ROLE OF SPATIAL FREQUENCIES ON THE MEMORY … · ROLE OF SPATIAL FREQUENCIES ... The spatial frequency analysis of stimuli is a ... low frequency grid mask of 2.2 cycles peT degree

C.P.C. : Cahiers de Psychologie CognitiveEuropean Bulletin of Cognitive Psychology1992, Vol. 12, n° 1, 79-98.

ROLE OF SPATIAL FREQUENCIES

AND EXPOSURE DURA TI ON lN FACE

PROCESSING: POTENTIAL CONSEQUENCESON THE MEMORY FORMAT OF FACIAL

REPRESENTATIONS

Christian Coin,l Rémy Versace,2and Guy Tiberghien 1

1. Laboratoire de Psychologie Expérimentale, URA CNRS 665Université Pierre Mendès France, BP 47 X, 38040 Grenoble Cedex, FranceE-mail: TIBERG @ FRGREN 81

2. Laboratoire de Psychologie Sociale, Université Blaise Pascal34 avenue Carnot, 63000 Clermont-Ferrand, France

Abstract. The aim of this research was to study the influence of exposureduration and the spatial-frequency composition of faces in a 'same-different'judgment task. Subjects had to match two faces presented successively. Therecognition rate depended on the spatial-frequency composition of the targetface, and on exposure du ration only for high-frequency stimuli. This effect wasobserved only for durations which were greater than Bloch' s psychophysicsthreshold and oniy concemed the 'same' face pairs. These results are quite con-sistent with the hypothesis that exposure duration has a differential effect on lowand high frequency integration. They are discussed in relation to the single anddual-process models of the 'same-different' judgment task (Farell, 1985).Potential consequences on the fonnat of facial representations in memory areproposed.

Key words: Representation of faces, exposure duration, spatial frequencies.Mots clés: Représentation des visages, durée d'exposition, fréquences spatiales.

80 Christian Coin, Rémy Versace, and Guy 1iberghien

INTRODUCnON

Two opposing conceptions have been used to understand the nature andformat of the mental representation of faces: * an analytic conception of faces

stored involving the fitting of elementary features, and a synthetic conception offaces stored as a whole and irresolvable into more elementary features (Tiber-ghien, 1983). This dichotomy is close to the distinction between the analogic(Kosslyn & Pomerantz, 1977) and the propositional (Anderson, 1978) represen-tation of visual information. Knowing which characteristics of the stimulus aretaken into account during perception is needed in order to have some indicationof the format in which fares are stored in memory. U sing filtered faces ormasks allows us to control the perceptual characteristics of the stimulus likely tobe encoded. On the other band, such an approach is compatible with neuro-physiological and psychophysical data suggesting that the visual system filtersdisplayed information into separate bands of spatial frequencies (De Valois &De Valois, 1980; Shapley & Lennie, 1985).

The spatial frequency analysis of stimuli is a method used by the visualsystem at the rellular and psychophysicallevels. It allows us to describe a prior1 representation 1 of the stimulus via properties called 1 stimulus efficace " on

which later processes can be performed. Evidence from research with animaisand humans indicates that physical attributes can play a Iole in the informationproœss at a high level. Indeed, neurophysiological research bas shown thatthere are neural relIs in the cat's lateral geniculate nucleus and visual cortex,and in the macaque's striate cortex (De Valois, Albrecht, & Thorell, 1982)which are sensitive to specific spatial frequency bands. Pollen, Nagler,Daugman, Kronauer, and Cavanagh (1984) observed that inferotemporal cells inthe monkey were selectively sensitive to one spatial frequency band. ln humans,Harvey, Roberts, and Gervais (1983) tested three models of visual perceptionwith a task involving the identification of letters and mountain scelles (modelsbased on spatial frequency analysis, on feature extraction, and on templatematching). They found that the mountain scelle identification task was betterdescribed by the spatial frequency model, which accounted for 85 % of thevarianre in the data. Analysis of the stimulus in spatial frequency terms is thususeful to understanding higher levels of processing.

Low spatial frequencies provide sufficient information about a spatialconfiguration (Shulman, Sullivan, Gish, & Sakoda, 1986) but do Dot permit a

* We thank J. Sergent, CI. Bonnet and an anonymous reviewer for their helpfulcomments and suggested revisions. We are also grateful to J.L. Embs for bis technicalcollaboration in carrying out fuis experiment.

Face processing 81

detailed representation of features (Sergent, 1986, 1989). ln contrast, high fre-quencies are needed to accurately specify features (Bruce, 1988). Several studiesusing masks have emphasized the importance of the low frequencies for facerecognition. Tieger and Ganz (1979) demonstrated that the superimposition of alow frequency grid mask of 2.2 cycles peT degree on a face --in which caseonly high frequencies are available --disrupts recognition more than masks ofhigher frequencies. Harmon (1973) observed that some faces could be recog-nized in spite of transformations eliminating information about features (carriedby the high frequency contents). By asking subjects to form a visual image froman original face and judge the visual similarity of that stimulus to their mentalimage of the memorized original face, Harvey (1986) showed that the mentalimage of a face was equivalent to a photograph having a central spatialfrequency of 3.8 cpd and a bandwith of 2.8 octaves.

However, in studies on face processing where the spatial composition offaces was manipulated, the contribution of the low frequencies was not alwaysgreater than that of the high frequencies. For example, Fiorentini, Sandini, andMaffei (1983) suggested that high frequencies conveyed the relevant facialinformation in identification tasks. By contrast, Ginsburg (1978, in Sergent,1986) used a matching task to show that the low frequency spectrum is suffi-cient for the processing of faces. Rather important procedure variables, includ-ing task demands, have been put forward to explain such discrepancies (e.g.,Sergent, 1986, 1989). Other factors may play a foie. With a recognition transfertask, O'Toole, Millward, and Anderson (1988) observed an increase in the highfrequency contribution with increased face familiarity.

Although different spatial frequencies convey different information, theyare not instantaneously available either. Low frequency integration, the earliestkind, allows for initial diffuse perception such as figure-ground discrimination.It is followed by high frequency integration where fine details become morediscemible (Breitmeyer, 1984; Eriksen & Schultz, 1979; Sergent, 1982b,1986). Exposure duration then bas a differential effect on low and high spatialfrequency integration; this bas been corroborated by several experiments (Breit-meyer & Ganz, 1977; Nachmias, 1967; Sergent, 1982a). Thus, brief exposureduration does not permit the processing of details (Paquet & Merikle, 1984;Sergent, 1982 a & b; Tolhurst, 1975). Increasing the exposure duration mainlyenhances the integration of high frequencies, while leaving the integration oflow frequencies relatively unchanged. By contrast, decreasing the exposureduration is more damaging to the perception of high frequencies (Nachmias,1967; Spitzberg & Richards, 1975; Tolhurst, 1975). Our first aim will be toconfirm the existence of this differential effect of exposure duration on low andhigh spatial frequency integration by manipulating the exposure duration and thespatial composition of the faces to be encoded.


The experimental paradigm adopted here is a 'yes-no' recognition task (de-layed matching). Used in particular by Smith and Nielsen (1970), this paradigmallows us to manipulate the spatial composition of the faces to be encoded,while avoiding similarity and interference problems. ln addition, it seems thatthe way in which multidimensional stimuli such as faces are processed (i.e.,what dimensions taken into account) depends on the task demands (Sergent,1984b). Now, a matching task was judged preferable to an identification taskbecause, unlike the latter, a matching task may be achieved equally weIl byprocessing the high or low frequencies (Sergent, 1986). On the other band, asemphasized by Hellige, Corwin, and Jonsson (1984), a categorization task (suchas male/female categorization) can be achieved without taking into accountinformation about local facial features (i.e., high frequencies), and solely on thebasis of relatively global characteristics such as face shape (i.e., low fre-quencies).

The memory representation generated from a face composed exclusively oflow frequency components can only retain relatively global information becausein this kind of stimulus, the details (a beauty mark, internaI features: eyes, nase,mouth) are not available. Faces composed of high frequencies may give rise to amemory representation containing information conveyed by low frequencies.Indeed, Ginsburg suggested that low spatial frequencies could be generated bythe visual system from an image which had been objectively high-pass filtered(1978, in Sergent, 1986, 1989).

If we consider the integration of spatial frequencies as an encoding processwhose output is sent to a comparison process (Paquet & Merickle, 1984), therecognition rate will be a function of the nature and quality of the encodedinformation. Since the integration of low spatial frequencies is not very sensitiveto exposure duration, the recognition rate for LP faces (low-pass filtered faces)should be relatively independent of exposure duration. ln contrast, for HP faces(high-pass filtered faces), the graduaI integration of high frequencies should leadto an improvement in performance as the exposure du ration increases. Finally,if the faces are actually represented in memory as low spatial frequencies,performance should be better for LP faces than for HP faces. Since highfrequencies are integrated gradually in time, the elaboration of a representationcomprising information conveyed by low frequencies will probably increase asexposure duration lengthens. Since the comparison process between therepresented stimulus and the test stimulus is based on low frequencies, adecrease in latencies may be expected when exposure duration increases. Onemust add that, in contrast, a feature-by-feature comparison would involve aserial self-terminating process (Sergent, 1984a; Smith & Nielsen, 1970). Such aprocess would require more and more time as the number of features to comparerises.

Face processing (m~1;, , ' 83

METHOD

Subjects

Sixty-six volunteer subjects participated in the experiment including elevenmales. They were recruited among students (first, second, third, and firth YeaTs)in the Department of Psychology at the University of Grenoble, France (meanage: 23 YeaTS 8 months; standard deviation: 4 YeaTS Il months). AlI had normalor corrected-to-normal vision in both eyes.

Material

Stimuli were derived from six black-and-white slides of male portraits.None of the portrayed men had a beard or wore glasses. The faces wereunknown to the subjects, as confirmed by a post-experimental control. Thebackground of the slides was neutral, white, and identical for each stimulus.Bach fare was presented in three different versions:-the original version (N), which is composed of the entire spatial frequencyrange;-the 'high-pass' version (HP), containing the highest frequencies;-the 'low-pass' version (LP), containing the lowest frequencies.

The HP and LP versions were obtained by filtering the Fourier transformof the original stimuli. The available spatial frequency spectrum for HP rares isthe part above 30 cycles peT face width and, for LP rares, up to 6 cycles peTface width. The slides were projected on a translucent screen 120 cm away fromthe subject's eyes. ln cycles peT visual angle, the LP version contained spatialfrequencies from 0 to 2 cycles peT degree (c/deg.) while the other version (HPfaces) contained spatial frequencies up to 10 c/deg. The experiment was fUn byan Apple lie microcomputer. The brightness and contrast of stimuli were keptconstant throughout the experiment. Head position was fixed using a chin Test sothat the fixation dot was constantly at the subject's eye level, avoiding anylateral or vertical motion.

Procedure

The experimental paradigm used was a recognition task (delayed matching)where two stimuli were presented successively: a target stimulus and then a teststimulus. 'Same' and 'different' stimuli had an equal probability of occurrence.A comparison was made at each trial. The spatial composition of the target stim-ulus was variable (HP, LP, or N). ln contrast, the test stimulus contained theentire spatial frequency range (N).


Subjects were tested individually. Each experimental session lasted about25 minutes. It began with a practire phase composed of 18 trials followed by thetest composed of 72 trials. The test trials consisted of three blocks of 24 trialsseparated by a two-minute rest period. SeveD exposure durations (controlled bya shutter) of the target stimulus were used (10, 20, 50, 150, 300, 500, and 1000fiS). The duration of the test stimulus was 1000 fiS. The inter-stimulus intervalwas 1000 fiS, since too fast a pare might have induced masking phenomena(Breitmeyer, 1984). As soon as the test stimulus appeared at the renter of thescreen, subjects were to respond as fast as possible by pressing one of tworesponse keys in front of them. Half of the subjects had to press the left key ifthe two fares were the same, and the other key if they were different. This set-up was reversed for the remaining subjects. Between each trial, a fixation dotwas projected onto the center of the screen for 500 fiS. To prevent the visiblepersistence phenomenon, its projection stopped 500 ms before the targetstimulus onset. The fixation dot appeared again as soon as the subject answered.Subjects were randomly assigned to each of the seveD duration conditions, withthe restriction that the target stimulus was presented to 6 subjects for 1000 fiS,and to 10 subjects for the other 6 conditions.

Unlike the target faces, which could be presented under three differentversions (HP, LP, N), the test faces were all normal (N). Therefore, there werethree different comparisons: LP-N, HP-N, and N-N, consisting of 24 trialseach. Half of the trials required a 'same' response and the remainder required a'different' response. Two orders of presentation, a direct order and a reversedorder, were counterbalanced among subjects. The stimuli presentation order wasrandomized with no more than 3 successive items, composed of the same faresand calling for the same answer, and no more than 3 target faces composed ofthe same spatial composition.

Design

The experiment involved two within-subject factors and two between-subject factors. The between-subject factors were presentation order (direct orreversed) and target stimulus duration. The within-subject factors were thesameness of the fares to be compared ('same' or 'different') and the spatialcomposition of the target face (HP, LP, N).

RESULTS

Correct responses

The percentages of correct responses obtained in each experimental condi-

Face processing 85

tion are presented in Table 1. Correct response latencies differing from themean by more than two standard deviations (computed for each experimentalcondition) were discarded: the means differed considerably between subjects andexperimental conditions. Less than 5 % of the total number of response wererejected for fuis reason.

TABLE 1. Correct response rates and latencies (ms) for each experimentalcondition (HP: high-pass filtered faces; LP: low-pass filtered fâces; N: normalfaces; same: same face; diff.: different faces).

.HP LP NDuratlon .

SaIne dlff. SaIne diff. SaIne diff.

10 ms 40.0 72.5 75.0 83.3 91.7 90.8 75.5(897) (854) (685) (713) (610) (650) (735)

20ms 49.0 64.2 88.3 75.8 94.2 90.8 77.1(887) (774) (664) (730) (555) (660) (712)

50 ms 60.0 65.8 77.5 84.2 93.3 92.5 78.9(830) (899) (643) (727) (671) (662) (739)

150 ms 56.7 69.2 82.5 80.0 90.8 87.5 77.8(871) (807) (678) (764) (656) (657) (739)

300 ms 75.8 65.0 88.3 80.0 95.0 91.2 82.6(841) (851) (705) (751) (600) (649) (733)

500ms 81.7 78.3 86.7 87.5 89.2 88.3 85.3(852) (941) (790) (876) (710) (804) (829)

1000 ms 90.3 66.7 84.7 88.9 91.7 81.9 84.0(785) (872) (775) (825) (687) (737) (780)

63.2 68.9 83.2 82.4 92.3 89.5(856-) (856) (702) (766) (639) (686)

66.1 82.8 90.9 79.9(856) (734) (662) (751)

TABLEAU 1. Pourcentages des réponses correctes et latences (ms) obtenues danschaque condition expérimentale (HP : hautes fréquences; LP : basses fréquences ;N : normal; same : visages identiques; diff. : visages différents).


The order of presentation did Dot have any reliable effect and did Dotinteract with any other factors. Because of the relatively small number oforiginal stimuli (i.e., 6), it is conceivable that the repeated presentation of samefaces might have produced a practice effect. However, the analysis of correctresponses indicated that performance remained constant during the whole test.

A three-way analysis of variance was fUn with one between-subject factor(exposure duration) and two within-subject factors: spatial composition of theface to be encoded (LP, HP, or N) and sameness (same or different).

The main effects involved spatial frequency, F(2, 118) = 198.57, MSe =304.37, p<.OOl, and exposure duration, F(6, 59) = 4.61, MSe = Il.37,p< .001. Performance was even better when exposure duration was longer (therecognition rates were 75;5% and 84.03% for exposure durations of 10 and1000 ms, respectively). Furthermore, a regression analysis displayed a log-linear relationship between exposure duration and recognition rate, F(l, 69) =21.08, MSe = 51.99, p< .001.

Figure 1. Correct response rates as a junction of exposure duration (logarithmic scale)and spatial frequency (HP: high-pass filtered faces; LP: low-pass filtered faces; N:nonnal faces). Only regression lines are shown.

% Correc1100 r~

9J f ~ ! ~ ~--

00 C'.'--'.-.'.9 'C-..,

?O

x60 xW

cLP

+N10 20 50 150 300 500

1CnJDJratioo

Figure 1. Pourcentages des réponses correctes en fonction de la durée de présentation(échelle logarithmique) et de la fréquence spatiale (HP: hautes fréquences, LP: basses

fréquences, N: nonnal). Seules les droites de régression sont représentées.

Face processing 87

Two two-way interactions were significant. The first, between exposureduration and spatial frequency, F(12, 118) = 5.53, MSe = 8.48, p< .001,showed that the effect of exposure duration varied according to the spatialfrequency range taken into account. It was significant on1y when the target farewas composed of high frequencies, F(6, 59) = 9.38, MSe = 24.48, p< .001.This fact showed that performance improved with an increase in exposureduration for HP faces (see figure 1). The second interaction, between exposureduration and sameness, F(6, 59) = 2.50, MSe = Il.11, p< .01, revealed that'same' judgments were expressed with 1ess errors when exposure duration waslonger, F(6, 59) = 5.65, MSe = 19.64, p< .001. ln contrast, the accuracy of'different' judgments seemed to be independent of exposure duration (F< 1).

A three-way interaction was found between exposure duration, spatial fre-quency and sameness, F(12, 118) = 2.32, MSe = 7.15, p<.05. The interactionbetween exposure duration and spatial frequency was significant for 'same'pairs, F(12, 118) = 5.27, MSe = 13.21,p<.001, but not for 'different' pairs,F(12, 118) = 1.14, MSe = 2.40, p> .05. As shown in Figure 2, the interactionbetween duration and sameness was significant for HP faces, F(6, 59) = 3.00,MSe = 20.66, p< .05, and not for LP faces, F(6, 59) = 1.54, MSe = 4.24,NS. 50, while recognition rate did not change noticeably with an increase inexposure duration for HP faces and 'different' pairs (F< 1), it increased in anearly continuous way for 'same' pairs, F(6, 59) = 7.92, MSe = 41.49,p< .001. These results showed that an increase in exposure duration enhancedperformance only for HP fares and 'identical' pairs.

ln order to check for the existence of the differential integration of spatialfrequencies as a function of exposure duration, an analysis restricted to HP facesand LP faces was carried out. A significant interaction occurred betweenexposure duration and spatial frequency, F(6, 59) = 3.06, MSe = 6.29,p< .05. On the other band, an increase in exposure duration increased thenumber of correct responses for HP faces, F(6, 59) = 9.38, MSe = 24.48,p< .001, but not for LP faces, F(6, 59) = 1.13, MSe = 2.39, NS. These resultsare consistent with the hypothesis that an increase in exposure duration improvesthe integration of high frequencies but leaves low frequency integration rela-tively unchanged. Furthermore, paired comparisons (Tukey test) revealed thatperformance was better for LP faces than for HP faces, whatever the exposureduration. However, the deviation of recognition rate was even smaller whenexposure duration was longer. For 10 mg, the deviation of recognition rate was23%, Q(l, 59) = 36.87, MSe = 75.62,p<.01, whileitreached 14%, Q(l, 59)= 13.27, MSe = 27.22, p<.Ol, and 9%, Q(l, 59) = 2.92, MSe = 6.00,

p< .05, for 300 and 1000 mg, respectively. These overall results closely fit thesecond hypothesis. They corroborate the idea that for unfamiliar faces, mainly10w spatial frequencies are stored in memory.

88 Christian Coin, Rémy Versace, and Guy nberghien

Figure 2. Correct response rates as afunction of exposure duration (logarithmic scale),spatial frequency (HP : high frequencies,' LP: low frequencies), and sameness (same:sameface; diff: differentfaces).

"/ r:- r ~~~ t/. JV , -'~i' ec:'"'onc:ec:100' ~... ~-

'1 .l' '"

1 80 '. 1/..'"..-!'" ,1 '"

", -'-" ,Jo,' "", ---o, " ,.l. l "", "---~ :" ~ ,l' ",,1

"60 -- ;"'

"'

/. -HP saine

/../-- HP ,ji f f40 .-LP same

LP di f f

10 20 50 150 300 500 1000

D'Jra~ ion

Figure 2. Pourcentages des réponses correctes en fonction de la durée de présentation(échelle logarithmique), de la fréquence spatiale (HP: hautes fréquences, LP: bassesfréquences) et de la similitude (same : visages identiques, diff : visages différents).

Latencies

Only the latencies of correct responses were retained. ln addition, for thesame reasons, as above for correct responses, latencies differing from the meanin each experimental condition by more than two standard deviations were re-jected. A five-way analysis of variance was performed, but solely for exposuredurations above 150 ms. By duration condition, the rates of discarded latencieswere 22%, 17%, 15%, and 18% (for 150, 300, 500, and 1000 ms, respec-tively). For shorter durations, the number of errors was too 10w and too variableto permit meaningful analysis. Indeed, latencies were the result of the means ofa variable number of correct responses for the HP modality of the spatialfrequency factor and the 'same' modality of the sameness factor. Thus, in the

Face processing 89

"10 ms duration, HP faces, 'same'" case, the value taken into account for thesecorrect responses was highly variable across subjects, ranging from one to tencorrect responses.

The order of presentation had no reliable effects and did Dot interact withany other factor. This factor will therefore Dot be mentioned in further analysis.Two significant main effects were found: spatial frequency, F(2, 64) = 60.20,MSe = 522070, p< .001, and sameness, F(l, 32) = 5.66'; MSe = 119102,

p< .001. Latencies were shorter for N faces (662 ms), intermediate for LP faces(734 ms) and longer for HP faces (856 ms). On the other band, 'same' judg-ments required less time than 'different' judgments (732 ms vs 769 ms). Thisresult, although surprising, has often been obtained (see Farel1, 1985, for areview). Furthermore, no interaction was significant (F ratios aliless than one).

DISCUSSION

An analysis of correct responses indicated a log-linear relation betweenexposure duration and recognition rate. This finding is quite similar to the Ellisdata (1981) for an identification task. ln addition, the regression line's slope wasrelatively moderate (a = 4.71). So, doubling the exposure duration scarcely im-

proved the recognition rate. It is worth noting that recognition rate was alreadyhigh when the target stimulus was presented for 10 ms (75.5%).1 Restrictingour analysis to normal face data (here we are approaching real life situations),the recognition rates reached 91.25%2 and 86.8%3 for 10 and 1000 msdurations, respectively. This means that, within the context of the recognitionmethod we used, subjects are able to extract relevant information permittingthem to process faces in a very short time as efficiently as if they were givenmuch more time. This result, which demonstrates the outstanding efficiency ofthe visual system, may be compared to the data obtained by Simpson andCrandall (1972). Those authors observed that one could ex tract informationabout facial expression with a 20 ms exposure duration as accurately as with amuch longer duration.

1. Postman's (1950) fonnula can be used to account for responses given by chance: Rc = B -M1 n-1, where B represents the number of correct recognitions, M the number of incorrectrecognitions, n the total number of proposed choices (here n = 2; 'same' or 'different'), and Rcthe corrected recognition mark. For the 10 ms exposure duration, Rc equals 51.1 %.2. The corrected recognition rate Rc equals 82.5 %.3. The corrected recognition rate Rc equals 73.6%.


The analysis of correct responses also indicated an exposure duration xspatial frequency interaction. An increase in exposure duration improved therecognition rate for HP fare stimuli but not for LP stimuli. It seems that highfrequency integration was improved by longer exposure duration. However, thisimprovement may be influenced by other factors, some of which were kept con-stant throughout the experiment. Thus, high frequency integration was facili-tated by an increase in stimulus energy (Sergent, 1987), which was itselfbenefiting from an increase in exposure duration.4 On the other band, contrastmay interact with duration and with the spatial composition of the stimulus: thecontrast of HP stimuli was higher than that of LP stimuli (see Sergent &Hellige, 1986, for a discussion). For example, Breitmeyer and Ganz (1977)found that contrast sensitivity benefited more from an increase in exposure dura-tion at high frequencies than it did at low frequencies. Moreover, latency anal-ysis showed that responding was significantly faster for HP faces than for LPfaces. These data are consistent with previous results showing the differentialintegration of spatial frequencies in time.

Recognition rates were higher when faces to be encoded were composed oflow spatial frequencies than when they contained high spatial frequencies. Anincrease in exposure duration did not lead to a significant rise in correctresponse rates for LP faces. These results are consistent with the hypothesis thatfaces may be represented in memory by low spatial frequencies. However, inthe present experiment, the inter-stimulus interval was rather short (i.e., 1000mg). The question remains whether such a pattern would be obtained withlonger inter-stimulus intervals.

Latency analysis did flot corroborate such a hypothesis. Thus, the effect ofexposure duration was not significant. Subjects did respond equally as fast,whether exposure duration was short or long (though with variable success, asshown by correct response analysis: overall recognition rates were higher forlong exposure durations). When the target fare was presented for a relativelylong time, subjects had more information at their disposai to compare and use torespond. Subjects may have tried to give a correct response by rechecking thesameness of the faces. Such a strategy generally leads to longer latencies whenthe stimuli to be compared are different, but bas no effect on response accuracy:'same' and 'different' processing is flot impaired (Farell, 1985; Krueger, 1978).Our data seem to suggest that such a strategy was used here. Latency analysisindicated a strong effect of the sameness factor: processing leading to a 'dif-

4. Light energy integration is described by Bloch's law such that IxT=K, in which 1 representslight energy, T duration, and K a constant (Buser & Imbert, 1987). This constant, for which thelight energy is completely integrated by the visual system, is about 300-400 ms for spatial acuitytasks and form perœption (Kahneman, 1964; Kahneman & Norman, 1964; Sergent, 1985).

Face processing 91

ferent' decision took longer on the whole than that leading to a 'same' decision.On the other hand, the recognition rates were unaffected by the nature of thecomparison: the sameness effect was not significant.

ln addition, the differential effect of duration on spatial frequency inte-gration was not confirmed by latency analysis. It is likely that this divergence isdue to individual differences. Indeed, Cooper (1980) showed that some subjects,especially rapid ones, preferentially use a holistic process (involving lowfrequencies?) whereas other subjects carry out an analytié process (perhapsinvolving high frequencies). Moreover, Hautekeete and Sockeel (1982) observedthat it is possible to induce both types of process in subjects. Finally, it is evenpossible to lead the subject to use a switching strategy by asking him/her toclearly indicate the differences -if any -between the stimuli (Cooper, 1980). Aconditional counting based on latencies of the 'N-N' comparisons (where thefaces to be compared contained the whole spatial frequency range) was carriedout. The number of correct responses by slower and faster subjects were thencomputed for each exposure duration condition, so as to obtain an equal numberof subjects in both cases. However a separate analysis of variance did not yieldsignificant differences between recognition rates.

The recognition rates and latencies obtained indicate that the task was easierfor LP faces than for HP faces. This result is consistent with the finding that thestimulus configuration (low frequencies) is more easily and quickly processed. Itis quite in agreement with the global precedence phenomenon so often observed(Navon, 1981; Paquet & Merikle, 1984; Pomerantz, Sager, & Stoever, 1977).However, faces composed of low frequencies may not be processed as config-urations. Kimchi and Palmer (1985) showed that for stimuli composed of a few,relatively large elements (in hierarchical patterns), the local and global levelsmay be processed as forfis. So, local elements are not necessarily integratedinto a global form. If we regard faces as stimuli composed of a few elements(eyes, nose, mouth, etc.), those making up LP faces could be processed as fea-tures (however different from the characteristics conveyed by high frequencies)and would not be automatically integrated into a global form. ln the presentexperimental situation, the test stimulus was projected at the same location thatthe target stimulus and the exposure of the two faces was the same. The taskcould be accomplished through feature matching, especially for the 'low fre-quency' features in the 'N-LP' condition. For instance, faces may matchthrough a feature like 'haiT' (the LP face's haiT and the N face's haiT beingeasily superimposed). Nevertheless, the contention that faces are composed of

.fewelements (eyes, nose, mouth, etc.) could be challenged if we add character-istics like those defined by Garner (1978): the size of the forehead, the relativeimportance of the mouth, the presence of wrinkles or beauty marks. Moreover,information about the spatial configuration is conveyed by the low spatial


frequencies (Shulman et al., 1986) and is very important for faœ recognition(Harmon, 1973; Harvey, 1986; Tieger & Ganz, 1979). For unfamiliar rares, itsee,ns that mainly low spatial frequencies are stored in memory (Harvey &Sinclair, 1985). Benton and Gordon (1971) observed a positive correlationbetween face recognition and discrimination between different patterns ofshading. These data show that, due to its importanœ for the processing of rares,the configuration conveyed by low frequencies is very probably taken intoaccount during the encoding process.

The analysis of correct responses revealed a three-way interaction betweenexposure duration, spatial frequency, and sameness. The most interesting resultobtained concerns HP faces. Recognition rate increased with exposure durationfor 'same' pairs, but. not for 'different' pairs. The representation constructedfrom the high frequencies was the same for the two conditions, since the subjectdid not know in advance whether the test face would be identical to or differentfrom the encoded face. The number of trials leading to 'same' responses beingequal to the number leading to 'different' responses, the hypothesis of a stim-ulus sampling bias in which 'same' pairs occur more frequently than 'different'pairs, must be discarded (Nickerson, 1978). The origin of these results must liein the comparison process.

ln order to account for the fast 'same' judgments in situations where thenumber of features (or attributes) varied -these 'same' decisions should belonger sinœ there is a larger number of features to be compared -the followingtwo major classes of models ttave been proposed (see Farell, 1985, for an exten-sive review): \

-Single-process model1 where the comparison between two stimulipresented simultaneously or suècessively is made analytically, feature-to-feature(Farell, 1985; Nickerson, 1978; Smith & Nielsen, 1970). Since the sameprocess is performed in both cases -whether the comparison bears on twoidentical stimuli or on two different stimuli -the fast-'same' effect can beattributed either to encoding facilitation (i.e., priming) when the two stimuli areidentical (proctor, 1981), or to an inhibitory effect ascribed to 'internai noise'leading to iterative feature-matching operations when the stimuli are different(Krueger, 1978).

-Dual-process models, where an analytic process underlies 'different' judg-ments (the associate comparison process is serial and self-terminating; as soonas a difference is detected, the process stops and the response is triggered) and aholistic process underlies 'same' judgments (Farell, 1985; Jones, 1982;Krueger, 1978; Nickerson, 1978; Smith & Nielsen, 1970). If we consider thatconfigural information conveyed by low frequencies is taken into account duringthe perception of an LP face, and that fine details are not available in such aface, it is possible to fit the data obtained to the two foregoing models.

Face processing 93

Our findings are Dot consistent with the former models. Thus, if analyticprocessing takes place regardless of faœ similarity, the recognition rate of HPfaces may be greater than that of LP faces. Indeed, as the encoding of detailscannot be carried out from LP faces, an analytic comparison cannot occur. Butour findings showed that the number of correct responses was the highest for LPfaces, for all exposure durations.

These results seem to be rather consistent with the dual-process models.According to the latter, an analytic proœss underlies 'different' comparisons. lnfact, the recognition rate of 'different' HP faces remained constant as theexposure duration increased since the faces differed in all details. Thus, a shortduration was sufficient to encode a feature of the target face which differedfrom the corresponding detail of the test faœ. A longer duration did Dot permitgreater accuracy due to the nature of the comparison process (serial self-termi-nating: as soon as a difference is detected the process stops) because even iflarger details were encoded, the process would Dot continue the review. Resultsshowed indeed that the recognition rate did Dot change significantly for 'dif-ferent' HP faces with an increase in exposure duration.

Furthermore, it seems that a holistic processing mode underlies the 'same'comparisons. Thus, for the 'same' HP face condition, an increase in exposureduration could allow for the elaboration of a facial representation composed oflow spatial frequencies from the face characteristics to be encoded, and lead tohigher correct response rates. However, for short durations (stiij for HP faces),when the pairs were composed of like faces, the recognition rate was relativelylow (40% and 49% for 10 and 20 ms durations, respectively). On the otherband, the recognition rate was clearly higher when the face pairs were different(72.5% and 64.2% for 10 and 20 ms durations, respectively). As already men-tioned above, a short exposure duration only enables incomplete high frequencyintegration (Breitmeyer, 1975; Sergent, 1982a; Tolhurst, 1975). Such a durationappears to be too brief for a representation composed of low frequencies to beconstructed -from the details available in HP faces. This explains why a holisticprocess may Dot be easily implemented and why the recognition rate wasrelatively low for 'same' HP faces. Conversely, such a duration seems sufficientto allow the subject to encode a limited number of details and distinguish thetest face from the target face. When the faces were different -they differed inall details -encoding only one detail was sufficient for the pairs to bedifferentiated and a 'different' response to be given.

Without questioning the analytic/holistic dichotomy relating to 'same'-'different' face pairs, this interpretation can still be challenged. It is at variancewith the widely observed fact that a brief exposure duration does Dot permit theprocessing of details (Breitmeyer, 1975; Sergent, 1982a; Tolhurst, 1975). But inthe present experiment for HP target faces, only the highest spatial frequencies


were available (i.e., the details). This interpretation assumes that encoding takesplare serially, though it is highly likely that the visual system proœsses imagesin para1lel. How can we explain why the recognition rate was relatively high for'different' HP faces when the exposure duration was very short (10 and 20 fiS)?It is possible that in this condition, when the task was difficult (the correctresponse rate barely exceeded the chance level), subjects were likely to respond'different' because there were more ways for stimuli to be different thanidentical (Hellige, Cox, & Livac, 1979).

Sinre the correct response rates for 'different' HP fares were not affectedby variations in exposure duration, it has been inferred that the comparison pro-ress operating on the test stimulus and the target stimulus was analytic. If thiswere the case, then the details of the encoded stimulus were available; thereforehypothetical facial representation composed of low frequencies elaborated fromhigh spatial frequencies is not irresolvable into more elementary parts. It ispossible that this availability may be inherent in the experimental scheme itself.It is conceivable that, with a longer retention interval, the characteristics of theencoded fare stayed within the low spatial frequency range.

A wide variety of studies have shown how important configural informationis to face processing. But different paradigms have been used: in some studies,chimeric faces have been obtained by cutting along the midline .(Young & Hay,1986; Wirsen, Klinteberg, Levander, & Schalling, 1990, for studying facialexpression) or by cutting along a horizontal line below the eyes (Young,Hellawell, & Hay, 1987). Other investigators have pointed out the importanreof configural information either indirectly (by inverted face representations;Carey & Diamond, 1977; Diamond & Carey, 1986) or by using schematic fares(Sergent, 1984 a & b; see Rhodes, 1985, for a review).

The spatial frequency approach offers numerous advantages which are com-patible with neurophysiological and psychophysical data. ln addition, it providesa face netric. The present study also shows the importance of configuralinformation to face processing. Our results are in agreement with other studiesin which spatial frequencies are directly manipulated (by filtering the stimuli) orindirectly manipulated (using masks) (Harmon, 1973; Harvey, 1986; Tieger &Ganz, 1979). Our knowledge about how facial information is represented inmemory remains incomplete. Further studies are necessary to determine in par-ticular if information extracted from LP faces is only configural (Shulman et al.,1986) or if parts of LP fares can be processed independently of configuration(Kimchi & Palmer, 1985).

Face processing 95

RESUME

Cette expérience avait pour but d'étudier l'influence de la durée de présentation etde la composition spatiale des visages à en coder dans une tache de reconnaissance"oui-non". Les sujets devaient comparer deux visages présentés successivement. Lesvisages-cibles variaient selon la gamme des fréquences spatiales et la durée deprésentation. Les paires de visages étaient constituées de visages identiques oudifférents. Le taux de reconnaissance est variable selon la composition spatiale desstimuli-cibles. Il dépend de la durée de présentation seulement pour les stimulicomposés de hautes fréquences. Cet effet n'apparaft que pour les durées supérieures auseuil psycho physique de Bloch et ne concerne que les paires de visages identiques. Cesrésultats confirment l'hypoth~se d'un effet différentiel de la durée de présentation surl'intégration des hautes et basses fréquences spatiales. Ils sont discutés en référence auxmod~les unitaire et dualiste concernant les comparaisons entre deux stimuli (Farell,1985). Des conséquences possibles sur le format des visages représentés en mémoiresont avancées.

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Received July 20, 1990Accepted July 19, 1991

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