Introduction to the Senses PSL 300 PDF

Introduction to the Senses PSL 300 University of Toronto  Question of the day How many senses do you have? - vast #ot variables  Outline n Receptors and Neurons n n Transduction n Stimuli n Coding n n Signaling Change n n...

Introduction to the Senses PSL 300 University of Toronto  Question of the day How many senses do you have? - vast #ot variables  Outline n Receptors and Neurons n n Transduction n Stimuli n Coding n n Signaling Change n n Temporal n Spatial n Higher Processing  The senses carry information about the body and surroundings to the CNS blance n We will focus on the 5 special senses (vision, hearing, equilibrium, taste, and smell) and 4 somatic senses (touch, temperature, proprio- ception, and nociception, itch i.e. pain and itch). skin pain& n We are usually at least partly conscious of data from these 9 senses. n We are largely unconscious of other sense data such as blood pressure, lung inflation, blood-glucose concentration, internal body temperature, pH etc. Receptors and Neurons  Every sensory system begins with receptors n These are cells which convert stimuli (e.g. light, sound) into electrical signals. The conversion is called transduction. n In some sensory systems (such as vision) the receptor cells are neurons; in others (such as hearing) they are non-neuronal epithelial cells. variety of strengh n A receptor cell converts stimulus energy into a graded change in membrane potential called a receptor potential. The receptor may then release neurotransmitter to affect a neuron. If the receptor is itself a neuron, it may fire action potentials. not (depends if it's a or cells neuron or not ( receptor Office workers saw purple “ghosts” because the ceiling fans shook their eyeballs.  Every type of receptor cell has an adequate stimulus n Its adequate stimulus is the form of energy to which it is most responsive, e.g. thermoreceptors respond most sensitively to temperature. n n But many receptors also respond to other forms of energy as well, if those other forms are powerful enough, e.g. some thermoreceptors also respond to certain chemicals. Office workers saw purple “ghosts” because the ceiling fans shook their eyeballs.  Receptors are classed according to their adequate stimuli n Chemoreceptors respond to specific molecules or ions, e.g. to glucose, or oxygen, or H+. respond to chemicals n Mechanoreceptors respond to mechanical energy such as pres- sure, vibration, gravity, and sound. n n Thermoreceptors respond to temperature. n n Photoreceptors respond to light.  Some receptors are very sensitive n Any receptor has a receptor threshold — the weakest stimulus that will cause a response in the receptor. not enough to percive smell n For example, some photoreceptors can detect a single photon of light. Chemoreceptors for smell may respond to a single molecule of an odorant (i.e. of a substance that gives off a smell). n n The perceptual threshold is different; it is the weakest stimulus that will cause a conscious perception in the organism, e.g. it takes ~40 odorant molecules for you to perceive a smell.  Sensory systems involve series of neurons info from get 10 neurons many Primary Secondary Tertiary n The first neurons in the system (either the receptors or the cells immediately downstream) are called primary sensory neurons. that De migh cell receptors n Primary sensory neurons synapse onto secondary sensory neur- ons, and these synapse onto tertiaries, and so on. n n At each stage, many presynaptic cells may contact any one post- synaptic cell. This convergence allows secondary and higher neur- ons to combine data from many receptors.  Sensory neurons carry information about many aspects of the stimulus what's the similius n One aspect is the stimulus modality, i.e. whether it is a light, a sound, a touch, etc. n Sensory systems indicate modality by labeled lines, meaning that the modality is revealed by which axons carry the signal, e.g. activity on neurons in the visual pathway means light; activity on neurons in the auditory pathway means sound.  Groups of neurons can represent intensity in 2 ways n Stronger stimuli may activate more neurons. This way of repre- senting stimulus intensity, by the number of active neurons, is called population coding of intensity. can mean diff things n Stronger stimuli may make the individual neurons fire at a faster rate. This is frequency coding. n Both mechanisms may operate together: a stronger stimulus may increase the firing rates of neurons and also cause more neurons to be active. Signaling Change  Receptors and neurons have dynamics zero activity Stimulus zcro activity Response Time n That is, their activities may depend not only on the stimulus right now, but on how it changes through time. n n e.g. when a stimulus suddenly increases or decreases, many receptors and neurons respond briefly and then fall silent again. n So these cells signal changes in stimuli, not steady levels. doesn't tell abot level ins the change  Different cells have different dynamics Stimulus Phasic Tonic sometimes called tonic Phasic-tonic Time n Phasic cells respond briefly to any change and then cease firing. n n Tonic cells maintain their activity when the stimulus is not chang- ing, signalling its present level. n Phasic-tonic cells react to change but don’t return all the way to zero firing when the stimulus is constant, so they also carry infor- mation about its steady level.  Many retinal cells are phasic n Therefore they report changes in your visual world, as when something moves. n n That is why you wave your arms to catch a friend’s attention in a crowded airport: your arm motion activates phasic cells in your friend’s retinas. n n In the next slide, if you hold your eyes fixed on the central “×” for 20 s, the pink clouds will disappear, because with near-stationary eyes you have little change going on in your visual world, and so your 1 phasic cells stop reporting. 1 Carpenter & Reddi (2012) Neurophysiology Stare at the black x  Phasic signals make communication more efficient n Because our world is fairly stable, it is more efficient to report changes than to repeat similar messages over and over. n n E.g. if you are gathering data from an array of weather stations, you don’t want them reporting “It is sunny... still sunny... still sunny...”. It is more efficient to have them call in only when there is a change in the weather. n n These kinds of changes through time, between one moment and the next, are called temporal changes.  It is also efficient to report spatial changes Lighter Ed ge Darker n Spatial changes are differences between neighboring regions in space, e.g. neighboring patches of retina or skin. n Spatial change is also called contrast, and locations where there is strong contrast are called edges. n n In our weather-station analogy, neighboring stations usually have similar weather, and so it is more efficient for them to report only when they are on an edge between 2 kinds of weather.  Sensory systems accentuate edges by lateral inhibition detect Primary Secondary Lateral inhibition Tertiary n Lateral inhibition means that cells inhibit their neighbors, or they inhibit the cells their neighbors excite. n n E.g. here, each secondary neuron inhibits the tertiary neuron that its neighbor excites (the inhibitory connections are drawn in red; all other connections are excitatory).  An example of an edge detected by sensory neurons Object Skin ressactive Primary moreactive Secondary Lateral inhibition Tertiary Edge between contact and no contact n If an object presses against the skin, primary and therefore sec- ondary neurons in the area of contact are activated whereas cells outside the contact area are at baseline (i.e. resting) activity (in the picture, darker means more active).  Now suppose the secondary neurons laterally inhibit the tertiaries Object Skin Primary A B Secondary Lateral inhibition C D E F G H Tertiary more less inhibition inhibition thonE than f n Cell A powerfully excites E and powerfully inhibits D and F. Cell B is less active than A, so its effects are weaker: it moderately excites F and moderately inhibits E and G. n n Hence E is more active than D, and F is less active than G.  Away from the edge, excitation and inhibition cancel out Object Skin Primary A B Secondary Lateral inhibition C D E F G H Tertiary n For cells C, D, G, and H, excitation and inhibition cancel out, and so these cells are near baseline activity. n n Hence the tertiary cells act as an edge (i.e. contrast) detector: only near the edge are there cells (E and F) whose activity is not at baseline. Higher Processing - eq object. recognition  Most sensory pathways run via thalamus to cortex diencephalon Somato- Gustatory sensory Auditory Olfactory Visual Thalamus n Most pathways run through thalamus, which is near the center of the brain, out to the sensory cortices on the surface of the cerebrum. n n Olfactory (smell) pathways are an exception: they don't project via thalamus. nose cortex > - n n Equilibrium pathways project mainly to cerebellum. thalamus mcb.berkeley.edu through  Sensory processing is inference educated guessing n Our senses evolved to guide our behavior — to steer us away from danger and toward food, shelter, a mate. A big part of this guidance is deducing what is going on around us, e.g. identifying things. n n That it is a hard job, because sense data are incomplete and ambi- guous, so the brain has to infer. “The brain is a detective and the retinal image is the crime scene.”1 n n The inference is unconscious and fast, e.g. we can identify things visually in 160 ms without conscious effort. 1 Adapted from Stone, Vision and Brain  Because it has to guess, the brain can be fooled echmmm t M n This picture shows flat (2D) quadrilaterals, but you see it as a 3D box, because your brain thinks that interpretation is the most likely. n n The shapes and colors are just what you would expect from a 3D box. If it weren’t a box, the quadrilaterals would have to have been cleverly arranged and colored, and your brain thinks that is unlikely.  The brain mistrusts coincidences n In the Kanizsa illusion, you see 2 overlapping triangles, but there is none in the image. n n The brain mistrusts coincidences — the wedges perfectly aligned with each other and with the edges of the 3 V’s — and explains them all by postulating a white triangle in front. Reading in Silverthorn’s Human Physiology n 8th edition: “Sensory Information is Processed into Perception” (page 291) and Chapter 10 up the end of Section 10.1 (pages 308–315). n 7th edition: “General Properties of Sensory Systems”, pages 310–317, and “Sensory Informa- tion is Processed into Perception”, page 294. n 6th edition: “General Properties of Sensory Systems”, pages 326–334, and “Sensory Informa- tion is Processed into Perception”, page 308.