Brief background
Adaptation
and plasticity are central to our nervous system’s ability to acquire
new abilities, adapt them to changing situations, and recover function
after injury. The role of synaptic plasticity in sensorimotor learning
and adaptation is the subject of much work described in several reviews [19,20,21,22].
Here, our main interest is the role of plasticity in the recovery of
function after injury for neurorehabilitation. For example, after
stroke, neurons re-wire connections both immediately surrounding the
injury and across distant brain areas [23,24,25,26,27,28], and these changes in connectivity are correlated with improvements in function with and without rehabilitation [27, 28].
Critically, adaptation and plasticity are not guaranteed to be fast,
well-guided, or beneficial (i.e., adaptive plasticity). Plasticity can
also be maladaptive [29, 30], a neurorehabilitation analog to focal dystonias [31, 32].
A
key goal of neurorehabilitation therapies is to promote plasticity
mechanisms that improve function while also mitigating maladaptive
changes to neural circuits. Many therapeutic approaches have been
proposed and attempted to achieve this goal—with varying degrees of
success—ranging from targeted behavioral training to implantable devices
that stimulate neural circuits.
Computational modeling of adaptation and plasticity can provide paths to maximize the impact of neurorehabilitation therapies [3, 33].
The plasticity and adaptation that occurs after injury span many levels
of the nervous system, from cellular processes (e.g., changes in ion
channel expression) [34], to network interactions (e.g., changes in synaptic connections) [27], to behavioral changes (e.g., compensatory strategies) [35]. As a result, a wide range of modeling methods have been used to describe plasticity at different levels [36].
In lieu of an exhaustive survey of existing models, we highlight some
useful categories of model types. For a particular phenomenon, such as
synaptic plasticity, models may focus on different levels of
abstraction. For example, phenomenological models like Hebb’s rule
describe input-output relationships between the rate/timing of neural
activity and connection changes, describing the computational principle
without directly modeling the biological implementation [37].
Biophysical models of spike-timing dependent plasticity, in contrast,
describe the physiological changes within neurons that give rise to
synaptic changes [38]. Data-driven models (i.e., machine learning) have also been employed for modeling a variety of plasticity phenomena (e.g., [39]).
A
variety of model types, spanning different levels of the nervous
system, have been used to describe how the nervous system will respond
to an intervention to inform rehabilitation therapies [3, 40].
Mechanistic models of how behavior evolves as we adapt to altered
dynamics like a split-belt treadmill, for example, informed training
interventions to improve gait post-stroke [41].
Phenomenological Hebbian plasticity models informed stimulation
protocols to increase the functional connections among regions in the
brain [11, 42, 43], and from the brain to muscles or spinal circuits [44,45,46,47].
Models describing nervous system changes over time are also valuable
for predicting outcomes and to guide clinical decision-making. Many
examples of these models rely on data-driven discovery from large
datasets. For example, the increasing prevalence of neural imaging
technologies in clinical practice have led to large datasets to develop
algorithms that predict functional recovery after stroke [48].
Machine learning approaches have also been used to assess whether
devices like non-invasive brain-computer interfaces will be effective [49, 50] and optimal parameters for therapies like deep brain stimulation [51].
These
examples highlight the diversity of plasticity models and applications
in neurorehabilitation. As with any other computational modeling effort [2],
decisions must be made about the level of abstraction and detail. The
challenge of these decisions is acutely clear in the realm of adaptation
and plasticity, where mechanisms span spatial scales from synapses to
behavior, and timescales from milliseconds to months [20].
Many existing models used for neurorehabilitation focus on a single
scale (e.g., describing behavioral changes). Models that bridge
neurological mechanisms of plasticity to behavior will likely be needed
to improve the precision of neurorehabilitation therapies. Such models
will require cross-disciplinary collaboration to develop and validate.
Similarly, most existing models focus on describing a particular
time-point, such as functional recovery after a certain time with a
particular, static therapy. The many time-scales of adaptation and
plasticity present challenges for modeling overall trajectories,
including the impact of interventions and changes in treatment over time
[52].
Extending
models of plasticity to span spatial and temporal scales could open new
ways to harness the power of computational methods in
neurorehabilitation. The dynamic nature of the nervous system creates a
variety of challenges for building therapies. Assuring an assistive
device provides meaningful functionality for extended periods of time
requires characterizing plasticity that may occur in response to the
device and developing devices that can adapt accordingly [53]. Similar considerations are needed for therapies where protocols may need to adapt over time as abilities change [54]—a form of meta-adaptation that mirrors meta-plasticity (i.e., ‘plasticity of plasticity’ [36]).
Achieving the goal of smart, personalized, and adaptive
neurorehabilitation therapies will require models that can capture the
dynamics of plasticity processes as well as human-device interactions
that will influence those dynamics. This will require new approaches to
bridge across models that predict how the nervous system will change in
response to a given intervention and those to describe how interventions
inpact the trajectory of changes in the nervous system and changes in
behavior over time.
Commentary
The
DARE workshop highlighted many fundamental challenges and opportunities
in modeling adaptation and plasticity for rehabilitation applications.
Multiple presentations (see Appendix for speaker summaries) speak to the
promise of using computational models to disentangle diverse learning
mechanisms used by the nervous system (e.g., Roth, Mariscal).
Mechanistic models such as those used by Roth shed light on the
neurophysiological underpinnings of disorders. Their findings, for
instance, suggest that Parkinson’s disease can lead to deficits in a
single learning mechanism while leaving others intact. Similarly,
data-driven methods to identify components of learning used by Mariscal
allowed them to characterize how learning generalized to new contexts
more precisely than past studies. A critical next step missing from most
workshop submissions is using these model-derived insights to directly
guide clinical therapies. Future work towards these efforts will have to
contend with challenges closely related to those faced in
personalization efforts (see below). For example, do models and their
parameters need to be estimated on populations of people or individuals?
Models may also require updating over time as learning proceeds,
closely mirroring challenges faced in human-device interactions.
Multiple
presentations (e.g., Liew, Orsborn, Hight, Schwock) aimed to
characterize plasticity that occurs as the result of therapies and
interventions or injuries. Schwock’s work highlights the potential
benefits of computational models to quantify changes between regions of
the nervous system when they are embedded within a large network (see
also [55]).
This work highlights the challenge of identifying the most useful
measures of nervous system plasticity, since nearly all metrics will be
approximations. Data-driven studies probing how physiological
measurements relate to clinical outcomes will likely be critical to
identify the most useful experimental and computational measures of
plasticity for neurorehabilitation. Collaborations between researchers
developing novel assays of plasticity and those using large clinical
datasets to predict clinical outcomes, such as discussed by Liew, will
be invaluable for future research and translation. Though such
collaborations will likely involve navigating the challenges of
measurement feasibility, such challenges highlight the potential promise
of extending neurorehabilitation ‘in-the-wild’ (see below) and research
into quantifying plasticity metrics.
Designing interventions that
induce plasticity is central to any rehabilitation effort. Data-driven
predictive models, such as those developed by Liew, provide methods for
predicting how someone may respond to an intervention dose. However,
these models have largely been used to predict a single endpoint, which
may miss dynamic interactions between plasticity and an intervention, as
highlighted by other presentations (e.g., Orsborn, Hight). Research
with brain-computer interfaces and cochlear implants demonstrate that
even interventions that intend to replace a function (rather than
rehabilitate) induce plasticity. This plasticity may be influenced by
how the device is designed (e.g., Orsborn’s investigations into
co-adaptation with brain computer interfaces), and could be further
manipulated by purposeful device interventions (e.g., vagus nerve
stimulation presented by Hight). User-device interactions to shape
plasticity open a huge opportunity to shape plasticity for
rehabilitation. Capitalizing on this opportunity, however, will require
improving models of how devices induce plasticity. Translating methods
to shape plasticity with devices into meaningful clinical therapies will
also require methods to predict functional outcomes.
Beyond these
examples of scientific challenges, we also noticed important challenges
and opportunities to create the scientific community needed to tackle
these challenges. All talks focused on plasticity, but we were
particularly struck by the topic diversity. For instance, the
presentations spanned upper limb movements (Roth, Orsborn), locomotion
(Mariscal), clinical sensorimotor function assessments (Liew), and
hearing/speech (Hight). There was also a diverse range of methods used
to quantify plasticity, from behavior (Roth, Mariscal, Liew), clinical
neuroimaging (Liew), and high-resolution electrophysiology (Orsborn,
Schwock). This breadth fostered rich discussions across sub-fields that
do not regularly interact. Integrating the knowledge gained from this
diversity of methods and applications and refining models of plasticity
and adaptation for rehabilitation will require bridges across these
communities and translating terminology between fields.
More at link.