IV-1

IV.

Resolution of Lexical Ambiguities in Ophthalmology

M. Coyaud

1.

Introduction
The content analysis procedures incorporated into the SMART

system are based in part on the words included in documents and searchrequests, and in part on various dictionary mapping and weighting processes.

No systematic attempt is normally made to eliminate all lexical

ambiguities; however, the procedures actually used will generally assign
large weights to those concepts which are semantically appropriate,while other, semantically inappropriate concepts are assigned much
lower weights.
A test is being performed to determine whether a complete
automatic text disambiguation procedure would produce substantiallybetter retrieval results than the standard text analysis methods now
incorporated into the SMART programs.

Specifically, a set of dis-

ambiguation rules has been generated for the field of ophthalmology
designed to replace each input item by a single, hopefully correct,
semantic identifier.

These rules are later to be incorporated into atest program operating with an appropriate document collection.

Thedisambiguation rules actually used are described in the present report.
In view of the specialization of ophthalmology in the field of
the medical sciences, one would expect that lexical ambiguities havingsome impact on the precision of the retrieval of documents would beIV-2

quite rare. To a certain extent, this assumption proves to be true as
far as can be verified using a small corpus of about 600 ophthalmologicalabstracts.

Upon examination, less than thirty index words were found withactual multiple meanings in a small corpus in ophthalmology.

As a com-

parison, more than 100 words selected as descriptors were previously foundto be ambiguous in a corpus of 500 abstracts in the field of physiologicalpsychology [1] . The classification used for the dictionary and for theoperation of the polysemy rules is provided by the Vision Information Centerof Harvard (it is based on the MESH dictionary used at the National Libraryof Medicine with a number of modifications).

2.

Procedures for Devising the Polysemy Rules
Two general approaches can be distinguished:

the first, a priori,

consists in obtaining by introspection a list of contextual criteria allowing
the automatic recognition of the meaning of a word in a given context; thesecond, a posteriori, consists in reading texts or rather concordances forlists of ambiguous words in given corpuses, so that the actual criteria
could be easily picked up from the concordance.
The introspective method contains the risk of being irrealistic.
The a posteriori, empirical method has therefore been chosen, but in certain
cases, some criteria provided by introspection were listed among the criteria
found in the concordances.
A first concordance was obtained from a corpus (later: corpus A)of 300 abstracts; rules were devised for the words actually ambiguous inthis corpus, as well as for words which are otherwise known to be ambiguous.

IV-3

A second concordance was obtained later, from a second corpus(corpus B) of 300 abstracts, in order to verify the rules elaborated
from the first concordance.
The original rules (part A) are first examined, followed by the
results of the confrontation with the concordances of the control corpus(part C) .

A)

The Rules Inspired by Corpus A.
a.

Exclusion List
Initially, an "exclusion list" was prepared, that
is, a list of "words" (= any individual sequence ofletters separated by two blank spaces) which areconsidered as unimportant for the retrieval of
documents.

This exclusion list (i.e. a kind of"antidictionary") consists of about 2500 words ofvarious types:

Grammatical words such as the, a_,only, does etc.; general words like high, little,
interest; and words which are not pertinent in
ophthalmology, like document, information, search etc.
The majority of the ambiguous words of corpus A
were included in the exclusion list.
b.

Dictionary (DIC)
About 2500 word forms and phrases were included in
the dictionary.

This list of words kept for indexationconsists of the following categories:i)-

nonambiguous wordsii)-

phrasesiii)-

ambiguous words appearing unambiguously
in corpus A.

iv)-

ambiguous words appearing with ambiguities
in corpus A.

IV-4

We will examine now the points ii), iii), and iv).
Phrases
A good many phrases were entered in the dictionary (DIC)
as separate entries in order to avoid the trouble ofgenerating a polysemy rule. The generation of phrases isthe best way of avoiding polysemies; whenever possible,
this method was used.

Sometimes, it is not sufficientto generate a phrase; then, one also prepares a polysemyrule.

For instance, acute refers, in corpus A, to theshape of an angle or to the critical state of a disease.
The phrase acute angle was entered in DIC. Many phrases
do not include ambiguous words; these phrases wereentered because they express a unique concept, like
ophthalmic artery; on the other hand, B irradiation,
vitamin B, B-adrenergic, B wave are phrases which arealways entered as separate DIC entries, since it isobvious that jB found alone is ambiguous.
Words Otherwise Ambiguous
A high number tabout 100) of words which are ambiguous
whenfioundin unspecified contexts boil down to unambiguous entries in the ophthalmological abstracts. Fora small number of these words, one can predict that they
will never occur with a different meaning than themeaning they usually have in this small subfield of
medical science; for instance, for the word cataract,
one can predict that it will never be found with themeaning "waterfall" in ophthalmology.

Although thewords of cataract type are not many, one has to admitthat whether they will occur in another sense than thatwhich was selected here is unpredictable.
such words is as follows:

The list ofIV-5

abberation (sight)
absence (not mental)
accommodation (sight)
acute (disease, not angle; acute angle is a phrase)administration (drug)alignment (with visual axis)
capsule (of eye)cataract (not in river)cell (physiological, not photoelectric)channel (lacrymal, not information)
circulation (of blood, not of documents, cars, etc.)
conduction (phyics)
concentration (chemical; high rate of; not camp)-te(NB: in the Medical Subject Headings, theonly meaning selected for concentration
was as a kind of camp)cone (eye cell)
contract (straighten)
convergence (optics)
crater (hole in body),
culture (biological)
decentralization (not economic)
density (energy)
deficit (not in budget)
deposition (chemical, physical)
depression (not mental; low pressure)detachment (retinal, vitreous, of membran, of layer)
diffusion (physical)
digital (.= finger; not computer)
disorder (not riot)
discrimination (visual, not racial)-tedistort (not truth)
diverge (not in opinion)-nt
dominant (hemisphere in brain)equator (lens)erosion (retinal)
exploration (transcranial...)expose (to light)-ure
explosive (onset of disease)field (visual)gas (as a source of disease)
globes (eye)heart (in body)
inclusion (particles in eye)intensity (optics)

invade (microbian)-sion (blood)irradiation (by rays; not physiological)
labyrinth (ear)nuclear (in cell)
operation (surgical)
operative (surgical),orange (color)
orbit (of eye, not of a spatial capsule)orbital (not flight)organic (chemical)
particle (not physical; small object, charcoal)plastic (equipment)pole (not geological nor electrical)
precipitate (physics)-ion
race (not run)radius (of circle; not the bone)reaction (physiological)
reactive (physiological)
receptor (sensorial)recipientred (not communist)
regime (diet)rehabilitation (medical)
resistance (physiological, not electric, nor politic)
response (physiological reaction)
retardation (mental)
retard (mental)
retraction (iris)
retract (iris)right (contrary of left)rod (eye cell),rotate (eye)rotation (eye)separation (of a part of body)
shot (disease)silver (color)sinus (anatomical, not geometrical)
solution (liquid)
star (in eye)stellar (figure on the surface of crystalline lens)
superior (location of muscles)
suspension (in liquid)
sympathetic (not kind)tear (liquid; not verb)
tension (of tissue, not political nor electrical)
tract (optic, in body; not political)

IV-7

trial (clinical, therapeutic)
tributary (vein, not river)
triton (a drug, not the animal)
uniform (measure, not cloth)version (movement)
vessel (in body)vortex (not eddy)
wall (of vessels in body)
weapon (not therapeutic, but real)
wing (of bone, not of bird nor of theater)

Words ambiguous in Corpus A
Rules were made for the following words:
angle
anterior
apparatusarm
attackchambercomplication
conception
contrastcorrection
current

extract
fibre
front
hand
left
link
light
paper
posteriorpotential
soundstressHere are examples of rules using natural language
parameters Cthe abbreviations for the classes aretaken from the Vision Information Center thesaurus):
1.

chamber = P(j)LNPRE, IN VITR(j) + STORED IN + M(j)IST ==0 * CHAMBER, A13
this means that chamber, when preceded by the words
listed after the code P(j)LNPRE, will not be taken
as a descriptor; in any other case, it will be takenwith the meaning of a part of the eye (anterior,posterior,..), class A13.

IV-8

2.

current = P(J)LNF(j)L, METH(J)D + TECHNIQUE =« (J) * CURRENT, H
this means that when followed by method, or technique,the word current will not be taken as a descriptor in
the electrical sense, as in any other case.

3.

correction = P(j)LN, LENS + SPECTACLE == CORRECTION, E04
this means that when words like lens or spectacle, appear
in the same sentence as the ambiguous correction, thenit is assigned a specific meaning in ophthalmology (class
E04 of equipments and therapeutic techniques).

4.

fibreCsI = P(j>L, A08 == FIBRE, A08 * P(j)L, A02 «— FIBRE, A02
when in the environment of fibre Cs) at least one concept(descriptor), appears related to the class of the nervous
system (A08), the meaning is nervous fibre; when thecontext includes one concept related to the class ofmuscles (A02) , then it is a muscular fibre.

5.

extract (ed+ingl = P(j)LIMPRE, D == EXTRACT, D
when immediately preceded by a concept belonging to
the class of chemicals (D\, the words extract, extracted,
extracting have a chemical meaning.

B)

Notes to the Polysemy Rulesa.

The parameters used to resolve the ambiguities are definedamong the English natural words, and among the concepts
symbols.
i)

Natural language (English) parameters:
POLN:
POLNPRE :POLNIMPRE:
POLNFOL:in the sentence
among the words preceding the ambiguous word
among the words preceding immediately the
ambiguous word
among the words following the ambiguous word.

IV-9

ii)

Concept parameters:
POL: in the sentence
POLIMPRE: immediately preceding the ambiguous word.

The symbols are as follows:= introduces a rule== introduces a translation into a concept symbol, introduces a parameter or the class of a concept+ introduces a new parameternew suffix* introduces a new rule(after a translation, * means that inany other case, the translation is soand sol(JL introduces suffixes

The forms of the ambiguous words are not suffixable.
For instance, current is the only ambiguous, form treatedhere Celectrical current, current method).; currents is
not ambiguous here.
In certain ca$e&, some, suffixed forms are ambiguous toethen, the suffixes are introduced between parentheses.
For example:

extract (ed + ing).

The exclusion lists include words used as parameters in
the polysemy rules, for instance:is, was, etc.

the, one, in, method,

It is necessary to eheck these words andtake them out of the exclusion list when the program
processes polysemies.
The dictionary takes priority over the exclusion list:i). Words exist which are put by mistake in the exclusion
list.
ii) Words have been put in the exclusion list which are
members of dictionary phrases. Example: color namingis in the dictionary but name (with its suffixes) isin the exclusion list.

IV-10f.

List of Rules for Corpus A
The class symbols refer to the VIC thesaurus (see annex)(1)

anterior=

P(j)LN, located + segment + part + portion+ surface + limit == ANTERIOR, A

apparatus=

P(j)L, E == APPARATUS, E * P(j)L, A + C + G ==APPARATUS, A

arm (s)=

P(|)L, E == ARM, E

attack (s)=P(j>L, C == ATTACK, C

angle(s)=P(|)L, A13 + C13 —chamber=

P(})LNPRE, in vitro + stored in + moist == 0

conception==pd)LN, foetal + uterus == CONCEPTION, GOl
P(j)L, GOl + A05 + C06 == CONCEPTION, GOl

ANGLE, A

complication =P(j)L, C ~COMPLICATION, C

contrast-

P(|)LNPRE, in * P())LNF(j)L, to == 0 * CONTRAST, X

hand

=

P(|)LNPRE, on other + on one == 0. * HAND, A

left

=

P(j)LNPRE, are, is, was:, were, be, Been, being== Q * LEFT, X

light

=

P(j)LNF(j)L, of experiment, result, technique== 0 * LIGHT, H

link (si=P(j>Lf D = LINK, D

paper (s.)_=P(j)LNPRE, this + that + his —posterior=

P(j)L, A == P(j)STERI(j)R, A

potential=

P(j)L, H + GOl == POTENTIAL, H

sound

=

P(j)LNPRE, LY == 0 * S(})UND, H

stress=

P(j)LNF(J)L, that 4- the importance == 0. *STRESS, F

0i(1)Preceded by an adverb, sound is, generally an adjective, and does notneed to be indexed. Ex: this method is neurologically sound.

IV-11C)

The Control of the Rules by Corpus B

The rules which were controlled are included in part ATa.

Words Otherwise AmbiguousSome of the words of the list for corpus A were foundambiguous: in corpus B:
Precipitation was found in corpus A only in the physicalsense:

fall of particles; in corpus B, it had the sense of"increase in rapidity"; "the common feature was the precipitation of the attacks by light falling on the patient".
In this sentence, there are not less than three ambiguous
words.

The polysemy of attack and light would have beensolved with the available table rules. Precipitation has tobe given a rule with parameters:

If it has in its environ-ment a word belonging to the class of chemicals, then it
should be translated by precipitation, as a chemicophysical process (.class H) . If not, it is to be translated as haste tin the common field, X).
solution always appeared as a liquid in corpus A. In
corpus B, it appeared in such sentences as: "thesolution is formulated to provide; solution of problem".
In this case, it was not indexed.

The suggested rule

is to look for words belonging to the chemical class inthe environment of solution.
silver was considered as a color in corpus A. In corpus
B, it was: found as a metal (."silver impregnation technique"}.

We. found no way of solving this polysemy with

our simple rules.
Except for these three cases, the list of "words otherwise ambiguous"
but supposed to be unambiguous: in ophthalmology was confirmed by an
examination of corpus B.

IV-12

Verification of the Rules with Parameters
Some parameters had to be added to the rules shown in part A.
In corpus B, the rules for contrast, correction, anterior,current and potential proved insufficient.

The following

ameliorations are proposed:c.

contrast:

add by_ as a parameter:
to be indexed.

by contrast is notcorrection:

did not necessarily refer to the spectacles,
but to surgery too ("surgical steps forcorrection of these complications; operation
for correction of dystopia").

current:

add the parameter criteria.

anterior:

add the parameter region, structure.

potential:

the parameters were insufficient. The
following contexts suggest new parameters:"evoked p., photoreceptor p., the p. appears
negative, transcorneal p., absence of anyp., ERG p.". In other contexts, potential
was not to be indexed (meaning "possible");"potential pathogens, the true nature andp. severity of rubella, there is a p.
histidine pump mechanism".

Polysemies intractable by Simple Means
Some abbreviations are ambiguous which seem very
difficult to solve when one cannot generate a phrase.
We have already seen, the example B (part A ) .

We havefound Qi meaning "candle unit" instead of "copper".
Besides silver, here are some polysemies unsolvable by
the means devised in this experiment:net (lattice, pure).

"the net effect of the corticofugal
influence".

rose (noun, verbI:"staining of cornea by rose bengal
dye; iodate levels of the fluids
rose after".

rest (lay, remain\:"the curvature of the cornea on
which it is to rest, an eccentric
rest position; the rest of the periphery".

IV-13

3.

Conclusion
In a first experiment of polysemy, resolutions made in the frameof a research on the SYNTOL Information Retrieval System [1], almost all
the rules were defined with reference to the classifications; in otherwords, these rules referred to the contexts of concepts (i.e. descriptors)
and not to the natural language contexts.

These rules worked well in

about nine cases, out of ten C10Q rules were applied 432 times) . For
improving these results, it was suggested to devise rules using certain
grammatical data 12], or rules using contextual parameters of the natural
language.

This second proposal, obviously simpler, was applied in thepresent experiment, on about 6Q0 abstracts of ophthalmology; the rules
seemed to work reasonably well, at least for this small corpus.
The remaining and essential question is that of the degree ofgenerality of these rules and of their applicability to other corpuses
in ophthalmology or other fields.
a.

Applicability in Ophthalmology
The only answer to this question is empirical; one has
to make further concordances in the same field, for aselection list of ambiguous words, in order to obtaina more reliable assessment of the general validity of
our rules.

b.

Applicability in Other Fields
The answer is very probably no.

One can see that the listof critical words in ophthalmology and in other subfieldsof medical science, (as epilepsyl or in related fields
CLike physiological psychology), are approximately thesame; but the preferred senses in each of these fields,

IV-14and consequently, the polysemy rules are often different.
Consequently, the rules presented here for ophthalmology areprobably not applicable to other medical subfields.

References[1] M. Coyaud and N. Siot-Decauville, LfAnalyse Automatiquedes Documents, Section 5 and Annex, Paris, Mouton, 1967.
[2] A. Borillo and J, Virbel, Etudes sur L'Indexation
Automatique, Centre National de la Recherche Scientifique,Centre dfAnalyse Documentaire, Marseille, France, 1966.

IV-16

Annex I
Outline of the Thesaurus of VIC

A.

Anatomy
A01
2
3
4
5
6
7
8
9
10
11
12
13

B.

Parts of body
Muscle, Skeleton
Digestion
Respiration
Urogenital
Endocrine
Cardiovasc.
NervesSense (except eye)
Tissues, Embryonic
Cells, Cells constitutents
Body Fluids
EyeD.

Drugs, Chemicals

E.

E.01
2
3
4
5
6
7

F.

B.01
2
3
4
5

Invertebrates
Vertebrates
Bacteria
Viruses
Fungi

Diseases
C.01
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

Infectious
Neoplasms, Cysts, Polyps
Muscular, Skeleton
Digestion
Respiration
Urogenital
Endocrine
Cardiovascular
Uremic, Lymphatic
Nervous
Sense (except eye)Intertegumentary
Eye
Injury, Poisoning, Shock
General Disease, Pathology
Nutrition, Metabolism

Psychology
G.01

BiologyH.

Diagnostic Techniques andMeasurement
Therapeutic
Anesthetic
Surgery (eye)Lab Techniques Equipment
Diagnostic Techniques Equipment
General Surgery Techniques
Equipment

Biological Functions

Physics(NB)

We added a class X for thegeneral words.