Central Auditory Pathway

Location of the auditory system in the skull

(Picture of neurons in auditory portion of mouse brain stem [based on expression of multiple fluorescent proteins, various combinations of which can produce ~90 different colours].) Variety of neurons in the auditory brainstem

Simplified version of the primary auditory pathway Note that most afferent fibers Decussate (cross contralaterally)

Fig CNCochlear nucleus SOCSuperior olivary complex MTBMedial nucleus of the trapezoidal bundle NLLNuclei of the lateral lemniscus ICInferior colliculus MGBMedial geniculate body

Primary Auditory Pathways The first relay of the primary auditory pathway occurs in the cochlear nuclei in the brain stem, which receive Type I spiral ganglion axons (auditory nerve). At this level an important decoding of the basic signal occurs: duration, intensity and frequency.

Primary Auditory Pathways The second major relay in the brain stem is in the superior olivary complex. The majority of the auditory fibers synapse there have already crossed the midline.

Primary Auditory Pathways The third relay carries the message up to the level of the inferior colliculus. The second and the third relays appear to play an essential role in the localization of sound.

Primary Auditory Pathways A final relay, before the cortex, occurs in the thalamus. Here an important integration of frequency, intensity and binaural information occurs.

Primary Auditory Pathway The final neuron of the primary auditory pathway links the thalamus to the auditory cortex. The message is recognized, memorized and integrated.

Primary Auditory Pathway The final neuron of the primary auditory pathway links the thalamus to the auditory cortex. The message is recognized, memorized and integrated.

Area 41: the primary auditory cortex (AI) Area 42: the auditory association area (AII)

Non-Primary (Extra-Lemniscal, non-traditional) Auditory Pathways The reticular pathway midbrain structure featuring several polysensory synapses. The auditory information is integrated with all the other sensory modalities. The reticular pathways participate with the states of consciousness and the motivation centers in selecting the information that should be treated as priority by the brain. Rostral to the reticular formation, the non-primary pathway leads to the thalamus, then to the polysensory cortex.

The Auditory CNS: Cochlear Nucleus (CN) It preserves and it may enhance information received from the auditory nerve All fibers arising from the cochlea synapse here ipsilaterally (a “must-synapse” station) Distributes auditory information to several different areas in the auditory pathway; the CN divides its output into several different tracts There are three nuclei of primary importance in audition: Anterior Ventral CN (AVCN); Posterior Ventral CN (PVCN), and Dorsal CN (DCN)

Cochlear Nucleus (CN) Each nerve fiber connects to all three divisions of the CN: anteroventral CN (AVCN), posteroventral CN (PVCN), and dorsal CN (DCN) First, each nerve fiber divides and one of the two branches terminates in the AVCN The other branch divides again before terminating in the cells of the PVCN and the DCN

Superior olivary complex (SOC) The SOC processes information about interaural delays and amplitudes (interaural time difference and interaural level difference) The SOC is composed of three major structures: medial and lateral SOC (MSO and LSO) and medial nucleus of the trapezoid body (MNTB) LSO: lateral superior olivary nucleus MSO: medial superior olivary nucleus MNTB: medial nucleus of the trapezoid body

The Auditory CNS: SOC Studies in which the SOC was selectively damaged have demonstrated that the SOC is essential in the localization of the source of sound It has been shown that lesions above the SOC (in the LL and IC) result in a loss of the ability to localize sound presented from the contralateral side Localization, therefore, is accomplished (cues are processed) at a level “lower” in the pathway than speech, or many other complex sounds (music, etc.)

Response to binaural stimulus presentation indicates that low frequency energy produces interaural phase differences, high- frequencies produce interaural level differences Cues for localization

Lateral Lemniscus (LL) The LL is a tract of axons traveling from just above the SOC and all the way to the IC Some of the axons synapse in a nucleus of cell bodies (NLL) Most continue directly to the IC The NLL have good temporal resolution; there are many cells with the onset response

The Auditory CNS: Inferior Colliculus (IC) Contains three nuclei (more distinct regions have been suggested); the largest and most well studied is the central nucleus (Central Nucleus of the IC – ICC) The IC is an integrative part of the system; it is responsive to temporal differences in the acoustic signal (ITD, ILD, AM and FM), also is very prominent in other species Thus, provides features needed to recognize speech, music, etc.

Bilateral interactions in the ICC Binaural information brought to the ICC by the SOC and from other structures Thus, the ability to localize the angle where a sound source is located becomes generalized at the level of the ICC

The Auditory CNS: Thalamus The thalamus is the last relay site on the way to the cortex for almost all sensory information, including auditory and visual It has been divided into a large number of nuclei; the primary nucleus involved in the auditory pathway is the Medial Geniculate Body (MGB)

Corpus Callosum Association areas are connected with each other by numerous fibers passing through the Corpus Callosum and the white matter of the hemispheres; these areas are responsible for the higher mental and emotional processes, such as memory, learning, speech, and the interpretation of sensations. The two hemispheres are joined by the Corpus Callosum

Development of the auditory pathways The central nervous system’s auditory pathways and centers develop more slowly and progressively, from the brain stem to the auditory cortex. This development ends only between the 4th to 8th years after birth.

Development of neurons in the auditory cortex Two micrographs, from the same area of the cat auditory cortex. The first stage (left) corresponds in man to 5 months in utero, when the cochlea starts functioning. The final stage (right) corresponds to a 6-8 year-old, fully developed, auditory cortex. Between the two stages, the development of dendrites (i.e. synapses) of neurons requires a fully functioning cochlea. Without normal stimulation, for example if there is congenital deafness, the auditory brain will remain at the immature stage. A cat at birth An adult cat

Role of stimulation During the first years of life, the brainstem and auditory cortex needs stimulation from the normal cochlea to achieve its development; any abnormality in cochlear function during this period may result in an abnormal development of the auditory brain. Performance 4.5 mos6 yrs The normally-developed cochlea facilitates maximum performance achieved by the auditory pathways When the cochlea does not develop completely, the auditory pathway and cortex does not reach its full developmental potential. Birth Rehabilitation for peripheral hearing loss imperative for development. Age

Plasticity of the brain in adulthood Plasticity of the brain occurs throughout life In adulthood, brain plasticity is still observed, e.g., learning and repair after lesion Learning by the adult/mature brain is much slower than during development A cochlear implant may yield positive results in a suddenly deafened adult; the brain shows a remarkable plasticity to adapt to stimulations sent by the implant that are different from those of a normal cochlea In presbycusis and as a result of noise exposure, high- frequency hearing loss may result in the auditory cortex modifying its tonotopic organization in order to facilitate detection and processing of lower frequencies

Is the auditory cortex necessary for hearing? Two hemispheres? The processing of speech involves more than the primary auditory cortex (AI) There are association areas (e.g., AII) that must be involved in speech processing Dominance of speech perception and production by the left hemisphere (differences for patients with a lesion of the left vs the right temporal lobe) The importance of two hemispheres is confirmed during the processing of novel (spectrally complex and/or time-varying; ie. speech) sounds presented in complex or distracting backgrounds (“cocktail party effect”)

RELE The dominant functions associated with each hemisphere Left: word recognition, letters, speech Right: figure recognition, emotional, numbers, rhythm Note: the fact that one hemisphere is dominant for a certain function does not mean that the other hemisphere cannot perform that function at all Tested using Dichotic listening Two different speech stimuli (e.g, words) are presented to the two ears simultaneously. The patient needs to repeat both words. In a nondichotic paradigm, stimuli can reach the auditory cortices by an ipsilateral route and a contralateral route. In dichotic speech testing conditions, the ipsi pathways are suppressed and the auditory system becomes essentially a crossed system.

Binaural Processing: Binaural summation Thresholds for stimuli presented binaurally are 3-6 dB better than those presented monaurally Neural representations are summed at some point in the CNS, probably at several levels from SOC and beyond