What is it?
Once you require your brain to inhibit the natural response to a stimuli, having it uninhibit that response takes some time. That delay is called the Negative Priming Effect. Your brain has been prepped to take extra time in producing a response, because that response was previously supressed.
How is it measured?
In psychological research experiments, negative priming is frequently evaluated with use of a variation on the Stroop test (Stroop, 1935. Journal of Experimental Psychology). The experiment participant is
presented with the name of a color printed in a different color font. The subject is told to ignore what the word reads, and to instead say the color of the font. On an immediately subsequent trial, the subject is presented with the name of a color printed in the previously suppressed color, and is again told to say the color of the font, as shown below.
Trial n (the "prime" trial)
On Screen: GREEN, printed in blue font
Trial n+1 (the "probe" trial)
On Screen: RED, printed in green font
Trial n+1 will have a longer response time than Trial n.
Why is this interesting?
It's common sense that when you ask your brain to undo something it just did, it takes a couple milliseconds. However, the Negative Priming Effect studies are intriguing because not all people show the effect. Schizophrenia patients, for example, show a much less potent effect than normal control subjects. In fact, their reaction times on trial n+1 are often equal to that of trial n.
It is hypothesized that this is because the schizophrenia patients have problems selectively attending to stimuli within their environment. This selective attention deficit has long been considered a plausible explanation for the hallucinations and delusions experienced by schizophrenics. The theory is basically that the patients have difficulty selectively attending to the font color in trial n, so they are less effective in inhibiting the color name. Because they don't as effectively inhibit it as normal controls, they don't show as much (or any) Negative Priming Effect on trial n+1.
Recent studies are looking at the correlation between frontal lobe brain activity and performance on negative priming tasks, and more specifically at the activity in the frontal lobes of schizophrenia patients. So far, results are looking a bit conflicted, and as with all academic research in search of grant money, additional studies are needed.
(for complete review) May, C., Kane, M., & Hasher, L. (1995) Determinants of Negative Priming. Psychological Bulletin, 118(1), 35-54.
Steel, C., Haworth, E.J., Peters, E., Hemsley, D.R., Sharma, T., Gray, J.A., Pickering, A., Gregory, L. Simmons, A., Bullmore, E.T., & Williams, S.C.R. (2000) Neuroimaging correlates of negative priming. NeuroReport,
vol. 12, 16, pg 1-6.