Neuronal activity in the brain has been traditionally separated into ongoing activity, which lacks a particular sensory stimulus or movement, and evoked activity, which is the response to a well-defined stimulus. Yet, studies have consistently shown that ongoing activity predicts stimulus response (Arieli et al., 1996; Tsodyks et al., 1999; Luczak et al., 2009) and behavioral outcome (Supèr et al., 2003; Womelsdorf et al., 2006) suggesting that both forms of activity are closely related. During the last decade, ongoing activity has been found to organize as neuronal avalanches (Beggs and Plenz, 2003) — scale-invariant activity cascades whose size, duration, waveform and inter-cascade intervals are governed by power laws (for review see Chialvo (2010) and Plenz (2012)). From microscale to macroscale organization, neuronal avalanches describe spontaneous firing of local pyramidal neuron groups in awake rodents in vivo (Bellay et al., 2015), the ongoing local field potential (LFP) and subthreshold population activity in rodents and nonhuman primates (Gireesh and Plenz, 2008; Petermann et al., 2009; Scott et al., 2014), as well as resting activity in humans observed using functional magnetic resonance imaging (fMRI) (Fraiman and Chialvo, 2012; Tagliazucchi et al., 2012; Haimovici et al., 2013), magnetoencephalography (MEG) (Palva et al., 2013; Shriki et al., 2013) and electrocorticography (ECoG) (Priesemann et al., 2014). Neuronal avalanches signify a cortical network at a critical or near-critical state (Chialvo, 2010; Plenz, 2012), which experiments (Shew et al., 2009; Gautam et al., 2015) and simulations (Beggs and Plenz, 2003; Bertschinger and Natschläger, 2004; Haldeman and Beggs, 2005; Kinouchi and Copelli, 2006; Rämö et al., 2007; Nykter et al., 2008) have shown to hold numerous advantages in information processing, such as maximum mnemonic repertoire size (Haldeman and Beggs, 2005; Shew et al., 2011; Fagerholm et al., 2016), information diversity (Nykter et al., 2008) and dynamic range (Kinouchi and Copelli, 2006; Shew et al., 2009; Gautam et al., 2015), optimized computational capabilities (Bertschinger and Natschläger, 2004), information transmission (Beggs and Plenz, 2003; Rämö et al., 2007; Fagerholm et al., 2016), sensory discrimination (Tomen et al., 2014) and learning (de Arcangelis and Herrmann, 2010). Yet, despite these potential advantages, evidence for neuronal avalanches outside the realm of ongoing activity, specifically, during behaviorally relevant motor and cognitive tasks, has been controversial.

Recent analysis of neuronal avalanches during sustained dynamic movie stimulation in the visual cortex of turtle (Shew et al., 2015) and visuo-motor tasks in human fMRI (Fagerholm et al., 2015) and EEG (Arviv et al., 2015) suggest transient dynamics that differ from scale-invariant avalanches. Similarly, neural models implied transient deviations from avalanche dynamics during strong external inputs (Millman et al., 2010; Taylor et al., 2013; Hartley et al., 2014; Stepp et al., 2015; Williams-García et al., 2014). Here, we hypothesize that transient changes in activity during evoked responses could simply reflect a change in the rate of avalanches rather than a transition to a different dynamical regime, for example a change in synchronization. Such a rate change likely requires modifications to standard avalanche analysis, as the standard was originally introduced for stationary rates of activity (Beggs and Plenz, 2003). In fact, by taking changes in activity rate into account, here we show in the behaving nonhuman primate that cortical dynamics during sensory and cognitive processing exhibit clear power law organization in line with the maintenance of neuronal avalanches. In line with these findings, our phase-synchronization analysis demonstrates that synchrony levels are maintained for ongoing and task-related activity. We expand on our experimental results using computational models and explore the sensitivity of this advanced approach to distinguish avalanche dynamics from states with changed synchrony, for example supercritical dynamics. Our results extend previous work on the dynamics of resting state activity and suggest that the optimization principles identified for critical dynamics during resting state activity extend to and sculpt cortical information processing during behavior.

In order to demonstrate avalanche maintenance during evoked activity that is independent of specific brain region or behavioral paradigm, we chose two types of tasks and two cortical areas in two adult macaque monkeys. High-density microelectrode arrays (MEA; 10 × 10; corner electrodes missing; inter electrode distance Δd = 400 µm) were chronically implanted in left prefrontal cortex (PFC) of monkey A and left premotor cortex (PM) of monkey B. We recorded extracellular unit activity (0.3–3 kHz) to confirm involvement of recording sites in behavioral tasks as well as the LFP (1–100 Hz), which is commonly used for avalanche analysis (Beggs and Plenz, 2003; Petermann et al., 2009; Scott et al., 2014).

Avalanche maintenance during a cue-triggered cognitive task

In our first behavioral condition, we asked whether scale-invariant dynamics can be found when strong transient activity changes due to cognitive processing are present. To this end, we analyzed avalanche dynamics in the dorsal-lateral prefrontal cortex (dlPFC; left hemisphere) for a visual-motor mapping task (monkey A; Figures 1–3). Monkey A applied a learned rule (>95% success rate) to two individually presented visual cues, which instructed the retrieval of food from a left feeder (left trials) or right feeder (right trials).

Figure 1

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Figure 2

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Figure 3

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It is known that the dlPFC is involved in such visual-motor mapping task (Asaad et al., 1998; Puig and Miller, 2012). Consistently, in all of 29 putative single-units recorded in the dlPFC (1 day, 1 hr recording session), unit activity was variable with transient increases or decreases in firing rate around cue on- and off-set that depended on left or right trials (Figure 1A; three examples shown). No significant change in Fano Factor (Churchland et al., 2010) was found during task execution (Figure 1B; see Materials and methods). On the other hand, the average LFP changed consistently for each electrode exhibiting two waveforms that differed between left and right trials (Figure 1C). We extracted peak times of the negative LFP (nLFP) at a threshold thr = –2 SD for further analysis (see Materials and methods), and observed a rapid ~8-fold increase in nLFP rate ~0.5 s after cue presentation, when the monkey was preparing to reach for the left feeder (Figure 1D, single electrode; Figure 1E, all electrodes). A much weaker, more delayed increase was observed for right trials. Based on the delayed increase in nLFP rate for left trials, we divided trials into a baseline period before cue-onset (BASE; –0.4–0 s), and an early (EARLY; 0–0.4 s) and late epoch (LATE; 0.4–0.8 s) after cue-onset.

To study neuronal avalanches, two 1 hr recording sessions from 2 consecutive days resulting in 322 trials (day 1: 139, day 2: 183) were combined. In short, for each trial successive nLFPs on the array were concatenated into contiguous clusters at temporal resolution Δt (Beggs and Plenz, 2003). A cluster started with the transition from an empty time bin of width Δt to a time bin with at least one nLFP on the array. The concatenation continued until a time bin with no nLFP was encountered (see also Figure 2A). The temporal resolution Δt was defined by the inverse of the average nLFP rate on the array and is not a free parameter (Beggs and Plenz, 2003). In line with standard avalanche analysis, we kept Δt fixed (‘fixed binning’) and determined its value from the full recording to be Δt = 4.75 ms for monkey A. We found that the probability density function of cluster size s, that is the number of nLFPs in a cluster, followed a power law, for both left and right trials, with slope α close to –1.5 and a cut-off for s > 96, the maximal number of MEA electrodes (Klaus et al., 2011; Yu et al., 2014) (Figure 1F, solid black and green lines for left and right trials, respectively; exponential vs. power law: ${\displaystyle p<{10}^{-5}}$; see Materials and methods). Importantly, these hallmarks of avalanche dynamics were destroyed by trial shuffling, which removes spatial trial-to-trial correlations while maintaining average rate changes (Figure 1F, broken lines; exponential vs. power law: ${\displaystyle 1-p<{10}^{-5}}$). This control demonstrates that the power law in cluster sizes, indicative of avalanche dynamics, does not simply arise from transient activity changes during behaviorally relevant periods.

By combining clusters from different behavioral epochs into one single distribution, however, transient deviations from scale-invariance could cancel each other, giving the appearance of maintained avalanche dynamics during behavior. Indeed, when separating clusters according to trial epochs BASE, EARLY and LATE, systematic differences in the corresponding avalanche raster were revealed. Specifically, the increase in nLFP rate during LATE for left trials yielded significantly larger avalanches during the period prior to reach (Figure 2B, red dots; Figure 2C, orange and purple segments indicate significantly high and low values, respectively; $p\le 0.05$; see Materials and methods). Accordingly, a preponderance of large avalanches was visible in the corresponding size distribution during LATE for left trials (Figure 3A and B, left), whereas size distributions remained close to a power law during BASE and EARLY and for right trials (Figure 3A and B; ${\displaystyle p<{10}^{-5}}$, all cases). In the case of right trials, where the transient changes in activity rate are much less pronounced, size distributions are more similar among all epochs, with a small tendency for larger avalanches during the LATE epoch (Figure 3A and B, right; ${\displaystyle p<{10}^{-5}}$, all cases).

These deviations from scale-invariance compared to BASE, on the other hand, were abated when the temporal resolution Δt to define clusters was obtained from the average nLFP rate for each trial i and epoch separately (${\displaystyle \mathrm{\Delta }{t}_i^{BASE,EARLY,LATE}}$; Figure 2D–F). We define this approach as ‘adaptive binning’. It decreases Δt during periods of high nLFP rate, while increasing Δt for low activity periods (Figure 2D). Indeed, adaptive binning obtained scale-invariant power laws in avalanche sizes that were similar for each epoch (Figure 3C,D; $p=0.06\left(0.08\right)$ and $p=0.11\left(0.1\right)$ for EARLY (LATE) compared to BASE, for left and right trials respectively; see Materials and methods), with a change in exponent consistent with previous results (Beggs and Plenz, 2003; Petermann et al., 2009; Priesemann et al., 2014) and a cut-off for s > 96 (Klaus et al., 2011; Yu et al., 2014). Comparing BASE and LATE avalanches for each trial, the bias toward larger ones in the latter epoch seen when employing fixed binning is significantly decreased after adaptive binning for left trials (Figure 3E, left), while during right trials, in which scale-free avalanches were obtained even for fixed binning, the bias does not change after adaptive binning. As can be seen in the corresponding raster plots and trial averages (Figure 2E,F), avalanche size decreased in LATE concomitant with an increase in avalanche rate (more prominently for left trials). We conclude that the apparent over-representations of large avalanches during behavioral epochs were due to a mismatched temporal resolution Δt by not accounting for systematic changes in nLFP rate during different epochs.

We further explored if nLFP rate within an epoch correlates with behavioral outcome. Indeed, reaction time, defined as the time it took the monkey to reach the reward after the visual cue was switched off, did not correlate with nLFP rate (left trials: R = 0.036, p=0.73; right trials: R = −0.039, p=0.71; p-values indicate the likelihood of obtaining a correlation at least as high by chance), avalanche rate (left trials: R = 0.041, p=0.68 for fixed Δt and R = −0.022, p=0.79 for adaptive Δt; right trials: R = −0.033, p=0.74 for fixed Δt and R = 0.012, p=0.87 for adaptive Δt) or avalanche size (left trials: R = 0.085, p=0.52 for fixed Δt and R = 0.062, p=0.66 for adaptive Δt; right trials: R = 0.058, p=0.63 for fixed Δt and R = −0.025, p=0.72 for adaptive Δt), in line with our finding of a scale-free regime regardless of rates for each trial. Given the monkey’s high performance (>95% successful trials), rates could not be analyzed for correct/incorrect trial outcome.

Avalanche maintenance during self-initiated movements

In monkey B, we studied activity changes during self-initiated movements. The monkey, without further cue or prompting, initiated the touching of a pad with her right hand, after which a reward was provided. Similar to monkey A, two 1 hr sessions were recorded in 2 consecutive days. Since results were highly consistent between different days, the data were pooled together, resulting in 663 trials analyzed (day 1: 319; day 2: 344). As the recording site was within the arm representation area within the PM, recorded activity clearly reflected task execution (Figure 4). About 200 ms before touching the pad, 18/43 (~42%) putative single-units increased in firing rate, whereas 5/43 units (~12%) decreased their firing rate around the time of touching (Figure 4A,B). Fano Factor of unit activity decreased in line with previous reports (Figure 4C) (Churchland et al., 2010). Similarly, the LFP transiently turned negative ~200 ms before touching for all electrodes (Figure 4D), together with a ~4-fold increase in the nLFP rate and a slight decrease thereafter (Figure 4E), which readily separated activity into a baseline epoch (BASE; –0.8 to –0.4 s relative to the touch), followed by a pre-touch (PRE; –0.4 to 0 s) and post-touch epoch (POST; 0 to 0.4 s) (Figure 4F).

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When all data across epochs was analyzed using fixed binning, that is Δt based on the full recording (Δt = 10.3 ms), avalanche sizes followed a power law with –1.5 exponent and cut-off at ~96 electrodes (Figure 4G; exponential vs. power law: ${\displaystyle p<{10}^{-5}}$), which was destroyed by trial shuffling (exponential vs. power law: ${\displaystyle 1-p<{10}^{-5}}$), again demonstrating that scale-invariant LFP clusters were based on the trial-by-trial correlations between cortical sites and did not simply arise from transient changes in nLFP rate.

Evaluation of avalanche raster plots based on epochs revealed systematic differences in the corresponding avalanche raster (Figure 5A,B). Specifically, the increase in nLFP rate during PRE yielded significantly more avalanches and of larger size during that period, whereas the converse was true during POST when nLFP rate decreased (Figure 5A; colored dots; Figure 5B; orange and purple segments respectively; $p\le 0.05$; see Materials and methods). The corresponding size distributions exhibited deviations from scale-invariance obtained for BASE (Figure 6A and B, left) with an excess of large avalanches for PRE and a deficit of large avalanches for POST, despite all distributions remaining close to power laws (exponential vs. power law: ${\displaystyle p<{10}^{-5}}$, all three epochs).

Figure 5

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Figure 6

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As in the case for the cognitive task, these systematic deviations in size distributions were abated when calculating Δt from the average nLFP rate for each trial i and epoch (${\displaystyle \mathrm{\Delta }{t}_i^{BASE,PRE,POST}}$; Figure 6A and B, right). Through adaptive binning we once again obtained scale-invariant power laws in avalanche size that were similar for each epoch ($p=0.177$, PRE vs. BASE; $p=0.05$, POST vs. BASE), with the bias towards larger avalanches during PRE being significantly reduced (Figure 6C). As can be seen in the corresponding raster plots and trial averages (Figure 5C,D), avalanche size decreased in PRE concomitant with an increase in avalanche rate confirming our findings in monkey A. During POST, under representation of large avalanches due to mismatched temporal resolution Δt can also be partially compensated for by adaptive binning.

Impact of ‘adaptive binning’ in capturing rate change vs. change in dynamics

We next illustrate the expected deviation from scale-invariance when rates change and its remediation using ‘adaptive binning’ in a simple schematic. In the hypothetical example shown in Figure 7A, we consider a baseline period with nLFP rate of 3 Hz per electrode, followed by a period with the rate increased to 9 Hz (Figure 7A, left). Fixed binning, which sets Δt according to the average rate of 6 Hz, underestimates the temporal resolution for baseline activity, whereas it overestimates Δt for the high activity period. The resulting power law size distributions will be steeper for the baseline period and shallower for the high activity period (Beggs and Plenz, 2003). The combined distribution will show an upward bend at the cross-over of the individual distributions (Figure 7A; middle, solid line) which seems to suggest a deviation from avalanche dynamics with an overabundance of large clusters similar to what would be expected for supercritical dynamics. Adaptive binning, that is calculating Δt for each period separately approximates the exponents for both power laws, resulting in a final distribution that matches a power law with an intermediate slope (Figure 7A, right) (Beggs and Plenz, 2003; Petermann et al., 2009; Priesemann et al., 2014). We note that simply using shorter time bins for all data would retain different slopes for different regimes and thus is not an alternative to adaptive binning.

Figure 7

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Although we illustrated this principle using two sequential periods with different rates, the same holds for repeat trials with different rates. In Figure 7B, we separated trials of monkey B into low- and high-rate responses based on PRE epochs (Figure 7B, left). Size distributions obtained from high-rate trials at fixed binning exhibit an overabundance of large avalanches (Figure 7B, middle, arrow). With adaptive binning, deviations from the baseline distribution decreased and all distributions approached power laws with a –1.2 exponent (Figure 7B, right). This demonstrates that trials in monkey B exhibit similar dynamics as those observed for monkey A or, more generally, that deviations from scale-invariance observed in the two experimental regimes can be explained by nLFP rate changes alone.

‘Adaptive’ thresholding as an alternative approach to ‘adaptive’ binning

Adaptive binning reduces Δt for periods of high rates, thereby reducing the number of nLFPs per time bin. Alternatively, one can increase the threshold for nLFP detection during these periods, likewise reducing the number of nLFPs encountered per time bin (see also [Petermann et al., 2009]). The reverse argument holds when periods of low rates are encountered. We tracked the mean and standard deviation of the LFP in successive windows of 100 ms width and used a moving threshold thr = mean – 2SD to identify nLFPs within each window (Figure 8A,C). Thus, activity modulations induced by the task were mainly reflected in nLFP amplitude, but not rate. This approach, which trades temporal resolution for local sensitivity, collapsed distributions from different periods (grey and orange areas in Figure 8B,C) in monkey A into a similar power law, indicative of sustained avalanche dynamics during task performance (Figure 8D,E).

Figure 8

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Our findings suggest that cortical avalanches are maintained in nonhuman primates during information processing, thus adding avalanche dynamics to the many links found between ongoing and evoked activities in the brain (Arieli et al., 1996; Tsodyks et al., 1999; Womelsdorf et al., 2006). We suggest that optimal information processing capabilities of avalanche dynamics (Beggs and Plenz, 2003; Bertschinger and Natschläger, 2004; Haldeman and Beggs, 2005; Kinouchi and Copelli, 2006; Rämö et al., 2007; Nykter et al., 2008; Shew et al., 2009; Tomen et al., 2014; Gautam et al., 2015) might guide sensory, motor and cognitive processing in the brain.

During ongoing activity, avalanche dynamics have been demonstrated in various brain areas including the primary visual (Hahn et al., 2010), primary motor (Petermann et al., 2009), somatosensory (Gireesh and Plenz, 2008), premotor and prefrontal (Yu et al., 2014) cortices, reflecting a common dynamic feature regardless of specific cortical areas. Therefore, we aimed to extract a similar rule that govern avalanches during evoked activity. To achieve this goal while minimizing the number of nonhuman primates used, we chose to record activity from different cortical sites and different behavioral tasks in two monkeys, while the common condition is that the recorded region was strongly activated by the corresponding task. Monkey A performed a visual-motor mapping task, in which the correct actions have to be applied to specific cues (Wise and Murray, 2000). This task is known to involve the dorsal-lateral prefrontal cortex (Asaad et al., 1998; Puig and Miller, 2012), in line with our finding of differential spiking as well as LFP responses for the left and right cues. Conversely, the target area in our self-initiated motor task for monkey B was the premotor cortex, which is involved in movement planning (Weinrich et al., 1984; Brasted and Wise, 2004) and self-initiated movement (Hoffstaedter et al., 2013). Consistent with previous studies, we found that, at both the single neuron and population level, the premotor cortex exhibited strong activation a few hundred milliseconds before the self-initiated touch. Our highly consistent results obtained for these different cortical areas and behavioral tasks suggest that avalanche dynamics might be a general principle governing cortical dynamics underlying various behaviors.

Our trial-shuffling control demonstrated that the power law in avalanche sizes reflects correlations that are maintained during behavior on a trial-by-trial basis and that this statistical feature did not simply arise from non-stationary rates. Recently, power laws, for example Zipf’s law, have been shown to arise when the range of event probabilities becomes enlarged, for example from underlying common correlations due to common inputs typically captured in latent variables (Schwab et al., 2014; Aitchison et al., 2016). Because trial-shuffling maintains such latent variables as captured in the average evoked response, yet, abolishes scale-invariance, our findings suggest that the scale-invariance encountered in our behavioral data reflects intrinsic correlation structure of trial-to-trial variability and not common correlations.

Originally introduced for resting or spontaneous activity (Beggs and Plenz, 2003), a fixed temporal resolution Δt assumes stationary rates, which is typically violated during sensory epochs or periods of movement when event rates change systematically. For avalanche analysis, the temporal resolution Δt is based on the inter-event interval distribution of the population, for example successive nLFP events on the array. This temporal scale Δt is not a free parameter. It further depends on the spatial scale, that is inter-electrode distance Δd, as well as local minimal event threshold thr. Importantly, all three parameters Δt, Δd and thr have been experimentally linked and shown to affect the slope of the power law in a predictable manner (Beggs and Plenz, 2003; Petermann et al., 2009). In our initial approach, that is adaptive binning, we kept the threshold thr and inter electrode distance Δd constant while changing Δt in accordance with the change in event rate, which in our experiments increased up to a factor of 10. This recovered power law statistics, which was confirmed by our simulation in which rate changes were created by increasing avalanche rate. These findings are in line with reported changes of the power law slope with changes in Δt (Beggs and Plenz, 2003; Petermann et al., 2009; Priesemann et al., 2014).

Our results highlight the importance of the principle of ‘separation of time scales’ (Bak et al., 1988; Vespignani and Zapperi, 1998; Plenz, 2012; Priesemann et al., 2014) for measuring avalanche dynamics. That is, the time scale at which cascading events unfold within individual avalanches, characterized by the avalanche duration, should be significantly shorter than the time scale at which different avalanches emerge, characterized by the inter-avalanche interval. Estimating the temporal resolution Δt from the inverse of the event rate provides a reasonable separation of time scales, because it balances two potential errors – false concatenation of separate avalanches and false separation of a single avalanche (Beggs and Plenz, 2003; Plenz, 2012).

In an alternative approach, we kept spatial resolution Δd and temporal resolution Δt fixed while changing threshold thr. This approach does not affect the power law organization, as demonstrated in nonhuman primates (Petermann et al., 2009) and human fMRI (Tagliazucchi et al., 2012). Particularly when Δt is difficult to change, changing thr might provide an alternative means to reestablish a separation of time scales, since large local events are less common than small local events. Our recovery of a power law for large local events during high activity periods demonstrates the analogous roles time and local event size play in avalanche dynamics. Our separate demonstration that synchrony does not change between BASE, EARLY/PRE and LATE/POST conditions further supports our finding that avalanches are maintained despite changes in event rates. Conversely, if thresholds are calculated individually for different temporal segments corresponding to different conditions of interest and the resulting distributions deviate from power laws along with concurrent changes in synchrony measures, then one has strong indication that the underlying dynamics deviate from criticality (Meisel et al., 2013).

It is important to emphasize that adaptive binning and thresholding are only tools to reveal the signatures of scale-free dynamics after they become blurred by increased drive and absence of separation of time-scales. They are not meant to be a perfect theoretical solution to the problem and have obvious limitations. For instance, there is no clear answer as to how large the window for calculating adaptive binning or thresholding should be. If it is too small, the inherent fluctuations of a critical system will be regarded as rate changes and therefore no power law dynamics can be observed. If it is too large, the rate changes can be averaged out inside them, and therefore the problem of the non-stationarity would remain. In the case of our experiments, we had a clear indication of when we should expect a rate change, which gave us a good parameter to choose the window size (~400 ms). Another problem arrives for extreme rate regimes. For very high drive, in which Δt would be much shorter than the time it takes for spikes to propagate from a neuron to its neighbors, it is unlikely that a power law can be recovered since activity becomes uncorrelated at that temporal resolution. On the other extreme, if there is almost no activity to be observed, a very large bin may require unrealistic long recordings to produce power laws. These two extreme cases may explain some of the residual error we found for left trials during PRE for monkey A (slightly larger avalanches even after adaptive binning) and during POST for monkey B (slightly smaller avalanches even after adaptive binning).

Another important point is that the advantages of criticality within a certain area allow it to optimally process information, regardless of the nature of the input received by that area. In fact, one of the advantages of criticality is maximal dynamic range (Kinouchi and Copelli, 2006; Shew et al., 2009; Gautam et al., 2015), which means that the downstream network which employs criticality will be able to respond properly for a larger range of different inputs. Nevertheless, it is possible downstream neurons employ a sort of adaptive binning mechanism. It has been suggested in simulations that the activity of an upstream network can influence how individual downstream neurons perform spatial and temporal integration at the synaptic level (Bernander et al., 1991; Rapp et al., 1992). The more input a cell receives, the more synchronized these incoming spikes have to be in order to produce a spike on the post-synaptic cell, effectively increasing the temporal resolution by which this downstream cell process its input (Bernander et al., 1991), similarly to how our proposed adaptive binning method works.

Neuronal avalanche dynamics are predominantly located in cortical layers 2/3 (Stewart and Plenz, 2006; Petermann et al., 2009) and exhibit strong non-linear components involving the interaction between cortical sites, that is the interactions of neurons or neuronal groups (Thiagarajan et al., 2010; Yu et al., 2011; Plenz, 2012). Accordingly, maintained avalanche dynamics during self-initiated and cue-related responses suggest non-linear interactions in superficial layers during cortical processing, which numerous studies have shown to be indeed the case. For example, evoked responses in superficial layers of visual cortex in the nonhuman primate have been found to be non-linear, potentially involving the local recurrent network (Williams and Shapley, 2007; Xing et al., 2012). Similarly, functional connectivity based on the interaction from neuronal firing in layer 2/3 of nonhuman primates has been found to contribute to motor coding (Hatsopoulos et al., 1998) and to be as robust or even superior over tuning curves in predicting motor outcome (Stevenson et al., 2012). With respect to their spatiotemporal spread, avalanches reveal a fractal dimension when analyzed individually (Plenz, 2012), with nearest-neighbor relationships in the average (Gireesh and Plenz, 2008; Yu et al., 2014), in line with compact spatiotemporal spreading of average evoked responses reported, for example, for premotor cortex (Rubino et al., 2006).

Our current findings may retroactively explain previous reports that found deviations from avalanche dynamics during stimulus presentation. Arviv et al. (2015) found power law avalanche size distribution when considering the full 1 s window for a visual detection task in human MEG recordings. This is consistent with our results when considering the entire period of task performance (~2 s). They also observed a trend toward supercritical dynamics during transient activity periods, just as we found predominantly large avalanches during high rate periods. Our analysis, though, shows adaptive binning as one potential approach to distinguish true supercritical dynamics from avalanche dynamics. Fagerholm et al. (2015) reported subcritical dynamics during focused attention in human EEG, in line with the observation of reduced correlations during such episodes (Cohen and Maunsell, 2009; Mitchell et al., 2009; Harris and Thiele, 2011). However, changes in activity levels were not reported in that study, and size distributions were evaluated outside their cut-off (Yu et al., 2014). Finally, early stimulus responses in the turtle’s visual system (Shew et al., 2015) have been reported to deviate from avalanches, a finding which might reflect the statistics of the stimulus used rather than intrinsic cortical dynamics.

Numerical simulations of systems under external drive or with non-zero spontaneous neuronal activity can never be truly critical, since the quiescent phase (and with it the transition connecting it to an active phase) disappears under these conditions (Taylor et al., 2013; Hartley et al., 2014; Williams-García et al., 2014). This effect is increasingly pronounced in simulations with small network size and large activity drive. Given the relatively small change in firing rate observed during behavior and the large size of the cortical network, our findings are in line with simulations that show weakly driven critical system to exhibit power law behavior when the temporal resolution is matched to the activity rate (Hartley et al., 2014).

In conclusion, our findings provide strong evidence that the scale-free organization of neuronal avalanches is maintained during evoked responses in premotor and prefrontal cortex, despite systematic fluctuations in firing rates. We therefore suggest that neuronal information processing during tasks might capitalize on various functional benefits shown for critical dynamics. This calls for further investigation into how critical dynamics may explain the execution of specific brain functions. Of equal importance, our results also shed new, methodological light on how to identify true deviations from avalanche dynamics under pathological conditions.

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Author details

1. Shan Yu

   Section on Critical Brain Dynamics, National Institute of Mental Health, Bethesda, United States

   Present address

   Institute of Automation, and Center for Excellence in Brain Science and Intelligence Technology, Chinese Academy of Sciences, Beijing, China

   Contribution

   Conceptualization, Data curation, Software, Formal analysis, Supervision, Validation, Investigation, Visualization, Methodology, Writing—original draft, Writing—review and editing

   Contributed equally with

   Tiago L Ribeiro

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-9008-6658

2. Tiago L Ribeiro

   Section on Critical Brain Dynamics, National Institute of Mental Health, Bethesda, United States

   Contribution

   Software, Formal analysis, Investigation, Visualization, Methodology, Writing—original draft, Writing—review and editing

   Contributed equally with

   Shan Yu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-3195-9284

3. Christian Meisel

   Section on Critical Brain Dynamics, National Institute of Mental Health, Bethesda, United States

   Contribution

   Software, Formal analysis, Visualization, Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

4. Samantha Chou

   Section on Critical Brain Dynamics, National Institute of Mental Health, Bethesda, United States

   Contribution

   Data curation, Writing—review and editing

   Competing interests

   No competing interests declared

5. Andrew Mitz

   Laboratory of Neuropsychology, National Institute of Mental Health, Bethesda, United States

   Contribution

   Resources, Data curation, Methodology

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-8045-0970

6. Richard Saunders

   Laboratory of Neuropsychology, National Institute of Mental Health, Bethesda, United States

   Contribution

   Resources, Data curation, Methodology

   Competing interests

   No competing interests declared

7. Dietmar Plenz

   Section on Critical Brain Dynamics, National Institute of Mental Health, Bethesda, United States

   Contribution

   Conceptualization, Resources, Software, Supervision, Funding acquisition, Validation, Investigation, Visualization, Methodology, Writing—original draft, Project administration, Writing—review and editing

   For correspondence

   plenzd@mail.nih.gov

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-0008-3657

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