Using brain-imaging techniques, researchers have performed empirical studies of the human brain to investigate humor from a neuroscience perspective and to learn more regarding the brain mechanisms underlying humor processing. Nonverbal humor is any material that uses visual stimulation to convey humorous information, whereas verbal humor is that which uses verbal stimulation to make people laugh. Within the field of visual gags, most studies of humor have used visual non-verbal cartoons as humorous stimuli. Functional magnetic resonance imaging (fMRI) techniques and non-invasive brain imaging instruments have allowed researchers to investigate the brain mechanisms of humor more efficiently. The process of comprehending and appreciating humor takes longer than milliseconds, so many of these studies have relied on fMRI using spatial resolution. Among the linguistic theories of humor, the leading ones are Raskin's Semantic-Script Theory of Humor (SSTH) and its revision, the General Theory of Verbal Humor (GTVH), by Raskin and Attardo.