Study Notes: Neurons and Perception

How are physiological processes involved in perception?
How can electrical signals in the nervous system represent objects in the environment?

Johannes Müller (1842): Formulated the "doctrine of specific nerve energies," which suggests that the nature of perception is defined by the pathway over which the sensory information is carried.
Camillo Golgi (1873): Developed a revolutionary method of staining specific neurons which allowed for the visualization of individual nerve cells under a microscope. He was awarded the Nobel Prize in 1906.
Edgar Adrian (1920s): Conducted seminal research by making recordings from single individual neurons.
The Modern Era (1950s): Marks the beginning of the contemporary era of brain research.
Key Figures: The transcript also identifies David Hubel and Torsten Wiesel as central figures in brain research.

Modular Organization: The brain operates through specialized modules. Different sensory modalities have primary receiving areas located in specific lobes:
    * Vision: Primary receiving area is in the Occipital Lobe.
    * Audition (Hearing): Primary receiving area is in the Temporal Lobe.
    * Tactile Senses (Touch): Primary receiving area is in the Parietal Lobe.
Frontal Lobe: Acts as an integration center, coordinating information received from two or more senses simultaneously.

Components of a Standard Neuron:
    * Cell Body: The metabolic center of the cell.
    * Dendrites: Branch-like structures that receive signals from other neurons.
    * Axon (Nerve Fiber): A long process that conducts electrical signals away from the cell body.
Specialized Receptor Neurons: Neurons that receive stimuli directly from the environment (rather than other neurons) have a specialized receptor at the terminal instead of a standard cell body structure.

Ion Mechanics:
* Sodium ( $Na^+$ ): These positive ions are normally pumped out of the cell to maintain balance. During the propagation of a neural impulse, they flood into the cell.
* Potassium ( $K^+$ ): These positive ions are described as "sluggish." They open more slowly than sodium channels. When they do open, potassium floods out of the cell to return it to its resting potential.
Electrical Phases:
    * Resting Potential: The difference in charge between the recording electrode (inside the nerve fiber) and the reference electrode (outside the fiber) is $-70\,mV$ .
    * Rising Phase: The permeability of the membrane changes, and $Na^+$ flows into the fiber, making the interior of the neuron more positive.
    * Falling Phase: $K^+$ flows out of the fiber, making the interior more negative.
    * Hyperpolarization: Potassium channels stay open slightly too long, causing the potential to dip below $-70\,mV$ . This is crucial as it keeps the action potential moving in one single direction.
Propagated Response: This physiological process travels the length of the axon.

Recording Methodology: Researchers use microelectrodes to record from single neurons. One electrode is placed inside the nerve fiber, and a reference electrode is placed outside.
Fundamental Properties of Action Potentials:
    * Propagated Response: Once triggered, the signal travels all the way down the axon.
    * Constant Magnitude: The size of the action potential remains the same regardless of the intensity of the stimulus.
    * Rate Coding: To represent increased stimulus intensity, the neuron increases its firing rate (number of impulses per second) rather than the size of the pulses.
    * Refractory Period: Each impulse has a refractory period of approximately $1\,ms$ .
    * Firing Rate Limits: The upper limit for neural firing is between $500$ and $800$ impulses per second.
    * Spontaneous Activity: Neurons show activity even in the absence of external stimulation.

Mechanism of Transmission:
    1. A signal travels down the axon and reaches the synapse.
    2. The impulse triggers the release of neurotransmitters from synaptic vesicles in the presynaptic (sending) neuron.
    3. Neurotransmitter molecules cross the synaptic gap and are received by the postsynaptic (receiving) neuron.
Lock and Key Metaphor: Neurotransmitters must match specific receptor sites like a key fits into a lock.
Function: Neurotransmitters act as triggers for voltage changes in the postsynaptic neuron.

Excitatory Transmitters:
    * Cause depolarization.
    * Make the neuron more positive.
    * Increase the likelihood of an action potential occurring.
Inhibitory Transmitters:
    * Cause hyperpolarization.
    * Make the neuron more negative.
    * Decrease the likelihood of an action potential occurring.
Summation of Input: The firing rate of a neuron is determined by the relative amount of excitatory vs. inhibitory input. If inhibition becomes stronger relative to excitation, the firing rate decreases and may eventually stop.

Simple Circuit (No Convergence):
    * Consists of excitatory inputs only.
    * Input into one receptor has no effect on neighboring circuits.
    * These circuits indicate only a single specific spot of stimulation.
Convergent Circuit (Excitatory Only):
* Input from multiple receptors summates into the next neuron.
* The output varies based on input size; increasing the stimulus size increases the response of the receiving neuron.
Convergent Circuit (Excitatory and Inhibitory):
    * Inputs from various receptors summate to dictate the final output.
    * Result in a weak response for single inputs or very long stimuli.
    * Result in a maximum firing rate for medium-length stimuli.

Definition: The area of receptors that affects the firing rate of a given neuron in a circuit.
Determination: Receptive fields are mapped by monitoring the response of single cells via electrodes.
Visual Research Example: In studies on anesthetized cats (Hubel & Wiesel, 1961), light stimuli are presented to the retina while responses in the optic nerve fiber are measured.
Center-Surround Antagonism:
    * An excitatory-center-inhibitory-surround receptive field shows different reaction levels based on light location.
    * The largest response occurs when the entire excitatory center is illuminated.
    * If the stimulus size increases to cover the inhibitory surround, the firing rate decreases.

Specificity Coding: The theory that each specific object (e.g., a specific face like Bill, Mary, or Raphael) is represented by the firing of one specialized neuron dedicated only to that object.
Distributed Coding: The theory that objects are represented by the pattern of firing across a large group of neurons. This is more efficient as three neurons can represent many different faces through varying combinations of firing patterns.

DLPFC: The Dorsolateral Prefrontal Cortex is studied in relation to anti-smoking advertisements.
Predictive Activity: Research indicates that baseline brain activity can predict a person's subsequent perceptual experience.