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\begin{titlepage}
\vspace*{2in}
\begin{center}
{\LARGE The Autonomy of
\medskip \\
Syntax and Semantics
\medskip \\
in Sentence Processing}
\bigskip\bigskip \\
{\Large (9.63 Laboratory in Cognitive Science)}
\bigskip\bigskip \\
{\large Juliet C. Bourne
\bigskip\bigskip\bigskip\bigskip\bigskip \\
May 5, 1992}
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\end{titlepage}

\begin{quotation}
{\footnotesize The purpose of this experiment is to determine whether
syntactic processing occurs independently of semantic processing. The
two independent variables are the legality of the syntax and the
plausibility of the semantics of stimulus strings. The task is a
same/difference task on the two stimulus strings which appear in each
trial. This method is similar to that described by Forster (1979). The
results of the experiment show a significant effect of syntax on the
reaction times for the same/difference task. No significant main effect
of plausibility or interaction between syntax and plausibility is found.
Thus, the conclusions to be drawn from these results are that syntactic
processing is used in the same/difference task and that syntactic
processing and semantic processing are autonomous (otherwise there would
have been an interaction between syntax and plausibility).}
\end{quotation}

\vspace*{.25in}

One model of sentence processing asserts that the different levels of
processing (lexical, syntactic, and semantic) are independent. (See
Forster (1979).) Each level of processing has its own ``module'' and
receives input from the level below it. So the semantic processing
module receives its input from the syntactic processing module, the
syntactic processing module receives input from the lexical processing
module, et cetera. This model, as opposed to models which do not assert
the autonomy of the different levels, provides good explanations for
phenomena in language processing. For example, people are able to
understand sentences even though they may not be plausible. Just because
the sentence ``Grumpy rocks are hard in the evening'' does not make much
sense semantically, that does not prohibit or hinder people from
understanding the sentence syntactically. A model of sentence processing
which did not assert the independence of the syntactic and semantic
processors might have a hard time explaining why the semantic absurdity
of the sentence does not affect the syntactic processing. However, the
module theory of sentence processing provides a natural explanation: the
syntactic processor creates the structure of the sentence, regardless or
its meaning, and gives it to the semantic processor. The semantic
processor may or may not be able to find a plausible meaning for it.
Thus, the autonomy of the different levels of sentence processing is an
attractive theory to explore.

Forster (1979) describes many methods and experiments in which the
various levels of processing and the interactions between them are
explored. The same/difference task is described in detail. This
experiment uses a similar task in order to show the effects of syntax
and semantics on the determination of the similarity of two strings of
words. Syntax is expected to have an effect on reaction time, since a
legal sentences are presumedly easier to compare than strings of words.
Based on Forster's results, plausibility should also have an effect on
reaction time, with plausible sentences taking less time to compare than
implausible sentences. If the modular model of sentence processing is
correct, no interaction between syntax and plausibility is expected.
That is, effect of syntax on the same/difference task should not vary
significantly with the plausibility of the sentence.

\sect{Method}

\subsect{Subjects}

The subjects for this experiment consisted of thirteen MIT students,
mainly undergraduates, enrolled in the Laboratory for Cognitive Science.

\subsect{Procedure}

Subjects were asked to judge whether a pair of sentences were the same
or different as quickly and accurately as they could. In each trial, two
sentences appeared on the computer screen, one above a fixation point
and one below. The sentences remained on the screen until the subjects
responded. The subjects pressed the \verb=z= key if the sentences
were different and the \verb=x= if they were the same.

\subsect{Design}

In this experiment there were two within-subject independent variables:
the syntax of the stimulus strings (``sentences'') and the semantics of
the stimulus strings. The syntax could be either normal or aberrant and
the semantics could be either plausible or implausible. Thus there were
four types of strings when the two variables were combined factorially: 

\begin{enumerate}
\item syntactically normal, plausible sentences
\item syntactically normal, implausible sentences
\item syntactically aberrant strings, implausible sources
\item syntactically aberrant strings, plausible sources
\end{enumerate}

There were a total of 61 pairs of strings\footnote{See {\bf Materials}
appendix.}. Forty of these pairs were identical, that is, the {\em same}
response was appropriate. These 40 pairs were divided evenly among the
four types described above. In addition, there were 21 ``filler'' pairs
in which the two strings differed from one another by a single word.

\sect{Results}

Only the 40 trials in which the strings are identical are considered for
data analysis. Of those, only the ones in which the subject responded
correctly (i.e. ``same'') are used. The error rates for the various
conditions are low, with an average of 1.9\% which is less than one
error per subject. (See figure 1.) There does not seem to be a
speed-accuracy trade-off between the conditions. However, since the mean
reaction time overall is 2.5 seconds, there might be a general
speed-accuracy tradeoff, meaning that subjects took more time on each
trial to avoid making errors.

\begin{figure}
\epsfbox{results.ps}
\caption{Error rates and mean reaction times for the four types of
stimulus strings.}
\end{figure}

The results show that a statistically significant main effect of syntax
is present, $F(1,12)=10.33, \,\,\, p < .01$. That is, for sentences
which are syntactically normal, the reaction times are significantly
faster than for sentences which are syntactically aberrant (see figure
1). No significant main effect of plausibility is found, $F(1,12)=4.59,
\,\,\, critF(1,12)=4.75, \,\,\, p<.05$. The means indicate that reaction
times for plausible sentences are faster than for implausible sentences,
but this difference does not appear to be significant. There does not
appear to be an interaction between syntax and plausibility,
$F(1,12)=2.98$.

\newpage
\sect{Discussion}

As expected, reaction times in the same/difference for syntactically
normal sentences are significantly faster than syntactically aberrant
strings of words. This shows that using syntactic processing to compare
strings of words is quicker than using lexical (i.e. word by word)
processing, since in the case where the syntax is aberrant lexical
processing must be used to compare the strings.. The fact that
plausibility does not have a significant effect is somewhat surprising.
One could reason that comparing two sentences for similarity using
the semantics would be quicker than using the syntax, because just the
meanings have to be compared. In the case where the semantics are
meaningless, syntactic processing should be used. In fact, the means
show that plausible strings have faster reaction times than implausible
string, but this difference is not large enough to be significant. From
the results one might hypothesize that the subjects were paying
attention to the semantics of the stimulus strings in few trials, thus
leading to the absence of a significant effect of plausibility.

As hoped and expected, the results show no significant interaction
between syntax and plausibility. Since an interaction would mean that
the effect of syntax on reaction times varies with the plausibility of
the stimulus strings, an interaction would signify that the syntactic
processing level can be affected by the semantic processing level. Even
though from figure 1 there seems to be a minor interaction between
syntax and plausibility, it is not statistically significant and
could have a number of explanations unrelated to theory that syntactic
and semantic processing are independent. Thus, from the results it can be
concluded that syntactic processing and semantic processing constitute
different stages or levels in sentence processing.

\sect{References}

Forster, K.I. ``Levels of Processing and the Structure of the Language
Processor'', Chapter 2, {\em Sentence Processing: Psycholinguistic
Studies Presented to Merrill Garrett}, William E. Cooper and Edward C.T.
Walker, eds. Lawrence Erlbaum Associates: 1979.

\newpage
\sect{Appendix: Materials}

\input sentences.tex

\newpage
\sect{Appendix: Statistics}

Calculations: $MS = \frac{SS}{df}$, $F = \frac{MS_Y}{MS_{Y x SUBJ}}$,
and $p$ values were obtained from tables of F distributions in {\em The
Essence of Statistics}, except in the case of $p < .001$.

\vspace*{.2in}
\noindent\hspace*{-.4in}
\begin{tabular}{lrrrrl}
Source & SS & df & MS & F & p \\ \hline
mean & 324616061.9445 & 1 & 324616061.9445 & 92.7545 & .001 \\
syntax & 421329.8245 & 1 & 421329.8245 & 10.3354 & .01 \\ 
plausibility & 341145.1060 & 1 & 341145.1060 & 4.5958 & NS \\
syntax x plausibility & 103901.6534 & 1 & 103901.6534 & 2.9805 & NS \\
subjects & 41996804.8928 & 12 & 3499733.7410 & \mbox{} & \mbox{} \\
syntax x subjects & 489189.4848 & 12 & 40765.7904 & \mbox{} & \mbox{} \\
plausibility x subjects & 890760.7367 & 12 & 74230.0614 & \mbox{} &
\mbox{} \\
syntax x plausibility x subjects & 418324.7460 & 12 & 34860.3955 &
\mbox{} & \mbox{} 
\end{tabular}

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