A researcher wants to determine the precise timing of neural responses to an auditory tone — specifically, whether the brain response occurs within the first 50 milliseconds. Which method is most appropriate?
AfMRI, because its millimeter spatial resolution can localize the auditory cortex precisely
BEEG, because it captures electrical changes on the millisecond timescale
CfMRI, because the BOLD signal is directly proportional to neural firing rate
DEEG, because it records individual action potentials from single cortical neurons
fMRI's BOLD signal peaks approximately 5–6 seconds after neural activity — orders of magnitude too slow to capture 50 ms responses. EEG captures summed postsynaptic potentials with millisecond resolution, making it the correct tool for studying response timing. Option 3 is also wrong: EEG does not record individual action potentials (too brief, too deep) but rather summated synaptic currents from thousands of synchronously active neurons. Option 2 is wrong about fMRI's measurement basis — BOLD reflects metabolic demand, not directly firing rate.
Question 2 Multiple Choice
A brain region shows elevated BOLD signal during a spatial navigation task. What does this most directly indicate?
AThat spatial memories are stored and retrieved specifically from that region
BThat neurons in that region are firing faster than in any other brain region
CThat there is increased local blood flow and oxygenation reflecting greater metabolic demand in that region
DThat the region is uniquely and exclusively responsible for spatial navigation
BOLD signal measures hemodynamic response — increased blood flow and blood oxygenation — which is an indirect proxy for metabolic demand. It does not directly measure neural firing (option 1), does not imply that region fires faster than all others (other regions may be active without being the experimental contrast), and does not support exclusivity claims about function (option 3). The inferential gap from 'elevated BOLD' to 'this region computes spatial navigation' requires careful experimental design, appropriate baseline contrasts, and statistical rigor — it cannot be read directly from the signal.
Question 3 True / False
EEG has excellent temporal resolution because it directly records action potentials from individual neurons near the surface of the cortex.
TTrue
FFalse
Answer: False
EEG's temporal resolution is indeed excellent (milliseconds), but the explanation in this statement is wrong. Individual action potentials are too brief (~1 ms) and too spatially distributed to produce detectable scalp voltages. EEG records the summed postsynaptic potentials (not action potentials) of thousands to millions of pyramidal neurons firing in synchrony. Their aligned dendritic currents sum to produce fields large enough to be detected at the scalp. EEG is also insensitive to neurons that fire asynchronously or in orientations perpendicular to the scalp.
Question 4 True / False
fMRI and EEG measure complementary aspects of brain activity, making simultaneous EEG-fMRI recordings more informative than either method alone.
TTrue
FFalse
Answer: True
fMRI provides spatial resolution (millimeters, telling you where metabolic demand increased) but poor temporal resolution (seconds, due to the slow hemodynamic response). EEG provides temporal resolution (milliseconds, capturing when electrical activity changes) but poor spatial resolution (the inverse problem makes source localization mathematically underdetermined). Combining them allows researchers to identify both when and where a neural process occurs. The tradeoff is technical: MRI's magnetic field distorts EEG signals, requiring specialized equipment and signal correction.
Question 5 Short Answer
Why is the phrase 'fMRI shows where thoughts occur' misleading, and what does fMRI actually measure?
Think about your answer, then reveal below.
Model answer: fMRI measures the BOLD signal — blood oxygenation changes reflecting increased metabolic demand in a region. It does not directly record neural activity or 'locate' cognition. The inferential leap from 'blood flow increased here' to 'this is where the thought is' requires careful experimental design: comparing active and baseline conditions to isolate the process of interest, statistical thresholding to separate signal from noise, and replication to confirm reliability. Additionally, most cognitive functions are distributed across multiple regions simultaneously, not located in a single spot. The phrase implies a precision and directness the measurement cannot support.
The hemodynamic response — the chain from neural activity to metabolic demand to blood flow to BOLD signal — introduces several layers of indirection. The BOLD signal reflects aggregate metabolic demand, not specific neural computations; it peaks seconds after the neural event; it varies with vascular health and baseline cerebral blood flow; and the region 'lighting up' is defined relative to a contrast condition, not as an absolute measure. Understanding these limitations is essential for critically reading neuroimaging literature.