Electrophysiological responses and the timecourse of perception and language processing

Neuronal Communication:

• Resting Potential: Neurons maintain a negative charge (-70mV) due to the balance of positive (e.g., Na⁺, K⁺) and negative ions across the cell membrane.

• Action Potential: When a neuron is activated, sodium (Na⁺) channels open, allowing Na⁺ to rush into the cell, making the interior more positive (depolarization). Potassium (K⁺) channels then open, allowing K⁺ to leave the cell, restoring the negative charge (repolarization).

• Neurotransmitter Release: Calcium (Ca²⁺) enters the axon terminal, triggering the release of neurotransmitters into the synaptic cleft, which then bind to receptors on the next neuron.

• Speed: The entire process of depolarization, repolarization, and neurotransmitter release takes 1-2 milliseconds (ms), making neural communication extremely fast.

Signal Degradation:

• The electrical signal generated by neurons weakens as it travels through brain tissue, the skull, and scalp. By the time it reaches the scalp, the signal is in the range of microvolts (μV), which is very weak compared to the original signal in the brain.

• Synchronized Activity: When many neurons fire simultaneously (synchronized activity), the signal becomes stronger and more detectable at the scalp. However, if opposing signals (positive and negative) occur at the same time, they can cancel each other out, making the signal harder to detect.

Electroencephalogram (EEG)Overview:

• Non-invasive Method: EEG measures electrical activity in the brain using electrodes placed on the scalp. It is widely used in both research and clinical settings.

• Temporal Resolution: EEG has high temporal resolution (millisecond precision), allowing researchers to track rapid changes in brain activity.

• Spatial Resolution: EEG has low spatial resolution because the signal is picked up from the scalp, making it difficult to pinpoint the exact source of activity in the brain.

• Sensitivity to Noise: EEG is highly sensitive to interference, such as muscle movements (e.g., blinking, head movements). However, modern filtering techniques have improved the ability to isolate brain activity from noise.

EEG Historical context and set up

Historical Context:

• Hans Berger (1924): First recorded EEG in humans. He distinguished between slow waves (about 90ms between peaks, associated with relaxation) and faster waves (about 35ms between peaks, associated with mental concentration).

• Development: Berger's work laid the foundation for modern EEG, which is now used to study a wide range of brain activities, from sleep to cognitive tasks.

EEG Setup:

• Electrodes: Typically, 64 electrodes are used, though the number can vary depending on the study. Electrodes are placed in specific locations on the scalp according to the 10-20 system, which standardizes electrode placement.

• Signal Amplification: The weak electrical signals picked up by the electrodes are amplified to make them detectable and analyzable.

• Continuous Measurement: EEG provides a continuous measurement of brain activity, making it ideal for studying processes that unfold over time, such as perception and language processing.

Event-Related Potentials (ERPs)

Overview:

• Stages of Processing: The brain processes information in stages, with each stage handling different aspects of the stimulus. ERPs are changes in the EEG signal that occur in response to specific events or stimuli.

• Manipulating Stimuli: By carefully manipulating stimuli and contrasting different conditions, researchers can identify different stages of processing in the brain.

Key ERP Components:

1. C1 and P1:

   • C1: An early visual processing component that occurs around 75ms after stimulus onset. It is associated with the primary visual cortex.

   • P1: A later component that occurs around 100ms after stimulus onset. It is also associated with the primary visual cortex but is modulated by attention to specific parts of the visual field.

2. N1:

   • A negativity that occurs around 170ms after stimulus onset. It is associated with attention and vigilance. For example, individuals with ADHD show a weaker N1 response compared to controls, suggesting differences in attentional processing.

3. Mismatch Negativity (MMN):

   • A response to unexpected changes in stimuli, occurring 150-250ms after the change. It involves both primary sensory areas (e.g., visual or auditory cortex) and frontal areas (cognitive control).

   • Example: If a series of identical sounds is played, and then a different sound is introduced, the brain generates an MMN in response to the unexpected change.

Time-Course of Visual Perception

Early Visual Processing:

• C1 and P1: These components reflect early visual processing in the occipital lobe (primary visual cortex). The P1 component is influenced by attention to specific parts of the visual field.

  • Example: If a stimulus appears in the lower vs. upper visual field, the P1 response will differ depending on where the participant is instructed to focus their attention.

• N1: This component is associated with attention and vigilance. It can distinguish between individuals with ADHD and controls, as those with ADHD tend to have a weaker N1 response.

Visual Expertise:

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• N200 (N2): A component associated with face perception, located in the fusiform gyrus (part of the temporal lobe). It is also present when contrasting real words with pseudowords (made-up words).

  • Chess Experts: Chess experts show increased activity in the fusiform face area when processing chess positions, suggesting that expertise can modulate visual processing. This area, typically associated with face recognition, can also be recruited for other types of visual expertise, such as recognizing chess patterns.

Time-Course of Language ProcessingSemantic Processing (N400):

• N400: A negativity that occurs around 400ms after stimulus onset, reflecting semantic processing. It is sensitive to unexpected words or semantic violations.

  • Example: In the sentence "The man drinks tea with shoes," the word "shoes" is unexpected and elicits a strong N400 response.

  • Modality Independence: The N400 is not limited to language; it also responds to unexpected information in other modalities, such as music. For example, if a melody contains an unexpected note, it can elicit an N400-like response.

Syntactic Processing (P600):

• P600: A positivity that occurs around 600ms after stimulus onset, associated with syntactic processing. It responds to grammatical violations.

  • Example: In the sentence "The woman persuaded to answer the door," the verb "persuaded" requires an object (e.g., "The woman persuaded the man to answer the door"). The absence of the object creates a grammatical violation, eliciting a P600 response.

  • Early Left Anterior Negativity (ELAN): A left-lateralized negativity that occurs 100-200ms after an ungrammatical stimulus. It is associated with local syntactic violations (e.g., missing words in a sentence).

Cross-Modal Processing:

• Arithmetic and Music: The P600 is also found in arithmetic tasks (e.g., detecting a violation in a sequence of numbers) and musical key violations (e.g., hearing a chord that doesn’t fit the key). This suggests that some brain processes are shared across different domains, such as language, arithmetic, and music.

ERP in Aphasia

Reduced ERP Amplitude:

• Individuals with aphasia (a language disorder caused by brain damage) show reduced N400 and P600 amplitudes, reflecting impaired semantic and syntactic processing.

  • Example: In a study by Friederici et al. (1999), individuals with left anterior cortical lesions (e.g., damage to Broca’s area) did not show the ELAN component, indicating a disruption in early syntactic processing.

Complex Thought in Severe Aphasia:

• Despite severe language impairments, some individuals with aphasia retain the ability to perform complex arithmetic tasks, suggesting that certain cognitive processes are preserved even when language is impaired.

Brain Oscillations

Different Rhythms:

• Neural activity occurs at different frequencies, each associated with different types of information processing:

  • Delta (2-4 Hz): Associated with prosody (rhythm and intonation in speech) and pauses.

  • Theta (4-8 Hz): Associated with speech sounds and syllables.

  • Alpha (10 Hz): Associated with auditory attention.

  • Beta (13-30 Hz): Associated with lexical processing (word recognition) and grammatical categories.

  • Gamma (30-50 Hz): Associated with grammatical processing and integrating complex information.

Key Insight:

• Faster oscillations (e.g., gamma) are associated with more complex cognitive processes, such as integrating grammatical information or recognizing complex patterns.

Summary

• Neural Communication: Neurons communicate via rapid electrical signals, involving ion movement and neurotransmitter release.

• EEG Strengths and Limitations: High temporal resolution but low spatial resolution; sensitive to noise but improved by modern filtering techniques.

• ERPs: Reveal stages of processing in the brain, with specific components (e.g., N400, P600) reflecting different aspects of perception and cognition.

• Cross-Modal Processing: Some brain processes (e.g., N400, P600) are shared across different domains, suggesting a general mechanism for integrating unexpected or complex information.

Key Takeaways:

• Neural Communication: Neurons communicate via rapid electrical signals, involving ion movement and neurotransmitter release.

• EEG Strengths and Limitations: High temporal resolution but low spatial resolution; sensitive to noise but improved by modern filtering techniques.

• ERPs: Reveal stages of processing in the brain, with specific components (e.g., N400, P600) reflecting semantic and syntactic processing.

• Cross-Modal Processing: Some brain processes (e.g., N400, P600) are shared across different domains, suggesting a general mechanism for integrating unexpected or complex information.