CHAPTER 2 - Basic Principles of Sensory Physiology

Question 1

Describe the basic structure of neuron

Answer 1

• the key component of neurons are:
  
  • the cell body, which contains mechanisms to keep the cell alive
  • dendrites, which branch out from the cell body to receive electrical signals from other neurons
  • axon, or nerve fiber, which is filled with fluid that conducts electrical signals
• also have sensory receptors

Question 2

Describe how to record electrical signals from a neuron

Answer 2

• Electrical signals are recorded from the axons using small electrodes to pick up the signals
• Two electrodes
  
  • A recording electrode (with its recording tip inside the neuron)
  • A reference electrode (located some distance away so it is not affected by the electrical signals)
• These two electrodes are connected to a meter that records the difference in charge between the tips of the two electrodes
• Difference displayed on a screen which shows electrical signals being recorded from a neuron
• When the axon is at rest, the difference in electrical potential between the tips of the two electrodes is -70 mV –> resting potential
• As the signal passes the recording electrode, the charge inside the axon rises to +40 mV
• As the signal continues past the electrode, the charge inside the fiber reverses course and starts becoming negative again, until it returns to the resting level –> action potential (lasts about 1 ms)

Question 3

What are some basic properties of action potentials?

Answer 3

• The action potential is a propagated response  once the response is triggered, it travels all the way down the axon without decreasing in size
  
  • Moving the recording electrode to a position nearer to the end of the axon, the electrical response would take longer to reach the electrode, but it would still be the same size (increasing from -70 to +40 mV) when it got there
  • Important property of the action potential because it enables neurons to transmit signals over long distances
• The action potential remains the same size no matter how intense the stimulus is
  
  • Changing the stimulus intensity does not affect the size of the action potentials but does affect the rate of firing
• Refractory period  the interval between the time one nerve impulse occurs and the next one can be generated in the axon
  
  • Refractory period for most neurons is about 1 ms, the upper limit of a neuron’s firing rate is about 500 to 800 impulses per second
• Spontaneous activity  action potentials that occur in the absence of stimuli form the environment
  
  • Establishes a baseline level of firing for the neuron
  • The presence of stimulation usually causes an increase in activity above this spontaneous level, but under some conditions, it can cause firing to decrease below the spontaneous level

Question 4

Describe what happens when an action potential travels along an axon. In your description, indicate how the charge inside the fibre changes, and how that is related to the flow of chemicals across the cell membrane

Answer 4

• Neurons are bathed in a liquid solution rich in ions (molecules that carry an electrical charge)
• Ions are created when molecules gain or lose electrons
• The solution outside the axon of a neuron is rich in positively charged sodium (Na+) ions, whereas the solution inside the axon is rich in positively charge potassium (K+) ions
  
  • This distribution of ions across the neuron’s membrane at rest is important to maintaining the -70-mV resting potential, as well to the initialization of the action potential itself
• As a signal approaches:
  
  • Positively charged sodium ions (Na+) rushes into the axon
    
    • Occurs because channels in the membrane that are selective to Na+ have opened, which allow Na+ to flow across the membrane and into the neuron
  • This opening of sodium channels represents an increase in the membrane’s selective permeability (the ease with which a molecule can pass through the membrane) to sodium
  • The inflow of positively charged sodium causes an increase in the positive charge inside the axon form the resting potential of -70 mV until it reaches the peak of the action potential of +40 mV
    
    • An increase in positive charge inside the neuron is called depolarization
    • This quick and steep depolarization from -70 mV to +40 mV during an action potential is referred to as the rising phase of the action potential
  • Once the charge inside the neuron reaches +40 mV, the sodium channels close and potassium channels open
    
    • Because there were more potassium ions (K+) inside than outside the neuron while at rest, positively charged potassium rushes out of the axon when the channels open, causing the charge inside the axon to become more negative
  • An increase in negative charge inside the neuron is called hyperpolarization
    
    • The hyperpolarization from +40 mV back to -70 mV is the falling phase of the action potential
  • Once the potential has returned to the -70-mV resting level, the K+ flow stops, which means the action potential is over and the neuron is again at rest
  • Sodium-potassium pump  keeps the build-up of sodium inside and the build-up of potassium outside the neuron from happening by continuously pumping sodium out and potassium into the fiber

Question 5

How are electrical signals transmitted from one neuron to another? Be sure you understand the difference between excitatory and inhibitory responses

Answer 5

• There is a very small space between neurons, known as a synapse
• Early 1900s, it was discovered that when action potentials reach the end of a neuron, they trigger the release of chemicals called neurotransmitters that are sorted in structures called synaptic vesicles at the end of the sending neuron
• The neurotransmitter molecules flow into the synapse to small areas on the receiving neuron called receptor sites that are sensitive to specific neurotransmitters
  
  • These receptor sites exist in a variety of shapes that match the shapes of particular neurotransmitter molecules
• When a neurotransmitter makes contact with a receptor site matching its shape, it activates the receptor site and triggers a voltage change in the receiving neuron
  
  • It has an effect on the receiving neuron only when its shape matches that of the receptor site
• When an electrical signal reaches the synapse, it triggers a chemical process that causes a new electrical signal in the receiving neuron
  
  • Nature of this signal depends on both the type of transmitter that is released and the nature of the receptor sites in the receiving neuron
  • Two types of responses can occur
    
    • Excitatory and inhibitory
  • Excitatory response occurs when the neuron becomes depolarized, and the inside of the neuron becomes more positive
    
    • This response is much smaller than the depolarization that happens during an action potential
    • If the resulting depolarization is large enough, an action potential is triggered
    • Depolarization is an excitatory response because it causes the charge to change in the direction that triggers an action potential
  • Inhibitory response occurs when the inside of the neurons becomes more negative, or hyperpolarized
    
    • Hyperpolarization is an inhibitory response because it causes the charge inside the axon to move away from the level of depolarization needed to generate an action potential
• Summary of both responses
  
  • Excitation increases the chances that a neuron will generate action potentials and is associated with increasing rates of nerve firing
  • Inhibition decreases the chances that a neuron will generate action potentials and is associated with lowering rates of nerve firing
• Since a typical neuron received both excitation and inhibition, the response of the neuron is determined by the interplay of excitation and inhibition
• Why does inhibition exist?
  
  • The function of neurons is not only to transmit information but also to process it, and both excitation and inhibition are involved in this processing

Question 6

What is a grandmother cell? Describe Quiroga and colleagues’ experiments on recordings from neurons in patients undergoing surgery for epilepsy

Answer 6

• Specificity Coding
  
  • Notion of a specialized neuron that responds only to one concept or stimulus
  • This idea that one neuron can represent one stimulus or concept, such as a face, fates back to the 1910s
  • Jerome Lettvin
    
    • Proposed that neurons could be so specific that there could be one neuron in your brain that fires only in response to, say, your grandmother
      
      • Grandmother cell –> would respond to your grandmother
    • Even just thinking about the idea of your grandmother, not just the visual input, could make your grandmother cell fire
  • Along this reasoning, you would have a “grandmother cell” for every face, stimulus, and concept that you’ve ever encountered
    
    • A specific neuron to represent your professor, one for your best friend, one for your dog, etc.
  • Maybe you have grandmother cells that respond to specific information in other sense as well
    
    • One neuron per song that you know or food that you’ve eaten
  • Quian Quiroga and colleagues (2005, 2008)
    
    • Recorded from the temporal lobe of patients undergoing brain surgery for epilepsy
    • Patients were presented with pictures of famous people from different viewpoints, as well as other things such as other faces, buildings, and animals  a number of neurons responded to some of these stimuli
    • However, some neurons responded to a number of ways of representing that person or building
    • These neurons were not just responding to the visual input of the famous person’s face, but also to the concept of that particular person
      
      • Other neurons were found that responded just to certain buildings and not any other buildings or objects
    • indicating that these specific cells can be found not just for people but for other objects as well
  • While this finding does seem consistent with the idea of grandmother cells, it does not prove that they exist
  • The idea of grandmother cells is not typically accepted by neuroscientists today
• Sparse Coding
  
  • 2008, Quiroga and coworkers proposed that sparse coding rather than specificity coding was more likely to underlie their results
  • Sparse coding occurs when a particular stimulus is represented by a pattern of firing of only a small group of neurons, with the majority of neurons remaining silent
  • One particular neuron can respond to more than one stimulus
  • There is evidence that the code for representing objects in the visual system, tones in the auditory system, and odors in the olfactory system may involve a pattern of activity across a relatively small number of neurons
• Population Coding
  
  • Population coding proposes that our experiences are represented by the pattern of firing across a large number of neurons
  • Advantage: a large number of stimuli can be represented, because large groups of neurons can create a huge number of different patterns
• Returning to the question, “how neural firing can represent perception”
  
  • Part of the answer: perceptual experiences – such as the experience of the aroma of cooking or the appearance of the objects on the table in front of you – are represented by the pattern of firing of groups of neurons
    
    • Sometimes the groups are small (sparse coding), sometimes large (population coding)

Question 7

What is the sensory code? Describe specificity, sparse, and population coding. Which types of coding are most likely to operate in sensory systems?

Answer 7

• The problem of neural representation for the sense has been called the problem of sensory coding, where the sensory code refers to how neurons represent various characteristics of the environment
• Specificity Coding
  
  • Notion of a specialized neuron that responds only to one concept or stimulus
  • This idea that one neuron can represent one stimulus or concept, such as a face, fates back to the 1910s
  • Jerome Lettvin
    
    • Proposed that neurons could be so specific that there could be one neuron in your brain that fires only in response to, say, your grandmother
      
      • Grandmother cell –> would respond to your grandmother
    • Even just thinking about the idea of your grandmother, not just the visual input, could make your grandmother cell fire
  • Along this reasoning, you would have a “grandmother cell” for every face, stimulus, and concept that you’ve ever encountered
    
    • A specific neuron to represent your professor, one for your best friend, one for your dog, etc.
  • Maybe you have grandmother cells that respond to specific information in other sense as well
    
    • One neuron per song that you know or food that you’ve eaten

Sparse Coding
- 2008, Quiroga and coworkers proposed that sparse coding rather than specificity coding was more likely to underlie their results
- Sparse coding occurs when a particular stimulus is represented by a pattern of firing of only a small group of neurons, with the majority of neurons remaining silent
- One particular neuron can respond to more than one stimulus
- There is evidence that the code for representing objects in the visual system, tones in the auditory system, and odors in the olfactory system may involve a pattern of activity across a relatively small number of neurons

• Population Coding
  
  • Population coding proposes that our experiences are represented by the pattern of firing across a large number of neurons
  • Advantage: a large number of stimuli can be represented, because large groups of neurons can create a huge number of different patterns
• Returning to the question, “how neural firing can represent perception”
  
  • Part of the answer: perceptual experiences – such as the experience of the aroma of cooking or the appearance of the objects on the table in front of you – are represented by the pattern of firing of groups of neurons
    
    • Sometimes the groups are small (sparse coding), sometimes large (population coding)

Question 8

What is phrenology, and what insight did it provide into neural representation?

Answer 8

• phrenology: bumps and contours on a person’s skull
• Phrenology was debunked, but it was the first proposal that different functions map onto different areas of the brain
• the idea that specific brain areas are specialized to respond to specific types of stimuli or functions is called MODULARITY, with each specific area called a MODULE

Question 9

Explain how neuropsychological case studies can support a modular view of neural representation, using Broca’s research as an example

Answer 9

• Left frontal lobe –> Broca concluded that this area was the speech production area, and it came to be known as Broca’s area
• Area in the temporal lobe –> involved in understanding speech, and which came to be known as Wernicke’s area
• Broca’s and Wernicke’s areas provided early evidence for modularity
• Neuropsychology  studies relating to the location of brain damage to specific effects on behaviour
  
  • Studying patients with brain damage is difficult for numerous reasons, including the fact that the extent of each patient’s damage can differ greatly

Question 10

Describe the technique of brain imaging. How can fMRI be used to study modularity?

Answer 10

• Brain imaging: a more controlled way that modularity has been studied –> recording brain responses in neurologically normal humans –> makes it possible to create pictures of the location of the brain’s activity
• METHOD: Brain Imaging
  
  • Magnetic resonance imaging (MRI)
    
    • This technique doesn’t indicate neural activity
  • Functional magnetic resonance imaging (fMRI)
    
    • Enabled researchers to determine how various types of cognitions, or functions activate different areas of the brain
    • Takes advantage of the fact that blood flow increases in areas of the brain that are activated
    • Measurement of blood flow is based on the fact that hemoglobin (which carries oxygen in the blood) contains a ferrous (iron) molecule and therefore has magnetic properties
    • Areas of the brain that are more active consume more oxygen, so the hemoglobin molecules lose some of the oxygen they are transporting, which makes them more magnetic and increases their response to the magnetic field
    • fMRI determines the relative activity of various areas of the brain by detecting changes in the magnetic response of the hemoglobin
    • fMRI is limited in that it can’t record activity from individual neurons
      
      • what’s being recorded  activity in subdivisions of the brain called voxels (small cube-shaped areas of the brain about 2 or 3 mm on a side)
      • not brain structures, but simply small units of analysis created by the fMRI scanner
    • fMRI results
      
      • usually “hotter” colours like red indicate higher activation, while “cooler” colours like blue indicate lower activation
      • these coloured areas do not appear as the brain is being scanned
        
        • determined by a calculation in which brain activity that occurred during the cognitive task is compared to baseline activity or a different task
• Belin and coworkers (2000)
  
  • Results: revealed an area in the temporal love – the superior temporal sulcus (STS), that was activated significantly more in response to vocal sounds than non-vocal sounds
  • This area was therefore dubbed the “voice area” of the brain

Question 11

What is distributed representation? Provide an example from one of the senses

Answer 11

• Distributed Representation
  
  • Geoffrey Hinton, James McClelland, and David Rumelhart
    
    • proposed that the brain represents information in patterns distributed across the cortex, rather than one single brain area
      
      • concept known as distributed representation
      • this approach focuses on the activity in multiple brain areas and the connections between those areas
  • one example of distributed representation
    
    • how the brain responds to pain
      
      • you might simultaneously experience the sensory component, an emotional component, and a reflexive component
      • these different aspects of pain activate a number of structures distributed across the brain
      • pain is a good example of how a single stimulus can cause widespread activity

Question 12

Discuss the difference between structural and functional connectivity. Which technique might be used if one was interested in studying the neural connections associated with a certain task, and why?

Answer 12

• Connections Between Brain Areas
  
  • Connections between brain areas may be just as important for perception as the activity in each of those areas alone
  • Two different approaches to exploring the connections between brain areas
    
    • Structural connectivity: the “road map” of fibers connecting different areas of the brain
    • Functional connectivity: the neural activity associated with a particular function that is flowing through its structural network
  • Distinction between the two:
    
    • MRI vs. fMRI example: the structure of the brain is measured using MRI and the functioning of the brain is measured by fMRI
  • One way of measuring functional connectivity involves using fMRI to measure resting state activity of the brain
    
    • Measuring someone’s brain activity as they are performing a task –> task-related fMRI
    • Measuring someone’s brain activity when they aren’t doing anything –> resting-state fMRI

Question 13

Describe how functional connectivity is determined. What is the resting-state method?

Answer 13

• METHOD: The Resting State Method of Measuring Functional Connectivity
  
  • 1. Use task-related fMRI to determine a brain location associated with carrying out a specific task. This location is called the seed location
  • 2. Measure the resting-state fMRI at the seed location. The resting-state fMRI of the seed location is called a time-series response because it indicates how the response changes over time
  • 3. Measure the resting-state fMRI at another location, which is called the test location. The response of the test location Somatosensory, which is located in an area of the brain responsible for sensing touch
  • 4. Calculate the correlation between the seed and test location responses. The correlation is calculated using a complex mathematical procedure that compares the seed and test responses at a large number of places along the horizontal time axis  high correlation = high functional connectivity, poor correlation = poor or no functional connectivity
• The test locations Somatosensory and Motor R are highly correlated with the seed response and so have high functional connectivity with the seed location
  
  • Evidence that these structures are part of a functional network
• Other ways of determining functional connectivity
  
  • Resting-state functional connectivity is one of the main methods
    
    • Example: functional connectivity can be determined by measuring the task-related fMRI at the seed and test locations and determining the correlations between the two responses

Question 14

How can functional connectivity provide insight into perception?

Answer 14

• Important note:
  
  • Saying that two areas are functionally connected does not necessarily mean they directly communicate by neural pathways
  • Functional connectivity and structural connectivity are not the same thing, but they are related
    
    • Regions with high structural connectivity often show a high level of functional connectivity
• Example of how functional connectivity can help us understand perception
  
  • It can be used to predict behaviour
• By examining the structural and functional connectivity between brain areas in a network, in addition to the activation in each brain area alone, researchers can get a more comprehensive picture of how the brain represents our perceptual experiences

Question 15

What is the mind-body problem? Why do we say that demonstrating connections between nerve firing and a particular stimulus like a face or a colour does not solve the mind-body problem?

Answer 15

• Mind-body problem: how do physical processes like nerve impulses (the body part of the problem) become transformed into the richness of perceptual experience (the mind part of the problem)?
  
  • These connections between electrical signals and perception provide a solution to the mind-body problem
    
    • FALSE –> they are all just correlations – demonstrations of relationships between neural firing and perception
  • The mind-body problem goes beyond asking how physiological response correlate with perception
    
    • It asks how physiological processes cause our experience