mechanisms of perception

• three fundamental types of the sensory areas of the cortex (principles of sensory system organization)

  • primary sensory cortex
  • secondary sensory cortex
  • association cortex
• primary sensory cortex - receives most of its input directly from the thalamic relay nuclei of that system
• secondary sensory cortex - made up areas of the sensory cortex that receives most of their input from primary sensory cortex or from other areas of the secondary system cortex of the same system
• association cortex - receives input from more than one sensory system
• three major principles of the interaction of three types of sensory cortex and among other sensory structures:

  1. Hierarchical organization

  • sensory areas of the cortex works in a hierarchical system.
  • receptors serve as the base of the system; perform the simplest and general analyses
  • the tops is the association cortex; performs the most complex and specific analyses
• 2. functional segregation

  • once assumed the areas of cortex were functionally homogenous (acted together to perform the same function)
  • three levels of cerebral cortex (primary, secondary, association) contains functionally distinct areas that specialize in different kinds of analysis
• 3. parallel processing

  • once believed the different levels of a sensory hierarchy were connected in a serial fashion
  • new evidence that systems work in parallel systems
  • this features parallel processing (simultaneous analysis of a signal in different ways by the multiple parallel pathways of a neural network)
• former model: hierarchical, functionally homogenous, serial
• current model: hierarchical, functionally segregated, parallel
• auditory system - perception of objects and events through the sound that they make
• sounds - vibrations of air molecules that stimulate the auditory system
• amplitude - perception of loudness
• frequency - perception of pitch
• complexity - perception of timbre
• pure tones (sine waves vibrations) - exists only in labs and sound recording studios
• sound is always associated with complex patterns of vibrations in real life
• fourier analysis - method helping in breaking down complex waves into sine waves

  • according to theory of audition, our auditory system performs a natural fourier-like analysis of complex sounds
• sound waves travel from the outer ear down the auditory canal and cauase the tympanic membrane (eardrum) to vibrate
• vibrations are transferred to three ossicles: malleaus (hammer), incus (anvil), stapes (stirrup)
• In this lesson, we will learn about the remaining four of the five exteroceptive sensory systems: the auditory (hearing), somatosensory (touch), olfactory (smell), and gustatory (taste) systems. In addition, we will also explore the mechanisms of attention: how brains attend to a small number of sensory stimuli despite being continuously bombarded by thousands of them.
• Visual system

  • By far the most thoroughly studied sensory system and as a result, the most well understood

  
• Types of sensory cortex

  • Primary Sensory Cortex
  • Secondary Sensory Cortex
  • Association Cortex
• Primary Sensory Cortex

  The area that receives most of its input directly from the thalamic relay nuclei of that system
• Secondary Sensory Cortex

  Comprises the areas of the sensory cortex that receive most of their input from primary sensory cortex of that system or from other areas of the secondary sensory cortex of the same system
• Association Cortex

  Area of the cortex that receives input from more than one sensory system
• Principles of Sensory System Organization

  • Hierarchical Organization
  • Functional Segregation
  • Parallel Processing
• Hierarchical Organization

  The sensory areas of the cortex are works in a hierarchical system. The receptors serve as the base of the system and they perform the simplest and most general analyses. At the top is the association cortex, which performs the most complex and specific analyses.

  
• Functional Segregation

  Each of the three levels of cerebral cortex -- primary, secondary and association -- contains functionally distinct areas that specialize in different kinds of analysis.
• Parallel Processing

  Sensory systems work in parallel systems which feature parallel processing -- the simultaneous analysis of a signal in different ways by the multiple parallel pathways of a neural network.
• Pure Tones (sine wave vibrations) exist only in laboratories and sound recording studios. In real life, sound is always associated with complex patterns of vibrations.
• Fourier analysis


  A way to break down complex sound waves into sine waves, and according to theory of audition, our auditory system performs a natural Fourier-like analysis of complex sounds.
• Aspects of sound perception

  • Amplitude (perception of loudness)
  • Frequency (perception of pitch)
  • Complexity (perception of timbre)
• From the world to the inner ear

  1. Sound waves travel from the outer ear down the auditory canal and cause the tympanic membrane (eardrum) to vibrate

  2. Vibrations are then transferred to three ossicles: malleus (hammer), incus (anvil), and stapes (stirrup)

  3. Vibrations of the stapes trigger vibrations of the membrane called oval window which transfers the vibrations to the fluid of the cochlea

  4. The cochlea is a long coiled tube with an internal membrane running almost to its tip called the organ of Corti (auditory receptor organ)

  
• Organ of Corti

  Composed of two membranes: the basilar membrane, where the hair cells (auditory receptors) are mounted, and the tectorial membrane, which rests on the hair cells
• When vibrations reach the organ of Corti, the shearing force on the hair cells increase firing in axons of the auditory nerve.
• Within the inner ear is the semicircular canals -- the receptive organ of the vestibular system. The vestibular system carries information about the direction and intensity of head movements, which help us maintain our balance.

  
• From the ear to the primary cortex

  1. As the auditory signals pass through the cochlea, the axons from the auditory nerves synapse in the ipsilateral cochlear nuclei, from which many projections lead to the superior olives on both sides of the brain stem

  2. The axons from the olivary neurons project via lateral lemniscus to the inferior colliculi, where the synapse of neurons that project to the medial geniculate nuclei of the thalamus, which in turn project to the primary auditory cortex

  
• Auditory pathways

  • Anterior auditory pathway (identifying sounds)
  • Posterior auditory pathway (locating sounds)

  
• Bilateral lesions to the auditory cortex were found to result to complete loss of hearing (loss of the ability to localize sounds and impairment of the ability to discriminate frequencies).
• Conductive deafness

  Damage to the ossicles