Key Points

• The auditory system is highly complex
• Initially captures airborne acoustic signals and converts them ultimately to a neural code that the brain can use
  • filtered through the mechanics of the basilar membrane of the cochlea and cochlear amplifier (outer hair cells)
• Auditory evoked potentials can provide a temporal (time) window into the auditory system, along with some of its many processes, structures and generating sources
• Specifically, they can proved information on the order of millisecond time units about maturation, aging, plasticity and neurobiologically impaired or altered auditory structures

General Neurophysiology Concepts

• What we know about AEPs has been inferred through
  • simultaneous comparison studies of recordings directly at the source (on a nerve or nucleus) and on the scalp surface
  • the study of abnormal auditory systems (lesions)
  • purposeful disrupting of structures, processes, or pathways (chemical or sectioning) in otherwise normal auditory systems (typically in laboratory animals)
  • sophisticated interpretations and interpolations of active sources from within and on the surface of the head using multiple channel recordings

Basic Neuronal Anatomy and Physiology

• Neurons are electrically excitable capable of generating action potentials

Potentials at the Source

• Evoked potentials begin with the neuroelectric activity of sensory cells
  • in the cochlea, these sensory cells are the outer and inner hair cells giving rise to gross cochlear potentials
  • outer hair cells are responsible for intensifying basilar membrane movements through electromotility which serves to sharpen auditory tuning curves
  • inner hair cells will then be able to perform their task of releasing neurotransmitters to the auditory nerve fibers
  • not unlike neurons, there are also movements of charged ions in and out of the outer and inner hair cells as they carry out their functions
  • AEPs that can arise from these hair cells include, the CM and the SP
    • The CM is likely a summed response of a large number of active outer hair cells in the basal region
• The CM follows the stimulus waveform
• The SP is likely a summed response of both outer and inner hair cells with possible neural contributions but it seems to reflect an electrical voltage shift caused by the basilar membrane not moving equally in both directions
  • Both of these potentials can be seen in electrocochleography recordings

Dipoles

• During depolarization positive ions rush into one region of a neuron (a sink) and cause the extracellular space in that region to become negative
• This initial inward flow of positive ions must eventually exit in another region of the neuron (a source) causing the extracellular space in that adjacent region to become positive
• As this current flow is generated by the sink and source, a separation of charges at different regions of the neuron is set up
• This separation of charges produces what is called a dipole
• A dipole looks a lot like a battery with its positive and negative ends
• With some exceptions, far-field potentials appear to be best recorded when electrodes are placed on opposite ends of the dipole
• Although APs do propagate along neurons, the dipoles they produce during AEP recordings are stationary
  • the averaged AP shown on a monitor is not an AP moving along a structure like the auditory nerve but an abrupt synchronous discharge of numerous auditory nerve fibers of the distal auditory nerve (closer to the cochlea) is what gives rise to the AP of the ECochG (or wave I of the ABR)
  • Wave II is the effedt of the surrounding tissue on the scalp-recorded AEp as the AP moves along the auditory nerve within the highly resistive internal auditory meatus in the temporal bone and exits to an area of low resistance within the skull
• The onset of synchronous discharges in progressively later auditory brainstem nuclei and fiber tracts are likely reflected in the other peaks of the ABR; stationary dipoles caused by the APs at different levels of the lower auditory system
• Dipoles are typically from pyramidal neurons that are oriented perpendicularly to the brain surface
  • the majority of these neurons are in layer V of the auditory cortex

Near and Far-Field Potentials

• Neural activity can be recorded directly from the surface of cranial nerves, fiber tracts, brainstem nuclei and the cortical surface as well as from muscles and organs
• It can also be recorded at great distances from the source (scalp or within the ear)
• These are near-field and far-field potentials, respectively
• Neither reflect activity from a single neuron, the extent depends on the arrangement of neurons

Synchrony

• To evoke, a stimulus with a very fast rise time is required, such as a 0.1msec (100usec) click
• The abrupt rise time associated with the click produces a wide band of energy that can broadly stimulate the cochlea and in turn cause numerous auditory nerve fibers to discharge synchronously at the onset of the stimulus
• click stimulus is not perfectly broadband
  • the traveling wave delas from the base to the apex in the cochlea
  • click stimuli do not provide frequency specific inforation
  • to obtain frequency-specific information, short duration tones (called tone bursts) are often used

Phase Locking

• attribute of neurons to discharge by time locking themselves more often to the same phase of a stimulus waveform
• based on the neurons characteristic frequency or frequency response range