Perception & Psychophysics1975, Vol. 17 (6),578-586

The span of the effective stimulusduring a fixation in reading

GEORGE W. McCONKIECornell University, Ithaca, New York 14850

and

KEITH RAYNERUniversity ofRochester, Rochester. New York 14627

A computer-based eye-movement controlled display system was developed for the study of perceptualprocesses in reading. A study was conducted to identify the region from which skilled readers pick upvarious types of visual information during a fixation while reading. This study involved making displaychanges, based on eye position, in the text pattern as the subject was in the act of reading from it, andthen examining the effects these changes produced on eye behavior. The results indicated that thesubjects acquired word-length pattern information at least 12 to 15 character positions to the right of thefixation point. and that this information primarily influenced saccade lengths. Specific letter- andword-shape information were acquired no further than 10 character positions to the right of the fixationpoint.

Psychologists have long been interested in thequestion posed by Woodworth (1938) in his review ofresearch on reading: "How much can be read in asingle fixation?" The investigation of this questionhas a long history, dating back to some of the earliestresearch on reading. Cattell, in 1885 (summarized byHuey, 1908), provided data on the number of lettersthat could be reported following a tachistoscopicpresentation of letter strings of different types. Sincethen, a number of studies have been conducted toinvestigate this question. If a person fixates a letter ona page of text, he often finds that he can recognizewords two or even three lines above and below the onebeing fixated, as well as words some distance to theleft and right. Beyond that, punctuation marks,capital letters, and the beginnings and ends ofparagraphs are visible. On the other hand, if a stringof random letters is presented visually by means of atachistoscope, a person is likely to be able to identify

The research described in this report was carried out at theArtificial Intelligence Laboratory at the Massachusetts Instituteof Technology. The authors wish to express their gratitude toDr. Marvin Minsky and Mr. Russell Noftsker for the use of theextensive computer facilities at the AI Lab. David Silver. a staffprogrammer there, did much ofthe programming involved. and hisassistance is greatly appreciated. Gary Wolverton, a graduatestudent at Cornell University, provided assistance with much of thedata analysis. The research was made possible by a SpecialFellowship from the National Institute of Mental Health to thesenior author and by Grant OEG-2-71-0S31 from the Office ofEducation. Portions of these data were reported at the 1973meetings of the Eastern Psychological Association and theAmerican Educational Research Association. Reprints may beobtained by writing to George W. McConkie, 214 Stone Hall,Cornell University. Ithaca. New York 14850.

578

only four to six letters. The answer to the question,then depends on the type of materials used and thepresentation and task conditions.

While the studies that have been conducted on thisquestion have yielded information about how much itis possible to see and to report from a single fixation.and how far into the periphery of the retina visualinformation can be identified, they fail to resolve theoriginal concern about reading. A better statement ofthe original question, as it relates to reading, wouldprobably be, "How far into the periphery are specificaspects of the visual stimulus typically acquired andused during fixations in reading?" This restatementdiffers from the original question in several respects.First. it recognizes that different types of visualinformation might be available and used by the readerin different retinal areas. Thus, there probably is nota single perceptual span. but a family of spansdepending on the aspects of the visual stimulus thatare being studied. Second, it notes that there may be adifference between what information is available tothe subject during a particular fixation and what heactually acquires and uses for a particular task. Thus.the perceptual span estimates that one obtains fromsubjects involved in different tasks are likely to differ.This leads to the third point. that in order to answerthe perceptual span question about reading. it isnecessary to study subjects who are engaged in the actofreading. The major concern for a theory of readingis not what people are capable of seeing during afixation. but what information they typically acquireand use as they read.

Previous studies of the perceptual span cannot be

SPAN OF EFFECTIVE STIMULUS IN READING 579

taken a, providing a definite answer to our restatedquestion for either of two reasons. Most of thesestud ies have involved subject'> in a task quite differentfrom normal reading. typically using singletuchistoscupic presentation'> from which the subject isasked to identify the letters. numbers. or wordspresented (Houma. 1473; Mackworth , 1965). Thosestudies that have involved the reading of passages. onthe other hand. have still used tasks which interferegreatly with normal reading behavior (Bouma &de Voogd . 1474; Newman. 1966; Poulton. 1962).

The research to be reported here was an attempt todevelop a method for obtaining information about thesize of the region from which specitic types of visualinformation are obtained during tixations while thesubjects are involved in reading a passage, with as fewconstraints on their reading behavior as possible. Aneye-movement controlled display system wasdeveloped which permitted a computer to frequentlysample the reader's eye position as he read from acom putcr-generated text display on a cathode-raytube (CRT). With this system. it was possible tomodify the text display on the basis of the reader's eyeposition. The research involved displaying anunreadable pattern on the CRT, with every letter ofthe original text replaced by another letter. Then.when the subject tixated the first line of text, thedisplay was moditied by replacing letters within acertain region to the left and right of the fixation pointon that line with the corresponding letters from theoriginal text. This created a "window" of normal textfor the reader on that fixation. When the reader madea saccade, the text in the window returned to itsmoditied form. and a window of normal text wascreated at the next fixation location. Thus, whereverthe reader looked. there was normal text for him tosee, allowing him to read quite normally. However,the experimenter could determine the size of theregion of normal text. as well as the nature of thepattern beyond this window region.

. The experiment involved having subjects read withdifferent-size windows and different peripheral textpatterns. The text patterns appearing in peripheralvision (outside the window) maintained or destroyedcertain visual characteristics of the original text. Itwas assumed that if the window were large enough forthe reader to obtain all useful visual informationwithin that region, the presence or absence of certainvisual characteristics in the region beyond the windowwould make no difference; that is, data on eyemovements and test performance would not beaffected. However. if the window were made smaller.a point would be reached where the deletion of thosevisual characteristics outside the window wouldinterfere with reading; that is, visual informationnormally obtained and used in reading had beendestroyed. Thus. the research strategy was tomaintain or destroy certain visual characteristics of

the original text in the peripheral text pattern. andthen to determine how small the window must bemade before a difference in reading was produced bythe nature of the peripheral text pattern used. Thiswould indicate how far into the periphery that visualaspect of the text was obtained and used in reading.

METHODSubjects

Six high school students participated as subjects in the study. Attwere seniors or juniors and were identified as being among the bestreaders in their school. They were paid for participation. receiving abase rate plus a bonus based on their performance on testquestions. Att had participated in an eartier study and were familiarwith the equipment and procedures used.

MaterialsSixteen 5(J()·word passages were selected from a high school

psychology text. None of the subjects had taken a course inpsychology. Each passage was divided into six approximatelyequal-size pages. which were displayed one at a time.double-spaced. during the experiment.

Six algorithms were used to substitute letters for characters in theoriginal text to produce modified versions of the text. cattedperipheral text patterns. Six peripheral text patterns were preparedfor each passage. conforming to a 2 by 3 design: XS. XF. CS. CF.NCS. and NCF. In the X versions. each letter in the original textwas replaced by an X. In the C versions. each letter was replaced bya letter visually similar (confusable) with it. The substitutions weretaken from confusability matrices developed by Bouma (1971) andHodge (1902). In the NC versions. each letter was replaced by aletter visually different (noncontusable) from it. with ascendingletters replaced by letters that did not extend above or below the lineor by descenders. and with descenders replaced by ascenders orletters that did not extend beyond the line of print. In addition. theS (spaces) form of each of these maintained spaces andpunctuation. whereas in the F (filled) form each space andpunctuation mark was replaced by an appropriate letter. an x in theXF version and other letters in the CF and NCF versions. Inaddition. in the XF version all replacement was done using capitalXs. thus eliminating capitalization characteristics. Figure 1 shows aline of normal text and the corresponding line after having letterssubstituted to produce each of the peripheral text versions.

Two multiple-choice test questions were prepared for each pageof text, thus yielding 12 questions for each passage. These questionstested retention of information clearly stated in individual sentencesin the passage. Each question had four alternatives from which thesubject was to choose.

DesignForty-eight display conditions were used in this study. produced

by factorially combining eight window sizes with six peripheral textpatterns. The six peripheral text patterns have already beendescribed: window sill'S used were: 13. 17.21. 25. 31. 37.45. and100 character positions. Thus. for the smaller windows. eachsucccssivclv larger window size extended the window by twocharacter positions at each end. As can be seen in Figure I. with awindow sill' of 17. the character being fixated and eight characterpositions to the left and right of the fixated character comprise thewindow.

Each subject read all It> passages (90 pages). with two of thepa"agl's being read under each of the 48 presentation conditions.All subjects read the passages in the same order. but the conditionorder was unique for each subject. An attempt was made to balanceprcscntution conditions over passage and page sequence order asfar as possible. Each presentation condition occurred once in thelirst eight passages. and again in the last eight. for each subject.

580 McCONKIE AND RAYNER

Graphology means personality diagno~is from hand writing. This is a

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FIgure 1. An example of a line of text andthe varjous peripheral text patterns derivedfrom it.

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Note.-On each line a window of size 17 is shown, assuming the readeris fixating the letter ~ in diagnosis.

ProcedureWhen a subject arrived for the experiment. a bite bar was

prepared tor him which served to reduce head movement during theexperiment. Then the eye-tracking sensors. mounted on glassesframes and held more securely with a headband. were placed onhim and adjusted. He then had the opportunity to warm up byreading two or three passages under conditions of various windowsizes and peripheral text patterns.

Prior to reading each passage. the subject was engaged in' acalibration task. A target spot appeared on the display. and thesubject was instructed to look d irectlv at it and press a lever locatednext tC1 his right hand. When the lever was pressed. the computerstored the signals being received from the eye-tracking equipment.and moved the target to a new location. This sequence occurred 15times. giving the computer the values needed for identifying fromthe eye-position signal where the subject's gaze was directed. Anadditional push ot the lever brought the first page of text onto theCRT. and later pages of a passage were also called by depressingthe lever. As the subject read each page. the computer kept acomplete record otthe position and duration ofeach fixation and ofthe time required tor each saccade. All times were recorded in oOthsof a second. and later converted to milliseconds. Alter reading allsix pages ofa passage. the subject came offthe equipment and tooka test tor that passage. Prior to reading the next passage. the testwas scored and he was informed of his score. In order to encouragesubjects to put their emphasis on understanding and rememberingthe content of the passages. they were given I cent tor each correctanswer on the test.

Each subject participated in the study tor two 2-h sessions.reading the tirst eight passages during the first session and the lasteight during the second session.

ApparatusThe equipment used tor this research has been described in detail

elsewhere (~cConkie&: Rayner. Note I). and is similar to a systemdescribed by Reder (19-31. It consisted of a Biometrics Model SGeye-movement monitor interfaced with a Digital EquipmentCorporation (DEC) PDP-o computer. permitting on-line recordingof eye movements, The computer also controlled a DEC Model 340display. which had a character generator for upper- and lowercaseletters. The PDP-o was also interfaced to a PDP-IO time-sharingsvstem. The experiment was conducted using the PDP-o. while thePDP-IO did all tile handling.

The CRT had a displayable area of 20.96 x 18.42 ern. capable ofdisplaying 40 lines of 80 characters each. Only 8 to 10 lines of textwere displayed at once. double spaced. The subject was 53.34 ernfrom the CRT. so a tulliine of text t72 characters) occupied about18 deg of visual angle. or about tour letters per degree. The CRThad a P-- phosphor. which is actuallv composed of two phosphors.one blue-white and ofshort persistence. the other yellow and of longpersistence. A dark-blue theater gel was used to filter out the yellowimage. leaving a rapid-decaying image having characteristicssimilar to those of a P-II phosphor. There was no noticeabletlu kvr.

The Biometrics eve-movement monitor uses a corneal reflectionmethod of monitoring eye position. There were two problems withih me, First. although it is relatively accurate in identifying eyeposition on the horizontal dimension. it is quite inaccurate on thevertical dimension. where it monitors position of the eyelid ratherthan of the eve itself. When a subject is asked to fixate a target. thecomputer indicates the eye as being directed to a letter no morethan three characters from the target. and almost always within twocharacter positions (', deg of visual angle). on the horizontaldimension. The vertical signal was so inaccurate that it was decidedtll ignore it and to use a heuristic to identify the line being read. It\\as avsu med that the reader began on the top line. and that a longvertical movement. or series of movements. totaling at least 45character positions. indicated that he was progressing to the nextline. The subjects were instructed to read line by line and not to tryto return to an earlier line. They were given practice with theequipment and encouraged to try violating these requests. toobserve the conditions under which the window would not followtheir eve positions. They experienced no difficulty on confirming tothese requests. though the number of regressions they typicallymade was likely reduced.

The second problem with the eye-movement monitoringequipment concerned the great amount of noise in the signal. Toreduce this. a filter built into the equipment was left engaged andanother added. Following the research, it was learned that the firstfilter de laved the signal substantially. Presenting one of thephotod iodcs in the sensors with a fast-rising infrared signal causedthe analogue output from the equipment to begin rising almostirnrnediatelv. reaching a maximum level in 4 to S msec. With thefilter engaged. however. although the output signal began risingalmost imrnediatelv. it reached maximum level only after 2S rnsecor more. The second filter delayed the signal less than '/, rnsec.

The computer was programmed to sample the signal from, theeye-movement monitor 60 times per second. On each sample. it wasdetermined whether the eye was moving or still. by comparing thechange in the signal since the last sample with a threshold value. Ifthe change was greater than the threshold value. the eye wasdeclared to be in a saccade: if less. to be fixated. When the e\"e wasfirst found to be fixated after a saccade. the CRT was turned offand the display instructions were changed to create a window at theappropriate location in the text. The CRT was then turned back on.The time the display was oil. with a window size of 17 characters.was 0 rnsec. short enough for no blink to be detected. For each20-character increase in the window size. the CRT off time wasincreased by 5 msec. The largest window size. 100 characterpositions. required the CRT to be off for 30 msec and producedvery nonceab lc-blinking of the display.

Since the display was changed each time a fixation was made. thequestion ot lag between the time the eye stopped and the detectionof the eve being stopped is particularly important. Two features ofthe system used made this lag substantially greater than wasrealized at the time the study was conducted. First. the tilter in theeve-movement equipment already mentioned increased the lagvubvtanriullv . Second. a relatively slow sampling rate (00 sec. or

SPAN OF EFFECTIVE STIMULUS IN READING 581

once every Ib msec) further increased the lag. With this samplingrate. it would be quite possible for the eye to be stopped for 18 msecprior to detecting the fixation. even with a fast-respondingeye-movement signal. If the fixation occurred at just the right time.it might be detected after only a few milliseconds. These twofactors. the tilter and sampling rate, resulted in a variable lag whichis difficult to assess exactly, but which could have been as much as40 msec at times. Identitication of the onset of a saccade wasprobably faster. Subjects did not have an experience of seeing onepattern change to another during a fixation. however. probably dueto a combined effect of visual suppression during and after saccadesand of visual mask ing (Haber & Hershenson, 1973).

RESULTS AND DISCUSSION

The DataFor each page of text read by each subject,

summary statistics were obtained for a number ofaspects of eye-movement behavior, in addition toscores on the retention test. Each fixation and saccadein the eye-behavior data was categorized as one offourtypes: a forward movement and fixation, a regressivemovement and fixation, a forward movement andfixation in a regression (if a regression had previouslybeen made and this forward movement failed to bringthe eye back to the point from which the regressionwas originally made>. or as a return sweep. On themovement or series of movements which advanced theeye to the next line, all movements were categorized asreturn sweep movements, and fixations bounded onboth sides by return sweep movements were identifiedas return sweep fixations. The fixation occurring atthe end of a return sweep, which was then followed bya forward movement, was identified as a forwardfixation.

Three types of eye-movement data were analyzed.The first was time data. The reading time per 100characters was computed for the page, and then thiswas broken down in two ways. First, it was brokeninto time spent in movement (movement time) vs.time spent in fixations (fixation time). Time spent inforward movements and in forward fixations was alsoobtained. Second, time was broken down by thecategories of movements and fixations. yielding totaltime per 100 characters for forward movements andfixations, regressions, forward movements andfixations in regressions, and return sweeps.

The second type of data consisted of simple countsof the number of fixations and movements per 100characters, both total for the entire page and brokendown into forward, regression, forward in regression,and return sweep, in order to have the number of eachof these types of movements and saccades thatoccurred.

The third type of data was measures of saccadelengths, saccade durations, and fixation durations.Saccade lengths were measured in number ofcharacter positions. First, second, and third quartileswere obtained from the distributions of all fixationdurations and forward-fixation durations for each

page. Second quartiles (medians) were obtained forthe distributions of fixation durations for regressions,forward movements in regressions, and return sweeps.The same statistics were obtained for the distributionsof saccade lengths and saccade durations, except thatno statistics were included for distributions of totalsaccade lengths and durations. Thus, there were 34dependent variables for which one score was obtainedper page for each subject.

Effects of peripheral text patterns. For each ofthese variables, eight three-way analyses of variancewere carried out, one for each window size. Thepurpose of each analysis was to determine whether atthat window size the peripheral text pattern had anyeffect on the dependent variable. The three factors ineach analysis of variance were subjects (six),letter-replacement algorithm used in producing theperipheral text pattern (called letter replacement,with three levels for C, NC, and X versions), andwhether the peripheral text pattern had spaces andpunctuation remaining or removed (called space vs.filled, with two levels for Sand F versions). This led toa large number of analyses, a total of 272, beingcarried out. Since this was bound to lead to a numberof significant effects on the basis of chance alone, thefollowing strategy was used to identify differences thatwere likely to be reliable. A difference was consideredto be reliable if it occurred at two successive windowsizes, with a significance level less than .10 at eachwindow size, and with the data pattern at bothwindow sizes being similar.

For most dependent variables, there was asignificant main effect for subjects, and for some,subjects interacted with other variables. These effectswill not be explored here, but attention will be given tomain effects for letter replacement and for space vs.filled. and interactions between these two variables.

For window sizes of 31 and greater, there were noreliable effects using the above definition ofreliability. Where significant effects were found at onewindow size, the data pattern observed at adjacentwindow sizes was not the same. Thus these effectswere assumed to be due to chance factors. It isconcluded that with a window size of 31, there is noevidence that the readers were acquiring eitherword-shape or word-length pattern information fromthe region beyond the window. Thus. there is noevidence that this information was acquired morethan 15 character positions from the point of centralvision by the subjects of this experiment. One possibleexception to this conclusion will be noted later.

A number of reliable main effects and interactionswere found at the smaller window sizes. These arelisted in Table 1. These could be broken down intofour categories: variables that influenced saccadelengths, those that influenced the duration offixations, those that influenced the number of"regressions. and those that were more gross measures

582 McCONKIE AND RAYNER

Table IA List of Reliable Effects Not Involving the Subject Factor

Window F SignificanceVariable Size Effect* Value df Level

Forward fixation time per 100 characters 17 S 31.60 1,5 .00321 S 4.14 1,5 .10

Forward movement and forward fixation time per 100 characters 17 S 30.33 1,5 .00421 S 4.47 1,5 .09

Number of fixations per 100 characters 13 S 5.91 1,5 .0617 S 11.49 1,5 .0213 SL 3.29 2,10 .0817 SL 3.41 2,10 .07

Number of forward fixations per 100 characters 13 S 19.06 1,5 .00817 S 41.53 1,5 .00221 S 4.60 1,5 .08

Number of regressive fixations per 100 characters 13 L 5.67 2,10 .0217 L 3.87 2,10 .0625 L 5.60 2,10 .02

Saccade length-second quartile 13 S 3.94 1,5 .1017 S 13.36 1,5 .02

Saccade length-third quartile 13 S 21.87 1,5 .00617 S 24.79 1,5 .00521 S 18.65 1,5 .00825 S 6.27 1,5 .06

Saccade length of regressions-second quartile 13 SL 8.48 2,10 .00717 SL 8.01 2,10 .009

Duration of forward movements-second quartile 13 S 5.91 1,5 .0617 S 9.35 1,5 .03

Duration of forward fixations-first quartile 21 SL 7.81 2,10 .00925 SL 8.00 2,10 .009

Duration of forward fixations-second quartile 17 L 3.51 1,5 .0721 L 4.47 1,5 .04

Duration of regressive fixations-second quartile 21 L 5.58 1,5 .0225 L 4.20 1,5 .05

*8 indicates significant effect for spaces vs. filled, L indicates significant effect for letter replacement, SL indicates a signif-icant interaction.

Figure 2. The length of forward saccades as a function ofwindow size and of the presence or absence of word·lengthinformation in the peripheral text pattern; first, second, andthird quartlles.

smallest window sizes. With word-length patterneliminated from the peripheral text pattern, saccadeswere of shorter duration. which of course simplyreflects the fact that saccades were of shorter distanceunder these conditions, as already noted.

The average length of regressive saccades also

Window Size (No. Character Positions)

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of eye behavior and hence were affected by the threecategories already mentioned.

Saccade lengths were affected only by the presenceor absence of spaces in the peripheral text pattern.Figure 2 presents the average first, second, and thirdquartiles for the distributions of saccade lengths forthe Sand F peripheral text patterns, together with anindication of the significant effects. As can be seen,when word-length patterns were eliminated from theperipheral text pattern by filling the spaces, thesaccades tended to be shorter. This effect is mostnoticeable at the third quartile. indicating that ittended primarily to reduce the number of longsaccades, thus constricting the distribution of saccadelengths at the high end, rather than simply shiftingthe entire distribution down. The difference betweenSand F versions is present at least up to a window sizeof 25. and may be present even further, though thedifference there is not significant. It appears, then,that word-length pattern information is acquired atleast as far as 12 character positions (3 deg) from thecenter of vision. and perhaps even farther, and may beused in guiding the eye during reading.

A significant main effect for spaces vs. filled wasalso found for saccade duration data at the three

SPANOF EFFECTWE STIMULUSIN READING 583

tended to be less when word-length information waseliminated from the peripheral text pattern, thoughthe difference was significant only at window size 17.The Spaces vs. Filled by Letter Replacementinteraction was significant at the .01 level forregression saccade length data at the two smallestwindow sizes. but the data pattern changed greatlybetween the two window sizes, so these interactionswill not be considered reliable or explored further.There was a tendency for the XS condition to produceparticularly short regressive saccades at the 17, 21,and 2S window sizes, with saccade lengths for thatcondition being more like those of the filled conditionsthan the other spaces conditions.

While lengths of saccades were intluenced primarilyby the presence or absence of word-length pattern inthe peripheral visual areas, the duration of fixationswas intluenced largely by letter replacement.Significant main effects for letter replacement werefound for the second and third quartiles of thedistributions offorward-fixation durations and for thethird quartile for total fixation durations at windowsizes 17 and 21. The forward fixation-duration dataare presented in Figure 3. Total fixation-durationdata are almost identical. From this figure, it can beseen that there tends to be a small difference in theduration of fixations between the C and NCletter-replacement conditions at the two smallestwindow sizes, but that this difference disappears atwindow size 21. At the smaller sizes, having improperword-shape patterns in the peripheral text patternintlates the fixation durations slightly. With windowsize 21, however, it seems to make no differencewhether the peripheral text pattern presents accurateor inaccurate word-shape information, suggesting

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Flgure 3. The duration of forward fixations as a function ofwindow size and of the presence or absence of word-shapeinfonnatlon in the peripheral text pattern: fIrst, second, andthird quartlles.

that general word-shape information is not acquiredby these readers as much as 10 character positionsfrom the point of central vision.

At the three smallest window sizes, the Xletter-replacement condition leads to the lowestfixation durations. The X condition has thecharacteristic that the boundaries of the windows arewell marked; the contrast between the normal text inthe window region and the homogeneous x pattern inthe peripheral area is very noticeable. Thus, it wouldseldom happen that a reader would make the error ofattempting to integrate letters from outside thewindow into the text pattern within the window itself.On the other hand, with the C and NCletter-replacement conditions, the boundaries of thewindow are not at all obvious, and the readerundoubtedly quite frequently picks up letters fromoutside the window area and attempts to integratethese with the normal text in the window, thusproducing some disruption. This disruption likelyleads to the longer durations for the C and NCconditions as compared to the X conditions. If this isso. then the fact that there is a difference in fixationdurations between the X and the C and NC conditionsat window size 21 suggests that the reader is pickingup specific letter information under the C and NCconditions as much as 10 character positions from hispoint of central vision. These differences disappear atwindow size 25, suggesting that this type of visualinformation is not being acquired by the readers at 12character positions from the point of central vision.

The first quartile forward fixation duration datashowed a Spaces vs. Filled by Letter Replacementinteraction at window sizes 21 and 25. However. thedata pattern was not consistent from one window sizeto the next, so these interactions will not beconsidered reliable.

This consideration of the fixation duration dataleads to two conclusions: first, the subjects acquiredspecitic letter information no more than 10 or 11character positions (2 1/ 2 deg) from the point of centralvision, and second, general word-shape informationwas not acquired any further into the periphery thanthis either. Thus. word-shape patterns. other thanword-length characteristics, appear to be acquired nofurther into the periphery than is specific visualinformation needed to identify letters.

The number of regressions made by subjects per100 characters was reliably affected by theletter-replacement pattern. Significant main effectsare shown in Figure 4. Here it is seen that the Ccondition produced the most regressions, whereas theX condition produced the least. The reason for thisdifference is not known, though it may havesomething to do with the naturalness of theappearance of the peripheral text pattern. The leastnatural-appearing pattern was the one which led tothe fewest regressions.

The differences in saccade length, fixation

584 McCONKIE AND RAYNER

13 17 21 25 31 37 45

Window Size (No. of Character Positions)

Window Size( Character Positions)

Figure S. Effect of window size on time to read 100 charactersof text.

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from a region extending more than about 10 or 11character positions from the point of central vision,and that they were not acquiring word-length patterninformation more than about 15 character positionsinto the periphery. If this is so, it is difficult toimagine what other visual characteristics of the textmight be acquired further into the periphery whichwere not present in the peripheral text patterns.Therefore, it will be assumed that the continued dropin the reading time and other curves with increasedwindow size has some other basis.

Among the many possible reasons why reducing thewindow size might produce the effects noted, twoseem particularly likely. One possibility is thatsubjects have the ability to control the size andlocation of the general area from which they acquirevisual information on each fixation during reading.Thus, as the window became smaller, they may havetended to constrict their field of visual attention to anarrower region. This may also have caused them to

duration, and number of regressions produced by thevariables resulted in differences in other more grossmeasures of eye behavior in reading. A significantmain effect for spaces vs. fil1ed was found at windowsizes 13 and 17 for total reading time per 100characters, total time spent in forward movementsand fixations per 100 characters, total number offixations per 100 characters, total number of forwardfixations per 100 characters, total time spent infixations per 100 characters, and total time spent inforward fixations per 100 characters. The lattervariable also showed a significant effect at windowsize 21. These data will not be presented in detail,since they simply resulted from the previously notedeffects of the variables on specific eye-behaviorrneasures and b.1 themselves adr nothing to theunderstanding of the reading proci''iS('S invo'ved.

General effects of window size, l-igure 5 shows theef k l.' l of window size on the time required to read thetext. The dependent variable is time req [fired to read100 characters. Reducing the window si/.l' caused thistime to rise from 4.33 to 6.92 sec, a 60% increase.This increase resulted primarily from an increase inthe amount of time the eye was fixated; total time inmovement rose 10%, while total time spent infixations rose 76%. As seen in earlier figures, thisincrease was a result of the subjects' making bothmore fixations (a 33% increase from 12.82 fixationsper 100 characters to 16.98) and fixations of longerdurations (a 31% increase from 245 msec medianforward fixation duration to 320 msec). The largernum bel' of fixations was entirely the result of readersmaking shorter forward saccades, with mediansaccade length dropping from 8.76 to 6.43 characterpositions. There was no increase in the number ofregressive movements as the window size was reduced.The median number of regressive movements per 100characters ranged from 1.1 to .9 for the differentwindow sizes. Thus, the increase in reading time withsmaller window sizes was not due to a change in thenumber of regressions, but was the result of changesin the normal forward saccade and fixation pattern.

As seen in Figure 5, reading time continued to dropas window sizes became larger, and reachedasymptote only at the largest sizes. Median saccadelengths and fixation durations showed the samepattern. This change may be due to either of two typesof influence: (1) either subjects were obtaining someuseful visual information from the normal text fromregions as wide as 45 character positions, which wasnot available in the peripheral text patterns, or(2) reducing the window size itself changed thereader's behavior or produced artifacts which werereflected in his eye-movement behavior. The first ofthese possibilities does not seem likely in view of theearlier-reported results from this experiment. Theevidence seemed to indicate that the readers were notobtaining word-shape or specific letter information

SPAN OF EFFECTIVE STIMULUS IN READING 585

make shorter saccades and perhaps even longerfixations as the window became smaller, but at thesame time would not have led to differences in readingbehavior as a fu notion of type of peripheral textpattern. If this explanation were in fact correct, itwould invalidate the previous conclusions about thesize of the region from which the subjects tended topick up visual information: the results found would betypical only of subjects reading under conditionswhich forced them to narrow their range of attentiondu ring each fixation.

The second possibility why smaller window sizesprod uced the effects noted is related to the lagdiscussed earlier in identifying the onset of a fixationand producing the display changes required for thisresearch. It was noted there that at times the displaychange might not have occurred for as much as40 rnsec following the actual eye-fixation onset. Eventhough this is within the time when visual maskingwas likely occurring as one stimulus pattern replacedanother, it is possible that the display changes takingplace in the periphery were sufficiently distracting toproduce changes in the eye-movement patterns,causing longer fixations and producing more shortsaccades. With small window sizes, these changeswould be taking place closest to central vision, wherethey might be expected to have the most disruptingeffect. As the window became larger, the changeswere being made further and further into theperiphery, where their effect, while still present. wasreduced. Thus, reading patterns may have beenaffected somewhat by these display changes takingplace cven some distance from the point of centralVISion.

At present. the authors tend to accept this latterexplanation. There are two reasons for this. First,Reder (personal communication) has conductedsimilar research under conditions with substantiallyless lag in producing the display change. and hereports that the curves asymptote much earlier in hisstudies. This suggests that the delayed asymptote inour research was not due to a narrowing of the field ofattention, since if it were, the same phenomenonwould be expected to be present in Reder's data.Rather, it is probably due to the relative slowness withwhich our display changes were taking place. Second,Rayner (1975) has reported a study which did notinvolve the type of display changes used in the presentstudy. and which would not tend to induce the readerto narrow his field of attention. He reported resultsconcerning the size of the perceptual span in readingwhich are quite compatible with the estimates fromthe present study. Thus. it is concluded that thewindow-size effects were probably due to artifactsproduced by the display change itself rather than tochanges in the width of the field of attention.

Retention test performance. The retention testswere constructed to have two questions taken from the

information on each page of text. This made itpossible to carry out a four-way analysis of variance onretention test scores to determine whether thevariables influenced the subjects' retention ofinformation from the passages. The only significanteffect was the Window Size by Subjects interaction (F= 1.553. P < .03. 35 and 288 df). Window size didnot have an effect on test performance (F = 1.371.P < .25, 7 and 35 df). The significant interaction wasplotted to determine whether there was a tendency forsome subjects. but not for others, to perform morepoorly with smaller window sizes. No such tendencycould be found. The interaction seemed to ariseprimarily from lack of complete counterbalancing inthe experiment due to the small number of subjects inrelation to the large number of passages. Curves forindividual subjects were very irregular across windowsizes, with no observable trends.

GENERAL DISCUSSION

This experiment has provided data which begin toanswer the question about the size of the perceptualspan during a fixation in reading. Although it may bepossible in tasks other than reading for subjects toidentify letters. word shapes, and word-lengthpatterns some distance into the peripheral areas, influent reading this information appears to be obtainedand used from a relatively narrow region. Thus. atheory of fluent reading need not suppose thatword-shape and specific letter information is obtainedfrom a region occupied by more than about three orfour words during a fixation. and perhaps not thatlarge if the span is not symmetrical around the pointof central vision, a question not tested in the presentstudy. Thus. it does not appear to be true that entiresentences are seen during a fixation; in fact. for mostfixations. not even a complete phrase will lie withinthis area.

It has often been suggested that poorcomprehension in reading can be the result of anarrow perceptual span. Smith (1970, for instance,suggests that having a narrow span, thus perceivingsmaller groups of words at a time, results in thecogn itive processing system having to deal with more"chunks" of information. This is said to produce aheavier load on the processing system, which reducesthe reader's ability to see relationships in the text,thus reducing comprehension of the text. It is ofinterest. then, that reducing the perceptual span ofthe readers in this study by reducing the size of thewindow. although it slowed their reading consider­ably. did not reduce their comprehension test scores.Thus, no evidence was found for the notion that areduced perceptual span produces reduced compre­hension.

The fact that word-length patterns are acquiredsomewhat further into the periphery than the more

586 McCONKIE AND RAYNER

specific letter and word-shape information. and thatthe presence or absence of this aspect of the visualpattern in the periphery is related to saccade length. isharmonious with the position that the eye is guidedduring reading on the basis of characteristics of theperipheral visual pattern (Hochberg. 1970). Thenature of this guidance is presently unknown. but it isof interest that we have found informally that peoplecan mark phrase boundaries in text fairly reliablywhen all letters have been replaced by xs. Thusword-length patterns are related to syntactic structureto some degree.

For the subjects studied here. saccades of medianlength carry the eye to a location just short of thefurthest point where letter- and word-shapeinformation tend to be acquired. This means that alarge proportion of the saccades carry the eye beyondthat point. Further studies need to explore whetherthe span for features of the text on whichinterpretation can be based is symmetrical orasymmetrical. which will indicate the degree to whichthis span overlaps from fixation to fixation. Finally.further study is also needed to investigate whetheruseful visual information is acquired from other linesthan the one being directly fixated in reading.

Although many questions about the nature of theperceptual spans during reading remain unanswered.the present study suggests that a technique involvingmanipulating the display on the basis of eye-positioninformation may have the power to provide answers tothese questions.

REFERENCE NOTE

I. McConkie, G. W .. & Rayner. K. Identifying the span ofthe effective stimulus in reading. Final Report OEG2-71-0531.

U.S. Office of Education. 1974. This report is available fromERIC Document Reproduction Service.

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(Received for publication January 24. 1975;revision received March 3. 1975.)