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8. Organization of the Sensory Systems.docx

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York University
PSYC 3530
Guy Proulx

8. Organization of the Sensory Systems Sunday, September 29, 2013 10:21 PM General Principles of Sensory-System Function  Each sensory system is organized on a similar, hierarchical plan  Sensory receptors are specialized cells that transduce, or convert, sensory energy (for example, light photons) into neural activity  We refer to people who lack receptors for parts of the usual visual spectrum as being color deficient or color-blind  For vision, light energy is converted into chemical energy in the photoreceptors of the retina, and this chemical energy is in turn converted into action potentials.  In the auditory system, air-pressure waves are converted into a number of forms of mechanical energy, the last of which eventually activates the auditory receptors, which then produce action potentials.  In the somatosensory system, mechanical energy activates mechanoreceptors, cells that are sensitive, say, to touch or pain. Somatosensory receptors in turn generate action potentials.  For taste and olfaction, various chemical molecules carried by the air or contained in food fit themselves into receptors of various shapes to activate action potentials.  For pain sensation, tissue damage releases a chemical that acts like a neurotransmitter to activate pain fibers and thus produce action potentials.  Every receptor organ and cell has a receptive field, a specific part of the world to which it responds  Rapidly adapting receptors detect whether something is there  Slowly adapting receptors adapt more slowly to stimulation  Exteroceptive receptors-- receptors that respond to external stimuli  Interoceptive receptors-- receptors that respond to our own activity o Tell us about the position and movement of our bodies  When we run, visual stimuli appear to stream by us, a stimulus configuration called optic flow  Auditory flow-- changes in the intensity of the sound that take place because of our changing location in relation to it  Optic and auditory flow are useful for telling us how fast we are going and in what direction we are moving  Some psychological conditions seem to be characterized by difficulty distinguishing between the self and others (e.g., hallucinations in schizophrenia)  Receptor density determines sensitivity  Two-point discrimination is highest on the parts of the body having the most touch receptors  Our sensory systems use different receptors to enhance our sensitivity under different conditions (e.g., rods and cones and color/black & white vision)  Fovea-- a small area of the retina where color photoreceptors are concentrated  Neural relays o All receptors connect to the cortex through a sequence of 3-4 intervening neurons o Neural relays also allow sensory systems to interact o Relays determine the hierarchy of motor responses  Periaqueductal gray matter (PAG)-- a region in the midbrain (brainstem) which contains relays from the pain pathway and is responsible for a number of complex responses to pain stimuli, including behavioral activation and emotional responses  The superior and inferior colliculus are responsible for detecting stimuli and locating them in space  Gating-- the inhibition of sensory information produced by descending signals from the cortex, through the periaqueductal gray matter, and on to lower sensory relays o Example: incurring an injury during a sport game only to realize later that it is more severe than originally thought  McGurke effect-- If a speech syllable such as “ba” is played by a recorder to a listener who at the same time is observing someone whose lips are articulating the syllable “da,” the listener hears not the actual sound ba, but the articulated sound da o Our perception of speech sounds is influenced by the facial gestures of a speaker  The presence of a stimulus can be encoded by an increase or decrease in the discharge rate of a neuron, and the amount of increase or decrease can encode the stimulus intensity  Topographic organization is a neural–spatial representation of the body or areas of the sensory world perceived by a sensory organ Vision  Rods, which are sensitive to dim light, are used mainly for night vision  Cones are better able to transduce bright light and are used for daytime vision  The photoreceptive cells synapse with a simple type of neuron called a bipolar cell and induce graded potentials in such cells. Bipolar cells, in turn, induce action potentials in ganglion cells. Retinal ganglion cells send ax
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