The recognition potential (RP) is an electrical response of the brain that is evoked by recognizable images, such as words, pictures, or faces (Rudell, 1991). Meaningless images do not evoke it. The RP peaks about 200 to 300 ms after image onset. Its latency indicates how rapidly an image is perceived. Image quality, word difficulty, and level of reading skill are some factors that affect RP latency. The RP occurs rapidly, well before a behavioral response can be made to a word. Short latency restricts the processes that it reflects to fast acting neural operations. This gives the RP immunity to non-perceptual factors that affect longer latency potentials and behavioral reaction time (RT).
The RP can manifest the processing of irrelevant items when behavioral data give no evidence of it (Rudell, 2000). Twelve subjects viewed a stream of unfamiliar non-letter strings (e.g., ) that changed every 200 ms. Intermixed with them were words (e.g., ), and non-word letter strings that were orthographically regular (e.g., ) or irregular (e.g., ). The subjects were asked to respond to valid words only, but they often mistakenly responded to non-word letter strings, especially if they were orthographically regular. Twelve other subjects viewed a similar image stream, but the targets were Gestalt figures (e.g., ) instead of words. Errors were no more likely for letter than for non-letter strings. Thus, the behavioral data suggested that word-detecting subjects were processing the irrelevant non-words, but Gestalt-detecting subjects were not. RP data indicated otherwise. Irrelevant letter strings evoked the RP in both subject groups and it was larger for the more regular items. This showed that both groups were processing the letter strings.
Higher scorers on the verbal portion of the Graduate Record Exam (GRE) showed shorter RP latency to words (Rudell and Hua, 1997). GRE performance was more strongly related to RP latency than it was to RT or to longer latency potentials. High scoring on the GRE probably required extensive reading experience. Years of frequent reading may have built up rapidly acting neural processes that facilitated the processing of words and hastened them into conscious awareness. Correlation results are subject to multiple interpretations, so advantage was taken of a natural experiment (Rudell and Hu, 2010). Some subjects (USA-educated) had extensive experience in reading English and others (China-educated) had lesser experience. The immunity of the RP to non-perceptual factors that affect RT suggested that the behavioral and electrophysiological results would differ in a specific way. Long-time readers of English were expected to show shorter RP latency to English words than less experienced readers of English, but no RP latency difference was expected for non-language Gestalt figures, with which neither group had greater experience. In contrast, due to speed accuracy tradeoff, the China-educated subjects were expected to show longer RT for both the words and the non-language images. The predictions were confirmed (see figure 2). China-educated subjects were more likely to incorrectly make a behavioral response under the impression that they had seen a genuine word. These mistakes caused them to become more careful, suppressing eagerness to make a fast response. The random intermixing of word and Gestalt targets prevented subjects from knowing if a word or a Gestalt target would be presented next, so the behavioral responses of the China-educated subjects were slowed down for both types of target. The amount of language experience that a person had was more strongly related to RP latency than it was to RT. This further validated use of the RP as a tool for investigating perception and demonstrated definite advantages that it has for studying acquired perceptual processes in humans.
Figure 1. Background and target images. The top array (A) contains 14 background images, one Gestalt figure target, and one English word target. Array B contains word targets. Array C contains Gestalt targets. (from Rudell and Hu, 2010, Int. J. Psychophysiol., 76, 158-168).
Figure 2. Behavioral reaction time versus recognition potential latency. The error bars indicate one standard error of the mean. (from Rudell and Hu, 2010, Int. J. Psychophysiol., 76, 158-168).
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