Sensory Processing and Perception

• Umwelt


  The perceptual world that is experienced and processed by an organism (coined by Jakob von Uxeküll in 1909)
• Umwelts differ across organisms, and they differ across development
• Sensory ecology


  The study of the sensory systems of different species & their interactions with their environment
• Sensory modalities


  • Vision
  • Smell
  • Hearing
  • Magnetic sensitivity
  • Electroreception
  • Hydrodynamic reception
• Sound


  Vibrations of objects that cause molecules in the surrounding medium to vibrate, creating pressure changes in the medium (sound waves)

  
• Sound waves


  • They are created by pressure fluctuations (e.g., in air)
  • When plotted on a graph, they form a sine wave

  
• Amplitude/Intensity


  Magnitude of displacement (increase or decrease) of a sound pressure wave, perceived as loudness

  
• Frequency


  Number of times per second that a pattern of pressure change repeats, perceived as pitch

  
• Decibels (dB)


  Unit of measure for the physical intensity of sound (sound pressure level)

  
• 20µPa (.0002 dyne/cm2) is the reference pressure for sound waves in air (defined as 0dB)

  
• Each 10:1 sound pressure ratio = 20 dB, so, a 100:1 ratio = 40 dB (we perceive a 10-fold increase in acoustic power as double the loudness)

  
• Hertz (Hz)


  Unit of measurement for the frequency of sound, 1 Hz equals 1 cycle per second, we hear from 20-20,000Hz

  
• Sine waves


  The simplest kind of sound, but not common in everyday sounds because not many vibrations in the world are so pure

  
• Complex sounds


  Best described as a spectrum that displays how much energy is present in each of the frequencies in the sound

  
• Complex sounds have multiple frequency components that vary in intensity (harmonic spectrum)

  
• The outer ear collects sounds from the environment and funnels them into the ear canal, enhancing certain frequencies and insulating/protecting the tympanic membrane

  
• The middle ear amplifies sound waves and transfers their energy to the cochlea through lever action and concentration of energy from the tympanic membrane to the smaller oval window

  
• Functions of the middle ear


  • Concentrates vibration, ossicles provide leverage to amplify vibrations, prevents loss of vibrational energy, acoustic reflex to protect the inner ear from loud sounds

  
• The inner ear is where changes in sound pressure are translated into neural signals

  
• Cochlear canals and membranes


  The cochlea has 3 canals filled with watery fluids: tympanic, vestibular, and middle
• Hair cells


  Converts sound waves into neural signals, made up of stereocilia that initiate the release of neurotransmitters when flexed
• The basilar membrane is lined with hair cells that terminate in stereocilia, their vibration sends a signal into the auditory nerve to the brain
• Inner hair cells


  Send information to the brain via afferent nerve cells
• Outer hair cells


  Receive information from the brain via efferent nerve fibers
• Characteristic frequency (CF)


  The frequency to which a particular auditory nerve fiber is most sensitive, clearest when sounds are very faint

  
• Two-tone suppression


  Decrease in the response (firing rate) of 1 auditory nerve fiber to 1 tone when a 2nd tone is presented at the same time, especially if that 2nd tone is lower in Hz
• Place theory of pitch perception


  Different 'places' along the basilar membrane of the cochlear partition are sensitive to different frequencies

  
• Rate saturation is the point at which an auditory nerve fiber's firing rate no longer increases with increasing sound intensity
• Outer hair cells


  Receive feedback from the brain and can make parts of the cochlear partition stiffen, making the responses of inner hair cells more sensitive and more sharply tuned to specific frequencies
• Two-tone suppression


  Decrease in the response (firing rate) of 1 auditory nerve (AN) fiber to 1 tone when a 2nd tone is presented at the same time, especially if that 2nd tone is lower in Hz
• Rate saturation


  The point at which an AN fiber is firing as rapidly as possible and further stimulation is incapable of increasing the firing rate
• By increasing intensity


  An AN nerve fiber's frequency selectivity is widened
• The stereocilia on hair cells are tuned to certain frequencies (CF), but this sensitivity can be overridden by the presence of harmonics and/or by changes in intensity

  
• Our auditory system relies on the responses of it's 14,000 AN fibers to determine frequency and perceive sound, much like the functioning of the visual system

  
• Auditory system


  • Large proportion of processing is done before primary auditory cortex (A1)
  • Differs from visual system where large proportion of processing occurs beyond V1
• Psychoacoustics


  Branch of psychophysics that studies the psychological correlates of the physical dimensions of acoustics in order to understand how the auditory system operates

  
• Loudness


  Psychological aspect of sound related to perceived intensity (amplitude)

  
• Pitch


  Psychological aspect of sound related mainly to the perceived frequency

  
• Audibility threshold


  Lowest sound pressure level that can be reliably detected at a given frequency

  
• Equal-loudness curve


  Sound pressure level vs. frequency for which a listener perceives constant loudness