ch. 2: Basic Principles of Sensory Physiology

neural processing

interaction of the signals of many neurons

• create our perceptual experiences

key components of the neuron

• cell body: keeps cell alive

• dendrites: receive electrical signals from other neurons

• axon/nerve fiber: filled with fluid that conducts electrical signals

sensory receptors

neurons specialized to respond to environment

what does recording from an individual neuron give us information about?

gives us information about what is happening in the nervous system

How does one record from a single neuron?

• two electrodes: a recording one, and a reference neuron → tip inside the neuron

• the electrodes are connected to a meter that record the difference in charge between the two neurons

axon at rest/ resting potential

• difference between inside and outside: -70mV

• the inside is more negative than outside (more k+)

explain the steps of the action potential

1. positively charged Na+ rush into the axon (attracted to interior negativity) which increases the permeability of the neuron.

2. rising phase: increase of positive charge in the neuron causes depolarization (+40mV)

3. +40mV cause Na+ channels to close and K+ channels open

4. falling phase: positively charged potassium rushes out of the neuron when the channels open, causing the inside of the neuron to become more negative (hyperpolarization)

5. once the potential returns to -70 mV, the k+ flow stops which means the action potential is over and the neuron is at rest

the basic properties of an action potential

1. propagated response: once a response is triggered, it travels all the way down the axon without decreasing in size, which enables neurons to fire long distances

2. the action potential remains the same size no matter how intense the stimulus is

3. changing the stimulus intensity affects the rate of firing (frequency)

4. have a refractory period of 1 ms—upper firing rate is 500-800 impulses per second

5. show spontaneous activity that occurs without a stimulus

ion

molecules that carry an electrical charge

permeability

the ease at which molecules can pass through the membrane

sodium-potassium pump

keeps buildup from occuring by continuously pumping sodium out and potassium into the fiber

how does one neuron transmit a signal to another neuron?

when action potentials reach the end of a neuron, they trigger the release of a chemicals called neurotransmitters

neurotransmitters are:

• released by presynaptic neuron from vesicles

• received by postsynaptic neuron on receptor sites

  • matched like a lock and key, into specific receptor sites

• used as triggers for voltage change in the postsynaptic neuron

what determines whether an AP will occur?

1. type of transmitter released (neurotransmitter)

2. nature of the receptor sites in the receiving neuron (either an excitatory or inhibitory response)

inhibitory vs excitatory responses

• inhibitory: occurs when the neuron is hyperpolarized and neuron is less likely to fire

• excitatory: occurs when the neuron is depolarized and neuron is more likely to fire

sensory coding

how neurons represent various characteristics in the environment

specificity coding

a neuron that responds to one type of concept or stimulus

Letvin’s tongue-in-check idea (grandmother cell)

neurons can be so specific that there could be one neuron that fires only in response to one stimulus (i.e. your grandmother)

• so strong that even the mere thought of your grandmother could make the grandmother cell fire

Quiroga et al., 2008 grandmother cell and patients with epilepsy

• recorded from temporal lobe of patients with epilepsy

• presented with pictures of famous people, faces, buildings, animals in order to see how the neurons responded

  • neurons responded to some of these stimuli

• neurons were found that only responded to famous actors (Steve Carrell)→ limited study tho

  • discovered that neurons were not just responding to the face but the concept of a particular person

sparse coding (Quiroga et., al)

• idea of sparse coding supported epileptic patients’ results

• sparse coding: 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

  • suggests that representing objects to visual systems, tones to auditory, etc, may involve a pattern of activity across a relatively small number of neurons

population coding

our experiences are represented by the pattern of firing across a large number of neurons

advantage of population coding

a large number of stimuli can be represented because large numbers of neurons can create a huge number of different patterns

Phrenology (Spurzheim)

35 diff mental faculties can be mapped onto the brain based on the bumps and contours of a person’s skull

• bump in back of the head might indicate what your strong mental faculties are

modularity

the idea that specific brain areas are specialized to respond to specific types of stimuli or functions

• module= each specific area

• ex. Broca’s area is specific to the production of speech (frontal lobe)

• ex. Wernicke’s area is specific to understanding speech (temporal lobe)

**can be studied using brain imaging in normal humans

specific brain area that responds when a voice is heard

superior temporal sulcus (STS)

(example of modularity)

task related fMRI vs resting state fMRI

• task related: measure brain activity while doing a task

• resting state: measure brain activity when not doing a task

what is the main method for determining functional activity?

resting-state fMRI

Perception can predict behaviour (Sadghani et al., 2015 )

• used fMRI to look at moment-to-moment functional connectivity

• participants had to detect a very quiet sound that was only perceptible 50% of the time (detection threshold)

• strength of functional connectivity immediately before detection task predicted how likely it was that the participant would hear the sound

mind-body problem

how do physical processes like nerve impulses (body) become transformed into the richness of perceptual experience (mind)?

distributed representation

idea that the brain represents information in patterns distributed across the cortex, not just one brain area

• focuses on the activity in multiple brain areas and the connections between those 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 the structural network

• the networks that are activated when someone is doing an action

How much more concentrated are sodium and potassium outside/inside ?

• sodium: 10x more concentrated outside

• potassium: 20x more concentrated inside

force of diffusion

when molecules move from an area of high concentration to low concentration