Studies on categorially ambiguous nouns and verbs in English have been carried out to support zero derivation and to determine the base category and the derivative from various perspectives (Marchand, 1964; Clark and Clark, 1979; Myers, 1984; Hale and Keyser, 1993, 1997; Borer, 2014). For example, from a morphological perspective, Myers (1984) pointed out that a zero-derived word can undergo a further process of inflection (e.g., [_N.pl [_N [_V walk]-Ø]-s]) but not derivation with a root-selecting suffix due to the intervening zero morpheme (e.g., [_ADJ [_N danger]-ous] vs. *[_ADJ [_N [_V walk]-Ø]-ous], morpheme ordering principle; Williams, 1981).

Syntactically, it is noted that verb-derived nouns cannot take verbal argument structure (e.g., [_V walked] the dog for three hours vs.*the [_N walk] of the dog for three hours; Borer, 2014, p. 131), and denominal (noun-derived) location verbs can only be transitive (e.g., She corralled her horses vs. *Her horses corralled; Hale and Keyser, 1993, 1997, p. 206). From a semantic perspective, Marchand (1964) pointed out that the analysis of a zero-derived word must be semantically dependent on its base counterpart (e.g., to [_V saw] = to cut something with a [_N saw]).

From the processing perspective, given that structure building incurs processing costs and building a more complex structure involves greater processing costs (Gibson, 1998, 2000), the zero-derived form should be more difficult to process than the base form of a categorially ambiguous word due to its greater morphological complexity, as shown in Lukic et al. (2023).

Lukic et al. (2023) conducted two experiments to investigate how morphological complexity affected processing of categorially ambiguous words in lexical access (Experiment 1) and online sentence processing (Experiment 2). In their Experiment 1 (forced-choice phrasal-completion task), they compared the selection rates and reaction times for the categorially (un)ambiguous nouns and verbs as in (3).

Conditions A and B were categorially unambiguous nouns and verbs, appearing only with the (3a) or to (3b), respectively. Conditions C and D presented categorially ambiguous words, where condition C presented words with the noun regarded as the base category, and condition D the verb regarded as the base category. Participants were asked to select between the and to to complete the phrase as in (3). They observed a “base-category bias” for the categorially ambiguous words: readers selected the for categorially ambiguous nouns (paint; 3c), and to for categorially ambiguous verbs (visit; 3d), in a way similar to the categorially unambiguous nouns (tray; 3a) and verbs (eat; 3b).

The same bias for base category was also found in their Experiment 2 (eye-tracking while reading task). They investigated categorially ambiguous noun/verb pairs in unambiguous contexts like “Rachel needed the/to paint since the house looked old” (Lukic et al., 2023, p. 9 (2)), and observed a reading time slowdown at the zero-derived word in both directions (e.g., noun-derived verb [_V paint], verb-derived noun [_N visit]). Based on these results, they suggested that the reader recognized the base category and was sensitive to the morphological complexity in the word structure.

The present study tests how covert structure may impact Korean categorially ambiguous adjectives and verbs in real-time sentence comprehension. The English study looked at nouns and verbs which were all tied up in theories about the representations of “objects” and “actions.” We examine Korean adjectives and verbs, which are subcategorized as “stative predicates” (Sohn, 1999; Kim, 2002; Koo, 2021). Additionally, English and Korean differ greatly in terms of typology. Korean tends to be agglutinative, allowing words to contain multiple morphemes, and each morpheme corresponds to a distinctive syntactic category and semantic meaning. English tends to be fusional, in which one morpheme can carry more than one syntactic and semantic property (Bauke and Roeper, 2012; Payne, 2017; Ramoo, 2021).² Therefore, if there is a zero morpheme that carries a specific syntactic category and semantic meaning as in Korean, we expect a zero morpheme in Korean to exhibit higher sensitivity than in English. If we find a similar effect with a different pair of categories in another language to that noted in previous studies of English for nouns and verbs, this can add crosslinguistic evidence to the role of zero derivation on categorially ambiguous words in real-time processing (Lukic, 2016; Krasuska and Yoshida, 2019; Lukic et al., 2023).

Previous analyses have sought to determine the base category of categorially ambiguous adjectives and verbs in Korean according to their morphosyntactic and semantic properties. For instance, Song (1988) noted that the derived form is semantically constrained compared to its base form, as shown in the comparison in (4a) and (4b).

In example (4a), [_ADJ balgda ‘bright’] can take discourse subjects including “day,” “room,” “color,” and “face.” However, in example (4b), [_V balgda ‘become bright’] can only take “day” as its subject. Therefore, [_ADJ balgda ‘bright’] is considered the base form, as it can take subjects belonging to a larger semantic field.

It is also pointed out that the phonologically unrealized zero morpheme can have semantic realization, adding a level of semantic complexity (Pesetsky, 1996; Harley and Noyer, 1999; Folli and Harley, 2007; Koo, 2021; also see Roeper, 2020 for a discussion of the thematic roles of zero morpheme): [_ADJ balgda ‘bright’] describes the status of an entity, whereas the zero morpheme in the adjective-derived verb [_V balgda ‘bright’-Ø ‘become’] adds a level of meaning indicating a process of the change of status. Moreover, the derivational suffix -a/eo.jida ‘become’ can be attached to the adjective stem [_ADJ balgda ‘bright’] to form a verb describing change or degree of the status, but not to the zero-derived verb [_V balgda ‘become bright’], as shown in the comparison in (5a) and (5b).

The incompatibility of -a/eo.jida ‘become’ being attached to [_V balgda ‘become bright’] in (5b) suggests that the zero morpheme realizes the meaning of “becoming.” This is compatible with Myers’s (1984) generalization that a suffix cannot be further attached to a zero-derived word due to the intervening zero morpheme (also see Harley and Noyer, 1999). As a base form, [_ADJ balgda ‘bright’] can take the derivational suffix -a/eo.jida ‘become,’ as in (5a), whereas the zero-derived verb [_V balgda ‘become bright’] cannot, as in (5b).

We follow these analyses and investigate the categorially ambiguous words with the adjectives being the base form in this study (Song, 1988; Koo, 2021).

Sixty native Korean speakers (age: 20–40) participated in the experiment. They were each paid ₩2000 as a reward.

We employed a 2 × 2 within-subjects design, where Category (adjectives vs. verbs) was crossed with Category Ambiguity of the words (categorially ambiguous vs. unambiguous). The categorially ambiguous words were adopted from Koo (2021), with adjectives regarded as the base form and verbs as the derived form based on morphosyntactic and semantic analyses. The categorially unambiguous words were those simply used only as adjectives or verbs, according to Pyojun-Gugeo-Daesajeon (Pyojun Korean Dictionary; Seoul: National Institute of Korean Language, 2019). Sixteen item sets were constructed as experimental stimuli. An example of the stimuli with four conditions is presented in Table 1.

In addition to the critical experimental items,³ we also included 36 filler items that were not pertinent to the present experimental manipulation.

Prior to the real-time processing experiment, we conducted an acceptability rating experiment (31 native speakers of Korean volunteered to participate in the experiment) to ensure that the processing difficulty manifested in longer reading times in our experiment would not be due to the unnaturalness of the inflectional structures used in our stimuli or to semantic anomaly. We expected that all conditions would be rated acceptable. The norming study revealed that all conditions were rated above 3.5 out of 7, suggesting that the acceptability of all sentences was above average.

The experiment was conducted on PC IbexFarm, which is a web-based demonstration platform (Zehr and Schwarz, 2018). All stimuli were pseudo-randomized in a Latin-square to ensure that the same condition in the identical item would not occur consecutively. Through the experimental link generated by PC IbexFarm, participants completed the experiment on their own laptops. Participants were instructed to press the space bar to read the sentences word-by-word at their own speed. After each sentence, there was a comprehension question (e.g., “Did somebody see anything?”), and participants were instructed to press ye ‘yes’ or aniyo ‘no’ to indicate their judgement.⁴

If these words are listed under separate lexical entries (Koo, 2010; Yang, 2015; for English, see Jackendoff, 1976; Lieber, 1980; ao), we expect greater processing costs for the verbs (conditions B and D) than adjectives (conditions A and C) in general, regardless of whether the word is categorially ambiguous or not, owing to the larger size of inflectional forms on verbs than on adjectives (cf. Traficante and Burani, 2003; also see Sereno and Jongman, 1997 for the greater processing difficulty in verbs than nouns).

Under the separate-entry account (e.g., Koo, 2010; Yang, 2015), another possibility is that we may see greater processing difficulty for categorially ambiguous words (conditions A and B) compared to unambiguous ones (conditions C and D) due to competition in word-activation (interactive activation model; Reicher, 1969; McClelland and Rumelhart, 1981; Rumelhart and McClelland, 1982). Under either possibility, we do not expect an interaction between Category and Category Ambiguity.

In contrast, following a theory that predicts that these words involve a process of zero derivation (Song, 1988; Koo, 2021; for English, see Marchand, 1964; Myers, 1984; Lukic, 2016; Krasuska and Yoshida, 2019; Lukic et al., 2023), the processing difficulty should be associated with word-internal structure building. Thus, we expect that adjective-derived verbs (condition B) would induce greater processing difficulty than the base adjectives (condition A) due to the morphological complexity and additional step in adding a covert category-changing morpheme in real-time processing. However, such morphological complexity effect should not be seen in categorially unambiguous verbs (condition D) compared to unambiguous adjectives (condition C), as they should be equally less complex in word-internal structure. In other words, we expect an interaction between Category and Category Ambiguity.

The results were analyzed with a linear mixed effect regression model using the lme4 package (Bates et al., 2015) with fixed effects for Category (adjective vs. verb) and Category Ambiguity (categorially ambiguous vs. categorially unambiguous) and random effects for both participants and experimental items (Baayen et al., 2008).⁵ Each model for individual region included the maximal random effect structure provided the model converged. Reading times for the critical region (Region 4), spillover region (Region 5), and second spillover region (Region 6) were log-transformed with the purpose of minimizing non-normality (Box and Cox, 1964). Figure 1 shows the mean reading times at the critical region (Region 4), spillover region (Region 5), and second spillover region (Region 6) for the four conditions.

We began by focusing on the critical region (Region 4). We found an interaction between Category and Category Ambiguity (β = 0.07, SE = 0.04, t = 2.03). Crucially, the planned comparisons showed a significant simple effect of Category within the categorially ambiguous adjective/verb conditions (conditions A and B) such that condition A was read faster than condition B (β = 0.05, SE = 0.03, t = 1.98). However, the same effect was absent in the categorially unambiguous adjective/verb conditions (conditions C and D); the reading times for condition C did not differ from those for condition D (β = −0.02, SE = 0.03, t = −0.87). We found no main effect of Category (β = 0.01, SE = 0.02, t = 0.81) or Category Ambiguity (β = −0.03, SE = 0.02, t = −1.79).

The interaction between Category and Category Ambiguity persisted into the spillover region (Region 5: β = 0.10, SE = 0.04, t = 2.20). Crucially, the planned comparisons showed a significant simple effect of Category within the categorially ambiguous adjective/verb conditions (conditions A and B), such that condition A was read faster than condition B (β = 0.14, SE = 0.03, t = 4.11). However, the same effect was absent in the categorially unambiguous adjective/verb conditions (conditions C and D); the reading times for condition C did not differ from that for condition D (β = 0.05, SE = 0.03, t = 1.59). Additionally, a significant main effect of Category was observed, such that the reading times for the adjectives (conditions A and C) were shorter than for the verbs (conditions B and D) (β = 0.09, SE = 0.02, t = 4.20). We found no effect of Category Ambiguity (β = 0.01, SE = 0.02, t = 0.41).

At the second spillover region (Region 6), we found a main effect of Category (β = 0.06, SE = 0.02, t = 2.99) such that adjectives (conditions A and C) were read faster than verbs (conditions B and D), and Category Ambiguity (β = −0.05, SE = 0.02, t = −2.36)⁶ such that categorially unambiguous adjective/verb sentences (conditions C and D) were read slower than the categorially ambiguous ones (conditions A and B). However, there was no interaction between these two (β = 0.04, SE = 0.04, t = 0.97).⁷

Table 2 illustrates the statistical by-region (regions 4, 5, and 6) analyses of the linear mixed model for the experiment.