Category specificity in the processing of color-related and form-related words: An ERP study
Introduction
Among the most intensely debated issues in the cognitive neuroscience of language is the question of the cortical “seat” of word meaning (e.g., Barsalou et al., 2003, Caramazza and Shelton, 1998, Humphreys and Forde, 2001, Kiefer, 2005, Kiefer and Spitzer, 2001, Martin and Chao, 2001, Pulvermüller, 1999, Tyler and Moss, 2001). Although there is little doubt that areas in left inferior frontal and superior temporal cortex – sometimes referred to as Broca's and Wernicke's regions – play a major role in language processing, the location of additional areas possibly contributing to semantic processing remains controversial. Most theories localize meaning-related mechanisms in areas anterior, inferior, and posterior to Wernicke's area, and they assume that these left temporal areas play a unique role in binding together the form and meaning of words (Hickock and Poeppel, 2000, Hodges, 2001, Price, 2000, Scott and Johnsrude, 2003). However, since most studies investigating the issue have focused on the cortical processing of highly imageable nouns and concepts related to their concrete object-related meaning, it is possible that other word types engage semantic representations in other cortical regions. Hemodynamic and neurophysiological imaging studies have compared words referring to objects with words that have a clear semantic relationship to actions, typically action verbs (Dehaene, 1995, Kellenbach et al., 2002, Preissl et al., 1995, Pulvermüller et al., 1996) or nouns referring to tools (Chao et al., 1999, Creem-Regehr and Lee, 2005, Ishai et al., 1999, Martin et al., 1996). For instance, Kiefer (2001) reports that words referring to artifacts elicit higher activation in fronto-central areas than words related to ‘natural kinds’. He attributes the fronto-central activation elicited by artifacts to the significance of actions for the representation of artifacts. These studies showed that words related to actions elicit strong frontal activation including premotor cortex, suggesting that the frontal activation might reflect aspects of the action-related meaning of action words (Martin and Chao, 2001, Pulvermüller, 1996). If so, the cortical locus of meaning processing could be, at least in part, determined by the general neuroscientific principle of Hebbian learning, according to which neuronal correlation is mapped onto connection strength (Hebb, 1949, Tsumoto, 1992). If word forms frequently co-occur with visual perceptions (object-related words), their meaning-related activity may be found in temporal visual areas, whereas action-related words frequently encountered in the context of body movements may produce meaning-related activation in fronto-central motor areas (Braitenberg and Pulvermüller, 1992, Pulvermüller, 1996, Pulvermüller, 2003, Martin and Chao, 2001). Earlier studies have provided evidence that word-meaning processing elicits specific action-related activity patterns in fronto-central areas, including motor and premotor cortex (Hauk et al., 2004, Hauk and Pulvermüller, 2004, Pulvermüller et al., 2000, Shtyrov et al., 2004). Action words referring to different parts of the body showed enhanced activation of fronto-central sensorimotor areas that were also active when actions were performed with the particular body parts to which the action words referred. These results provide evidence for a close link between the cortical processing of actions and the processing of the meaning of words that refer to those same actions. In essence, fronto-central action-related areas may play a role in category-specific semantic processing.
Whether or not frontal areas also contribute to the processing of other word categories is still a matter of debate. According to the cell assembly model of Pulvermüller and Preissl (1991), words of all kinds are organized by neuron populations distributed over superior temporal and inferior frontal perisylvian cortex, which processes acoustic and articulatory information. This view, although well supported by empirical data (cf. Pulvermüller, 2003), is questioned by some models coming from the aphasiology tradition (Hickock and Poeppel, 2000, Wernicke, 1874). These models postulate separate modular processors in the frontal and temporal lobes, selectively contributing to language comprehension and production, respectively. Word reading would, accordingly, only activate posterior areas but leave frontal areas unaffected. Findings about frontal cortex activation elicited by reading or even passively hearing words, especially action-related words such as action words and artifactual objects like tools, cast doubts on this type of models (Hauk et al., 2004, Kiefer, 2001, Pulvermüller et al., 2003). However, action words and artifacts form a small subset of the vocabulary, and it may be asked whether activation of specific frontal lobe areas is a more general phenomenon shared by other word types too. In this study, we investigate word categories that relate to different kinds of visual perception: words that have strong semantic associations to a color (e.g., “red”, “grass”) and words that are related to a distinctive form or shape (e.g., “arc”, “ball”). We hypothesize that, although both word categories should activate specific inferior temporal areas in the ventral where-stream of visual processing, form-related words would elicit additional activity in frontal cortex. This additional activation in frontal lobe would reflect the fact that an abstract form is always related to a distinct action pattern, a sequence of movements that can be performed with different body parts to outline the shape. In addition, form-related words could also activate occipital areas, such as lateral occipital cortex (LOC) or V1, that have been reported to be involved in the processing of visual shapes (Grill-Spector et al., 2001, Murray et al., 2002). By the same principle, we also expect color-related words to elicit a higher activation in middle temporal and/or occipital areas (i.e., V4) as these areas have been described to be involved in the perception of colors (Zeki and Marini, 1998).
To investigate whether such a differentiation could be observed, we performed an event-related potential (ERP) experiment similar to the one described by Hauk and Pulvermüller (2004). The usage of event-related potentials (ERPs) extracted from multi-channel EEG recordings enables us to investigate the precise time course of the differentiation of the words according to their semantic categories. The minimum norm source estimation technique described by Hauk (2004) was used to pinpoint the cortical origin of word-evoked neurophysiological activity on a standard brain surface.
We assume that all word types alike should activate areas in the left inferior temporal lobe. It has been claimed that this activation could reflect the engagement of a center processing the meaning of words, similar to Lichtheim's “concept area” (Price, 2000). However, other authors have postulated that a posterior area in the left inferior temporal lobe specializes in the prelexical processing of visual word forms (VWF area; Cohen et al., 2000, Cohen et al., 2002, Dehaene et al., 2002, Fiez and Pedersen, 1998, McCandliss et al., 1997, McCandliss et al., 2002, Polk and Farah, 2002), and, finally, others have claimed that inferior temporal areas house word-category-specific processes (Martin and Chao, 2001). Given the limited spatial resolution of EEG, we expected a general word-related activation in inferior temporal lobe initially followed by spreading of activity either in temporal lobe primarily (color words) or from temporal to frontal cortex (form words).1 Interestingly, the activation of the VWF area is reported to occur within 150 ms and 200 ms following the presentation of visual stimuli (Bentin et al., 1999, Helenius et al., 1999, McCandliss et al., 2002, Tarkiainen et al., 2002), which temporally overlaps with the activation of category-specific cortical areas that Hauk and Pulvermüller (2004) locate at around 200 ms from stimulus onset. In fact, even earlier activation of areas related to the meanings of words has also been reported to occur at around 100 ms from stimulus onset (Pulvermüller et al., 2001). This suggests that the early activation of inferior temporal areas might in fact correspond not only to the activation of the orthographic features of the words, but also to their meanings. In this respect, the ERP methodology allows us to investigate whether there are distinct temporal windows in which semantic and specifically orthographic processes occur.
Section snippets
Materials and methods
Ten monolingual native speakers of English participated in the study. Their age varied between 20 and 39 years (mean age 26.4 years, standard deviation 6.26 years). They spent a minimum of 15 years on basic and higher level education. All had normal or corrected-to-normal vision and reported no history of neurological illness or drug abuse. Neuropsychological testing (Oldfield, 1971) revealed that all of them were right-handed. Informed consent was obtained from all subjects, and they were paid
Identification of peaks in the ERPs
The bottom part of Fig. 2 shows the root mean squares (RMS) of the ERP amplitudes at all recorded electrodes for each of the three types of stimuli in our experiment: ERPs to hash marks (green line), ERPs to color-related words (red), and ERPs to form-related words (blue). The graph shows three clear peaks in the RMS, centered approximately at 106 ms, 154 ms, and 254 ms from stimulus onset. At around 202 ms from stimulus onset, one of the stimulus categories, form-related words, elicited an
Discussion
The present study investigated the neurophysiological correlates (ERP and minimum norm estimates) of visual lexical processing. Our discussion focuses on the spatio-temporal patterns of cortical processes related to word-form and letter information and on category-specific semantic information.
Acknowledgments
This research has been funded by the Medical Research Council (U.K.) and the European Community under the “Information Society Technologies Programme” (IST-2001-35282).
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