81 research outputs found
Coexistence of critical sensitivity and subcritical specificity can yield optimal population coding
The vicinity of phase transitions selectively amplifies weak stimuli,
yielding optimal sensitivity to distinguish external input. Along with this
enhanced sensitivity, enhanced levels of fluctuations at criticality reduce the
specificity of the response. Given that the specificity of the response is
largely compromised when the sensitivity is maximal, the overall benefit of
criticality for signal processing remains questionable. Here it is shown that
this impasse can be solved by heterogeneous systems incorporating functional
diversity, in which critical and subcritical components coexist. The subnetwork
of critical elements has optimal sensitivity, and the subnetwork of subcritical
elements has enhanced specificity. Combining segregated features extracted from
the different subgroups, the resulting collective response can maximise the
tradeoff between sensitivity and specificity measured by the
dynamic-range-to-noise-ratio. Although numerous benefits can be observed when
the entire system is critical, our results highlight that optimal performance
is obtained when only a small subset of the system is at criticality.Comment: 7 pages, 4 figure
Diversity improves performance in excitable networks
As few real systems comprise indistinguishable units, diversity is a hallmark
of nature. Diversity among interacting units shapes properties of collective
behavior such as synchronization and information transmission. However, the
benefits of diversity on information processing at the edge of a phase
transition, ordinarily assumed to emerge from identical elements, remain
largely unexplored. Analyzing a general model of excitable systems with
heterogeneous excitability, we find that diversity can greatly enhance optimal
performance (by two orders of magnitude) when distinguishing incoming inputs.
Heterogeneous systems possess a subset of specialized elements whose capability
greatly exceeds that of the nonspecialized elements. Nonetheless, the behavior
of the whole network can outperform all subgroups. We also find that diversity
can yield multiple percolation, with performance optimized at tricriticality.
Our results are robust in specific and more realistic neuronal systems
comprising a combination of excitatory and inhibitory units, and indicate that
diversity-induced amplification can be harnessed by neuronal systems for
evaluating stimulus intensities.Comment: 17 pages, 7 figure
Mechanisms of Zero-Lag Synchronization in Cortical Motifs
Zero-lag synchronization between distant cortical areas has been observed in
a diversity of experimental data sets and between many different regions of the
brain. Several computational mechanisms have been proposed to account for such
isochronous synchronization in the presence of long conduction delays: Of
these, the phenomenon of "dynamical relaying" - a mechanism that relies on a
specific network motif - has proven to be the most robust with respect to
parameter mismatch and system noise. Surprisingly, despite a contrary belief in
the community, the common driving motif is an unreliable means of establishing
zero-lag synchrony. Although dynamical relaying has been validated in empirical
and computational studies, the deeper dynamical mechanisms and comparison to
dynamics on other motifs is lacking. By systematically comparing
synchronization on a variety of small motifs, we establish that the presence of
a single reciprocally connected pair - a "resonance pair" - plays a crucial
role in disambiguating those motifs that foster zero-lag synchrony in the
presence of conduction delays (such as dynamical relaying) from those that do
not (such as the common driving triad). Remarkably, minor structural changes to
the common driving motif that incorporate a reciprocal pair recover robust
zero-lag synchrony. The findings are observed in computational models of
spiking neurons, populations of spiking neurons and neural mass models, and
arise whether the oscillatory systems are periodic, chaotic, noise-free or
driven by stochastic inputs. The influence of the resonance pair is also robust
to parameter mismatch and asymmetrical time delays amongst the elements of the
motif. We call this manner of facilitating zero-lag synchrony resonance-induced
synchronization, outline the conditions for its occurrence, and propose that it
may be a general mechanism to promote zero-lag synchrony in the brain.Comment: 41 pages, 12 figures, and 11 supplementary figure
A mechanism for achieving zero-lag long-range synchronization of neural activity
Poster presentation: How can two distant neural assemblies synchronize their firings at zero-lag even in the presence of non-negligible delays in the transfer of information between them? Neural synchronization stands today as one of the most promising mechanisms to counterbalance the huge anatomical and functional specialization of the different brain areas. However, and albeit more evidence is being accumulated in favor of its functional role as a binding mechanism of distributed neural responses, the physical and anatomical substrate for such a dynamic and precise synchrony, especially zero-lag even in the presence of non-negligible delays, remains unclear. Here we propose a simple network motif that naturally accounts for zero-lag synchronization of spiking assemblies of neurons for a wide range of temporal delays. We demonstrate that when two distant neural assemblies do not interact directly but relaying their dynamics via a third mediating single neuron or population and eventually achieve zero-lag coherent firing. Extensive numerical simulations of populations of Hodgkin-Huxley neurons interacting in such a network are analyzed. The results show that even with axonal delays as large as 15 ms the distant neural populations can synchronize their firings at zero-lag in a millisecond precision after the exchange of a few spikes. The role of noise and a distribution of axonal delays in the synchronized dynamics of the neural populations are also studied confirming the robustness of this sync mechanism. The proposed network module is densely embedded within the complex functional architecture of the brain and especially within the reciprocal thalamocortical interactions where the role of indirect pathways mimicking direct cortico-cortical fibers has been already suggested to facilitate trans-areal cortical communication. In summary the robust neural synchronization mechanism presented here arises as a consequence of the relay and redistribution of the dynamics performed by a mediating neuronal population. In opposition to previous works, neither inhibitory, gap junctions, nor complex networks need to be invoked to provide a stable mechanism of zero-phase correlated activity of neural populations in the presence of large conduction delays
Estimating the impact of structural directionality: How reliable are undirected connectomes?
Directionality is a fundamental feature of network connections. Most
structural brain networks are intrinsically directed because of the nature of
chemical synapses, which comprise most neuronal connections. Due to limitations
of non-invasive imaging techniques, the directionality of connections between
structurally connected regions of the human brain cannot be confirmed. Hence,
connections are represented as undirected, and it is still unknown how this
lack of directionality affects brain network topology. Using six directed brain
networks from different species and parcellations (cat, mouse, C. elegans, and
three macaque networks), we estimate the inaccuracies in network measures
(degree, betweenness, clustering coefficient, path length, global efficiency,
participation index, and small worldness) associated with the removal of the
directionality of connections. We employ three different methods to render
directed brain networks undirected: (i) remove uni-directional connections,
(ii) add reciprocal connections, and (iii) combine equal numbers of removed and
added uni-directional connections. We quantify the extent of inaccuracy in
network measures introduced through neglecting connection directionality for
individual nodes and across the network. We find that the coarse division
between core and peripheral nodes remains accurate for undirected networks.
However, hub nodes differ considerably when directionality is neglected.
Comparing the different methods to generate undirected networks from directed
ones, we generally find that the addition of reciprocal connections (false
positives) causes larger errors in graph-theoretic measures than the removal of
the same number of directed connections (false negatives). These findings
suggest that directionality plays an essential role in shaping brain networks
and highlight some limitations of undirected connectomes.Comment: 29 pages, 6 figures, 9 supplementary figures, 4 supplementary table
Tracking the distance to criticality in systems with unknown noise
Many real-world systems undergo abrupt changes in dynamics as they move
across critical points, often with dramatic and irreversible consequences. Much
of the existing theory on identifying the time-series signatures of nearby
critical points -- such as increased signal variance and slower timescales --
is derived from analytically tractable systems, typically considering the case
of fixed, low-amplitude noise. However, real-world systems are often corrupted
by unknown levels of noise which can obscure these temporal signatures. Here we
aimed to develop noise-robust indicators of the distance to criticality (DTC)
for systems affected by dynamical noise in two cases: when the noise amplitude
is either fixed, or is unknown and variable across recordings. We present a
highly comparative approach to tackling this problem that compares the ability
of over 7000 candidate time-series features to track the DTC in the vicinity of
a supercritical Hopf bifurcation. Our method recapitulates existing theory in
the fixed-noise case, highlighting conventional time-series features that
accurately track the DTC. But in the variable-noise setting, where these
conventional indicators perform poorly, we highlight new types of
high-performing time-series features and show that their success is underpinned
by an ability to capture the shape of the invariant density (which depends on
both the DTC and the noise amplitude) relative to the spread of fast
fluctuations (which depends on the noise amplitude). We introduce a new
high-performing time-series statistic, termed the Rescaled Auto-Density (RAD),
that distils these two algorithmic components. Our results demonstrate that
large-scale algorithmic comparison can yield theoretical insights and motivate
new algorithms for solving important practical problems.Comment: The main paper comprises 18 pages, with 5 figures (.pdf). The
supplemental material comprises a single 4-page document with 1 figure
(.pdf), as well as 3 spreadsheet files (.xls
Dwelling Quietly in the Rich Club: Brain Network Determinants of Slow Cortical Fluctuations
For more than a century, cerebral cartography has been driven by
investigations of structural and morphological properties of the brain across
spatial scales and the temporal/functional phenomena that emerge from these
underlying features. The next era of brain mapping will be driven by studies
that consider both of these components of brain organization simultaneously --
elucidating their interactions and dependencies. Using this guiding principle,
we explored the origin of slowly fluctuating patterns of synchronization within
the topological core of brain regions known as the rich club, implicated in the
regulation of mood and introspection. We find that a constellation of densely
interconnected regions that constitute the rich club (including the anterior
insula, amygdala, and precuneus) play a central role in promoting a stable,
dynamical core of spontaneous activity in the primate cortex. The slow time
scales are well matched to the regulation of internal visceral states,
corresponding to the somatic correlates of mood and anxiety. In contrast, the
topology of the surrounding "feeder" cortical regions show unstable, rapidly
fluctuating dynamics likely crucial for fast perceptual processes. We discuss
these findings in relation to psychiatric disorders and the future of
connectomics.Comment: 35 pages, 6 figure
Signal integration enhances the dynamic range in neuronal systems
The dynamic range measures the capacity of a system to discriminate the
intensity of an external stimulus. Such an ability is fundamental for living
beings to survive: to leverage resources and to avoid danger. Consequently, the
larger is the dynamic range, the greater is the probability of survival. We
investigate how the integration of different input signals affects the dynamic
range, and in general the collective behavior of a network of excitable units.
By means of numerical simulations and a mean-field approach, we explore the
nonequilibrium phase transition in the presence of integration. We show that
the firing rate in random and scale-free networks undergoes a discontinuous
phase transition depending on both the integration time and the density of
integrator units. Moreover, in the presence of external stimuli, we find that a
system of excitable integrator units operating in a bistable regime largely
enhances its dynamic range.Comment: 5 pages, 4 figure
- …