The function of the commissure of the inferior colliculus in auditory processing

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Neural signals generated by the ears in response to sounds feed into two mirror image pathways on either side of the brainstem. While both left and right divisions receive inputs from both ears, the right side is dominated by information about sounds on the listener's left side and vice versa. This application proposes that the localisation and recognition of sound sources depend on the activity in one side being compared and referenced to the other. This interaction overcomes the problem that the outputs of neurons on one side alone are ambiguous because they don't respond exclusively to one attribute of sound, thus a change in a neuron's firing can reflect a change in the loudness of a sound, a shift in its position, or both. We propose that the large bundle of fibres interconnecting the two inferior colliculi (the auditory centres in the midbrain where the brainstem pathways culminate) provides the basis for mediating this left- right comparison. Since all signals reaching the higher cortical centres are processed en route within the inferior colliculi they are ideally situated to facilitate such an interaction. Furthermore, the bundle of fibres (the commissure) that connects one colliculus to the other is one of the largest inputs each inferior colliculus receives, and it terminates within the colliculus in a highly organised fashion. We seek to investigate the functional role of the commissural connections by temporally deactivating the inferior colliculus on one side of the brain and investigating what effect this has on the neural responses to sounds in the other inferior colliculus. We have developed and tested a means of rapidly deactivating the colliculus by cooling it. This is achieved by placing a small loop of stainless steel tubing on its surface through which we pump a coolant at subzero temperature. We can check that that the cooling effect is limited to the intended structure by measuring brain temperature with a tiny thermocouple, but as a double check, will also apply a method that enables us to infuse drugs that block neural activity into the inferior colliculus. We will also examine the effect of cutting the commissural fibres directly. The experiments will be performed in the anaesthetised guinea pig so that we can record the neural activity elicited by sounds. We will measure different aspects of brain activity, including the responses of single neurons (recorded simultaneously in different regions of the inferior colliculus), and electrical signals that represent the activity of large populations of neurons. By comparing these fine grain and summary responses to different types and combinations of sound stimuli before, during, and after deactivation of the other inferior colliculus we will discover how one inferior colliculus influences the other in different situations. We predict, that blocking commissural input will influence several facets of sound processing - a view supported by pilot studies. Specifically, we expect to observe changes in responses to features of sound that signal its location. Sounds in different locations give rise to differences in the level and timing of the sound between the ears. Recent discoveries about how the brain computes sound position from these differences suggest that referencing of left and right activity is important.We know little about the organisation of commissure of the inferior colliculus, and even less about its function in hearing. The outcomes of this research will, therefore, provide information of fundamental importance about the role of these connections in hearing, and help us better to understand the mechanisms of sound perception. The inferior colliculus is also being assessed for implanting an auditory- brain prosthesis to benefit patients who, through trauma or disease, have lost the nerves between the ears and the brain. Success in developing such a device depends on a greater understanding of the inferior colliculus.