Background Noise Improves Gap Detection in Tonically Inhibited Inferior Colliculus Neurons

Abstract

Single units in the inferior colliculus (IC) in the C57Bl/6 inbred mouse strain were tested for their temporal processing ability as measured by their minimum gap threshold (MGT), the shortest silent interval in an ongoing white-noise stimulus which a unit could encode. After ascertaining the MGT in quiet, units were re-tested in various levels of background noise. The focus of this report is on two types of tonically responding units found in the IC. Tonically inhibited (TI) units encoded gaps poorly in quiet and low levels of background noise as compared with tonically excited (TE) units. In quiet, the MGTs of TI units were about an order of magnitude longer than the MGTs typical of TE units. Paradoxically, gap encoding was improved in high levels of background noise for TI units. This result is unexpected from the traditional viewpoint that noise necessarily degrades signal processing and is inconsistent with psychophysical observations of diminished speech and gap detection processing in noisy environments. We believe the improved feature detection described here is produced by the adaptation of inhibitory input. Continuous background noise would diminish the inhibitory efficacy of the gap stimulus by increasing the latency to the onset of inhibition and decreasing its duration. This would allow more spontaneous activity to "bleed through" the silent gap, thus signaling its presence. Improved feature detection in background noise resulting from inhibitory adaptation would seem an efficient neural mechanism and one that might be generally useful in other signal detection tasks.

Changes in the Response Properties of Inferior Colliculus Neurons Relating to Tinnitus

“…The mean MGDT for sustained units was considerably shorter at 7.45 ms (±1.43 ms; n = 126). Offset units were excluded from analysis due to their low incidence in the sample ( n = 6 across both groups), although the mean MGDT of offset units was considerably longer at 30.5 ms (±10.81 ms; n = 6), this is consistent with the results of other previous studies ( 16 , 41 ).…”

The superior paraolivary nucleus shapes temporal response properties of neurons in the inferior colliculus

“…Properties of neurons in the central nucleus of the IC have been studied extensively with regard to responses to temporal sound features, including those of amplitude-modulated sounds (Langner 1992; Burger and Pollak 1998; Frisina 2001; Caspary et al 2002; Zhang and Kelly 2003; Joris et al 2004) and gap detection (Walton et al 1997; Barsz et al 1998; Wilson and Walton 2002; Allen et al 2003). Because SPON neurons, like IC neurons, exhibit a high degree of sensitivity to similar ranges of sound envelope modulation frequencies and gaps in ongoing sounds (Walton et al 1997; Yang and Pollak 1997; Krishna and Semple 2000; Zhang and Kelly 2006; Kadner and Berrebi 2008), it is reasonable to investigate how SPON inhibition modulates temporal responses of IC neurons.…”

Transient gain adjustment in the inferior colliculus is serotonin- and calcium-dependent

“…Although the kinetics of the slice are slow compared to in vivo conditions, our findings may point to the cellular bases of sensitivity to stimulus order and context in IC neurons, particularly on the time scale of 100s of ms. Stimulus context has proven important for neural coding in the IC, as shown with studies of increased sensitivity to gap detection in background noise, reflecting a form of IC gain control (Wilson and Walton 2002), or the temporary enhancement of spike rate depending on the interval between two sequential tones (Finlayson, 1999). Indeed, temporal order of sound stimuli does affect our understanding of speech components (Holt and Lotto 2002).…”