Prinzmetal Attention

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This version is similar, but not identical, to a paper by the same title published in the Journal ofExperimental Psychology: Human Performance and Perception

The Phenomenology of Attention

Part 1: Color, Location, Orientation, and Spatial Frequency

William Prinzmetal, Hedy Amiri, Kristin AllenUniversity of California, Berkeley

Tami EdwardsYale University

The effect of attention on the phenomenal appearance of objects wasinvestigated in the domains of color (hue), location, line orientation, and

spatial frequency. Observers indicated the appearance of a brieflypresented above-threshold stimulus by selecting a matching stimulusalong a sensory continuum (e.g., color) . Attention was manipulatedwith a dual task that involved letter identification. Attention had littleeffect in changing the way objects appeared in terms of observers' meanresponse. However, in each stimulus domain, attention reduced thevariability of responses. It is argued that attention should be viewed interms of reducing uncertainty.

Kurt Koffka (1935) suggested thatthe main question in the study of visual

perception was "Why do things look as theydo?" Before answering the question of whythe world appears as it does, one needs adescription ofhow the world appears. In thestudy of visual attention we are at aconsiderable disadvantage because we havelittle knowledge of how attention affects theappearance of objects. In the researchreported here we address this deficit byasking how attention affects the appearanceof objects. Does attention make objectsappear more intense, brighter, longer, or moreclear than they would otherwise appear?

How does attention affect the perceived color,location, or orientation of objects?

attention (for reviews see James, 1890;Pillsbury, 1908; Titchener, 1908). Most

conclusions about how attention affects thephenomenology of perception were drawnfrom the investigators' own introspections.This methodology led to disagreements. Forexample, Mach and Stumpf were interested indetermining whether attention to oneparticular pitch in a chord strengthens thatpitch. Sitting at a harmonium in thepsychology laboratory in Prague, theyconducted the critical experiment (seeTitchener, 1908, pp. 215-216). Stumpfreports "While Mach could clearly hear thestrengthening/amplification (of the pitch), I

could not at all." (quoted from Titchener,1908). Mindful of such disagreements,James concluded his review on whetherattention increases the intensity of a stimulusby stating, "The subject is one which wouldwell repay exact experiment, if methods couldbe devised" (1890, p. 426). This reportdescribes our attempt to conduct exact

Nineteenth century sensoryphysiologists and psychologists wereconcerned with the phenomenology ofattention. Investigators, including Mach,Fechner, Wndt, Titchener, Mller, Jamesand others, were concerned with thephenomenal quality imparted to objects by


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experiments on the effect of attention on thephenomenology of perception.

experiments do not provide a very goodindication of how a stimulus appears to theobservers. Consider a typical experiment onattention in which an observer is asked toclassify the color of a briefly presentedstimulus, e.g., "red," "blue," or "green" (e.g.,

Ashby, Prinzmetal, Ivry, & Maddox, 1996;Prinzmetal, Presti, & Posner, 1986).Suppose that on a trial the stimulus is actuallyred, but it appears magenta or purple to theobserver. Given the nature of the experiment,the observer has no way of indicating theexact appearance of the stimulus because itdoes not fit one of the response categories.

The last half of the twentieth centuryhas witnessed a burgeoning of research onattention. Surprisingly, practically none ofthis research has investigated how attention

affects "the way things look" (Baars &Banks, 1992). For example, an impressivebody of research demonstrates that attendedobjects are responded to faster thanunattended objects (e.g., Eriksen & Hoffman,1973; Posner, 1980). Related research, thatdates to Wndt's laboratory (see James, 1890,chapter 11), demonstrates that an attendedstimulus may be perceived before anunattended one (e.g. Stelmach & Herdman,1991). Nevertheless, both of these researchtraditions asks the question of when astimulus is perceived, not how it appears.

Likewise, there is a plethora of research usingthe visual search task that investigates thefactors that affect the time it takes to find astimulus (e.g., Neisser, 1964; Treisman &Gelade, 1980). However, the visual searchtask does not clarify what the stimulus lookslike once it is found.

In the experiments that we describehere, we made it possible for observers toeasily and naturally report the appearance ofthe stimulus, regardless of whether or not itfit a predesignated category. We

accomplished this by investigating simplestimulus continua (e.g., hue, brightness,orientation). Under various conditions ofattention, stimuli were briefly presented andobservers were asked to match the appearanceof the stimulus along one of the continua.For example, an observer might be presentedwith a colored stimulus and then asked toselect the best matching color out of acontinuous palette of colors. As we willdemonstrate, our method has severaladvantages over those employed by previousinvestigators of attention. Besides making it

difficult to infer how a stimulus appears,previous methods may be insensitive tomanipulations of attention, and they may givespurious results due to the classificationprocess itself. Each of these problems will bediscussed later.

A second research area has been ofthe effect of attention on the detection ofstimuli that are near threshold or are heavilymasked. Observers in these studies had todetermine whether a stimulus was present ornot (e.g., Bashinski & Bacharach, 1980;

Downing, 1988). The general finding wasthat detection performance was better withattention than without attention (but see Shiu& Pashler, 1994). These studiesdemonstrated an impressive role of attentionin perception, but they did not shed muchlight on the issue of the subjective experienceand attention. We are specifically interestedin how attention affects the appearance ofabove-threshold stimuli. It does not makesense to consider the phenomenology ofstimuli that cannot be seen (or stimuli that arenearly subliminal). In all of the experimentsreported in this paper, we carefullydetermined whether the stimulus was abovethreshold.

Thus, despite a substantial researcheffort, there have been only a few studies thatdirectly address the question of how attentionaffects the appearance of objects. Forinstance, Tsal and his colleagues haveinvestigated the effect of attention onbrightness contrast, line length, and location(Tsal, Shalev, Zakay, & Lubow, 1994; Tsal &Shalev, 1996). In the research reported here,we investigated the effect of attention of theappearance of objects in terms of color,location, orientation, and spatial frequency.In all of the experiments, observers were freeto indicate the stimulus appearance anywherealong the continuum under investigation.Care was taken to insure that the stimuli were

Finally research has shown thatobservers are more accurate in classifying astimulus in terms of brightness, orientation,or form when attending to the stimulus thanwhen not attending to it (e.g., Downing, 1988;Shaw & Shaw, 1977). Classification


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above threshold. Finally, to provideconverging operations, we manipulatedattention in several different ways, asdescribed below. Experiments 1 to 4investigated the effect of attention on color(hue) appearance. These experiments

illustrate our basic methods. Experiments 5and 6 investigated the effect of attention onlocation. Finally, Experiments 7 and 8investigated the effect of attention onorientation and perceived spatial frequency,respectively.

different objects (the letters and the dot) at thesame time. Successive presentation shouldbe less demanding because observers canfirst devote their attention to the letters andthen attend to the dot. Several investigatorshave found that performance is better with

successive presentation (e.g., Eriksen &Spencer, 1969; Hoffman, 1978, 1979;Prinzmetal & Banks, 1983).

Attention could have at least twoeffects on the distribution of color responses.First, attention might decrease the variabilityof responses. A decrease in variability couldbe considered as a decrease in uncertainty(see, e.g., Maddox & Ashby, in press). On atrial with attention, for example, an observermight be very certain that the stimulus was anexact shade of green (e.g., chartreuse) .However, without attention, the observer

might have some uncertainty about theprecise stimulus color, only know that it was"greenish." Over trials, this effect of attentionwould translate into greater variability ofresponses when attention is diverted.

Experiment 1The goal of the first experiment was

to determine how attention affects the color(hue) of an object. We obtained color judgments from observers in the followingmanner. Throughout the experiment in thecenter of the monitor (always visible), was a

palette of colors that formed a ring. Amonochrome illustration of the stimulus andpalette is shown in Figure 1. (Note that thenumbers shown in Figure 1 were not part ofthe palette but are included as a reference tothe color specification in Table 1). Therewere 254 colors in the palette. The paletteappeared as a continuous gradation of colorsthat differed in hue, with brightness andsaturation nearly constant. During a trial, astimulus dot would briefly appear either onthe left or on the right side of the monitor.Observers then indicated the color of the dot

by moving a screen cursor, with a mouse, to alocation on the palette that matched the colorthat they experienced, and then pressed themouse button to register their response.

The second influence that attentioncould have on the distribution of responses isto the shift the average percept. For example,without attention, the perception of a colormight shift toward a 'prototypical' color(Rosch, 1975). We do not know of specifictheories of attention that predict a shift in thecolor of an object from one hue to another

with varying amounts of attention.In contrast, Treisman and her

colleagues (e.g., Treisman & Gelade, 1980;Treisman & Schmidt, 1982) proposed thatbasic dimensions, such as color andorientation, are registered automaticallywithout attention (e.g., Treisman & Gelade,1980; Treisman & Schmidt, 1982). Hence,according to Treisman's theory, attentionshould not alter the perception of color eitherin terms of the mean response or thevariability of responses. Most of theevidence for this proposal comes from searchtasks with reaction time as the dependentvariable. Treisman and others have foundthat reaction time to detect a target colorincreased only slightly as the number ofnontarget items increase in the display. Onthe other hand, Nagy and Sanchez (1990)found that reaction time did increase with thenumber of items provided the nontargetcolors were similar to the target color.

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Attention was manipulated by givingobservers a second, dual task which involveddetermining whether a centrally placed matrixof letters had contained the letter F or T.Attention was manipulated by having theletters and the colored dot appearsimultaneously (at the same time) orsuccessively (one at a time). With successivepresentation, the letters were first brieflypresented, and 0.5 seconds later the coloreddot was presented. Simultaneouspresentation should demand more attentionbecause observers must attend to two


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There have been a few experiments inwhich observers classified the color of atarget with classification accuracy as thedependent variable. In a "color" (black vs.white) classification task, Nakayama andMackeben (1989) found that precuing the

target location did not improve accuracy(Experiment 1, "simple" condition). On theother hand, Prinzmetal, Presti, and Posner(1986) found precuing the target location didimprove the accuracy of color classification.The latter results are consistent with the ideathat precuing the target location enablesobservers to preferentially attend to thatlocation. Our approach allowed us to accountfor these discrepancies.

know in which of the two locations thecolored dot would appear, they could notmove their eyes in anticipation of the dotlocation.

Each block contained 104 trials: 96trials on which a dot appeared, and 8 trials on

which no dot appeared (catch trials). Onthese target (dot) absent trials, observers wereinstructed to move the cursor to the absentbutton and indicate which target letterappeared (F or T). (Note that the letter matrixwas present on every trial). Reaction timewas not measured and observers were urgedto take their time and be as accurate aspossible. We were interested in determiningprecisely how the stimulus appeared, not howfast observers could respond. Each blocktook about 6 minutes, which means thatobservers took 2 to 3 seconds to execute each

response.

To insure that observers were indeedperceiving the briefly presented dots, weincluded catch trials. On catch trials, only the

letters were presented but the dot was notpresented. If the dots are above threshold,observers should be able to discriminate catchtrials from target dot-present trials.

The following feedback was given toobservers during a trial. When observersresponded with the incorrect letter, thecomputer emitted a brief tone. If the observerresponded that a dot was present when it wasnot (false alarm) or responded that the dotwas not present when it was (miss), thecomputer emitted a 2-tone sequence thatsounded like a foghorn. Observers were notgiven any feedback during a block about theircolor accuracy, but between blocks they weretold their average absolute color precision for

the block (i.e., the average number of palettesteps for the stimulus to the response color).

Method

Procedure. Throughout theexperiment, the color response paletteremained in view in the center of the monitor.A trial began and ended with a fixation dot inthe center of the palette. On a simultaneoustrial, a 3 x 3 matrix of letters was presentedfor 67 ms. in the center of the color palette(see Figure 1). At the same time, a colored

dot was also presented at one of two fixedlocations in the periphery for 67 ms.Following termination of the stimulus, across-shaped cursor, appeared on the screen.Observers were instructed to move the cursor,with a mouse, to the location on the responsepalette that most closely matched the color ofthe dot. They then pressed the left button onthe mouse if the letter matrix contained theletter F, and the right button if the matrixcontained the letter T. After the observerresponded, the cursor disappeared and thefixation dot reappeared. The next trial beganafter 0.5 seconds. The location of the coloreddot was randomly determined on each trial.

Each observer was given a minimumof 2 blocks of practice, one with simultaneouspresentation and one with successivepresentation. Data were collected over 6blocks of trials. Half of the observers firstran 3 blocks of simultaneous presentationfollowed by 3 blocks of successivepresentation. The other half of the observersbegan with 3 blocks of successivepresentation. The experiment lastedapproximately 1 h.

Stimuli. The displays werepresented on a 13-in. Apple monitor

controlled by a Macintosh II computer.1 Themonitor had a screen resolution of 72 pixelsper inch (approximately 28 pixels per cm).Observers sat 40 cm from the monitor withtheir heads restrained by a chin rest. Thestimuli were presented on a whitebackground. The color palette was created by

Successive trials were similar exceptthat the letter matrix appeared first for 67 ms.Following a blank interval of 0.5 seconds, thecolored dot briefly appeared for 67 ms. Thusthe exposure duration was the same for bothsimultaneous and successive trials. Onsuccessive trials, since observers did not


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continuously modulating the Macintosh"Hue" parameter in 254 steps around thepalette. The CIE coordinates, as measuredwith a Minolta Chroma meter (modelCS100), of 8 locations on the palette aregiven in Table 1.

Figure 2, for example, is a frequencyhistogram of all of the responses of allobservers to one of the stimulus colors (color3, palette position 64). We examined thedistribution of responses for the other sevenstimulus colors and they all appeared very

similar: The distributions were fairlysymmetric and mesokurtic.3 In each of thedistributions, there was greater variability withsimultaneous than successive presentation.

There were 8 stimulus colors, andthese colors were taken from the 8 locations

marked on Figure 1.2 Each color was usedan equal number of times in a block of trials.Observers were not told that there were only8 stimulus colors. Figure 1 is drawn to scale,with 1 degree of visual angle indicated.

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---------------------------------------------- To statistically verify the reduction invariability with attention, we calculated theaverage absolute deviation for each condition.The average absolute deviation is preferred tothe standard deviation as a measure ofvariability because it is robust to violation of

the assumptions of analysis of variance(Keppel, 1991, p. 102). To calculate thismeasure, first we calculated the mean errorseparately for each observer and eachstimulus color. (As reported below, some ofthese means were positive, some negative.)The average absolute deviation is the numberof palette steps from these means to eachresponse. Observers' had less variability forsuccessive presentation than for simultaneouspresentation, 19.1 vs. 11.6 color steps. Thisdifference was reliable, F(1,11) = 21.82, p p > .05. Nevertheless,there was no indication of a tradeoff betweenthe two tasks.

Results and Discussion

The twelve observers included in theanalysis reported here clearly coulddiscriminate target-dot present from thetarget-dot absent trials. The mean hit rateswere .996 and .995 for easy and difficult

noise conditions, respectively. The meanfalse alarm rates were .014 and .011 for easyand difficult conditions, respectively. Theaverage d's were 4.46 vs. 4.50, for easy anddifficult noise, t(11) = 0.43.

Like color, the main effect of attention

is to make the perceived location of objectsmore veridical: When observers can attend toa stimulus, the perceived location of thatstimulus is more accurate. Attending to astimulus has a similar effect on its perceivedlocation and color. Next, we examined the absolute

precision of location responses (i.e., thedistance from the stimulus to the observer'sresponse). This average distance was greaterwith the difficult noise condition than theeasy noise condition, 1.31 vs. 1.18 degrees ofvisual angle (26.3 vs. 23.8 screen pixels).This difference was reliable, F(1,11) = 13.36,

p < .05. In terms of response precision, theresults replicate and extend the findings inExperiment 5.

Experiment 6We wanted to determine whether

other manipulations of attention would have asimilar effect on the perception of location.We felt that the spatial manipulation ofattention used in Experiment 3 mightinadvertently affect perceived location in that

the letters near the target dot might provide aspatial frame or anchor for location judgments. Therefore, in Experiment 6 wemanipulated attention by simultaneouslypresenting the dot and letters, but varying thedifficulty of the letter task (as in Experiment2). With an easy letter task, there should bemore attention available for dot localizationthan with the difficult letter task.

As in Experiment 5, we examined theresponses to see if there were any systematicspatial shifts in observers' responses from theactual stimulus location (i.e., mean shifts).First, we analyzed the data to see if there wasa mean shift in responses toward the center ofthe visual field, as suggested by Wolford(1975). There was a small shift toward thecenter of the field that averaged 0.14 degreesof visual angle. However, only 6 of the 12observers in the analysis had a meanresponse closer to the center of the visualfield. In plots of observers' responses wecould discern no other shifts in locationresponses.

The method was identical toExperiment 5 except that we used onlysimultaneous presentation in both attentionconditions. The letter matrix and dotappeared exactly at the same time for 33milliseconds (unlike Experiment 4). On halfof the blocks of trials the nontarget letters inthe letter matrix were selected randomly fromall of the letters of the alphabet (difficultnoise condition), and for the remaining blockthe nontarget letters were always the letter O(easy noise condition). Half of the observersbegan with 3 blocks of the easy noise

Not unexpectedly, observers weremore accurate in target letter identification (Fvs. T) in the easy noise condition than the


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difficult noise condition, 98.8% vs. 89.8%,F(1,11) = 27.34, p < .05.

The response histograms from ourexperiments could be thought of as whole-brain tuning curves as opposed to single-celltuning curves. One goal of Experiment 7 wasto determine how attention affected theorientation tuning in the intact human.

The results of Experiments 5 and 6,together with the results of Tsal and Meiran(1993), clearly demonstrate that attention hasan effect on the perceived location of an

object: When observers attended to astimulus, their perception of the location ofthe stimulus is more precise than when theydo not attend. Thus, the consequences ofattention on color and location are similar.This finding, however, does not mean thatattention to a feature (e.g., color) is the sameas attention to a location. Our experimentsdid not address this issue.

Our second reason for examiningorientation is that it is a domain with clearbiases. Observers are quite accurate in judging the orientation of lines that areexactly vertical or horizontal. However, linesnear vertical or horizontal are consistentlyjudged to be further from these axis than theyactually are (see Schiano & Tversky, 1992,for a review). One way of describing thesebiases is that they are shifts away fromboundaries that describe salient regions (i.e.,up-down boundary; left-right boundary).Huttenlocher, Hedges, and Duncan (1991)

described a similar effect that occurred whenlocalizing a dot in a circle. Observersimplicitly imposed horizontal and verticalboundaries that divide the circle, and werebiased in localizing the dot away from theseboundaries. We wanted to see what effect, ifany, attention would have on these biases.

Finally, these experiments provideevidence for the proposed reconciliation ofTreisman's early theory of feature integration(e.g., Treisman & Schmidt, 1982) with the

idea that feature integration errors are due tothe misperception of the location of features(e.g., Ashby, Prinzmetal, Ivry, & Maddox,1996; Hazeltine, Prinzmetal, & Elliot, inpress; Prinzmetal, 1995). It may be thatfeatures that occur at the same perceivedlocation are combined, but that attentionaffects the precision of location perception.(Other factors also affect location perception,see e.g., Carrasco & Chang, 1995; Prinzmetal& Keysar, 1989). We now have conclusiveevidence in support of the latter proposition.

Method

We manipulated attention bycomparing simultaneous and successivepresentation of a centrally located lettermatrix and a peripherally presented line.Simultaneous trials began with the

presentation of a fixation point. After .25seconds, the fixation point was replaced by a3 x 3 matrix of letters in the center of themonitor and a line at one of 4 locations in theperiphery. The 4 locations for the stimulusline formed the corners of an imaginarysquare centered on the monitor. The line andletter matrix remained in view for 100milliseconds. One-half second after thestimulus presentation, a cursor reappearedalong with an adjustable comparison line inthe center of the monitor. This comparisonline was centered on the screen and wouldrotate clockwise as the observer moved themouse to the left, and counter clockwise asthe observer moved the cursor to the right.The observer could spin this comparison linearound like a propeller. Observers wereinstructed to move the mouse back and forthuntil the orientation of the comparison linematched the orientation of the brieflypresented stimulus line. When the

Experiment 7

Experiment 7 investigated theinfluence of attention on the perceivedorientation of a line. We were interested inthis question for two reasons. First, at thephysiological level, there is evidence thatattention affects the behavior of singleneurons. In particular, Spitzer, Desimone,and Moran (1988) recorded from cells in areaV4 in macaques when the monkeys wereengaged in either an easy or difficult match-to-sample task. Presumably, the monkeyswere devoting more attention to the task in thedifficult than easy condition. Bothwavelength and orientation tuning weresharper in the difficult than easy condition.That is, cells were more selective for theorientation or color to which they wouldrespond when the animal devoted greaterattention to the task. Attention increased theprecision in the cell response tuning curves.In fact the tuning curves resembled ourresponse histograms (i.e., Figures 2 and 3).


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orientation matched the stimulus, observerswere instructed to press the left mouse buttonif the letter array contained the letter C, andthe right mouse button if it contained theletter G. When the observer responded, theresponse propeller and cursor disappeared.


In measuring angular error, there isalways a 180 degree ambiguity. For example,if the orientation of the stimulus line was 0degrees (vertical), orienting the propellerclockwise 15 degrees could be an angular

error of 15 degrees clockwise or 165 degreescounterclockwise. We used the smallerangular error so that the angular error couldbe from -90 degrees to 90 degrees from thestimulus (negative numbers indicate theresponse was counterclockwise from thestimulus). The distribution of all the errors isshown in Figure 6. Overall, observers werequite accurate in their orientation judgments,98.5% of the responses were within 45degrees of the stimulus orientation. To theextent that the stimuli were below threshold,responses would have been random and as

many responses would have been outside aswithin 45 degrees from the stimulus. Theaccuracy of responses assures us thatobservers did perceive the stimulus lines.

Successive trials were identical tosimultaneous trials except that the lettermatrix appeared first for 100 milliseconds.Following an interval of .5 seconds, thestimulus line appeared in the periphery for100 milliseconds. The location of thestimulus line was randomly determined oneach trial.

There were 96 trials in a block. Halfof the trials in a block tested simultaneouspresentation and half tested successivepresentation. Furthermore, there were 12stimulus line orientations and these occurred

equally often within a block. The 12 lineorientations tested, clockwise from vertical,were as follows (in degrees): 0 (vertical), 9,17, 45, 63, 81, 90 (horizontal), 99, 117, 135,153, 171. The order of trials within a blockwas randomly determined.

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The stimulus lines, letter matrix, andresponse propeller were black on a whitebackground. The stimulus line subtended avisual angle of 5 degrees (100 pixels) andwas 1 pixel wide. The center of the stimulusline was located 8.5 degrees (170 pixels)from the center of the fixation point. The

propeller subtended a visual angle of 2degrees in length (40 pixels). The propellerwas made using an anti-aliasing technique sothat there would be no obvious cues forsetting it to precisely vertical or horizontal(Tanner, Jolicoeur, Cowan, Booth, &Fishman, 1989). The exposure duration andeccentricity of the stimulus line were suchthat aliasing was not visible so that this anti-aliasing technique was not used on thestimulus line. The target letters were theletters 'C' and 'G'.

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examining the response histograms such asFigure 6. First, there is less variability withsuccessive presentation than withsimultaneous presentation. The averageabsolute deviation for simultaneous and

successive presentation was 9.2 and 6.5degrees, respectively. This difference wasreliable, F(1,11) = 9.3, p < .05. For eachangle tested, observers had less variabilitywith successive than simultaneouspresentation. Furthermore, each observer hadless variability with successive presentation.

There was a significant differencebetween the 12 stimulus orientations in termsof variability, F(11,121) = 7.09, p < .05.Observers were most accurate by thismeasure when the stimulus was 0 or 90degrees, with average absolute deviations of5.3 and 4.7 degrees, respectively. Theaverage absolute deviation for the remaining12 stimulus orientations averaged 8.4 degrees(range 7.8 to 9.1 degrees). These results aresimilar to others in orientation perception (seeSchiano & Tversky, 1992). The interactionbetween the attention condition and stimulusorientation did not approach significance,F(11,121) = 0.89.

There were no catch trials in thisexperiment. Pilot research indicated that thestimulus lines were clearly above threshold.Hence, we did not include catch trials so as toinclude as many target present trials aspossible in order to sample a larger numberor orientation. Analysis of the responsesconfirmed that the stimuli were abovethreshold.


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---------------------------------------------- Hock, in press; Mackeben & Nakayama,1992; Nakayama & Mackeben 1993). Onthe basis of these results, it has been claimedthat attention improves "spatial resolution."Of course, there are several types of spatialresolution. The finding that attention

improves vernier acuity could be explained bythe effect of attention on location (e.g.,Experiment 5 and 6) or the effect of attentionon line orientation (i.e., a vernier offsetcreates a slanted line). There is an additionalconstrual of spatial resolution. Spatialresolution can be understood in terms ofchannels tuned to different spatial frequencies(e.g., see DeValois & DeValois, 1990). Bythis interpretation, attention might affectspatial resolution by facilitating high spatialfrequency channels relative to low spatialfrequency channels. In Experiment 8 we

tested whether attention would shift theapparent spatial frequency of a stimulus.Alternatively, as in the previous experiments,attention might reduce the uncertainty (i.e.,variability) of the spatial frequency of thestimulus.

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As mentioned earlier, previousinvestigators (e.g., Schiano & Tversky, 1992)have found considerable biases in orientation

perception. Lines near vertical (but notexactly vertical) are reported as having agreater slant than they do. Also, lines nearbut not quite horizontal are reported to befurther from horizontal. As shown in Figure7, we observed these biases. For example, astimulus line 9 degrees from vertical wasreported as being an average of 19 degreesfrom vertical a mean error of 10 degrees.An ANOVA of mean shift showed asignificant effect of stimulus orientation,F(11,132) = 30.00, p < .05. However, theattention manipulation did not influence the

magnitude of the bias. The interaction ofattention and mean shift did not approachsignificance, F(11,132) = 1.34, ns. As can beseen in Figure 7, simultaneous and successivepresentations are barely distinguishable.

The average percent correct for targetletter identification was 95% and 94% forsimultaneous and successive presentation,respectively. These were not significantlydifferent (F < 1.0); there was no evidence fora trade-off between tasks.

A second issue modivated thisexperiment. Every major early researcherconcerned with attention concluded thatattention affects the "clarity" of perception.For example, William James stated that"there is no question whatever that attentionaugments the clearness of all that we

perceive" (1890, p. 426). Edward Titchenerclaimed that "attention is identical withsensory clearness" (1910, p. 267). Althoughthere was near unanimity on this point, therewas some uncertainty about what was meantby "clearness." Pillsbury commented "wemay say that attention increases the clearnessof the sensations attended to but it is verydifficult to describe what is meant byclearness" (1908, p. 2). (Also see, forexample, James, 1890, p. 426; Woodworth,1938, p. 694). Experiment 8 was our attemptat an explicit definition of "clearness" interms that relate to concrete concepts inmodern vision research. One mighthypothesize that clearness has something todo with apparent spatial frequency.

In general, the effect of attention onperceived orientation is similar to the other

domains we have explored: The phenomenalimpression of orientation is, on average, moreveridical with increased attention. We alsofound systematic biases in perceivedorientation that were consistent with pastresearch (e.g., Schiano & Tversky, 1992).Observers reported lines near a cardinalorientation (vertical and horizontal) as furtherfrom the actual orientation. Huttenlocher etal. (1991) suggested that these biases weredue to categorical effects in memory. Thissuggestion is intriguing because it relatesorientation (and location) perception to otherperceptual domains that are physicallycontinuous, but may have a psychologicalstructure that is in some respects categorical(e.g., color, speech perception). Whatever thecause of the orientation distortions, they werenot affected by attention.

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Experiment 8 ----------------------------------------------Several investigators have found that

attention improves vernier acuity (e.g., Balz &The relation between clearness and

spatial frequency is illustrated in Figure 8.


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The right panel is the original image, it is ablack-and-white version of the work bySalvador Dali called Gala Looking at the Mediterranean Sea (also see Harmon, &Julesz, 1973; Uttal, Baruch, & Allen, 1995).The left panel is a low pass version of that

image. One might describe it as "blurred" or"not clear." This image was created by low-pass filtering of the original image. Ifattention operates by affecting clearness inthe manner illustrated in Figure 8, perhaps theright panel will look more like the left panelwhen attention is diverted.

the letters appeared first and the peripheralstimulus appeared with a stimulus onsetasynchrony of .5 seconds. All otherprocedural details were identical to theprevious experiments.

Stimuli. The palettes were created as

follows. We began with a white line, 16pixels wide (0.8 degrees) and 405 pixels long(19.4 degrees). This line was convolved witha difference of gaussians (DOG) filterdefined in the following manner (see Bergen& Wilson, 1979). Let G1 and G2 be thepositive and negative going gaussians,respectively.Method

Procedure. We manipulatedattention by comparing simultaneous andsuccessive presentation with a dual taskprocedure, as before. One task was todetermine whether a centrally placed matrix

contained the target letter F or T. The othertask was to match a briefly presentedperipheral line that had been band-passfiltered. Observers responded by moving thecursor to the location of one of the twopalettes that they felt most closely matchedthe stimulus they had observed. The palettesremained in view throughout a block of trials.A sample simultaneous stimulus is shown inFigure 9.

G1i = exp(-1 * * (i/Width1)^2)G2i = exp(-1 * * (i/Width2)^2)

Width2 was set at 1.75 * Width1. Thus the

only parameter for the gaussians is Width1(width of positive going gaussians). Thesetwo functions were combined as follows:

DOGi = (G1i - (c * G2i)) / Width1

c is a scaling constant that made the positiveand negative gaussian functions have thesame total area. The output of theconvolution was scaled so that the point(s)with the highest value would be the brightestwhite (see Table 1) and points correspondingto the value zero would be the middle gray onthe monitor. The palettes were created bycontinuously varying Width1 from 1.0 at thetop of the palette, to 18.0 at the bottom of thepalette in a linear fashion.

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To insure that the peripherallypresented stimuli were above threshold, weused a slightly different procedure.Observers were instructed to respond usingthe palette on the same side of the screen asthe stimulus appeared. The location of thestimulus was randomly determined. Weassumed that if observers could correctlylocate the visual field in which the stimuluswas presented, it was above detectionthreshold.

There were 8 stimuli in thisexperiment and they were taken from 8evenly spaced locations along the palette; i.e.,stimulus 1 was 1/9 from the top of the palette,stimulus 2 was 2/9 from the top, etc. (seeFigure 9). The length of the stimulisubtended a visual angle of 7.4 degrees.

After completing a minimum of 2blocks of practice, each observer participatedin 5 blocks of 80 trials. Half of the trialswithin a block used simultaneouspresentation and half used successivepresentation. The order of trials within ablock was randomly determined. Theexposure duration for both the letters andperipherally presented stimulus was 67milliseconds. Both stimuli either appeared atthe same time (simultaneous presentation) or


As in the previous experiments,attention had a significant effect on thevariability of responses. The averageabsolute deviation from the mean of eachcondition was 38.5 palette steps forsimultaneous presentation and 29.8 palettesteps for successive presentation. Thisdifference was reliable, F(1,11) = 42.10, p

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Prinzmetal Attention