4Neural Synchrony, Bursting and Hyperexcitability

FBeyond rate — the temporal signature of a phantom

Module 3 described how the auditory system, starved of input, ratchets up its spontaneous firing rate. But a population of neurons all firing faster at random times would tend to average out into noise — the cortex is rather good at ignoring uncorrelated background chatter. What turns hyperactivity into a percept is the temporal patterning of that firing.

Two changes matter. First, individual cells switch from firing single, evenly spaced spikes to firing tight clusters of spikes — bursting. A burst is a far more potent signal to a downstream neuron than the same number of spikes spread out, because the postsynaptic potentials summate. Second, neighbouring cells begin to fire together — their activity becomes synchronised. The brain reads coincident input across many fibres as evidence that “something real is out there.” [2004]

TWhy synchronous activity is “heard”

Normal sound is encoded partly by which fibres fire and partly by when they fire relative to one another — cross-fibre coherence is one of the cues the auditory brain uses to bind a stimulus into a single perceived object. After cochlear injury, that coherence-detector is exploited by the system’s own noise: spontaneous spikes that have become correlated across a band of frequency channels masquerade as a genuine, externally driven event. [2004]

Crucially, the firing pattern of hyperactive dorsal cochlear nucleus fusiform cells comes to resemble the response those same cells would give to a real tone. Internally generated activity that mimics the statistics of sound-evoked activity is, from the cortex’s point of view, indistinguishable from sound. [2005] Wu, Martel and Shore showed directly that increased synchrony and increased bursting of these fusiform cells — not merely their average rate — tracked the behavioural evidence of tinnitus in animals. [2016]

THyperexcitability up the neuraxis

These changes are not confined to the brainstem. As aberrant activity ascends, each station can add its own gain and its own loss of inhibition, so synchrony and bursting are progressively elaborated through the inferior colliculus, the thalamus and the cortex. In the medial geniculate body of the thalamus, single-unit recordings in awake, behaviourally tinnitus-positive rats reveal exactly this profile — elevated firing, increased bursting and abnormal temporal structure that correlate with the behavioural readout of tinnitus. [2014]

The driver of bursting in thalamic neurons is instructive: when a cell is chronically hyperpolarised (because it has lost excitatory drive), low-threshold calcium (T-type) channels de-inactivate, and the cell rebounds in rhythmic bursts. The same membrane mechanism that lets the thalamus generate sleep rhythms is, in the deafferented state, hijacked to generate the pathological low-frequency bursting that seeds cortical dysrhythmia — a theme module 8 develops in full.

CFrom a measurable signature to a treatment target

If the perceptually relevant variable is synchrony rather than rate, then a rational therapy is one that desynchronises the offending circuit rather than simply quietening it. This is the explicit logic of bimodal (auditory + somatosensory) stimulation: precisely timed pairing of sound with electrical stimulation of somatosensory inputs to the cochlear nucleus drives spike-timing-dependent plasticity that reduces fusiform-cell synchrony — and produced measurable tinnitus reduction in both guinea pigs and humans. [2018]

For the clinician, the take-home is conceptual but consequential: a patient’s tinnitus loudness does not have to map onto “how fast” neurons are firing. It may map onto how coherently they fire. Interventions that retune the timing of neural activity — rather than ablate it — follow directly from this insight. [2016]