Notes

Brainstem and Central Auditory Pathway

1. Cochlear Nucleus (start sound feature processing, frequency and sound onset/offset).
2. Superior Olives (Interaural Intensity Differences).
3. Inferior Colliculi (Somatosensory connections, multisensory).
4. Medial Geniculate (Integration and connecting to A1).
5. Auditory Cortex (A1).

Mapping of Elementary Auditory Functions in the Cortex

• Spectral and Temporal Auditory Processing:
  • Spectral processing involves the analysis of the frequency content of a sound, which is related to the perception of pitch.
  • Temporal processing involves the analysis of the timing and sequencing of sound events, which is related to the perception of rhythm, tempo and sound onset/offset.
  • The left auditory cortex favors the extraction of information from short temporal integration windows, whereas the right counterpart primarily relies on long integration windows.
• Intensity Coding
  • It refers to the way that the auditory system represents the loudness or amplitude of a sound. Neurons respond to different levels of sound intensity, with some neurons specifically sensitive to low-level sounds and others responsive to high-level sounds. The brain uses this information to construct a representation of the sound intensity and to determine the loudness of the sound.
• Timbre and Complexity Coding
• Auditory Object Recognition and Spatial Location ("What" and "Where" Streams)
  • "What" and "Where" streams in the auditory cortical system of primates (and humans). Similar to "what" and "where" streams in the visual system.
• Motor Perception
  • Moving vs Stationary auditory objects.

Speech Processing and Neural Oscillations

• Speech processing is mediated by neural tracking, which refers to the alignment of neural oscillations with the speech signal.
• High excitability phases of neurons align with information in speech (e.g., syllables, words, regularities, etc.).
• Neural oscillations track different linguistic units of the speech signal. Neural tracking of speech takes place at different time scales. Different methods, for example cross-correlation between the envelope of the speech signal and the EEG signal, phase coherence, etc.

Speech Segmentation Speech is a continuous acoustic signal without reliable gaps in between words or linguistic entities. The challenge is how to recognize word boundaries when no lexicon is available for word recognition. Once the word forms have been recognized, meaning assignment can be achieved through associative and contextual learning.

• Statistical Learning: Computation of transitional probabilities between adjacent syllables.
• Prosodic Bootstrapping: Detection of word boundaries based on rhythm, intonation or lexical stress cues.

The Vestibular System: Rotational Movements and Linear Accelerations Functions of the Vestibular System: register body motions, postural control (Vestibulo-spinal tract), maintain upright posture, measure gravity field, keep eyes still when head moves. Two organs in the inner ear (labyrinth):

• Semicircular tubes filled with fluid:
  • Oriented in three planes, one for each dimension in which we move the head (pitch, roll, jaw). Responsible for the coding of rotational movements.
• Otolith organs containing hair cells, and consisting of utricle and saccule:
  • Responsible for the coding of linear accelerations.
  • Infer body position in relation to gravity.
  • Detect changes in the direction and speed of movements.

Conclusions The auditory system enables to perceive air pressure fluctuations (frequency and amplitude) through complex processing steps in the outer, middle and inner ear. The cochlea is characterized by an excellent spectral (tonotopy) and temporal resolution. The neural codes are transmitted from the cochlea to the cortex via brainstem and central auditory system. Several specialized cortical modules contribute to different aspects of hearing (intensity, timbre and complexity coding, auditory object identification, speech and language processing, etc.). The vestibular system is fundamentally involved in balance and posture processing.