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Intercollicular commissural connections refine the representation of sound frequency and level in the auditory midbrain

  1. Llwyd David Orton
  2. Adrian Rees  Is a corresponding author
  1. Newcastle University, United Kingdom
Research Article
Cite this article as: eLife 2014;3:e03764 doi: 10.7554/eLife.03764
6 figures


Microdialysis of procaine into the IC produced a rapid, reversible deactivation of spiking.

(A) Schematic coronal image of the setup for recording the effects of procaine on neuronal activity in the left IC. (B) Mean (±SD) firing rate of multi-units recorded by an electrode adjacent to the probe, each point represents the average spikes per stimulus value in response to 100 repetitions of a CF tone at 20 dB above threshold. Procaine caused an immediate and persistent deactivation throughout infusion. After switching back to aCSF, firing recovered to near control levels after 20 min. (C) Multi-unit FRA recorded from an electrode adjacent to the microdialysis probe. (D) Procaine infusion abolished firing at virtually all frequencies and levels. (E) Response area shape and firing rate recovered similar to control.

Deactivation of the contralateral IC influenced the firing rate of IC neurons with V responses but had little effect on their shape or area.

Columns (A, D, G, J) show four examples of IC units in control (top), deactivated (middle) and recovery (bottom) conditions with responses for each unit normalized to the maximum firing rate across conditions. While firing rate changes within V FRAs were common, the tuning and shape of these FRAs were unaffected by deactivation of the contralateral IC by either cooling or MDP. All changes in firing rate during deactivation (B, E, H, K) recovered following cessation of cooling or procaine infusion (C, F, I, L).

Deactivation of the contralateral IC influenced firing rate as well as the receptive field area and shape for nonV responses.

(A) Low tilt FRA with high spontaneous rate. (B) Procaine in the contralateral IC caused a reduction in both driven and spontaneous firing and FRA area. (D) Narrow FRA which (E) reduced in area on contralateral cooling but fired sporadically over a wider frequency range. (G) Closed FRA which (H) expanded in area and increased in firing rate. (J) Broad FRA which (K) reduced in rate and area on contralateral cooling. (C, F, I, L) All changes in firing rate and response area reversed on recovery.

Differential changes in firing rate and receptive field area by FRA class.

(A) Total spikes in FRA on deactivation of the contralateral IC as a function of control value. A wide range of changes in firing rate were noted for both V and nonV receptive field classes with deactivation. (B) Cumulative distribution of change index (see text) for total spikes showing a similar range and distribution for V and nonV FRAs. (C) Receptive field area on deactivation as a function of control. (D) Cumulative distributions of change index of FRA area. The range of area change for nonV units was three times greater than for those with V response areas.

Changes in the shape and half maximum firing rate of RLFs on deactivation of the contralateral IC.

(AC) Control RLFs (black) with non-monotonic response functions with increasing sound level. With deactivation (red) RLFs became more monotonic and in some (B and C) the curves shifted to higher sound levels. These changes reversed on recovery (blue). (DF) Normalizing the functions in the top row shows the sound level required to elicit half maximal firing increased during deactivation. (G) The population of half maximum firing rates increased on deactivation of the contralateral IC. (H) Changes in half maximum firing rates did not correlate with changes in threshold, indicating that the dominant effect of commissural input on firing rate was at supra-threshold levels.

Deactivation of the contralateral IC reduced sound level discriminability in the IC.

(A) Control RLF of an IC neuron (median ± IQR). (B) ROC pairwise analysis of the responses in (A) with colors of ROC curves matching the corresponding paired values in (A). (C) An example RLF (black) which showed a reversible reduction in rate and slope on deactivation (red) that recovered to control values (blue). (D) A ‘discriminability index’ (DI) was calculated from the change in area under the ROC (see text) for each adjacent stimulus pair in each condition. The reduction in rate and slope in (C) on deactivation resulted in a reduction in DI from 1.5 to 1.2 with recovery to 1.4. (E) The median DI for the population of RLFs declined on deactivation of the contralateral IC, indicating a reduction in discriminability of sound level on in the absence of commissural input.


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