The recognition potential is an electrical response of the brain to recognizable images. Meaningless or unrecognizable images do not evoke it. The peak of the RP occurs about 200 to 300 ms after word presentation, depending on the conditions. RP latency is sensitive to factors affecting the speed of perception, such as image degradation, complexity, word priming, word frequency, and word difficulty. The RP occurs before a behavioral response can be made to a word. Its short latency narrows down the time at which critical processes occur. They must have operated by the time the RP is generated. The short latency of the RP is also important for its immunity to non-perceptual factors.
Sutton et al. discovered the vertex-positive component called P3 or P300. It was described as an endogenous event-related potential because it depended more on internal conditions than on external features of stimuli. P3 latency for printed words ordinarily exceeds 450 ms. Kutas and Hillyard described a shorter latency response. N400 was a vertex-negative potential occurring about 400 ms after words that ended sentences in a semantically incongruous fashion. Stimulus expectation affects P3 and N400. The size of P3 increases when the probability of an oddball stimulus decreases and when inter-stimulus intervals are increased. The incongruous ending of a sentence is a surprising event. Both the P3 component and N400 seem to reflect post-lexical processes.
The RP has shorter latency than N400 and P3 and has a different topography. The largest responses are recorded from occipital areas. It is not sensitive to surprise or probability of occurrence. Both spoken and visually displayed words evoke N400 and P3, but only visually displayed words evoke the RP. The failure of auditory stimuli to evoke the RP suggests that its sources are restricted to elements of the visual system.
Behavioral reaction time (RT) manifests the processing of irrelevant words in special cases, such as the Stroop color-word test. Incongruity of the words with stimulus features increases RT. A failure to find effects on RT does not imply that irrelevant words are not processed. It only indicates an absence of interference with RT. The recognition potential manifests the processing of irrelevant words in the absence of incongruity. It also reveals the processing of irrelevant non-words and reflects how closely they resemble genuine words (Rudell, A. P. & Hu, B. (2000). Behavioral and brain wave evidence for automatic processing of orthographically regular letter strings. Brain and Language 75, 137-152).
The orthographic regularity of non-words was manipulated using the bigram positional frequency (BGPF) metric of Massaro et al. It indicates the probability of a particular pair of letters occurring at a given position in words of specified length. Subjects in one group detected words in a stream of unfamiliar non-letter strings, such as 
. Intermixed with them were 128 words (e.g., 
), 128 high BGPF non-words (e.g., 
), and 128 low BGPF non-words (e.g., 
). The instructions required responses to valid words only. Conditions were identical for a second group of subjects, except that the targets were 128 different Gestalt figures (e.g., 
) instead of words. As shown in the accompanying figure, subjects often erroneously responded to non-word letter strings in the word detection task, especially when they were high in BGPF. Errors seldom occurred in the Gestalt detection task. The behavioral data suggest that word-detecting subjects processed the irrelevant non-words, especially ones that were high in BGPF, but Gestalt-detecting subjects did not process them, since they had no more effect on behavior than unfamiliar non-letter characters did.
RP data contradict that conclusion. Irrelevant letter strings evoked the RP regardless of the task. The effect of orthographic structure was apparent in both tasks, the RP being larger for more regular items. The RP was larger for subjects who detected words. Automatic processes might be more likely to operate when subjects specifically look for words. The importance of the RP results is that they show that the brain automatically processes irrelevant non-words, even though that processing is not revealed by behavioral measures. The orthography effect on the RP identifies a factor that is critical for the processing
Selected Publications
Simon, J. R. and Rudell, A. P. (1967). Auditory S-R compatibility: The effect of an irrelevant cue on information processing. Journal of Applied Psychology 51, 300-304.
Fox, S. S. and Rudell, A. P. (1968). Operant Controlled Neural Event: Formal and systematic approach to electrical coding of behavior in brain. Science 162, 1299-1302
Amassian, V. E., Cracco, R. Q., Maccabee, P. J., Cracco, J. B., Rudell, A. P., and Eberle, L. P. (1989). Suppression of visual perception by magnetic coil stimulation of human occipital cortex. Electroencephalography and Clinical Neurophysiology 74, 458-462.
Rudell, A. P. and Hua, J. (1997). The Recognition Potential, word difficulty, and individual reading ability: On using event-related potentials to study perception. Journal of Experimental Psychology: Human Perception and Performance 23, 1170-1195.
Rudell, A. P. and Hu, B. (2001). Does a warning tone accelerate the processing of sensory information? Evidence from recognition potential responses to high and low frequency words. International Journal of Psychophysiology 41, 31-42.
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