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{\LARGE Up and Down the Garden Path} 
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{\Large 9.59 Psycholinguisitics} 
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{\large Juliet C. Bourne
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December 16, 1991}
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\section{Introduction}

The Garden Path theory of sentence processing asserts that sentences are
processed serially, i.e. only one syntactic analysis is entertained at
a time, that initial analyses are based on structural criteria, and that
there is a process of reanalysis which uses both syntactic and
referential criteria when the initial analysis is wrong. The theory
maintains that the context or discourse information does not effect the
first analysis of a sentence at all. The importance of this theory and
its claims lies in its support of the modularity theory. If it can be
shown that the Garden Path theory is correct that no semantic, world, or
other non-syntactic knowledge is used in the first analysis of
sentences, then there will be a strong argument for the modularity of
language.

\section{Experiments}

Many experiments have been done to show the garden path effects that
appear in sentences with ambiguous (in terms of location of attachment)
prepositional phrases and reduced relatives. The principle of Minimal
Attachment predicts these effects. However, these experiments have been
done mostly with sentences presented with no context. In order to
determine whether or not context has any effect on eliminating garden
path effects, sentences must be presented in a context which gives the
ambiguous region a preferred reading (by the Referential Theory). 

\subsection{Perfetti et al}

Perfetti did four experiments involving garden path effects (and the
removal thereof by discourse) in sentences with ambiguous prepositional
phrases and reduced relatives.\footnote{``Parsing in discourse:
Contextual effects and their limits'' by Charles A. Perfetti and M. Anne
Britt, University of Pittsburgh, Keith Rayner, University of
Massachusetts, and Simon Garrod, Glasgow University. Presented at the
Psychonomic Society Annual Meeting, New Orleans, November 1990.}

Experiments 1 and 2 tested garden path effects in sentences with
prepositional phrases. The method for experiment 1 was self-paced,
word-at-a-time reading, and measurements of reading time were taken. In
experiment 2, eye movement measurements were taken. The same materials
were used in both experiments.\footnote{There was some additional
material in experiment 2 which made a distinction between lexical and
syntactic garden path sentences.} Sentences were provided in null,
neutral, or biasing contexts. The results showed a strong garden path
effect in null or neutral contexts, but {\em no} effect in biasing
contexts.

Experiments 3 and 4 tested garden path effects in sentences with reduced
relatives. The method for experiment 3 was the same as experiment 1. The
method for experiment 4 was the same as experiment 2. The same materials
were used in these two experiments and sentences were presented in
neutral or biasing contexts. An additional distinction was made in
experiment 4 between contexts in which the referent was ``in focus'' and
in which it was not. The results showed a strong garden path in {\em
both} neutral and biasing contexts.\footnote{Focussed contexts produced
slightly less of an effect than non-focused contexts.}

\subsection{Confetti}

``Confetti1'' was an experiment done in Molly Potter's lab at the MIT
Brain and Cognitive Sciences department during the summer of 1991. It
took selected materials from the experiments done by Perfetti {\em et
al}, attempting to control for variables like the number of words in a
sentence and the difference in the number of words between the noun and
verb attachment versions of each sentence. The idea here was to attempt
to reproduce the results given by Perfetti using a method involving
Rapid Serial Visual Presentation (RSVP) rather than self-paced or eye
movement methods.

In the method involving RSVP, words of a sentence are rapidly presented
one at a time in the center of the computer screen. The subject is
usually asked to recall the sentence immediately following its end. The
subject's response is compared to the actual sentence presented. General
accuracy counts are done and used as a way to define a cut-off point in
performance. Subjects performing below this cut-off point are not used
in the final analysis. Scoring the subject's responses depends on the
particular experiment being performed. 

In the case of Confetti1, the sentences presented each had an ambiguous
region. The subjects' responses were scored based on the changes made to
the ambiguous regions. The possible scores were G (garden pathed), Q
(questionably garden pathed), O (omitted the ambiguous region), H
(omitted half of the ambiguous region), S (solved the ambiguity), and X
(no changes). An example of getting a score of G is responding ``The
ship tied up by the pier and sank'' when the stimulus sentence was ``The
ship tied up by the pier sank''. An example of solving the ambiguity (S)
is responding ``The ship that was tied up to the pier sank'' to the
above stimulus sentence. 

The ambiguous sentences from the Perfetti experiments were gathered and
put into RSVP format. The sentences were presented with no context
whatsoever and fillers helped to hide the pattern of the ambiguities.
When the experiment was first run, it was discovered that a per-word
duration of 100ms was too slow; subjects were recalling the sentences
nearly perfectly. So, the per-word time was reduced to 83ms.
\footnote{This shows that recovery is very quick. For a method like RSVP
it is necessary to shorten the stimulus duration in order to see an
effect (since no timing measurements are taken).} Subjects no longer
performed perfectly, but most of them made it past the general accuracy
cut-off.

Data was gathered from 16 subjects. The results seem to show a
significant difference between the noun and verb attachment versions of
the sentences -- i.e. there seems to be a garden path effect. Reduced
relatives show the strongest effect. The analysis of the data is
complicated due to all of the possible scores and also the omission of
the ambiguous region (or parts thereof), which could be due to either
garden pathing or simply not reading quickly enough. A careful and
well-thought-out analysis should be done to take into account all
possible explanations for the results. However, as a first pass, it
seems that there is a garden path effect.

Future experiments should be done using the RSVP method again, but also
providing a context for the sentences. With these experiments, the
possibility of eliminating garden path effects by discourse can be
explored. If it is found that garden path effects for ambiguous
prepositional phrases are also eliminated as in the Perfetti
experiments, then the conclusion could be drawn that the hypothesis that
only syntactic knowledge is used in parsing is wrong. However, if garden
path effects persist even with context, then the conclusion could be
drawn that the Garden Path theory is correct and that RSVP provides a
truly on-line look at the parser in action.

\section{Conclusions}

Given Perfetti's results which show the elimination through context of
garden path effects with ambiguous prepositional phrases, the
Referential Theory, which claims that all syntactic analyses are
computed and the semantic analyzer chooses one of them based on the
context, seems to be correct. However, this does not explain why the
garden path effects for reduced relatives did not disappear given a
biasing context. 

Thus, there is a lot more to explore where the Garden Path and
Referential Theories are concerned. The results of such explorations
should give a clearer picture as to how the process of parsing sentences
proceeds. Beyond just explaining why people are ``garden-pathed,'' these
theories have implications for the structure of the language module and
how its sub-modules interact. Is there autonomy among the sub-modules?
Or does semantics influence parsing? Actually, the two theories might
not be incompatible. The Referential Theory explains how ambiguities are
resolved in everyday use, whereas the Garden Path theory attempts to
explain how syntactic parsing proceeds independent of semantics. If
garden path effects can be shown, even with disambiguating context,
using the on-line measurements that RSVP provides, then it can be said
that syntactic analysis is separate from semantic analysis. And thus it
could be concluded that modularity exists {\em within} the language
module.

\section{Epilogue}

There are good reasons for end-of-term regulations. It is entirely fair
and possible to make the paper due a week before the end of classes.
From ``Regulations of the Faculty'', section 2.52, for future reference:

For each subject with a final examination: no examination shall be given
and no assignment, term paper, or oral presentation shall fall due in
the six days preceding the reading period. For each subject without a
final examination: at most, either one in-class examination may be
given, or one assignment, term paper, or oral presentation may fall due,
in the six days preceding the reading period. An in-class examination
given during those six days is limited to one normal class period (or to
one and one-half hours, whichever is shorter).

No assignment, term paper, or oral presentation for any subject shall
fall due after the last day of class exercises scheduled for that
subject.


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