Outline: Auditory Pathways and Spatial Localization

Hair cells to

spiral ganglia (cell bodies of cells which do spike) to

auditory/cochlear nerve to

dorsal and ventral cochlear nucleus

Use of lateral inhibition, tighter tuning, on & off cells

Ventral CN to superior olivary complex (in the pons/brainstem)

First point of binaural combination, including spatial localization cues

Dorsal CN and olive to nucleus of the lateral lemniscus to

inferior colliculus in the midbrain (direction selectivity) to

medial geniculate nucleus in the thalamus to

auditory cortex

All preserve tonotopic organization

Efferent pathways all the way back to the hair cells (especially outer)

Direction Cues

ILD (interaural level differences)

Due to shadowing by the head

Available for tones above 3-5 KHz (low frequencies can bend around the head, effectively)

ITD (interaural timing differences)

Due to difference in distance to the two ears

Two sub-cues

Transient sounds: onset time difference

Sustained sounds: phase difference

Thus, requires volley coding, and phase difference

must be less than 1/2 cycles, so frequency must be lower than 1.5 KHz

Neither cue varies substantially with distance

Both cues are ambiguous:  cone of confusion

Use head movements to disambiguate

Filtering by the pinnae - frequency/spectral pattern matching - Head-related transfer function (HRTF)

Direction errors decrease (i.e. improve) above and below 3 Khz

Shows transition from ITD (low frequency) to ILD (high frequency)

Distance Cues

Echoes and reverberations - cues to distance and to environment

Loudness constancy

Physiology

Owl - spatial receptive fields (center surround)

Superior olivary nuclei

E-E and E-I cells (E = excitatory, I = inhibitory, one for each ear)

E-I computes ILD

Both can be tuned for ITD

Inferior colliculus

Tuning for direction of spatial movement for click trains

Based on changing ITD