You must have seen these plots before, where the temporal resolution of various methods of probing brain function is plotted along one axis and their spatial resolution on the other. Spatial resolution is often approximated in terms of units of the nervous system (from dendritic spines through neurons and cortical columns all the way to lobes and hemispheres). Similarly, temporal resolution is indicated with easy-to-understand labels, from milliseconds to hours and beyond.
Thus, functional MRI, which can resolve brain activity down to the millimeter and to the second, occupies a different position on the plot than EEG, whose temporal resolution is much more accurate (within the millisecond) but whose spatial resolution is more on the scale of centimeters. The techniques that boast the highest resolution in both space and time are generally more invasive: intracerebral micro-electrode arrays are a prime example.
These plots also present a crucial piece of information when assessing methods: the extent to which they can sample the brain’s function. This is generally done by drawing an area for each technique that depicts the span that it covers in both dimensions. This notion of sampling or coverage is critical given our current understanding of major cerebral functions being subtended by large-scale networks of neurons that link remote areas into cohesive units. Thus, micro-electrodes can resolve individual neurons, but it is practically impossible to record from more than a few small patches of brain at a time.
The same sampling problem applies to time: micro-electrodes can record high-quality brain signals for days to weeks to a few months, but scarring around the implanted material tends to alter the properties of the signals in the very long term. By contrast, there is no a priori technical hurdle to placing the same subject in the fMRI scanner every day for their entire life.
Using a third dimension
In addition to resolution and coverage, some of the plots go further and use a third dimension (pseudo-3D isometric plots or colors) to represent another feature of the method. For instance, Walsh and Cowey illustrate whether a given method allows interfering with the function of the brain; examples include microstimulation and transcranial magnetic stimulation (TMS).
In another example, Devor and colleagues nicely use color to show how different optical imaging methods are able to penetrate through the thickness of the brain.
My final example, taken from an article by Mehta and Parasumaran, uses the third dimension to represent how much a given method forces the subject (human, in this case) to remain immobile. Obviously, fMRI and MEG, where the sensors are fixed to heavy machinery, and not attached to the subject’s head as are EEG electrodes of NIRS sensors, ideally require perfect immobility. This is likely to become a fundamental aspect of neuroscience methods, as neuroscience moves further towards more naturalistic, ecologically valid experimental paradigms.
To sum up, any given approach to investigating brain function has a number of dimensions that affect its performance and its adequacy to address a particular question. Spatial-temporal resolution plots for neuroscience methods are a good way of representing this complex dimensionality and a nice example of how one well-designed image can transmit a wealth of information.
Grinvald, A., & Hildesheim, R. (2004). VSDI: a new era in functional imaging of cortical dynamics Nature Reviews Neuroscience, 5 (11), 874-885 DOI: 10.1038/nrn1536
Walsh, V., & Cowey, A. (2000). Transcranial magnetic stimulation and cognitive neuroscience Nature Reviews Neuroscience, 1 (1), 73-80 DOI: 10.1038/35036239
Devor, A., Sakadžić, S., Srinivasan, V., Yaseen, M., Nizar, K., Saisan, P., Tian, P., Dale, A., Vinogradov, S., Franceschini, M., & Boas, D. (2012). Frontiers in optical imaging of cerebral blood flow and metabolism Journal of Cerebral Blood Flow & Metabolism, 32 (7), 1259-1276 DOI: 10.1038/jcbfm.2011.195
Mehta, R., & Parasuraman, R. (2013). Neuroergonomics: a review of applications to physical and cognitive work Frontiers in Human Neuroscience, 7 DOI: 10.3389/fnhum.2013.00889