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Lecture

Week8 outline

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Department
Psychology
Course
PSYCH 3A03
Professor
Paul Faure
Semester
Fall

Description
This outline summarizes major points covered in lecture. It is not intended to replace your own lecture notes. Threshold Tuning Curves  Threshold tuning curves display SPL of signal required to cause cell to evoke firing  Threshold at any frequency is the SPL that evokes firing above the spontaneous rate.  Each cell has a different Characteristic Frequency (CF) or Best Excitatory Frequency (BEF).  The effective range of tuning curve reflects the dynamic range of the neuron (see I/O function).  Effective bandwidth of tuning curve increases with CF across cells.  Low CF fibers have a narrower bandwidth of tuning than fibers with a higher CF.  Shape and width of tuning curve reflects bandpass filter properties (frequency selectivity) of mechanical tuning of basilar membrane. ANF Spiking Is Phase Locked To Stimulus  Spike (action potential) timing to frequencies < 5 kHz is non-random.  Spikes occur at specific times (phases) of stimulus. Such spikes are said to be phase locked.  ANF spikes only to positive portion of stimulus waveform (pressure condensation).  Rectification: unit responds only to one direction of stimulus waveform.  Stimulus rectification reflects directional selectivity of hair cell stereocilia.  Phase locked discharges of ANFs important for encoding and distinguishing sound frequency.  Phase locked spikes do not have to occur on every cycle of stimulus waveform.  Upper frequency limit of phase locking (ca. 5kHz) determined by refractory period of neuron.  At low frequencies, ANFs may fire multiple spikes to peak pressure of stimulus, but at higher frequencies ANFs fire fewer spikes and they may miss one or more cycles due to refractory period.  Behaviorists and electrophysiologists graph spike timing using time domain displays re: stimulus.  Coefficient of synchronization (or Vector Strength) is index of magnitude of cell’s phase locking.  CS = 0 represents no phase locking; CS = 1 represents perfect phase locking. Graphic Displays of Neural Responses  Used to interpret behavior and relationship between behavior and activity of nervous system  Period Histogram – spiking over time, shown relative to stimulus period (or phase)  Post-Stimulus Time Histogram – summary of spiking response number in bins of time  Interspike Interval Histogram – summary of time between spikes in response o Peak height represents the most commonly observed ISIs. o ISI spacing of phase locked responses on PSTH decreases as frequency increases. o Smallest evoked ISI can’t be smaller than absolute refractory period. o When using a click stimulus, the ISI Histogram can inform you about the CF of ANF  Coefficient of synchronization (or Vector Strength) is index of magnitude of cell’s phase locking. Measuring Impulse Response of ANF Using Click Stimulus  Response of ANFs to an acoustic transient or click stimulus reveals information on transduction process.  Periodic peaks in ISI histogram indicate that biomechanical processing of click is similar to passing the click through an electrical bandpass filter.  Periodic oscillations of ANF is ringing of neural responses after passing through cochlear bandpass filter  Period of temporal oscillations in ISI Histogram can be used to determine CF of ANF  Example: ISI Histogram reveal period between spikes to click stimulus = 2.22 ms, therefore:   = 1 / Frequency ; Frequency = 1 /   Frequency = 1 / 2.22 10 s-3  Frequency = 450 Hz (i.e. the CF of the ANF = 450 Hz)  Time shift in neural response (i.e. latency difference) to 180° phase reversed clicks reflects directional selectivity of hair cell stereocilia to motion of basilar membrane Click Interspike Interval Histogram (review)  Periodic oscillations of ANF is “ringing” of neural responses after passing through cochlear bandpass filter.  Period of temporal oscillations ANF spiking can be used to determine the CF of ANF. Psych 3A03 28 October 2012 Week 8 Dr. Paul A. Faure  Time shift in neural response (i.e. latency difference) to 180° phase reversed clicks reflects directional selectivity of hair cell stereocilia to motion of basilar membrane Post-Stimulus Time Histogram (PSTH)  Post-Stimulus Time Histogram displays neural responses to repeated stimulus presentations during and after the stimulus.  Used to compare driven (evoked) activity and post-stimulus effects of stimulation. Peri-Stimulus Histograms (PSTH)  Peri-Stimulus Time Histogram displays neural responses to repeated stimulus presentations before, during and after the stimulus.  Used to compare spontaneous activity to driven (evoked) activity, and post-stimulus effects Monaural Two-Tone Suppression  Tone 1 is presented at the CF of the ANF; Tone 2 is presented off of the CF.  Varying the frequency and SPL of Tone 2 re Tone 1 shows suppression of the neural response  Cause of two-tone suppression is nonlinear motion of basilar membrane vibrations to 2 tones.  One must be cautious in trying to predict responses of a neuron to a complex sound based on its responses to simple sounds.  Can also measure two-tone suppression effects on other spike parameters such as spike latency, coefficient of synchronization, spike rate, etc. Binaural Two-Tone Suppression  Function of efferent connections to Peripheral Auditory Nervous System (PANS) not fully understood  Crossed Olivocochlear Bundle (COCB) fibers from Medial Superior Olive (MSO) are inhibitory  Binaural (both ears) stimulation of an auditory neuron can result in suppressed neural response relative to monaural (one ear) stimulation  In vitro experiments have shown that stimulation of COCB fibers inhibits excitatory responses from ipsilateral afferent fibers Encoding Frequency  Place theory of hearing: frequency of sound input is encoded by noting which region or place within the cochlea (or auditory nerve) was stimulated (or fires action potentials).  Temporal or Volley Theory of hearing: periodicity of the volley of neural discharges (spikes) can be used to reconstruct the frequency of sound input (up to limit ca. 5 kHz; neural refractory period). Encoding Amplitude  Amplitude or Intensity of stimulus encoded by number of spikes and/or discharge rate of ANFs.  Dynamic range of single ANF is limited to 30-50 dB (point of saturation), so this cannot account for the dynamic range of hearing that we measure behaviorally.  Combinin
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