3Spontaneous Hyperactivity After Deafferentation

FThe paradox of more firing from less input

The defining electrophysiological signature of tinnitus in animals is an increase in spontaneous firing rate — activity recorded in silence — in central auditory nuclei after the cochlea is damaged [2004]. It seems backwards: cut the input and the output goes up. The resolution is that central neurons are not passive relays; they defend a target activity level, so reduced afferent drive provokes a compensatory rise in excitability [2014].

This spontaneous hyperactivity is the leading candidate for the neural correlate of the tinnitus signal: a steady barrage of firing in the absence of sound that the rest of the brain can read as sound [2010].

TFirst stop: the dorsal cochlear nucleus

The earliest and most robust hyperactivity appears in the dorsal cochlear nucleus (DCN). Kaltenbach and Afman showed that intense sound exposure raises spontaneous firing in DCN fusiform cells, and that this elevated activity resembles tone-evoked responses in normal animals — as if the nucleus were generating a phantom tone [2000]. The behaviour correlated with psychophysical evidence of tinnitus, making the DCN an early generator [2005].

Mechanistically the rise reflects disinhibition: loss of GABAergic and glycinergic restraint lets fusiform cells fire freely, and converging somatosensory inputs are up-weighted — the substrate for somatic modulation of tinnitus [2019].

TClimbing the pathway: inferior colliculus and cortex

Hyperactivity is not confined to the brainstem. Mulders and Robertson recorded increased spontaneous firing in the inferior colliculus after acoustic trauma and, importantly, found it became progressively independent of ongoing cochlear input — a ‘centralisation’ of the hyperactivity over time [2009]. The generator migrates upward and inward.

At the cortex, Seki and Eggermont recorded primary auditory cortex after tone-induced loss and found increased spontaneous firing and neural synchrony in the deprived frequency region [2003]. Homeostatic plasticity is sufficient to drive this: silencing input and blocking compensatory up-scaling can prevent the tinnitus-like phenotype in animal models [2011].

CTime course after noise trauma

The hyperactivity has a developmental arc. Immediately after trauma, spontaneous activity can briefly drop as the cochlea is shocked; over hours to days it rises and then climbs further over weeks as homeostatic gain and disinhibition take hold [2003]. The early DCN response can appear within days; midbrain and cortical changes mature later and become less reversible [2009].

This time line matters clinically. It frames an early window in which restoring or enriching input might blunt the maladaptive cascade before it centralises — the rationale behind early acoustic enrichment and prompt amplification [2014].

CRelation to tinnitus pitch

Spontaneous hyperactivity is not uniform across the tonotopic axis; it peaks in neurons tuned to the frequencies bordering the hearing loss. Because perceived pitch tracks the locus of maximal aberrant activity, this links the physiology to the percept: the brain ‘hears’ the frequency region where its spontaneous firing is loudest [2009].

That coupling is the through-line of this chapter. A peripheral edge of deafferentation produces a corresponding edge of central hyperactivity; downstream networks then synchronise, amplify and emotionally tag that signal into the conscious experience of tinnitus [2010].