Theories of Hearing Place Volley Theories of Hearing Place - Helmholtz Frequency - Rutherford Resonance Auditory nerve firing codes frequency Volley Principle - Wever nerve fibers work as group Resonance Theory (Helmholtz, 1857 Perception of pitch depends on selective vibration of a specific place on the BM. Telephone Theory (Rutherford, 1886) Entire basilar membrane vibrates Cochlea not frequency-specific along its length Pitch is related to firing pattern of auditory nerve fibers Frequency related to rate of firing Problems with Telephone Theory Presumes ability of neurons to fire synchronously with all frequencies of stimulus Refractory = 1 ms Maximum firing rate = 1000 x per sec Does not account for high frequency hearing Does not account for hearing loss due to BM damage at specific places i.e. damage to basal end = high frequency hearing loss Volley Principle In this example: Nerve fiber responds every 5 cycles Theories of Hearing Place - Volley Theory Place = high frequency (~4k Hz & above) Rate (volley) = low frequency (~400 Hz & below) Combination = mid frequencies Theories of Hearing Traveling Wave - Bekesy ▪ BM Characteristics ▪ ▪ ▪ ▪ Not under tension Widest at apex Stiffness gradient BM acts as low pass filter ▪ TW travels from high to low impedance (base- to apex) Movement of stapes in OW causes pressure wave in cochlear fluids and displacement of basilar membrane. Traveling Wave Auditory Nerve Frequency & Intensity Coding Reissner’s memb. Ganglion Cells * Stria Vas. Spiral Ligament Osseous Spiral Lamina *Habenula Perforata: between 2 plates of OSL: nerve fibers Auditory Nerve Fibers Spontaneous activity Threshold All or none Refractory period ~1 ms Maximum discharge rate Minimum stimulus = increase in firing rate ~1000 spikes/sec Compound AP Sum of many fibers Action Potential Spontaneous discharge: No stimulus Increase firing rate: Stimulus present More intense stimulus Frequency Coding Tuning Curve CF Sensitivity of Auditory Nerve Fibers Characteristic frequency Smallest amount of intensity required to cause firing Preservation of frequency selectivity in AN Frequency Coding Narrow tuning 25 dB/octave below CF 100 dB/octave above CF Low frequency tail Reflects frequency analysis of cochlea Tuning curves for single units with different CFs Low frequency AN broad tips High frequency AN sharp tips PST Histogram TW fast at base and slows toward apex Rapid, synchronous discharge from basal fibers Reflects travel time of TW Shorter latency for high freq CF Longer latency for low freq CF PST Histograms Responses of neurons with different CFs Latency mirrors TW delay Frequency Coding Temporal theory (phase locking) Low frequency tones Phase Locking Timing of response (spikes) synchronized with the stimulus. Spike occurs at the same point along stimulus waveform Period Histogram Single AN fiber to different frequency stimuli Spikes at various time intervals Spikes time locked w/freq Neural firing timed to the period of the tone Period of spikes decrease with increase in frequency Stim frequency < 1kHz > 1k Hz Intensity Coding Intensity Coding Action Potential Spontaneous discharge: Increase firing rate: No stimulus Stimulus present Faster firing rate: More intense stimulus Intensity Coding Dynamic Range Threshold to Saturation Dynamic Range of hearing = 140 dB Dynamic Range of AN neurons 20 to ~50 dB Single fiber cannot account for intensity coding Rate-Level Effect of stimulus intensity on firing rate Rate increases with stim intensity over a range of 20 – 50 dB above neuron’s threshold Firing rate of AN fiber as function of increase in intensity Firing rate saturates ~ 45 – 50 dB asymptotes Firing rates of AN fibers with similar CFs and different thresholds. Intensity Coding Across Fiber Tuning Curves As stimulus intensity increases more and more fibers on both the low frequency & high frequency sides of CF are activated. Intensity Coding: Rate + # Neurons Increase firing rate of more fibers over wider frequency range with increase in stimulus intensity Phase locking required to maintain frequency coding. Intensity Coding: Rate + # Neurons Increase firing rate of more fibers over wider frequency range with increase in stimulus intensity Phase locking required to maintain frequency coding.