20 th Feb 2019 Review the visual pathway Pathway begins at the physical apparatus of the eye Works its way through connecting nerves Lateral geniculate nucleus LGN is an important junction ID: 1038611
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1. The perception of soundNisheeth20th Feb 2019
2. Review: the visual pathwayPathway begins at the physical apparatus of the eyeWorks its way through connecting nervesLateral geniculate nucleus (LGN) is an important junction Terminates in primary visual cortexhttps://en.wikipedia.org/wiki/Visual_system
3. Review: retina can calculate derivatives
4. Review: simple and complex cells in V1
5. Feature searchXXXXXXXXXXXXXXXXXXOXOOOOOXX Conjunction searchTreisman & Gelade 1980Review: visual search
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7. FIT problems: what is a feature?LinesLines in circlesLines + circles
8. Review: perception as inferenceWant to know θ, get to see φ
9. Review: Perceptual learning as neuronal realignment(Law & Gold, 2009)
10. Processing soundWhat are we sensing?
11. Sound Wave:Amplitude and Frequency (Hz)Sound Pressure is measured in units called Pascals1 Pascal (Pa) = 1 Newton of force/m21 atmosphere = 100,000 PaHuman absolute hearing threshold = 0.00002 Pa = 20 microPaFrequency measured in cycles/sec = Hertz (Hz)Nominal range of sensitivity: 20 – 20,000 Hz
12. Inverse-Square LawArea of sphere = 4πr2acoustic power = pressure2
13. Decibels (dB)The decibel is a logarithmic unit used to describe a ratio (i.e., log (x/y))dB = 10 log(Observed Power / Reference)
14. dBSPLThe transducers (microphones) on sound level meters measure sound pressure (i.e., N/m2 or Pascals).Pressure needs to be converted to power prior to calculationof the decibel equivalent….i.e., acoustic power = pressure2 Finally, we need to agree upon a Reference value.By convention, we use 20 microPa (i.e., the hearing threshold)Thus:dB = 10 log (Observed Pressure2 / 20 microPa2)However……..
15. dBSPL (continued) Prior to the advent of hand-held calculators and computers(circa 1970), performing a squaring operation was computationally expensive and prone to error.To reduce computational demands, hearing science adopted a somewhat confusing convention in the specification of thedBSPL unit:dBSPL = 20 log (Observed Sound Pressure / 20 microPa)+6 dBSPL = doubling sound pressure +20 dBSPL = 10x pressure+3 dBSIL = doubling acoustic power +10 dBSIL = 10x acoustic power
16. Typical Sound Amplitude Values
17. Working with decibelsJND for sound intensity is about 1 dBSPL for most of normal range of hearingIf one machine emits 80 dBSPL then how much sound amplitude would be expected from two machines side-by-side? Convert from dBSPL back to raw pressure, sum the pressures, then convert sum to dBSPL 80 dBSPL antiLog(80/20) 10,000 20 log (10,000+10,000) ~ 86 dBSPL
18. Sound Stimuli are Complex
19. Complex Sound = Sum of SinesJ.B.J. Fourier(1768-1830)
20. Speed of SoundAcoustic energy results from atraveling wave of rhythmic “compression” through a physical medium (e.g., air; water; steel).It is the “compression” that travels not the medium, per se.The characteristic speed of this travelling wave varies as a function of the medium (elasticity; density).The speed of acoustic energy through the air (aka “sound”) is331 m/sec (or 742 MPH) at 0-deg C(Faster at higher temperatures).
21. Processing soundHow are we sensing it?
22. Ear anatomy
23. The Cochlea
24. The Organ of Corti3000-3500 Inner Hair Cells (IHC)12,000 Outer Hair Cells (OHC)
25. Auditory Hair CellsThree rows ofOuter Hair CellsOne Row of Inner Hair Cells
26. Hair cellsInner hair cells support auditory transductionConvert sound energy into neural responsesAuditory transduction and encoding modelsOuter hair cells act as amplifiersUse electromotility to selectively amplify quiet soundsCochlear amplification models (probably won’t talk about these)https://en.wikipedia.org/wiki/Hair_cell
27. Auditory Transduction
28. Basilar membrane tongues the hair cellsNote: K+ ion concentration gradient across sensory hair cells (see pink cavities)
29. IHC Stereocilia “Tip Links”“tip link” connects gate to adjacent cilia.Shearing motion forces gate to open.Mechanical open-and-close ofgate modulates influx of potassium ions (much FASTER than slow chemical cascade in visual transduction).K+ depolarization of IHC triggers release of glutamate at cochlear nerve fiber synapse.
30. Innervation of 3000 IHCsversus 12,000 OHCs30,000+ fibers in cochlear nerve. Nearly 10:1 fiber-to-IHC innervation ratio.Sparse number of fibers carry info from OHC to brain.Small number of fibers descend from brain to OHCs.Role of OHC’s? Mechanical gain otoacoustic emission
31. Sound Amplitude CodingMultiple nerve fibers for each IHC.Each nerve fiber tuned to a different 40 dB “range” of stimulus intensity.Intensity-level multiplexing
32. Tuning Specificity of Cochlear NerveFibers “Broadens” with Increased Intensity
33. Outrageously clever intensity modulation
34. Auditory Frequency CodingHow are sounds of different frequencies recognized?
35. Basilar Membrane Dynamics
36. The Simple Beginningsfor von Békésy’s Nobel Prize
37. Basilar Membrane Response
38. Cochlea as a Fourier transformer
39. Functional Aspectsof Hearing
40. Species-Specific Frequency Range
41. Human “Earscape”
42. Minimum Audibility CurveAverage detection threshold for 18-yr-olds for 1KHz tone at sea level is20 microPa (μPa)Minimum occurs at approx. 3 KHzBinaural intensity thresholds are 3 dB lower than mono
43. Loudness is non-linear
44. LoudnessStevens’ SONE SCALEof Loudness PerceptionPerceptual Anchor:1 sone = loudness of 1 KHz at 40 dB (40 phons)Find the dB level that is twice as loud (2 sones) or half as loud (0.5 sones), etc. and construct a scale.[i.e., Magnitude Estimation]The psychological magnitude of sound (i.e., “Loudness”) grows at a slower rate than the physical magnitude of the sound stimulus.
45. LoudnessUsing magnitude estimation techniques, S.S. Stevens has quantified this nonlinear relationship as: L = k * P0.6 = k * I0.3L=loudness; P=sound pressure (µPa)I=sound intensity (pW/m2)Stevens’ Power Law; Linear in log-log plot; slope ≈ exponent log(L)=log(k)+0.3 log(I) straight line log(L)≈0.3 log(I) Hence, a log unit increase (10dB) of intensity yields 0.3 log (100.3 or 2-fold) increase in loudness.Note: Binaural presentation perceived as approx. 2x more loud than monaural equivalent.
46. Sone Scale LandmarksNormal conversation 1-4Automobile @ 10m 4-16Vacuum cleaner 16Major roadway @ 10 m 16-32Long-term hearing damage dosage 32+Jackhammer @ 1m 64Brief-exposure hearing damage 256Pain threshold 676
47. Equal Loudness ContoursFrequency differentiation is flattened at high amplitudes; Speech and music sounds “tinny” at high loudness levels; Remember change in cochlear nerve tuning at higher intensity levels.
48. Tonal Masking:Psychophysical Tuning CurvesFixed test tone (e.g., 1KHz @ +10 dB)Frequency of masking tone variedHow intense must masking tone be in order to make the test tone indiscriminable?Plot of masking intensity thresholds reveals frequency tuning of underlying auditory processing channel(s)
49. Multiple “Frequency Channels”Revealed by Masking Curves
50. MEL Scale of pitchReference unit of perceived PITCH: 1000 Hz = 1000 MelsPerceived pitch increases “linearly” with stimulus frequency below 4KHz; but grows at a much slower rate at 4KHz and above.Semi-Log PlotLinear Plot