The Quarterly Journal of Experimental Psychology Section A

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Scrutinization, Spatial Attention, and the Spatial Programming of Saccadic Eye Movements John M. Findlay & Zoi Kapoula To cite this article: John M. Findlay & Zoi Kapoula (1992) Scrutinization, Spatial Attention, and the Spatial Programming of Saccadic Eye Movements, The Quarterly Journal of Experimental Psychology Section A, 45:4, 633-647, DOI: 10.1080/14640749208401336 To link to this article: http://dx.doi.org/10.1080/14640749208401336

Published online: 29 May 2007.

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THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY,1992.45A (4) 633-647

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Short Report: Scrutinization, Spatial Attention, and the Spatial Programming of Saccadic Eye Movements John M. Findlay and Zoi Kapoula University of Durham, Durham, U.K . Laboratoire de Physiologie Neurosensorielle, CNRS, Paris, France

Results are presented from an experiment in which subjects’ eye movements were recorded while they carried out two visual tasks with similar material. One task was chosen to require close visual scrutiny; the second was less visually demanding. The oculomotor behaviour in the two tasks differed in three ways. (I) When scrutinizing, there was a reduction in the area of visual space over which stimulation influences saccadic eye movements. (2) When moving their eyes to targets requiring scrutiny, subjects were more likely to make a corrective saccade. (3) The duration of fixations on targets requiring scrutiny was increased. The results are discussed in relation to current theories of visual attention and the control of saccadic eye movements.

Scrutinization is a term used to describe a mode of looking behaviour in which particularly careful attention is given to detail. The very existence of a specific word for this type of behaviour suggests it is distinguished in some way from more casual, everyday, looking. However, this difference does not appear to have been investigated scientifically. We present results here from an experiment in which we record eye scanning and show how looking behaviour changes when a task demands scrutiny. It is possible, of course, that as well as changing the overt pattern of eye movements, scrutinization also involves covert changes in the way visual information is processed. Requests for reprints should be sent to John M. Findlay, Department of Psychology, University of Durham, South Road, Durham D H l 3LE, U.K. We gratefully acknowledge the assistance of a Twinning Grant from the European Science Foundation. We are also grateful to Ala Hola for comments on an earlier version of this paper.

01992 The Experimental Psychology Society

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Looking behaviour is characterized overtly by a sequence of fixations and saccadic eye movements. During each fixation the eye is stationary, with gaze directed to one location in the visual field. The eye remains at this point until a saccadic movement takes it rapidly to a new gaze location. The saccadic eye movement constitutes a miniature behavioural act that requires the programming of decisions about when the eye movement is to occur and where the eye will land. We examine how these decisions are affected in a task requiring scrutiny. We make use of the global effect that occurs when saccades are made to an extended target or target pair. Saccades to such targets tend to be directed to the “centre of gravity” of the target configuration (Coren & Hoenig, 1972; Findlay, 1982; Ottes, van Gisbergen, & Eggermont, 1984). The effect occurs when the targets are quite widely separated (several degrees) but not when the separation is very large (Ottes et al., 1984) or when the target elements straddle the original fixation point (Findlay, 1983). The effect has been related to the fact that the neural pathways directing the visual signal to the oculomotor centres use a form of distributed coding (Findlay, 1987; Lee, Rohrer, & Sparks, 1988; Wise 8t Desimone, 1988). He and Kowler (1989) have suggested that the global effect results from a high-level cognitive strategy. However, if this were so, it is difficult to understand why it is so universally found. The global effect tendency is subject to modification by the influences of attention or expectation (Coeffi & O’Regan, 1987; Findlay, 1981, 1985; Findlay & Crawford, 1983; He & Kowler, 1989; Vitu, 1990), but in none of these studies has the global effect been eliminated when the subject makes a short latency saccade to a newly appearing target. An alternative suggestion, made by Ottes, van Gisbergen and Eggermont (1985) is that the global effect represents an automatic default option for the saccadic system, but one that may be modulated by other factors. In this paper we report an experimental comparison of eye movement behaviour in two conditions, one requiring detailed accurate fixation of targets. The stimuli presented in each condition were almost identical. This allowed us to attribute any changes in saccade characteristics to the task instructions. We wished to find out whether the global effect would be modified, or even abolished, by instructions that required scrutinization of the stimuli. Our design allowed us to study both saccades made to a newly appearing stimulus and also subsequent scanning saccades.

Method Subjects. Five laboratory workers, including the authors, with ages ranging from 25 to 45, served as subjects. All had normal, or corrected to normal, vision. Subjects SC, AH, and MV had had previous experience

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of eye movement experiments but had no knowledge of the specific hypotheses under investigation. Stirnuii. The stimuli were generated on a Hewlett-Packard 1321 computer, using an X-Y display driven by a CED Alpha computer. The display had a P31 phosphor and was refreshed at a rate of 100 Hz. The viewing distance was 84 cm. The stimuli, which always appeared to the right of the fixation point, consisted of one or more small squares (formed by 16 computer pixels). The side of each square was 1 cm (0.7"). In the interior of these squares was displayed a number of isolated spots (single pixels). In one experimental condition, designated "normal", this number was either one or two. In the second condition, designated "accuracy", the number was three or four. The spots were displayed using a routine that ensured that they did not always occupy the same locations relative to the square frame, so that a particular number of spots would appear in a variety of configurations. The generating routine also ensured a minimum separation between the spots, so that in foveal vision each spot was clearly separated from its neighbours. The stimuli were very similar to those used by Findlay and Crawford (1983). In that paper, it is demonstrated that the discrimination between squares containing 4 and 5 spots can only be made reliably if the visual axis is positioned within about half a degree of the target. Discrimination between one and two spots can be made over a much wider range of eccentricities. Thus the accuracy condition required close scrutiny, whereas the normal condition did not. On each trial, one, two, or three squares were presented in various positions chosen from a set of 14 possibilities (single squares at 2", 4", 6" or 8";double squares at 2 + 4", 2 + 6", 2 8", 4 6", 4 + 8", or 6 + 8";

+

+

3 Normal condition

I+

Accuracy condition FIG. 1 . Examples of target stimuli in the two conditions. The subjects' task was to count the number of spots inside the squares.

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FINDLAY AND KAPOULA

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+ +

+ +

+ +

or triple squares at 2 4 6", 2 4 8", 4 + 6 + 8", or 4 6 10"). By using such a variety of target configurations, several aims were achieved. (1) The single targets provided necessary controls. (2) The triple targets allowed data to be collected on scanning activity subsequent to the first saccades, addressing the question of whether these saccades are subject to the global effect. The variety of target possibilities was chosen to ensure that the target arrangement was unpredictable in the 2O-8" range, with the 4 + 6 + 10" combination introduced to study the possible effect of stimulation in an unexpected location.

Design. Each subject performed one block (comprising 8 practice trials and 112 experimental trials) under each condition (the two conditions were run on separate days). Subjects SC and AH performed the normal condition, followed by the accuracy condition; the other three subjects performed the conditions in the reverse order. Target Presentation. The targets were presented as part of the following task. The fixation point appeared near the left edge of the screen until subjects pressed a key indicating that they were ready for the trial to begin. After 1 sec the fixation point was replaced with a small (0.3") indicator digit (or pair of digits) for a further 500 msec, following which the indicator disappeared and the target(s) were displayed. The subjects were asked to read and memorize the indicator number, then count the spots inside the square(s) and respond with a two-button keypad whether the count was equal to or different from the indicator. The indicator number was programmed to be equal to the spot count on half the trials; on the remaining half of the trials, it differed by one from this count. Thus in the normal condition the indicator number was between 1 and 6, and in the accuracy condition it was between 4 and 15. Eye Movement Recording. The eye movements were recorded with a scleral search coil device similar to that described by Collewijn, van der Mark, and Jansen (1975). Because of the principle used, this device produces a signal uncontaminated by head movements. The accuracy of the recording was thus only limited by system noise, and this was always 0.1" or better. Figure 2 shows a typical record. Calibrations were taken at the beginning and end of each block. The reproducibility of these calibrations was within 2% of the full range (8").

Results 1. First Saccades: Landing Site and Latency. When the records were analysed, it was discovered that subjects had not always maintained fixation as well as anticipated. Despite the instructions to maintain fixation at the

SCRUTlNlZATlON AND SACCADIC EYE MOVEMENTS

c

.-c

t-

-

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.-

637

:1 4

2

0

1 .O

0.5

15

Time after target onset in seconds

FIG. 2. Example of an eye movement record. The targets on this trial were three squares at 2". 4". and 6". The first saccade ( S l ) shows global overshoot and is followed by a small corrective saccade (Slc) back to the 2" target. Following this, saccades S2 and S3 take the fixation to the 4" and 6" targets, respectively. Subsequent saccades go back to the first and second targets.

fixation point until the targets appeared, the position at the start of the first saccade was displaced towards the targets on an appreciable number of trials. Usually this displacement was small, but on occasions a more substantial displacement occurred, particularly for two of the subjects. Consequently trials were discarded unless the registered position at the start of the first saccade was within 1" of the fixation point. This resulted in the rejection of some 30% of the data for subjects SC and MV and a few trials for each of the other subjects. Analysis of the start position of the remaining trials shows a rightwards bias in the start position. Table 1 shows the mean start position under each TABLE 1 Mean Position of the Eye for Each Subject and Condition at the Start of the Analysed Trials

Normal

Accuracy

MV

0.64 0.04 0.48 0.36 0.38

0.22 0.21 0.24 0.76 0.62

Mean

0.38

0.42

AH

ZK JF

sc

Note: The measure is in degrees with rightwards displacements scored positive and leftwards displacements scored negative. Trials in which the eyes were initially more than 1" displaced from the fixation point were excluded. Note that in all cases, the mean start position is displaced from the fixation point towards the side on which the targets appear.

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condition for each subject, indicating that on every occasion the fixation bias is rightwards-that is, in the direction towards which the targets will appear. It is known that anticipation of a saccade produces a drift of the eye in the anticipated direction (Kowler & Steinman, 1979), but the magnitude of these drifts is generally small. It seems more likely that the subjects were making anticipatory saccades before the targets appeared and also adopting a slightly eccentric fixation position. Because of these biases, the analyses have been based on the landing position at the end of the saccade, rather than the saccade size. The coil recording method allows an accurate estimate of fixation position (see later) within the limits of the calibration accuracy. Table 2 shows the mean landing point (together with its standard deviation) at the end of the first saccade for each subject in the single-square conditions. Analysis of variance (ANOVA) on the landing position to single squares shows no systematic effects of experimental condition. The variability in the landing positions may be a slight overestimation of the actual variability because of the possibility of small calibration errors (discussed in greater detail in Findlay, 1982), but the idiosyncratic individual variability of saccade gain is greater than could be accounted for by this factor.

TABLE 2 Saccade Landing Points on Trials in which Single Squares Were Presented, Centred at 2". 4", 6", and 8" from the Fixation Point

Normal

Accuracy

2

4

6

8

2

4

6

8

AH

2.17 (0.13)

4.04 (0.40)

6.13 (0.47)

8.27 (0.29)

1.94 (0.18)

3.84 (0.27)

5.88 (0.59)

8.24 (0.23)

ZK

1.81 (0.32)

3.49 (0.36)

5.41 (0.41)

7.03 (0.65)

2.23 (0.30)

3.47 (0.49)

8.05 (0.36)

JF

2.26 (0.23)

4.49 (0.21)

6.46 (0.25)

8.61 (0.64)

1.98 (0.14)

3.84 (0.27)

6.20 (0.60) 6.14 (0.48)

sc

1.87 (0.27)

3.96 (0.35)

6.17 (0.26)

7.72 (0.42)

1.87 (0.27)

4.17 (0.30)

6.25 (0.27)

8.11 (0.43)

MV

2.56 (0.58)

4.13 (0.41)

6.41 (0.43)

7.96 (0.53)

2.45 (0.42)

4.51 (0.09)

6.07 (0.04)

8.52 (0.75)

Mean

2.13 (0.31)

4.02 (0.35)

6.11 (0.36)

7.92 (0.51)

2.09 (0.26)

3.97 (0.28)

6.11 (0.40)

8.26 (0.42)

Subject

8.36 (0.33)

~~~~

~

Note: The values show the mean landing point, in degrees from the fixation point, across the analysed trials (up to eight), together with the standard deviation of this measure in degrees.

0.11

0.29

MV

Mean

-0.11

0.27

0.16 0.37 0.26 0.40 0.14

0.80

0.79 1.03 0.68 0.92 0.61

2+4+6 vs 2

*

0.35

0.75

0.21 0.12 0.31

0.61

0.29 0.47 1 .07 0.58 0.62

2+4+8 vs 2

0.33

0.12 0.75 0.17 0.26

0.46

0.53 0.54 0.35 0.23 0.63

4+6 vs4

0.64

0.10

0.30

0.22

8

0.38

-0.02 0.68 -0.18 0.13 0.48

0.26 0.39 0.17

0.60 0.63 0.65 0.52 0.82

4+6+8 vs 4

0.00 -0.12 0.14 0.20 0.27

4+8 vs4

0.10 -0.60 0.47 0.03 0.16 0.03

0.27

0.18

0.52

0.15 0.62 -0.03 0.13 0.48

0.13 0.08 0.52 0.07 0.11

6+8 vs6

0.55 0.30 0.72 0.45 0.56

4+6+10 vs4

Note: The individual values give the difference in degrees between the mean landing position to multiple configurations and that to single squares, with the square in the same position as the left-hand square of the multiple targets. Positive values indicate that the saccades to multiple squares landed to the right of those to single squares (the global effect), and negative values indicate that the multiple-square saccades land to the left of single-square saccades.

0.06

-0.02 0.65

sc

AH ZK JF

0.21 0.13 0.13 0.52 0.34

Accuracy condition

-0.06 -0.27 -0.12 -0.11 0.00

Mean

-0.04 -0.10

0.61

0.49

0.54

MV

sc

AH ZK JF

0.02 0.32 0.95 1.57 0.21

2+8 vs2

0.13 0.90 0.76 0.21 0.47

2+6 vs2

0.26 0.81 0.81 0.36 0.48

Normal condition

2+4 vs 2

TABLE 3 First Saccades to Multiple Target Configurations

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Table 3 compares the amplitudes for double and triple target configurations with those to single targets. The measure shows the difference between the landing position of saccades to the particular configuration and that of saccades to control single squares in the first position. This difference can be interpreted as a measure of the influence exerted by the more distant square(s) in the display on the first saccade, assuming that, at some level, this saccade could be described as being aimed at the nearest square. The results in Table 3 show that the global effect occurs throughout in the normal condition (all but two of the cells are positive). The average magnitude of the global effect overshoot is 0.49". In the scrutinization condition, the global effect is reduced overall to a magnitude of 0.21". An ANOVA shows that the overshoot is different in the two conditions, F( 1, 4) = 19.44, p < 0.01. The reduction in the accuracy condition is particularly marked in cases where the target elements are widely separated. For example, Figure 3 shows the magnitude of the global effect in all the double square configurations with one target at 2". The global effect magnitude shows a decline across the three values of the accuracy condition, F(2, 12) = 3.68, p = 0.05. The 2"+8" configuration is interesting in suggesting a negative effect-that is, the presence of the distant target results in a small reduction in saccade size. Four of the five individuals show this reduction; for the fifth subject the 8" target does not show any effect. The latencies of the first saccades were identical in the two conditions (overall mean latency in the normal condition was 13S+S msec and in the accuracy condition 1355~4msec. These short latencies may be attributed to the fact that the trials were self-initiated and temporally uniform,

Normal

b

.

-0.2

3

1

4

,

,

5

. , . , . , 6

7

8

.

I

9

Second square position, dogs

FIG. 3. The global effect on 2 square trials with the near stimulus at 2". The graph shows the mean overshoot (or undershoot) produced when the far stimulus is in different positions.

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2 . Corrective Saccades. Table 4 shows the proportion of first saccades that were followed by a detectable corrective saccade. This measure was made by eye from the displayed eye record. In the great majority of cases, the corrective saccades were small (less than lo).In about 1% of cases, a more substantial secondary saccade occurred, often because the first saccade fell well short of the target; thus a staircase pattern was generated. For single-target configurations, each subject shows a greater tendency to make corrective saccades in the accuracy condition. Table 2 shows that the first saccade amplitudes were identical in each condition, and so the difference between conditions in corrective saccade frequency must be attributed to a genuine task difference. For multiple target configurations, no clear picture emerges. It is possible that a second competing factor enters into play here. Because of the larger global effect, first saccades to multiple targets land further from the target in the normal condition, which will render a corrective saccade more likely.

3. Scanning Patterns. In the great majority of cases, saccades could clearly be identified which took the gaze to each target presented (cf. Figure 2), although frequently with a small corrective saccade intervening. Again in the great majority of cases, the sequence of fixating the targets was in left-to-right order. However, in a few cases (2 for subject SC, 4 for JF, and 5 for MV), a scanning pattern occurred in which the targets were not fixated in the left-right order. The presence of corrective saccades, together with the occasional non-sequential patterns, demonstrates that the motor program for scanning is not rigid and can be overridden opportunistically. 4. Saccades to the Second Target. Table 5 shows the mean landing position and its standard deviation for saccades to the second target in the TABLE 4 Proportion of Trials in Which a Corrective Saccade Followed the Initial Saccade for Each Subject and Condition

Single Targets

Multiple Targets

Normal

Accuracy

Normal

Accuracy

MV

25 16 25 24 31

66 34 34 34 64

17 52 30 5 9

24 40 27 22 44

Mean

24

47

23

32

AH ZK JF

sc

4.39 (0.24)

JF

6.30

6.80 (0.54)

5.73 (0.u))

6.85 (0.23)

8.54

8.71 (0.57)

8.31 (0.23)

9.02 (0.36)

7.97 (0.56)

8.69 (0.33)

2+8

6.40

6.65 (0.76)

6.18 (0.35)

6.30 (0.14)

6.05 (0.57)

6.81 (0.23)

4+6

8.66

8.90 (0.28)

8.20 (0.19)

9.20 (0.28)

8.16 (0.44)

8.85 (0.11)

4+8

8.89

9.31 (0.33)

*

8.85 (0.12)

8.28 (0.24)

9.14 (0.27)

6+8

4.35

6.68

7.17 (0.60)

(0.20)

(0.28) 5.00 (0.23)

6.48 (0.35) 6.70

6.53 (0.33)

6.51 (0.51)

2+6

4.25 (0.44) 4.10

4.25 (0.26)

4.13 (0.25)

2+4

The values give mean landing position in degrees, together with the standard deviation.

4.34

Mean

Note:

4.91 (0.25)

MV

3.85 (0.14)

4.12 (0.25)

ZK

sc

6.49 (0.23)

4.45 (0.37)

AH

5,65 (0.53)

2+6

2+4

Normal Condition

8.52

8.48 (0.43)

(0.52)

8.69 (0.24) 8.59

8.35 (0.33)

8.51 (0.74)

2+8

6.72

7.16 (0.38)

(0.19)

6.41 (0.15) 6.81

6.58 (0.36)

6.66 (0.12)

4+6

Accuracy Condition

8.74

8.76 (0.43)

(0.26)

8.79 (0.34) 9.09

8.53 (0.39)

8.55 (0.19)

4+8

8.75

9.28 (0.16)

(0.21)

8.47 (0.18) 9.05

8.51 (0.41)

8.42 (0.23)

6+8

TABLE 5 Mean Landing Position of Saccades That Take the Gaze to the Second Square Position for Two-Square Configurations

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cases of two-square trials where this target was the final one to be fixated. In contrast to the data in Table 2, the saccades in this table are generated as the second saccade or (where a corrective saccade has occurred) the third saccade in the scanning sequence. A consistent difference in the amplitude between the two cases is evident. For each target location, saccades in Table 2 are smaller by about 8% than those in Table 5. It appears that the immediate target-elicited saccades are reasonably accurate, whereas those generated during scanning overshoot the target. No differences in saccade precision (as measured by the standard deviation of landing positions) are evident. Table 6 shows the difference in amplitude between saccades to the second square in three-square conditions and those in the equivalent twosquare condition with no third square. It is thus comparable with Table 3 in showing the perturbing influence of the third square. Table 6 also shows for each subject and condition the time elapsing between the appearance of the squares and the saccade that took the eye to the second square. In all subjects, the scanning saccades to the second target occur at a later point in time in the accuracy condition than in the normal condition, indicating that longer was spent in fixating the first target square. In contrast to the systematic global effect overshoot seen in the normal condition of Table 3, the differences shown in Table 6 do not show a consistent global effect in either the normal condition or the accuracy condition.

DISCUSSION Three aspects of oculomotor behaviour have been shown to differentiate normal scanning from scrutinizing. (1) Saccades to a target appearing in the visual field were affected differently by the simultaneous appearance of neighbouring targets. (2) For single targets, corrective saccades were more likely to occur under conditions of scrutiny. (3) Fixation durations were longer under conditions of scrutiny, although the latency of the first saccade was not different. The first finding is of particular interest, because it provides a link between the processing of visual material and the voluntary direction of the eyes. The global effect shows that, for target elicited saccades, the region over which stimulus information can influence the saccade is remarkably large. This is probably due to the population coding employed in the oculomotor generation process (Findlay, 1987). However, the present results show that this region can be modulated by the intention to scrutinize. The voluntary direction of attention in the absence of eye movements has formed a very active research area in recent years. Posner (1980)

0.00

0.07

Mean

0.35

0.68

0.51

0.53

-0.11

0.14

4+6+8 vs4t6

-0.01

-0.11

8

0.11

-0.06

0.02

4+6+10 vs4+6

438

389

377

420

545

459

Latency msec ~~~~~~~~

0.04

-0.14

-0.03

0.16

-0.11

0.30

2+4+6 vs2+4

0.05

0.01

-0.07

-0.02

0.13

0.20

2+4+8 vs2+4

-0.01

-0.16

-0.34

-0.20

-0.61

-0.02

0.11 -0.08

-0.35

-0.02

4+6+10 vs4+6

-0.33

-0.13

4+6+8 vs4+6

Accuracy Condition

695

529

497

562

1019

869

Latency msec

Note: The measure is identical with that used in Table 3 and shows the difference in degrees between the saccades in the three-square configuration and those in the two-square configuration with squares in the same leftmost two positions. Positive values indicate a rightward displacement. The final column for each condition shows the mean time between the onset of the targets and the occurrence of these saccades.

-0.26

-0.18

MV

0.36

-0.28

0.45

0.27

0.07

sc

-0.34

ZK

0.11

2+4+8 vs2+4

JF

0.15

AH

2+4+6 vs2+4

Normal Condition

TABLE 6 Mean Landing Position of Saccades That Take the Eye to the Second Square in the Three-Square Configurations

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introduced the metaphor of a searchlight, selecting an area of the visual field for preferential processing. Selection of an area of the visual field is also a necessary component of the programming for a voluntary saccadic eye movement, and it seems likely that there is a relationship between the two selection processes. The reading task provides an instructive illustration. When a saccade is made in normal reading, it is centrally programmed to move in a rightwards direction along the line of text. Nevertheless, the exact size of the saccade depends on the pattern of visual input (McConkie, Kerr, Reddix, & Zola, 1988). The effect may be modelled by assuming that the eye is directed by the stimulus information within some attentional window (Vitu, 1991). This work on attention in the absence of eye movements has emphasized that an attentional beam can be directed to different spatial regions. More recently, it has been suggested that the size of the hypothetical beam is variable, and the metaphor of a zoom lens has been suggested (Eriksen & St James, 1986; Laberge & Brown, 1989). Our results may be interpreted in terms of an attentional window of variable size if it is assumed that only material within the attentional window is involved in the weighting operation when a saccade is programmed. Scrutiny would then correspond to a narrowing of the beamwidth of the window, or “zooming in”. In the scrutiny condition there appears to be a graded change in effectiveness of the far stimulus of a stimulus pair as its distance from the near stimulus is increased, as shown in Figure 3. One interpretation of this result is that the window has a graded profile at the edges, rather than a sharp one. There are, however, alternative possibilities, such as a window whose size varies from trial to trial or a window whose size shrinks gradually during the course of a trial, so that more distant stimuli fall outside the window faster than do closer ones. A particularly intriguing result is the consistent small negative perturbing effect that emerges for a stimulus at 8” when paired with a 2” one. This suggests that, rather than having no influence, stimuli just outside the attentional focus generate some form of inhibitory signal in the programming of saccade amplitudes. The second finding of the study was that corrective saccades were more likely to occur under conditions of scrutiny. This result does not seem to have been previously reported. The study of corrective saccades has generally been restricted to oculomotor researchers who have shown, for example, that the probability of a corrective saccade occurring depends largely on the error signal remaining after the first saccadic movement (Becker, 1989). Work in this tradition has only recently begun to be linked with more cognitively oriented research. The difference in the number of corrective saccades between the condition was found in all subjects when single targets were presented. Such a systematic difference was not found with multiple targets. However, for multiple targets there is a confounding

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factor, as, because of the increased global effect in the normal condition, the first saccades to multiple targets in this condition land further from the near target than in the scrutiny condition. The probability of a corrective saccade occurring increases as the error at the end of the primary saccade becomes larger (Becker, 1989). The third finding was that fixations are generally longer under conditions of scrutiny. This is in line with previous findings and with intuitive considerations about speed and accuracy. Much work has shown that the duration of fixations in scanning is directly influenced by the cognitive load imposed by the scanned item (Gould, 1973; Just & Carpenter, 1988). The greater perceptual demands of the accuracy task result in longer fixations on the items, although the exact magnitude of the increase differs considerably between the subjects tested.

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Manuscript received 27 June 1992

Scrutinization, spatial attention, and the spatial programming of saccadic eye movements.

Results are presented from an experiment in which subjects' eye movements were recorded while they carried out two visual tasks with similar material...
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