<P> This strong assumption was first studied in 1996 by Boynton and colleagues, who checked the effects on the primary visual cortex of patterns flickering 8 times a second and presented for 3 to 24 seconds . Their result showed that when visual contrast of the image was increased, the HDR shape stayed the same but its amplitude increased proportionally . With some exceptions, responses to longer stimuli could also be inferred by adding together the responses for multiple shorter stimuli summing to the same longer duration . In 1997, Dale and Buckner tested whether individual events, rather than blocks of some duration, also summed the same way, and found they did . But they also found deviations from the linear model at time intervals less than 2 seconds . </P> <P> A source of nonlinearity in the fMRI response is from the refractory period, where brain activity from a presented stimulus suppresses further activity on a subsequent, similar, stimulus . As stimuli become shorter, the refractory period becomes more noticeable . The refractory period does not change with age, nor do the amplitudes of HDRs . The period differs across brain regions . In both the primary motor cortex and the visual cortex, the HDR amplitude scales linearly with duration of a stimulus or response . In the corresponding secondary regions, the supplementary motor cortex, which is involved in planning motor behavior, and the motion - sensitive V5 region, a strong refractory period is seen and the HDR amplitude stays steady across a range of stimulus or response durations . The refractory effect can be used in a way similar to habituation to see what features of a stimulus a person discriminates as new . </P> <P> Researchers have checked the BOLD signal against both signals from implanted electrodes (mostly in monkeys) and signals of field potentials (that is the electric or magnetic field from the brain's activity, measured outside the skull) from EEG and MEG . The local field potential, which includes both post-neuron - synaptic activity and internal neuron processing, better predicts the BOLD signal . So the BOLD contrast reflects mainly the inputs to a neuron and the neuron's integrative processing within its body, and less the output firing of neurons . In humans, electrodes can be implanted only in patients who need surgery as treatment, but evidence suggests a similar relationship at least for the auditory cortex and the primary visual cortex . Activation locations detected by BOLD fMRI in cortical areas (brain surface regions) are known to tally with CBF - based functional maps from PET scans . Some regions just a few millimeters in size, such as the lateral geniculate nucleus (LGN) of the thalamus, which relays visual inputs from the retina to the visual cortex, have been shown to generate the BOLD signal correctly when presented with visual input . Nearby regions such as the pulvinar nucleus were not stimulated for this task, indicating millimeter resolution for the spatial extent of the BOLD response, at least in thalamic nuclei . In the rat brain, single - whisker touch has been shown to elicit BOLD signals from the somatosensory cortex . </P> <P> However, the BOLD signal cannot separate feedback and feedforward active networks in a region; the slowness of the vascular response means the final signal is the summed version of the whole region's network; blood flow is not discontinuous as the processing proceeds . Also, both inhibitory and excitatory input to a neuron from other neurons sum and contribute to the BOLD signal . Within a neuron these two inputs might cancel out . The BOLD response can also be affected by a variety of factors, including disease, sedation, anxiety, medications that dilate blood vessels, and attention (neuromodulation). </P>

Is an fmri scan useful for detecting structures of function