Even in the absence of sensory stimulation the brain is spontaneously active. This background “noise” seems to be the dominant cause of the notoriously high trial-to-trial variability of neural recordings. Recent experimental observations have extended our knowledge of trial-to-trial variability and spontaneous activity in several directions: 1. Trial-to-trial variability systematically decreases following the onset of a sensory stimulus or the start of a motor act. 2. Spontaneous activity states in sensory cortex outline the region of evoked sensory responses. 3. Across development, spontaneous activity aligns itself with typical evoked activity patterns. 4. The spontaneous brain activity prior to the presentation of an ambiguous stimulus predicts how the stimulus will be interpreted. At present it is unclear how these observations relate to each other and how they arise in cortical circuits. Here we demonstrate that all of these phenomena can be accounted for by a deterministic self-organizing recurrent neural network model (SORN), which learns a predictive model of its sensory environment. The SORN comprises recurrently coupled populations of excitatory and inhibitory threshold units and learns via a combination of spike-timing dependent plasticity (STDP) and homeostatic plasticity mechanisms. Similar to balanced network architectures, units in the network show irregular activity and variable responses to inputs. Additionally, however, the SORN exhibits sequence learning abilities matching recent findings from visual cortex and the network’s spontaneous activity reproduces the experimental findings mentioned above. Intriguingly, the network’s behaviour is reminiscent of sampling-based probabilistic inference, suggesting that correlates of sampling-based inference can develop from the interaction of STDP and homeostasis in deterministic networks. We conclude that key observations on spontaneous brain activity and the variability of neural responses can be accounted for by a simple deterministic recurrent neural network which learns a predictive model of its sensory environment via a combination of generic neural plasticity mechanisms.
Our brains are always active, even when we rest or sleep. This may seem somewhat surprising given the high metabolic costs associated with neural activity [1]. It seems there should be some important function associated with such spontaneous brain activity to justify the associated metabolic expense. It might be that keeping the brain spontaneously active is much like keeping the engine of your car running while waiting at a red light—allowing you to take off faster once the light turns green. On the other hand, without the constantly reverberating neural activity, wouldn’t we be like automata that only jump to action if triggered by an external event? Maybe spontaneous brain activity, rather than being some form of “noise” that keeps the engine running, is the very core on which our minds are constructed. The problem of spontaneous activity is closely tied to the notoriously high trial-to-trial variability of cortical responses to identical stimuli [2]. Such variability is commonly interpreted as resulting from internal noise. However, while neurons seem to be quite noisy under frequently used laboratory conditions (DC input currents and room temperature), the noise is absent [3] or reduced [4–6] in more realistic conditions, and cannot account for the full trial-to-trial variability of neural responses [7,76]. Instead, spontaneous activity prior to stimulus onset seems to be the underlying cause of trial-to-trial variability. This was shown by Arieli et al. in a seminal study [8] almost 20 years ago using voltage-sensitive dye imaging (VSDI). They demonstrated that trial-to-trial variability can be almost perfectly predicted from the spontaneous activity prior to stimulus onset through a simple linear combination of the spontaneous activity prior to stimulus onset and the stimulus-triggered-average response. They concluded that “the effect of a stimulus might be likened to the additional ripples caused by tossing a stone into a wavy sea. ” This encouraged many follow-up studies shedding new light on the relation between spontaneous activity and evoked sensory responses (see [9] for review). Of particular interest are the following four recent findings: The findings on neural variability and spontaneous activity discussed above raise three important questions. First, do these findings reflect independent phenomena or are they somehow related? Second, what neurophysiological mechanisms give rise to these findings? Third, is there a functional meaning to these findings and, if so, what is it? There are two complementary ways of approaching these questions: bottom-up and top-down. A bottom-approach may try to identify a minimal network model whose mechanisms are modelled after biological findings that is able to reproduce the observed effects and explain how they are rooted in basic physiological mechanisms. A top-down or normative approach will start with an assumed function and investigate whether and how (approximately) optimal solutions to this function resemble the observed physiological findings. In the end, the goal is always to reconcile both views into a unified picture. Our approach follows the bottom-up route. We hypothesize that three properties of cortical circuits might underlie all these findings on spontaneous activity and neural variability: Evidence for the first property comes from a range of studies. First of all, neural structures that do not receive recurrent input, such as the retina, display much lower response variability [21,22] suggesting that neural variability is induced by fluctuations of recurrent input. Second, there is anatomical evidence that V1 receives less than 5% bottom-up input synapses and more than 95% of synapses stem from neurons within V1 and higher-level areas [23]. Retrograde labelling and paired recordings show that neurons responding to the same grating or natural stimulus are both more likely to be connected and have stronger connections than expected by chance, suggesting that these recurrent connections enforce specific neural populations to be coactive [24,25]. Another link between anatomy and function was obtained by comparing long-distance axonal projections and spontaneous activity patterns [26]. The authors found that spontaneous and evoked activity followed the axonal pathways. Similar results were obtained by comparing resting state activity in fMRI and anatomical pathways determined by diffusion tensor imaging [27]. Finally, purely functional studies show that the order of activation of individual cells is similar during spontaneous and evoked activity suggesting that it is constrained by the underlying recurrent connectivity [28]. Taken together, there is broad evidence that recurrent connectivity shapes the common structure of spontaneous and evoked activity. Second, evidence from developmental studies suggests that the statistics of features in the visual world get translated into cortical connectivity by means of Hebbian plasticity [29,30]. A recent study demonstrates that such self-organization is still present in adult animals as early as in V1 [31]: In this experiment, adult mice were stimulated with repeated sequences of gratings of different orientations. The authors then recorded activity from neurons in V1 and found that the circuit formed a predictive representation of these spatiotemporal sequences. In particular, the circuit “recognized” a sequence by responding stronger to it than to the reversed sequence and showed predictive behaviour by “filling in” activity when parts of the sequence were left out. Pharmacological controls suggest that the underlying plasticity was local to V1. Together with the observation that the statistics of spontaneous activity match the statistics of the environment of the organism [12,14–16], this suggests that the statistics of the environment are stored in the recurrent connectivity of the neural circuit and that the structure of spontaneous activity is simply an image of the learnt circuit structure that reveals itself when the network receives no external drive. Evidence for the third property comes mainly from computational neuroscience studies. While it is known that homeostatic plasticity is essential to restore a balance between excitation and inhibition after perturbation by different forms of input deprivation (reviewed in [32]), there are only few experimental studies on the direct interaction between homeostasis and learning (see, e. g. , [33,34]). Theoretical studies on neural plasticity, on the other hand, have long argued that homeostasis is needed to “tame the beast” of Hebbian learning by enforcing stable network dynamics [35]. These theoretical predictions are consistent with clinical findings from different brain pathologies. For example, dysfunctional homeostasis has been linked to autism spectrum disorder and intellectual disabilities [36]. To test if these three properties are indeed sufficient to account for the many findings on spontaneous activity and neural variability, we capture them in a minimal neural network model: The self-organizing recurrent neural network (SORN, [37]) is a network of interconnected excitatory and inhibitory populations of McCulloch & Pitts model neurons. These neurons have binary states determined by thresholding the input, are updated synchronously, and are fully deterministic. The excitatory neurons are recurrently connected and receive structured input sequences. A binarized form of spike-timing dependent plasticity (STDP) shapes these recurrent excitatory connections. The network dynamics are kept in check by two forms of homeostatic plasticity: Synaptic normalization ensures that the summed synaptic weights from and onto each neuron stay the same during STDP. Intrinsic plasticity regulates the thresholds of the excitatory neurons on a slow timescale to avoid silent or overly active neurons. SORN networks have been shown to have powerful sequence learning abilities. For example, on challenging sequence learning tasks involving hidden states and long temporal dependencies they greatly outperform analogous reservoir architectures without plasticity in the recurrent network [37]. More recently, they have also been shown to produce state-of-the-art results in learning artificial grammars with their performance resembling that of human subjects [38]. Furthermore, such networks have also been shown to capture experimental data on the statistics and fluctuations of synaptic efficacies [39]. These features make them interesting candidates for modelling experimental findings on spontaneous activity and neural variability. In the following we show how the SORN model reproduces the key findings on spontaneous activity and neural response variability discussed above. To capture the essence of the experimental situations above, we define two learning tasks: a sequence learning task and an inference task. Both are deliberately simple compared to previous SORN studies [37,38] but suffice to capture the key findings. The sequence learning task is inspired by [31] and consists of presenting the network with one or more discrete sequences of input stimuli. After replicating this study, which demonstrated the learning of predictive models in V1 of adult mice, we use similar tasks to study the similarity of spontaneous and evoked activity. The inference task is inspired by [20], where subjects are presented with an ambiguous face-vase stimulus and their FFA activity is used to predict their perceptions. Apart from replicating this specific result, the trial structure of this task allows us to study the effect of spontaneous activity on evoked activity. By carefully analysing the SORN’s behaviour in these two tasks, we show how it readily reproduces the key experimental findings on spontaneous activity and neural variability without requiring any internal noise. On the other hand, adding small amounts of noise to the network produces similar findings (see Discussion). Taken together, our results suggest that the experimental findings on neural variability reflect the dynamics of a largely deterministic but highly adaptive cortical network learning a predictive model of its sensory environment. Our findings also support the claim that the notorious variability of neural responses is largely due to spontaneous network dynamics. It is noteworthy that a simple model such as the SORN can account for such a diverse set of findings that were independently obtained in a range of different species with a range of different recording techniques (multi-electrode and single cell recordings, optical imaging, functional magnetic resonance imaging) in a number of different labs. It suggests that the SORN model as a minimal instantiation of the three properties introduced above captures some fundamental principles of cortical learning and information processing in a distilled form.
We begin by characterizing some dynamic properties of the network. For this, we stimulate the network with random alternations of ten letters, where each letter presentation corresponds to stimulating a subset of excitatory neurons (see Methods for a detailed description of the model and stimulation paradigms). After the fraction of excitatory to excitatory connections converges, we deactivate STDP and stop stimulation to observe the spontaneous activity. This spontaneous activity develops as the intrinsic plasticity of each neuron shifts the excitability thresholds to compensate for the missing input. Fig 1b and 1c shows that the activity of individual neurons of the network is well described by an exponential inter-spike interval (ISI) distribution and can be summarized by a coefficient of variation distributed around 1. These indicate that the individual neurons fire irregularly with Poissonian statistics, a key feature of neural variability in neocortex [10,40]. Additionally, while the network converges to a stable fraction of connections, the individual weights keep fluctuating (Fig 1d and 1e) as observed experimentally [41]. Finally, the network approaches a lognormal-like weight distribution after learning (Fig 1f), which is a documented property of cortical circuits [42]. Taken together, these results demonstrate that some essential features of neocortex taken as evidence for a “noisy” brain are readily captured by our deterministic model. Next, we replicate the sequence learning paradigm from [31]. In their study, the authors stimulated mice with a discrete set of gratings of different orientations. For example, they stimulated their mice with the sequence “ABCD”, which refers to four distinct gratings presented in succession. They then recorded activity from neurons in V1 and found that the circuit formed a representation of these spatiotemporal sequences. We modelled this experiment by stimulating our network with a similar set of stimuli. We model visual input to V1 by stimulating a small subpopulation of our excitatory units when a stimulus is presented. This subpopulation can be thought of as having receptive fields that align with the grating in the original study. To replicate the first experiment from [31], we stimulated our network with the sequence “ABCD” during an initial period of self-organization. After the stimulation, we deactivated STDP, stopped the stimulation, and ran the network only with intrinsic plasticity to get a period of spontaneous activity, similar to the sleep phase in the original study. Finally, we tested the network with either “ABCD” (same sequence) or “DCBA” (reversed sequence). The network matched the effects reported in Fig 1 of [31] in that it responded with a higher mean rate to “ABCD” than to “DCBA” indicating that the trained sequence was “recognized” (Fig 2a). In a control experiment we trained the network on all permutations of the original sequence (i. e. “ABCD”, “ABDC”, …, “DCBA”) and similar to the matching control from [31], the mean rate was similar for both test sequences. In a further experiment in [31], the authors then showed that V1 can “predict” upcoming sequence items by training the mice with “ABCD” and testing with “ABCD”, “A_CD” and “E_CD”. They showed that V1 responded more strongly to the first two test sequences than to the third one and more strongly to a “B” or a blank following an “A” than to the blank after an “E” (cp. Fig 4 of [31]). As before, our simulations qualitatively replicate these results (Fig 2b). Note that much more sophisticated sequence learning abilities in SORN have already been demonstrated in [37,38]. These two experiments demonstrate that our simple model captures key experimental findings on the physiology of sequence learning. With this as a backdrop, we can investigate the properties and interactions of spontaneous and evoked activity in similar learning experiments in the next sections. Spontaneous activity is structured in space and time [12,13] and revisits those states, i. e. neuronal activation patterns, more often that correspond to overrepresented features in natural stimuli, such as horizontal and vertical bars in [12]. It has been proposed that this is a result from adapting the spontaneous activity to the statistics of the evoked activity during development [16]. In order to model these findings, we adapted the sequence learning paradigm: During an initial self-organization phase, we stimulate the network with random alternations of two sequences: “ABCD” and “EFGH” with different relative probabilities. In a second phase, we deactivate STDP to study the evoked activity of the adapted network. Finally, we stop the input and observe the network’s spontaneous activity. Due to the intrinsic plasticity in the model, our network compensates for the missing input and produces spontaneous activity in the absence of noise. After observing the properties of spontaneous activity, we next investigate the interaction between spontaneous activity and evoked activity in an inference task with trial-like interleaved spontaneous and evoked activity. By inference task, we mean that the task involves integrating previously acquired knowledge with an ambiguous sensory input to arrive at a unique interpretation of that stimulus. This non-formal usage of the term inference has a long tradition going back at least to Hermann von Helmholtz who used the German expression “unbewusster Schluss”, which is usually translated as unconscious inference. This task is used to model the studies demonstrating that the neural variability significantly drops at the onset of a stimulus [10], that the evoked activity can be linearly predicted from the spontaneous activity [8], and that the spontaneous activity before stimulus onset predicts the decisions when a noisy [19] or ambiguous [20] stimulus is presented. We model these findings by training our networks on a task inspired by [20]. In their setting, an ambiguous face-vase stimulus is immediately followed by a mask. After the mask, the subjects have to decide whether they perceived a face or a vase. This trial structure allows us to study the interaction between spontaneous activity before stimulus onset and the evoked activity following the presentation of a stimulus. We model the task as follows: during training, the network is presented with two randomly alternating input sequences represented as “AXXX_ _ _ …” and “BXXX_ _ _ …” with different probabilities. “A” and “B” stand for the face and vase, respectively, the common “XXX” for the mask, and “_ _ _ …” for periods without input corresponding to a blank screen. During the initial self-organization the network learns to represent these sequences and their prior probabilities. After self-organizing to these stimuli, STDP is switched off and a linear classifier is trained to postdict whether “A” or “B” has been presented based on the neural activity at the first blank stimulus, “_”. We define the network “decision” as the postdiction of this classifier for either “A” or “B” similar to subjects’ “decision” whether they just saw a face or a vase. The mask ensures that the decision is based on an internal representation and not simply on a direct input-output mapping. During subsequent testing, the network is stimulated with ambiguous mixtures of “A” and “B”, which we indicate as “A/B”, followed by the mask. Similar to human subjects reporting on their ambiguous percept, the network then has to decide with the classifier for “A” or “B” in light of the ambiguity of “A/B” and the prior probabilities of “A” and “B” during training. Different levels of ambiguity are modelled by stimulating fA × 10 input units of “A” and (1 − fA) × 10 input units of “B” where fA is a fraction. Further model details are given in the Methods. As in the previous section, we will first investigate the qualitative behaviour of the model self-organizing to these stimuli and compare them to the experimental findings. Thereafter, we will describe how the network performs inference for the ambiguous stimuli. Given all these data, the question arises how the underlying mechanisms interact to give rise to this variety of features. To better understand the dynamics of the neurons, we determine the conditional probability for neuron xi to spike given that neuron xj spiked at the prior time step when no input is presented. This will elucidate both how the excitatory connectivity affects the network dynamics and how the network dynamics affect the connectivity via STDP.
We chose a bottom-up modelling approach to address the question of the origin of neural variability and spontaneous activity. Ultimately, our goal is to reconcile such an approach with top-down functional approaches. Several features of neural variability and spontaneous activity have an elegant interpretation in terms of sampling theories of perceptual inference [45,48]. In such theories, neural activity is interpreted as samples from a posterior probability distribution over the states of certain variables of interest given a sensory input. Correspondingly, spontaneous activity in the absence of sensory input is interpreted as samples from a prior probability distribution over these variables. This view elegantly captures several of the key experimental observations. For example, in this view the decrease in response variability after stimulus onset simply reflects that the posterior distribution over properties of the stimulus once it has been observed will be “narrower” compared to the prior distribution before stimulus presentation. The same holds for the outlining of evoked responses by spontaneous activity. We showed that a fully deterministic SORN model can exhibit certain features expected from sampling-based inference (see, e. g. , [45]). It may appear surprising that the behaviour of a fully deterministic network exhibits certain features of a probabilistic sampling strategy, but recent work has shown that Boltzmann Machines can be approximated by a sufficiently chaotic system [58]. It is well-known that such chaos can arise in deterministic neural networks similar to the SORN, for example by balanced excitation and inhibition [59]. Previous work with SORN models has also shown that they can exhibit dynamics close to the critical transition point between ordered and chaotic dynamics [60]. In principle, therefore, it cannot be ruled out that the SORN can be related to network implementations of sampling algorithms. Hence, it is insightful to contrast our study with top-down modeling studies that tried to construct neural implementations of the sampling theory. Most prominently, the authors of [61] showed in a rigorous analysis that in a feed-forward-like structure with winner-take-all circuits, STDP can be used to approximate a sampling-version of expectation maximization and thereby learn a generative model of the input. Similarly, [62] demonstrated that Markov chain Monte Carlo sampling can be approximated with symmetrically connected spiking neurons. A different group showed that spatio-temporal patterns can be entrained in a network with an artificial importance-sampling rule [63]. A more biological approximation of this again resembles STDP. Finally, the model in [61] was recently extended to demonstrate that sequence learning in a Hidden Markov Model can again be approximated by using only a local STDP rule [64]. Therefore, recent top-down modelling studies demonstrate that STDP is a good candidate to learn appropriate weights to represent statistics of the input (see also [65]). This suggests that it may be possible to reconcile the bottom-up model presented here with top-down approaches for probabilistic inference by sampling, leading to a more complete understanding of neural variability and spontaneous activity. In this context it is worth pointing out that the top-down models discussed above make heavy use of intrinsic noise to achieve these sampling-effects (but see [66]). Our results, together with [58], suggest that this may not be necessary. In fact, our work casts some doubt on the heavy use of noise in other network models where it is used to stabilize networks in irregular regimes or to avoid oscillations or epileptic-like behaviour. This practice is usually justified by referring to data on neural variability in the cortex. Here we demonstrated that key findings on neural variability can be accounted for by a completely deterministic network learning a model of its sensory environment. Additionally, there is experimental evidence suggesting that action potential generation is highly deterministic and synaptic transmission becomes essentially deterministic as long as experiments are performed under realistic conditions [3–5]. Therefore, we propose that the common practice to make heavy use of noise in neural simulations should not be taken as the gold standard. Nevertheless, we also tested the robustness of our results to noise by adding Gaussian white noise with zero mean on the excitatory units and tracking the amount of spikes added and suppressed by the noise. We found that setting the variance of the noise to disturb up to 10% of the activity does not relevantly alter the results presented here. So just how noisy is the brain? What do our results imply for the amount of noise we should expect to find in the brain? And what processes deserve to be called “noise” in the first place? From an information theoretic perspective, a natural assumption when theorizing about sensory processing is that the brain is trying to maximize the conditional mutual information MI (W; R | S) between sequences of states of the outside world W (or relevant parts thereof) and the internal representation R of this state sequence in terms of spatio-temporal cortical activity patterns, given the animal’s current brain state S. Conditioning on S expresses that the spatio-temporal activity patterns elicited by sensory inputs will depend on the current brain state of the animal, which is the product of its life-long experience and recent history of interaction with its environment. In any case, this mutual information can be decomposed as MI (W; R | S) = H (R | S) - H (R | W, S). Thus, in order to maximize MI (W; R | S) the brain should try to maximize the entropy of its internal representation of the world H (R|S) while at the same time trying to minimize the conditional entropy of this representation given the future sequence of states of the world and its current brain state H (R|W, S). Maximizing the first term also implies maximizing a lower bound of H (R), which means that the brain should make use of a maximally rich and diverse set of responses that, to an observer who does not have access to the true state of the world and the full brain state, should look like noise. In fact, the more efficient the code, the more the brain activity should look like noise. We speculate that this is the true reason why the activities of neurons are often so well-described by simple stochastic models. Minimizing the second term means that the ideal encoding should be deterministic, i. e. , R is a deterministic function of W and S, because then H (R|W, S) reaches its smallest possible value of zero. Put simply, the best coding mechanism will have as little intrinsic noise as possible. Note that this does not imply that the neural response to a repeated stimulus will necessarily be the same (or even similar!). It only implies that it is a deterministic function of the current sensory input and the animal’s constantly evolving brain state. On the surface, the great response variability of neurons in sensory cortices seems to be at odds with the idea of a deterministic encoding. However, our results show that there exists a simple way of learning a fully deterministic coding scheme that is consistent with the key features of neural response variability observed in the brain. This raises the possibility that the brain may be using a highly efficient deterministic coding strategy and that for many years neuroscientists have mistaken this deterministic neural code for an inefficient and noisy one.
Our model is based on the self-organizing recurrent neural network (SORN) [37]. Pilot studies related to the work presented here have been presented at a conference [67]. For more detailed information and a validation of the results, the code is available on github: https: //github. com/chrhartm/SORN. In this study, we compare the behaviour of our model to a variety of experimentally reported results. While our network is operating on a timescale of roughly 25ms (the width of a typical STDP-window) and on the spiking level with rates around 4Hz, the experimental data are fMRI-BOLD signals, optical imaging data and multi-unit activity spike trains and have effects on time scales that range between milliseconds and seconds. Comparing these partly very different data can therefore sometimes be only done on an abstract and qualitative level. In the following section, we outline how each comparison was performed.
Neural recordings seem very noisy. If the exact same stimulus is shown to an animal multiple times, the neural response will vary substantially. In fact, the activity of a single neuron shows many features of a random process. Furthermore, the spontaneous activity occurring in the absence of any sensory stimulus, which is usually considered a kind of background noise, often has a magnitude comparable to the activity evoked by stimulus presentation and interacts with sensory inputs in interesting ways. Here we show that the key features of neural variability and spontaneous activity can all be accounted for by a simple and completely deterministic neural network learning a predictive model of its sensory inputs. The network’s deterministic dynamics give rise to structured but variable responses matching key experimental findings obtained in different mammalian species with different recording techniques. Our results suggest that the notorious variability of neural recordings and the complex features of spontaneous brain activity could reflect the dynamics of a largely deterministic but highly adaptive network learning a predictive model of its sensory environment.
Where’s the Noise? Key Features of Spontaneous Activity and Neural Variability Arise through Learning in a Deterministic Network