The studies reviewed in section 2.2 all used visual materials. There are also studies
employing auditory stimuli to investigate the N400 effect and to see if there is difference in
the ERP results between written and spoken language processing. For example, Holcomb and
Neville (1991) conducted a study on the N400 effect by using natural speech as stimuli. In
their experiments, there were three conditions: best completion, semantically related
anomalies and semantically unrelated anomalies. They used 135 English sentences, and
controlled the anomalous target words so that they did not share the same initial phoneme as
the best completions. Subjects were required to decide if the spoken sentence stimuli made
sense. The results were in agreement with previous visual studies showing the N400 effect in
both related and unrelated anomalies, with a relatively larger effect in unrelated condition.
The N400 effect was found to be largest over occipital, Wernicke's and posterior sites, with
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the left hemisphere more negative than the right. It was especially noted that the ERPs
difference between the best completion and the unrelated anomaly appeared quite early at
around 50-100 ms, and they considered it as the early onset of N400 effect. This earlier onset
of N400 effect found in auditory stimuli clearly pointed out the discrepancy in semantic
processing between visual modality and auditory modality. Further, to see if there was any
difference between naturally connected speech and speech with words spoken in isolation,
they extended the experiment by adding a 750-ms inter-stimulus interval (ISI) as independent
variable. However, there was no early onset of N400 effect in this extended experiment as in
natural speech stimuli. On this absence of early onset of N400, Holcomb and Neville (1991)
suggested two possibilities. For one, they thought that there was an interaction between the
nonsemantic between-word cues(i.e. prosody and coarticulation) and contextual information
in natural speech stimuli but not in speech spliced with intervals. This interaction enabled the
information about the final words to be provided rapidly to the subjects and resulted in an
earlier N400 effect accordingly. For the other, the rate of stimuli presentation was concerned
that natural speech stimuli was presented at a relatively high rate. However, regarding this
factor, Kutas’ (1987) visual study showed a contradictory finding in that an earlier ERP effect
appeared in the experimental condition with slow stimuli presentation. Thus, Holcomb and
Neville (1991) suggested that further studies were necessary to tease these factors apart.
Later in Friederici et al’s (1993) study, they investigated the effects of semantic,
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syntactic and morphosyntactic violations in natural speech processing with 160 German
sentences. For semantic violation, they manipulated the experimental sentences to contain
selectional restriction violation (i.e. the mismatch between the preceding noun and the
sentence-final verb). A probe-verification paradigm was employed and subjects were asked to
judge whether the probe words was part of the preceding sentences they just heard. As
predicted, the results revealed a classical N400 pattern evoked by the semantic violation,
which was broadly distributed over both hemispheres.
Connolly and Phillips (1994) designed an experiment to test the relationship between
Phonological Mismatch Negativity (PMN) and word expectancy developed by contextual
information. For PMN, it is measured to occur at around 175-225 ms and to peak at around
25-75 ms later. In their experiment, they used 160 English sentences as auditory stimuli and
constructed four conditions which have 40 sentences for each: Phoneme Mismatch-Semantic
Mismatch, Phoneme Mismatch-Semantic Match, Phoneme Match-Semantic Mismatch, and
Phoneme Match-Semantic Match. For semantic match, the sentence-final target word is the
one with highest cloze probability for the sentence. As for the manipulation of phoneme
match, the sentence-final target word shares the same initial phoneme as the word with
highest cloze probability, and vice versa. The results conformed to their prediction: the
elicitation of both PMN and N400 in Phoneme Mismatch-Semantic Mismatch condition, the
elicitation of PMN but no N400 in Phoneme Mismatch-Semantic Match condition, N400 but
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no PMN in Phoneme Match-Semantic Mismatch condition, and finally none of the
components in Phoneme Match-Semantic Match condition. Topographically, the N400 effect
was found more frontally distributed and symmetric in both hemispheres, and PMN was more
evenly distributed. This frontal distribution of the N400 effect in Connolly and Phillips' (1994)
experiment was obviously different from the posterior distribution pattern found in previous
visual (Marta Kutas & Hillyard, 1980b, 1984b) or other auditory experiments (Holcomb &
Neville, 1991). On this discrepancy, Connolly and Phillips (1994) argued that it is probably
because no overtly behavioral response were required from the subjects in their auditory
experiment, resulting in the inconsistent results to the previous findings regardless of
modality differences. Nevertheless, they suggested that more researches are awaiting to
confirm this assumption. Also, Connolly and Phillips (1994) pointed out that in Phoneme
Match-Semantic Mismatch condition, the N400 effect had a delayed peak latency due to the
same initial phoneme shared by the eliciting word and expected word. In other words,
compared to Phoneme Mismatch-Semantic Mismatch condition, there was no separation for
the expected and unexpected words at the initial phoneme in phoneme match condition. Thus,
they concluded that this acoustic-phonological processing, PMN, is at the lexical selection
stage where the contextual effect begins to influence. Unlike Holcomb and Neville’s (1991)
study which suggested an early onset of N400, Connolly and Phillips (1994) argued an early
negativity PMN elicited before N400 in auditory language processing.
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In a more recent study, Hagoort and Brown (2000) examined natural speech processing
with 120 Dutch sentences. In their experiments, target word anomalies were controlled to be
at either sentence-final or sentence-medial positions. They found the N400 effect elicited in
both experimental conditions, which is similar to that elicited by visual materials. The scalp
distribution of this N400 effect was larger over posterior sites and slightly larger over left
hemisphere than over right hemisphere, which was similar to Holcomb and Neville's (1991)
findings. On this pattern of N400 different from the previous visual studies, Hagoort and
Brown (2000) indicated two possibilities. For one, they suggested that there might be
non-overlapping neural generators for the two different input modalities that result in such
discrepancy. For the other, they reported that in fact the distribution of N400 varied in some
of the previous visual studies; therefore, it is also possible that the left hemisphere
preponderance of the auditory N400 is not deviant from the pattern of the visual N400.
Aside from the N400 effect, Hagoort and Brown (2000) also observed an early
negativity (N250) in both of the sentence-final and sentence-medial position conditions,
while this early negativity was not shown in previous visual experiments. To interpret this
early negativity, Hagoort and Brown (2000) considered it as an index of lexical selection,
representing the mismatch between the expected word forms and the actual cohort activation
of the target words. Addressing the lack of early negativity in visual stimuli, they ascribed it
to the fact that the lexical information of the target words was available upon presentation in
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visual materials. Thus, the early negativity components would not be standardly seen as in the
condition with visual stimuli. With the two different negativity effects shown in their
experiments, their study was different from previous research that either showed no early
negativity (Friederici et al., 1993) or an earlier onset of the N400 effect in speech processing
(Holcomb & Neville, 1991). In spite of different interpretations on the earlier negativity,
Hagoort and Brown’s (2000) finding could be compared with PMN discussed by Connolly
and Phillips (1994). Even though the two negativity components were not measured to be in
the same latency range, they were both considered functionally independent from the N400
effect (i.e. not just as an early onset of the N400 effect), and were at the stage of lexical
selection. In addition, the early negativity components were only found in auditory rather
than visual materials. Finally, the finding of two negativity components in both Connolly and
Phillips’ (1994) and Hagoort and Brown’s (2000) studies evidently indicates a discrepancy
between the processing in auditory and visual stimuli, with the latter representing the
monophasic negativity.