A drug selectively reduces P3 amplitude in a cognitive task while leaving N400 amplitude unchanged. What can you infer?
AThe drug impairs overall cognitive processing, since P3 and N400 reflect the same general attention resource
BThe drug specifically impairs context updating (P3 function) while leaving semantic integration (N400 function) intact
CThe drug affects early sensory processing, because both P3 and N400 reflect perceptual gating
DNo inference is possible because ERP components are too variable to support single-component interpretations
This is dissociation logic in action. The P3 reflects context updating — revising an internal model of the task situation when a significant event occurs. The N400 reflects semantic integration — how easily a word fits its context. Because the two components are independently modulated, a manipulation that selectively reduces P3 without touching N400 specifically impairs the context-updating stage while leaving semantic processing intact. This precision of inference is exactly what makes ERPs valuable beyond behavioral measures.
Question 2 Multiple Choice
In a spatial attention experiment, attended stimuli show enhanced P1 amplitude (80–130 ms) compared to unattended stimuli. What does this demonstrate about attention?
AAttention operates after the stimulus has been fully identified, selecting relevant items from a complete perceptual representation
BAttention modulates sensory gain in early visual cortex before detailed stimulus analysis — amplifying the signal at attended locations from the very start of processing
CThe P1 enhancement reflects conscious awareness of the stimulus, not attention per se
DAttention can only modulate processing after 200 ms, so P1 enhancement must reflect baseline differences between conditions
The P1 peaks at 80–130 ms over occipital scalp — too early for detailed object identification. Its enhancement by spatial attention demonstrates that the brain amplifies sensory signals at attended locations before it 'knows' what the stimulus is. This is sensory gain modulation: attention acts like a spotlight that brightens input from attended regions in early visual cortex. Option A describes a 'late selection' theory that the P1 evidence contradicts. Option C is wrong because subjects can show P1 enhancement without conscious awareness of stimuli.
Question 3 True / False
Attention can independently modulate P1 amplitude without changing N1 amplitude, demonstrating that early sensory gain and attentional filtering are separable processing stages.
TTrue
FFalse
Answer: True
This is one of the key empirical demonstrations of dissociation in ERP research. P1 (80–130 ms) reflects sensory gain modulation at attended locations; N1 (150–200 ms) reflects discrimination and filtering of stimuli at those locations. Experimental manipulations can selectively enhance P1 without changing N1 (or vice versa), which proves these are not a single 'attention effect' but genuinely distinct stages. If they were the same process, you could not modulate one without the other.
Question 4 True / False
The N400 component is larger (more negative) for words that are semantically congruent with their preceding context than for incongruent words.
TTrue
FFalse
Answer: False
This is backwards — and it is a very common confusion. The N400 is LARGER (more negative amplitude) for semantically INCONGRUENT or unexpected words (e.g., 'I take my coffee with cream and dog'). The N400 indexes the difficulty of integrating each word into the ongoing meaning representation: congruent words are easy to integrate (small N400), while incongruent words are costly to integrate (large N400). Larger N400 = harder semantic integration = more surprising word.
Question 5 Short Answer
Why do cognitive neuroscientists often use ERP recordings alongside fMRI rather than relying on either method alone?
Think about your answer, then reveal below.
Model answer: ERPs provide millisecond temporal resolution that reveals the timing and sequence of processing stages (e.g., P1 at 80 ms vs. N1 at 150 ms), but offer limited spatial resolution. fMRI provides precise localization of active brain regions but lacks the temporal resolution to separate processes occurring hundreds of milliseconds apart. Together, they provide complementary information: ERPs reveal *when* each processing stage occurs and whether stages can be dissociated; fMRI reveals *where* the activity is generated. Neither method alone tells the full story of neural computation.
The complementarity of ERPs and fMRI is a practical expression of a deeper principle: understanding cognition requires knowing both the spatial organization of processing (which brain regions are involved) and the temporal dynamics (in what order, and with what timing). ERP dissociations like P3-vs-N400 tell you that two processes can be selectively impaired; fMRI tells you where each process lives. When both methods converge on an answer, the evidence is much stronger than either alone.