 1970s, defining an "affordance" as an "opportunity for action" (Gibson, 1977). This concept offered novel means to understand the role of individual actors in constructing their experience. For explaining action and re...

This edited collection provides a comprehensive and empirically informed discussion on affordances and their role in studying goal-directed behavior, covering philosophical, experimental psychological, neuroscientific, and applied perspectives. Showcasing the work of expert contributors from different backgrounds, the book inspires new directions...

An enduring controversy in biological and psychological sciences has centered on the tension between the role of random multiplicative cascade processes and the sometimes independent, additive effects of seemingly modular, scale-dependent component processes. The former supports the fluid interactivity of an organism's faculties for perception, act...

Anomalous diffusion processes, characterized by their nonstandard scaling of the mean squared displacement, pose a unique challenge in classification and characterization. In a previous study (Mangalam et al., 2023, Physical Review Research 5, 023144), we established a comprehensive framework for understanding anomalous diffusion using multifractal...

Accounts of speech perception disagree on how listeners demonstrate perceptual constancy despite considerable variation in the speech signal due to speakers’ coarticulation. According to the spectral contrast account, listeners’ compensation for coarticulation (CfC) results from listeners perceiving the target-segment frequencies differently depend...

Movement variability may reflect a fluid cascade of interactive factors. Instead of an assemblage of independent components recruited as needed in separate synergies, the movement system could embody a cascade of contingency, with contextual pressures at one scale shaping the degrees of freedom at another scale. Modeling cascades has been challengi...

A hallmark of dexterous, context-sensitive behavior is the capacity to blend and integrate information across various scales. Seamlessly pursuing multiple goals and navigating multiple task constraints involves a range of concerns across many scales: brief or bottom-up sensory corrections, long-term or top-down switches in intention and attention t...

Biological and psychological processes have been conceptualized as emerging from intricate multiplicative interactions among component processes across various spatial and temporal scales. Among the statistical models employed to approximate these intricate nonlinear interactions across scales, one prominent framework is that of cascades. Despite d...

Affordances are opportunities for action. They offer entry into a lawful account of direct perception in which full-bodied organisms are causal players in perceptual, cognitive outcomes. However, the ecological-psychological discourse has split on the issue of affordance’s proper scale. Viewing affordances as native to a so-called scale of behavior...

Stimulation and movement interact nonlinearly across multiple scales—a point empirically, quantitatively available through multifractal structure. Multifractal movements might implicate multifractal stimulation. Previous correlational modeling of accelerometry-measured torso movements during blindwalking indicated that multifractality in movement p...

When measuring physiological data, the central limit theorem typically implies a consistent variance, resulting in data that closely follows a Gaussian distribution. However, physiological measurements often deviate from this expectation, increasing variance due to nonlinear correlations across various scales. The challenge lies in testing these ta...

An enduring controversy in biological and psychological sciences has centered on the tension between the role of random multiplicative cascade processes and the sometimes independent, additive effects of seemingly modular, scale-dependent component processes. The former supports the fluid interactivity of an organism's faculties for perception, act...

Individuals with Parkinson's disease exhibit tremors, rigidity, and bradykinesia, disrupting normal movement variability and resulting in postural instability. This comprehensive study aimed to investigate the link between the temporal structure of postural sway variability and Parkinsonism by analyzing multiple datasets from young and older adults...

Any reliable biomarker has to be specific, generalizable, and reproducible across individuals and contexts. The exact values of such a biomarker must represent similar health states in different individuals and at different times within the same individual to result in the minimum possible false-positive and false-negative rates. The application of...

Natural and behavioral sciences increasingly model measurement time series as random multiplicative cascades---nonlinear processes that divide, branch, or aggregate structures across generations, creating multiplicative interactions that break ergodicity. Multifractal formalisms provide a framework for studying cascades, where multifractal spectrum...

A rich and complex temporal structure of variability in postural sway characterizes healthy and adaptable postural control. However, neurodegenerative disorders such as Parkinson's disease, which often manifest as tremors, rigidity, and bradykinesia, disrupt this healthy variability. This study examined postural sway in young and older adults, incl...

Walking exhibits stride-to-stride variations. Given ongoing perturbations, these variations critically support continuous adaptations between the goal-directed organism and its surroundings. Here, we report that stride-to-stride variations during self-paced overground walking show cascade-like intermittency—stride intervals become uneven because st...

When measuring physiological data, the central limit theorem suggests that a consistent variance confines the data within a narrow range resembling a Gaussian distribution. However, in the case of physiological measurements, the variance often increases due to cascade-like interactivity, resulting in distributions with heavier tails and nonlinear c...

Biological and psychological processes routinely break ergodicity, meaning they fail to have stable means ($Mean$) and independent variation over time that we might find in additive white Gaussian noise (awGn). One possible reason for this failure of ergodicity is the failure of biological and psychological processes to exhibit independence across...

Anomalous diffusion in various complex systems abounds in nature and spans multiple space and time scales. Canonical characterization techniques that rely upon mean squared displacement break down for nonergodic processes, making it challenging to characterize anomalous diffusion from an individual time-series measurement. Nonergodicity reigns when...

Any reliable biomarker has to be specific, generalizable, and reproducible across individuals and contexts. The exact values of such a biomarker must represent similar health states in different individuals and at different times within the same individual to result in the minimum possible false-positive and false-negative rates. The application of...

Interference between a walking task (target speeds on a self-paced treadmill) and dual visual and tactile-visual response time task was investigated. Ambulatory dual-task scenarios reveal how attention is divided between walking and additional tasks, but the impact of walking speed and dual-task modality on gait characteristics and dual-task perfor...

Walking exhibits stride-to-stride variations. Given ongoing perturbations, these variations critically support continuous adaptations between the goal-directed organism and its surroundings. Here, we report that stride-to-stride variations during self-paced overground walking show cascade-like intermittency---stride intervals become uneven because...

Any reliable biomarker has to be specific, generalizable, and reproducible across individuals and contexts. The exact values of such a biomarker must represent similar health states in different individuals and at different times within the same individual. The application of standard cutoff points and risk scores across populations hinges upon the...

Anomalous diffusion in a variety of complex systems abounds in nature and spans multiple space and time scales. Canonical characterization techniques that rely upon mean squared displacement (MSD) break down for non-ergodic processes, making it challenging to characterize anomalous diffusion from an individual time-series measurement. Non-ergodicit...

An adaptive response to unexpected perturbations requires near-term and long-term adjustments over time. We used multifractal analysis to test how nonlinear interactions across timescales might support an adaptive response following an unpredictable perturbation. We reanalyzed torque data from 44 young and 24 older adults who performed a single-leg...

Turing inspired a computer metaphor of the mind and brain that has been handy and has spawned decades of empirical investigation, but he did much more and offered behavioral and cognitive sciences another metaphor—that of the cascade. The time has come to confront Turing’s cascading instability, which suggests a geometrical framework driven by powe...

Ergodicity breaking is a challenge for biological and psychological sciences. Ergodicity is a necessary condition for linear causal modeling. Long-range correlations and non-Gaussianity characterizing various biological and psychological measurements break ergodicity routinely, threatening our capacity for causal modeling. Long-range correlations (...

An adaptive response to unexpected perturbations requires near-term and long-term adjustments over time. We used multifractal analysis to test how nonlinear interactions across timescales might support an adaptive response following an unpredictable perturbation. We reanalyzed torque data from 44 young and 24 older adults who performed a single-leg...

Biological and psychological processes routinely break ergodicity, meaning they fail to have stable means ($Mean$) and independent variation over time that we might find in additive white Gaussian noise (awGn). One possible reason for this failure of ergodicity is the failure of biological and psychological processes to exhibit independence across...

Ergodicity breaking is a challenge for biological and psychological sciences. Ergodicity is a necessary condition for linear causal modeling. Long-range correlations and non-Gaussianity characterizing various biological and psychological measurements break ergodicity routinely, threatening our capacity for causal modeling. Long-range correlations (...

The brain-as-computer metaphor has anchored the professed computational nature of mind, wresting it down from the intangible logic of Platonic philosophy to a material basis for empirical science. However, as with many long-lasting metaphors in science, the computer metaphor has been explored and stretched long enough to reveal its boundaries. Thes...

The creativity and emergence of biological and psychological behavior tend to be nonlinear—biological and psychological measures contain degrees of irregularity. The linear model might fail to reduce these measurements to a sum of independent random factors (yielding a stable mean for the measurement), implying nonlinear changes over time. The pres...

Turing inspired a computer metaphor of the mind and brain that has been handy and has spawned decades of empirical investigation, but he did much more and offered behavioral and cognitive sciences another metaphor—that of the cascade. The time has come to confront Turing’s cascading instability, which suggests a geometrical framework driven by powe...

The stochastic processes underlying the growth and stability of biological and psychological systems reveal themselves when far from equilibrium. Far from equilibrium, nonergodicity reigns. Nonergodicity implies that the average outcome for a group/ensemble (i.e., of representative organisms/minds) is not necessarily a reliable estimate of the aver...

The stochastic processes underlying the growth and stability of biological and psychological systems reveal themselves when far from equilibrium. Far from equilibrium, nonergodicity reigns. Nonergodicity implies that the average outcome for a group/ensemble (i.e., of representative organisms/minds) is not necessarily a reliable estimate of the aver...

The ubiquity of tool use in human life has generated multiple lines of scientific and philosophical investigation to understand the development and expression of humans’ engagement with tools and its relation to other dimensions of human experience. However, existing literature on tool use faces several epistemological challenges in which the same...

We see the computer metaphor of the brain as a holdover from premodern scientific traditions hoping to anchor the mind’s computational ability in a material anatomical part. Despite having prompted decades of valuable empirical insights, the computer metaphor has likely outgrown its usefulness. Brains are context-sensitive and capable of adapting t...

The ubiquity of tool use in human life has generated multiple lines of scientific and philosophical investigation to understand the development and expression of humans’ engagement with tools and its relation to other dimensions of human experience. However, existing literature on tool use faces several epistemological challenges in which the same...

Speech perception and memory for speech require active engagement. Gestural theories have emphasized mainly the effect of speaker's movements on speech perception. They fail to address the effects of listener movement, focusing on communication as a boundary condition constraining movement among interlocutors. The present work attempts to break new...

The creativity and emergence of biological and psychological behavior tend to be nonlinear—biological and psychological measures contain degrees of irregularity. The linear model might fail to reduce these measurements to a sum of independent random factors (yielding a stable mean for the measurement), implying nonlinear changes over time. The pres...

When humans handle a tool, such as a tennis racket or hammer, for the first time, they often wield it to determine its inertial properties. The mechanisms that contribute to perception of inertial properties are not fully understood. The present study’s goal was to investigate how proprioceptive afferents contribute to effortful perception of heavi...

Quiet standing exhibits strongly intermittent variability that has inspired at least two interpretations. First, variability can be intermittent through the alternating engagement and disengagement of complementary control processes at distinct scales. A second and perhaps deeper way to interpret this intermittency is through the possibility that p...

Speech perception and memory for speech require active engagement. Gestural theories have emphasized mainly the effect of the movements of the speaker on speech perception. They fail to address the effects of listener movement, focusing on communication as a boundary condition constraining movement among interlocutors. The present work attempts to...

Modern biomedical, behavioral and psychological inference about cause-effect relationships respects an ergodic assumption, that is, that mean response of representative samples allow predictions about individual members of those samples. Recent empirical evidence in all of the same fields indicates systematic violations of the ergodic assumption. I...

Visually guided postural control emerges in response to task constraints. Task constraints generate physiological fluctuations that foster the exploration of available sensory information at many scales. Temporally correlated fluctuations quantified using fractal and multifractal metrics have been shown to carry perceptual information across the bo...

The ‘quiet eye’ (QE) approach to visually-guided aiming behavior invests fully in perceptual information’s potential to organize coordinated action. Sports psychologists refer to QE as the stillness of the eyes during aiming tasks and increasingly into self- and externally-paced tasks. Amidst the ‘noisy’ fluctuations of the athlete’s body, quiet ey...

We tested for transposition effects (TEs) in Hindi (a Modern Indo-Aryan language) using unprimed lexical decision. TEs are less accurate and slower responses to transposed-nonwords (e.g., ‹PSATE›, formed from base-word ‹PASTE›) than corresponding replaced-nonwords (e.g., ‹PLUTE›). In Hindi’s orthography, letters map transparently to phonemes (excep...

Intermittency is a flexible control process entailing context-sensitive engagement with task constraints. The present work aims to situate the intermittency of dexterous behavior explicitly in multifractal modeling for non-Gaussian cascade processes. Multiscale probability density function (PDF) analysis of the center of pressure (CoP) fluctuations...

Standing still and focusing on a visible target in front of us is a preamble to many coordinated behaviors (e.g., reaching an object). Hiding behind its apparent simplicity is a deep layering of texture at many scales. The task of standing still laces together activities at multiple scales: from ensuring that a few photoreceptors on the retina cove...

Healthy human postural sway exhibits strong intermittency, reflecting a richly interactive foundation of postural control. From a linear perspective, intermittent fluctuations might be interpreted as engagement and disengagement of complementary control processes at distinct timescales or from a nonlinear perspective, as cascade-like interactions a...

Intermittency is a flexible control process entailing context-sensitive engagement with task constraints. The present work aims to situate the intermittency of dexterous behavior explicitly in multifractal modeling for non-Gaussian cascade processes. Multiscale probability density function (PDF) analysis of the center of pressure (CoP) fluctuations...

As a hallmark of dexterity, intermittency warrants explicit acknowledgment in models of dexterous behavior. However, intermittency poses a major challenge to modeling itself. The present work aims to situate investigation of intermittency on a firmer multifractal footing. In a previous study (Furmanek et al., 2020), multiscale probability density f...

Modern biomedical, behavioral and psychological inference about cause-effect relationships respects an ergodic assumption, that is, that mean response of representative samples allow predictions about individual members of those samples. Recent empirical evidence in all of the same fields indicates systematic violations of the ergodic assumption. I...

The ‘quiet eye’ (QE) approach to visually-guided aiming behavior invests fully in perceptual information's potential to organize coordinated action. Sports psychologists refer to QE as the stillness of the eyes during aiming tasks and increasingly into self- and externally-paced tasks. Amidst the ‘noisy’ fluctuations of the athlete’s body, quiet ey...

Vector autoregression (VAR) modeling allows probing bidirectional relationships in gender/sex development and may support hypothesis testing following multi-modal data collection. We show VAR in three lights: supporting a hypothesis, rejecting a hypothesis, and opening up new questions. To illustrate these capacities of VAR, we reanalyzed longitudi...

Standing still and focusing on a visible target in front of us is a preamble to many coordinated behaviors (e.g., reaching an object). Hiding behind its apparent simplicity is a deep layering of texture at many scales. The task of standing still laces together activities at multiple scales: from ensuring that a few photoreceptors on the retina cove...

A growing consensus across otherwise disparate perspectives on perception and action is that visually guided postural control emerges from within task constraints. Task constraints generate physiological fluctuations across various parts of the body. These fluctuations foster exploration of the available sensory information. For instance, standard...

Research into haptic perception typically concentrates on mechanoreceptors and their supporting neuronal processes. This focus risks ignoring crucial aspects of active perception. For instance, bodily movements influence the information available to mechanoreceptors, entailing that movement facilitates haptic perception. Effortful manual wielding o...