Pupillometry as a method of measuring workload is described. Pupillometric measures provide an indication of momentary fluctuations in central nervous system excitability that occur as cognitive operations are performed; the magnitude of these changes may serve as a sensitive indicator of the workload imposed by cognitive tasks

Beatty, J.

English

NASA Technical Reports Server

] 82-13666 '
PUPILLOMETRICMEASUREMENTOF OPERATORWORKLOAD
Jackson Beatty
Department of Psychology
and
Brain Research Institute
University of California, Los Angeles
Operator worklo,,Jand its assessment is a major issue in human
performance theory. It is intuitively compelling to _old that some
mental operations demand more of the operator than do others. We speak
easily of attention-demanding tasks and tasks that can be performed "in
one's sleep." Yet we know very little about what these demands might be,
wi_atthe structure or structures are upon which such demands are placed,
and, not surprisingly, we have reached little concensus as to how
processing load might be measured. In this paper, I shall describe one
approach to the workload measurement, pupillometry. Pupillometric
measures p_ovide an indication of momentary fluctuations in central
nervous system excitability that occur as cognitive operations are
performed:, the magnitude of these changes may serve as a sensitive
indicator of the workload imposed by cognitive tasks.
PUPILLOMETRY
Pup111ometry utilizes optical measurement methods to determine th,:
diameter of the pupil of the eye. Photographic measuren}ents were
emplnyed in early pupillometric experiments; now electronic video
pupillometry is commonly used. These instruments, such as the G and I.;
Applied Science Laboratories television pupillometer, process an
infrared video image of the eye to extract either vertical pupillary
diameter or, in some cases, pupil area. This value is recomputed 30
times each second. In simpler systems a headrest is used to stablize
the positien of the pupil in space; more complex systems allow free
head movement and track the position of the eye using a second, larger
video image to control a servomechanism that aims the pupil video
camera. Such _nstruments have been installed recently in aircraft
cockpit simulators.
In our laboratory, the output of the pupillometer is sampled and
storo_d by computer for later analysis. Three standard programs are
_tilized: the first performs an inspection of the data, correcting
trials with minor artifacts and rejecting trials with more seriously
contaminated data; the second program averages trials together, sorting
by stimulus and response codes as appropriate; the third computes the
changes in pupillary diameter over specified segments of the trial from
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the averaged task-evoked oupillary resp)nses. Similar procedures are
employed in other laboratories•
PHYSIOLOGICAL CONTROL OF PUPILLARY MOVEMENTS
' Pupillary diameter is deter_ ned by the relative strengths of
contraction of the two opposing muscle groups of the iris, the dilator
and sphincter pupillae. The dilator muscles are radially oriented band5
of smooth muscle that are innervated by the sympathetic branch of the
autonomic nervous system. Contraction of th_se muscles dilates the
pupil• Conversely, the sphincter pupillae are innervated by the
parasympathetic system and act to close the pupil when activated•
Pupillary dilation, therefore, can result from either sympathetic
excitation or parasympathetic inhibition• Both these pathwayc are
affected by activation of nuclei comprising the reticular activating
system of the brainstem; thus, pupillary movements are used in
classical neurophysiology to measure the activation of these reticular
nuclei (_1oruzzi,1972).
Although the details of the interaction of the reticular core with
higher brain regions are only poorly understood at present (Hobson and
Brazier, 1980), t_ere is little doubt that cortico-reticular and
reticulo-cortical _nteractions play a major role in determining the
dynamics of higher brain functions. Most commonly the reticular system
is viewed as serving an energizing function for the cortex (l.uria,
1973), but it is perhaps wise to regard this viewpoint as a metaphor.
Nonetheless, the ideas that pupillary movements reflect reticular
activation and that retlcular activation controls the dynamics of
cognitive processing provide a theoretical basis for the use of
pupillometry in the study of mental workload.
WITHIN-TASK VARIATIONS IN PROCESSING LOAD
The amplitude of the task-evoked pupillary response has been shown
to reflect variations of processing load within a wide variety of
cognitive tasks. Some examples are the following.
Memory. One of the First clear demonstrations that pupillary
diameter varys with processing load was found in the study of short-term
memory (Ka_neman and Beatty, 1966; Kahneman, Beatty and Pollack, 1967).
In simple short-term recall of digit strings, as when being told a
telephone number and then dialing it, a characteristic pattern of
pupillary movements are observed. Pupil diameter increases
systematically as each digit is heard, reaching a maximum dilation in
the interval before report (subjects in these experiments repeated the
digit strings at the rate of 1/sec rather than dialing a telephone).
Furthermore, the size of the peak dilation between listening and report
is a monotonic function of the number of items heard (3 to 7 digits).
During report, the pupil constricts w_th each digit spoken, reaching
baseline diameter at the completion of the task. Such effects are
highly reliable; they occur in every subject and are remarkably
1982005792-016
• consistent in magnitude. The dilation for 7 digits is approximately 0.5
nlm,
The slope of these task-evoked pupillary responses is determined by
the difficulty of the to-be-remembered information. For example, the
memory span for unrelated nouns is shorter than that for digits; _t is
said, therefore, that the nouns place larger demands on the processing
• system than do digits. Thus, it is not surprising thct the magnitude of
pupillary dilation for individual items is greater for nouns than digits
(Kahneman and Beatty, 1966).
A third important feature of the task-evoked pupillary response in
the short-term memory task is that it increases with items presented
only within the range of possible performance; when further demands are
placed upon the processing system no additional dilation is observed•
This important finding was first reported by Pearler (1974), who
measured the task-evoked pupi!lary response for string lengths greater
than the memory span. For n_st individuals, by the way, the span for
digits is _bout 7 items. When longer strings are presented, errors
begin to occur (Miller, 1956). Peavler found that when superspan
strings were presented, the pupil dilated with each item unLil the
seventh; subsequent items elicited no further dilation. This
saturation of the pupillary response at the limi*_ of capacity has been
verified subsequently by other workers.
Tne short-term memory task provides a particularly clear
demonstration of the properties of the pupillary response as a measure
of mental workload. Manipulations that should increase load, here item
type and number of items, increase the amplitude of the re;ponse in an
orderly manner. Increases in processing demands beyond the capacity of
the system to respond are not reflected in pupillary movements. The
task-evoked pupillary response indexes the processing demands being met
by the system, not the demands placed upon the system by the task.
Thus, it may serve as a measure of mental work executed, at least within
the confines of a particular task.
Language. A number of levels of language processing have been
studied pupillometrically. Beatty and Wagoner (1978) showed that the
degree of processing required in a simple letter matching task was
faithfully reflected in the magnitude of the task-evoked pupillary
response. The smallest dilations occured when only matching of physical
details was require to reach a judgement of "same", as fo_"the letter
pair "AA". When name code extraction was required (as for "Aa") the
pupillary response was significantly enhanced. But in both cases the
response was relatively small, on the order of 0.1 mm.
Larger responses wc_e observed by Ahern and Beatty (1981) in a word
matching task, in which individuals heard two words and were required to
determC,_eif they shared the same meaning. The word pairs, which were
orawn from psychometric tests of intelligence, v_ried in difficulty.
Difficulty level was reflected in the amplitude o+ the task-evoked
pupillary responses.
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The largest responses for language processing were obtained with a
grammatical reasoning task (Baddeley, 1968). Subjects l_stened to i
sentences of the form "A follows (precedes) B" and then heard an i
exemplar letter pair ("BA"); the task was to determine if the _entence
correctly describes the pair. The sentences varied in grammatical form,
beir active or passive and positive or negated. A significant effect
of ammatical complexity was observed. Response were approximately 0.5
. mm._ in amplitude (Ahern and Beatty, 1981).
Other tasks. Pupillometric investigations have also been reported
for a variety of other types of tasks, including simple sensory and I
motor tasks. Although space does not permit a summary of these results, !
they _upport the cenclusion that the task-evoked pupillary response is a
sensitive and accurate physiological measure of within task variations i
in processing load. _ _
BETWEEN-TASK VARIATIONS IN PROCESSING LOAD
But any useful measure of operator workload must do more than
reflect within-task variations in processing load; it must, in
addition, provide a measure of workload imposed by qualitatively
different mental operations. Only in this way can operator workload be
assessed in complex man/machine systems.
Evidence that the task-evoked pupillary response is sensitive to
variations in processing load imposed by qualitatively different mental
operations has been provided in a detailed review by Beatty (in press).
From the published literature, all data were employed that met two
criteria; there were no motor responses occuring during measurement and
the published figures permitted estimation of the maximum value of the
pupillary response. The resulting data were remarkably consistent,
leading to the conclusion that the task-evoked pupillary response
provides a reliable and reasonable indication of the processing load
imposed by cognitive tasks that differ markedly in their internal
structures.
BETWEE_J-INDIVIDUALVARIATIO_ISIN PROCESSING LOAD "
Finally, there is evidence that the amplitude of the task-evoked
pupillary response provides an indication of differences in processing
load imposed by the same task on individuals who differ in cognitive
abilities. Ahern and Beatty (1979, 1981), for example, measured
pupillary responses while solving multiplication problems in university
student with high and low Scholastic Aptitude Test scores. The
high-scoring students showed s,gnificantly smaller pupillary responses
at all difficulty levels. Further, the amplitude of these responses
correlated only with ability measures and not with personality variables
_ or psychometric measures of state or trait anxiety. Finally, there was
_, no difference between groups in the amplitude of the light end dark
pupillary reflexes. These data support the view that solving the same
_j objective multiplication problem (e.g., 6 time 8) is a _re demanding
!.
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task for the less able students.
SUMMARY
All available evidenc_ supports the view that the amplitude of the
task-evoked pupillary response provides a sensitive indicator of the
workload imposed by mental operations. However, much remains to be
learned. _Je do not understand with any certainty the physiological
mechanisms linking the autonomic periphery with the highest levels of
central nervous system function. We know little about the ways in which
these responses, or other similar physiological measures, may be applied
to the solution of practical design problems facing human factors
engineering. But the future appears promising; pupillary movements may
provide one key for providing a solid empirical basis for the problem of
operator workload assessment.
REFERENCES
Ahern, S.K., and Beatty, J. Physiological signs of information
processing vary with intelligence. Science, 1979, 205, 1289-1292.
Ahern, S.K., and Beatty, J. Physiological evidence that demand for
processing capacity varies with intelligence. In M. Friedman, j.P.
Dos and N. O'Connor (Eds.), Intelligence and learning. New York:
Plenum, 1981.
Baddeley, A.D. A three-minute reasoning test based on grammatical
transformation. Psychonomic Science, 1968, 10, 341-342.
Beatty, J. Task-evoked pupillary responses, processing load, and
the structure of processing resources. Psychological Bulleti__n_n,i
press.
Beatty, J., and Wagoner, B.L. Pupillometric signs of brain
activatior,vary with level of cognitive processing. Science, 1978, 199,
1216-1218.
Hobson, J.A., and Brazier, M.A.B. (Eds.), The reticular formation
revisited: Specifyin9 function for a nonspecific s_,ystem.New York:
Raven, 1980.
Kahneman, D., and Beatty, J. Pupil diameter and load on memory.
Science, 1966, 154, 1583-1585.
Kahneman, D., Beatty, J., and Pollack, I. Perceptual deficit
during a mental task. Science, 1967, ]57, 218-219.
Luria, A.R. The workin9 brain. New York: Basic Books, 1973.
_liller,G.A. The magical numt_erseven, plus or minus two: Some
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limits of our capacity for processing information. PsYchological
Review, 1956, 63, 81-97.
\
Noruzzi, G. The sleep-waking cycle. Reviews of Physiology:
Biochemistry and Experimental Pharmacology, New York: Springer-Verlaq,
1972.
Peavler, W.S. Individual differences in pupil size and
performance. In M. Janisse (Ed.), Pupillary dynamics and behavior.
New York: Plenum, 1974.
NOTE
This research was soonsored by the Environmental Physiology Program
of the Office of Nav_] ResEarch under Contract N00014-76-C-0616.
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Pupillometric measurement of operator workload

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