The Auditory System

The Auditory System

Information Processing in the Nervous System

Our fundamental interest is to understand the mechanisms used by the nervous system to process and analyze information about the sensory environment. Information processing in the nervous system operates on a number of fundamental levels: the dynamic electrical signalling of individual neurons (regulated by ion channels and their distribution within the cell membrane), the communcation between neurons that takes place largely at synapses, and the collective behavior of sets of neurons within neural circuits that are excited (or inhibited) according to external stimuli and internal conditions. Neural information processing is neither static nor uniform; rather at all levels, from molecules to networks, it depends on preceeding states, including developmental interactions with the environment, lifelong learning and training experience, and the immediate cellular and behavioral context. The sensory systems (hearing, vision, touch, smell, taste, and balance) have proven to be excellent experimental systems to study neural information processing. Sensory information processing is often easily traced from one level to the next, the external stimulus can be well controlled, and the constraints of the system and requirements for specific types of processing are often readily discernable. Previous and ongoing work in the laboratory has focussed on electrical signalling and ion channels in individual neurons, and synaptic communication between cells. A new direction that we are taking is to examine neural circuit organization and synaptic plasticity in cortex.

Why the auditory system?
To study information processing in groups of neurons, it is helpful to use a system with well defined inputs, and well defined anatomy. There are several systems that meet these criteria, including various invertebrate preparations. However in vertebrates the requirement for well defined inputs is met best at the peripheral levels of sensory systems.

The auditory system has two principal advantages over the other sensory systems for the study of information processing mechanisms:

The representation of sound in the auditory nerve has been well studied over the past 40 years, and responses to simple sounds, such as tones, noises and clicks, as well as sounds as complex as portions of speech, have been sufficiently well characterized that sophisiticated computational models exist that capture many features of the nerve’s responses. The auditory nerve represents these sounds in a fairly consistent way from one fiber to the next. Each auditory nerve fiber receives input from a single cochlear inner hair cell, which imparts sensitivity to a limited range of frequencies, similar to a bandpass filter. Individual fibers representing one frequency vary in their sound level threshold (the intensity of sound at which they change their firing rate), and this is (inversely) correlated with their spontaneous activity in silence. Thus, the input to the brain, although not completely homogeneous, is well characterized and can be reasonably represented computationally.
The first stage of auditory information processing occurs in the cochlear nucleus, where the relatively uniform discharge of auditory nerve fibers is transformed into a set of distinct ascending parallel pathways that emphasize different features of the sensory environment. These transformations can be understood in terms of the types of analysis that the auditory system must perform to accomplish its numerous perceptual tasks. This divergence of processing underlying these transformations is supported by defined subsets of cochlear nucleus neurons that have distinct patterns of afferent synaptic connection, dendritic architecture, neurotransmitter receptor expression, and ion channel expression. The ability to examine the mechanisms used by different cell types to process an essentially common input allows a comparative approach, and has helped us to understand the importance of different neuronal mechanisms in accomplishing different information processing tasks. The apparent direct correspondence between ion channel expression and the types of integration carried out by particular auditory neurons has particularly helped us understand the mechanisms of information processing in these cells.

The major research efforts of the laboratory are focussed on the roles of ion channels, their kinetics, and cellular distribution, in shaping neural integration in the dorsal and ventral cochlear nucleus.
We have an additional interest in the roles of dendrites in the processing and integration of synaptic inputs.
We are also studying short term and long term modulation ion channel function in cochlear nucleus neurons.
We are also studying the effects of deafness on the cellular mechanisms of information processing in the cochlear nucleus. In these experiments, we characterize the processing capabilities of the residual auditory system, and associated changes in ion channel expression, with the long term goal of proposing stimulation strategies to accommodate or overcome changes in these capabilities due to changes in available cellular mechanisms.
A future direction is to examine long-term synaptic plasticity and circuit organization in auditory cortex.
Experiments are complemented with biophysically realistic compartmental models of channels and cells, to relate cellular mechanisms to the formation of neural information codes.