Brains, Neurons, and Neural Codes

Vertebrate Brains: The Basic Plan

The development of vertebrate brains can be understood through embryonic development, which reveals the six major divisions of the adult central nervous system (CNS).

  1. Cerebrum: The largest part of the brain, responsible for higher-level cognitive functions.
  2. Thalamus and Hypothalamus: Involved in sensory relay and homeostasis.
  3. Midbrain: Part of the brainstem, involved in motor control and sensory processing.
  4. Pons and Cerebellum: Coordinate movement and balance.
  5. Medulla: Controls vital functions like breathing and heart rate.
  6. Spinal Cord: Relays signals between the brain and the body.

Brains and Neurons

  • Cell Theory: In the 1830s, Schwann's development of cell theory laid the groundwork for understanding the nervous system.
  • Neuron Doctrine: In the early 1880s-1900s, Ramon y Cajal argued for the neuron doctrine, applying cell theory to the nervous system and positing that the neuron is the fundamental structural and functional unit.

Spikes and Synaptic Potentials

The relationship between voltage, spikes, and synaptic potentials is crucial for neural communication. When excitation reaches a certain threshold, it triggers an action potential (spike). Inhibition, on the other hand, reduces the likelihood of an action potential. These action potentials propagate signals between neurons, forming the basis of neural processing.

Neural Processing and Neural Code

Sensation and perception depend on neural processing whose nature depends on the neural code.

Neural Codes: Basic Building Blocks

Neural circuits are composed of interconnected neurons that communicate through specific synaptic connections. These connections form motifs such as:

  • Feedforward excitation
  • Feedforward inhibition
  • Convergence/divergence
  • Lateral inhibition
  • Feedback/Recurrent inhibition
  • Feedback/Recurrent excitation

Neural Coding

Individual action potentials are uniform and do not carry distinct information. Therefore, the code must be based on:

  • Space: Which neurons are active.
  • Time: The precise timing and sequence of spikes within a spike train.

Neural Codes: Space

Neural circuits and networks are formed by specific synaptic connections between nerve cells.

  • Scale:
    • Human brain: 86 billion neurons.
    • Human cerebral cortex: 17 billion neurons.
    • Cerebellum: 69 billion neurons.
    • Synaptic contacts per neuron: Up to 10,000.
    • Implied synapses: On the order of 101410^{14}.

Neural Codes: Time

Time: spike trains

  • Rate code: Average spike frequency over some integration time. However, it is difficult to determine which time period is appropriate.
  • Temporal code: Precise timing of spikes within a spike train is important.

Neural Codes: Spike Timing

  • Rate code: Average spike frequency over some integration time. However, it is difficult to determine which time period is appropriate.
  • Temporal code: Precise timing of spikes within a spike train is important.

Example calculations:

  • 5 spikes in 100 ms = 5/0.1 s-1 = 50 s-1
  • ISI = 5 ms = 1/0.005 s-1 = 200 s-1
  • ISI = 25 ms = 1/0.025 s-1 = 40 s-1

Where ISI is the interspike interval.

Timescale Considerations

  • Conduction Velocity: Action potentials travel along nerves at varying speeds, ranging from
  • Generation Rate: Action potentials can be generated at rates of 500-1000 spikes per second, implying the fastest action potentials can occur within 1-2 ms (including the absolute refractory period).

Helmholtz and the Speed of Thought

By the mid-19th century, the elements were in place for the neuroscientific study of perception.

Helmholtz demonstrated that nerves transmit signals at a finite velocity, measured in tens of meters per second which was considered slow at the time.

Visual Perception Pathways

The visual perception involves several stages:

  1. Retina: photoreceptors, bipolar cells, ganglion cells
  2. Thalamus (LGN)
  3. Primary visual cortex
  4. 'Higher' visual cortices

The processing time at each stage contributes to the overall time required to generate a visual percept.

Recording Electrical Activity

Overview of techniques:

  • EEG (electroencephalography): Measures summed activity of many neurons via voltage changes on the scalp.
  • ECoG (electrocorticography): Electrodes placed directly on the cortical surface to record local field potentials, but cannot resolve individual neural activity.
  • Extracellular Recording: Microelectrodes in extracellular space pick up spikes from nearby neurons.
  • Intracellular Recording: (highest resolution) Microelectrodes inserted through cell membrane records voltage difference between intra- and extracellular space.

The image shows the differences between the signals recorded by different resolution techniques, along with the setup of each.

Signal Averaging: Event Related Potentials (ERP)

Using EEG to study responses to sensory stimulation

  • The signal from a single electrode is due to activity of thousands to millions of neurons, any response is lost in the noise due to activity of many neurons unrelated to the stimulus.
  • However, if the stimulus is repeated many times and averaged, random activity tends to cancel out, revealing a signal related to the stimulus

Early Discoveries in Neuroscience

  • Nature of electricity and ‘animal electricity’ worked out in parallel, from 1790s on​
  • Early 1900s: electrical resting potential of cells measured
  • 1920s: all-or-none nature of action potential (Adrian)
  • 1940s-50s: theory of neuronal electrical activity worked out