Methods of Psychological Research Online 2000, Vol.5, No.4
Institute for Science Education
Internet: http://www.mpr-online.de
© 2000 IPN Kiel
DEVAT : Design of Experiments for Visual Attention Tasks Jesus Mª Alvarado12 and Carmen Santisteban12
ABSTRACT We describe here a program for the Design of Experiments for Visual Attention Tasks (DEVAT). This program incorporates an extended version of the Eriksen response-competition paradigm. It allows the operator to make experiments with visual stimuli where a target letter is presented surrounded by noise letters in two-dimensional configurations. Reaction times and error rates are measured and then compared with the predictions made by such one- or two-dimensional visual attention theories as the CTVA and CTVA-2D models. The program is written in C++ and runs in the DOS operating system on IBM-PC compatibles with a 486 or later processor. An example with some results provided by DEVAT is given and a demo running in WINDOWS is also included.
Keywords: visual attention; spatial models; object-based models; program, C++.
1
Unidad de Resonancia Magnética Nuclear, Universidad Complutense de Madrid, Paseo Juan XXIII, 1,
28040 Madrid, Spain 2
Departamento de Metodología de las Ciencias del Comportamiento, Facultad de Psicología, Universidad
Complutense de Madrid, Campus de Somosaguas, 28223 Madrid, Spain. Tel: 34 91 394 32 57, Fax: 34 91 394 32 45, E-mail:
[email protected],
[email protected].
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INTRODUCTION The Theory of Visual Attention (TVA; Bundesen, 1990) is a general theory of selec-
tive visual attention in which both visual recognition and the selection of elements in the visual field depend upon making perceptual categorisations. The COntour DEtector theory (CODE; van Oeffelen and Vos, 1982, 1983) is a theory of perceptual grouping by proximity. The CODE Theory of Visual Attention (CTVA; Logan, 1996) combines both these theories and explains a wide range of spatial effects in visual attention. Logan (1996) applied CTVA to a study into how perceptual grouping and spatial distance between items may affect reaction times (RT) and error rates in visual attention tasks. Among other things his findings included the effects of distance between target and distractors in the flankers task (Eriksen and Eriksen, 1974). We have recently extended the flankers task from linear stimulus arrays to 2-dimensional configurations (Alvarado, Santalla and Santisteban, 1998b). We have also extended CTVA to two dimensions, the so-called CTVA-2D model, in which we have assumed that the location of each element in the display can be represented by a 2D Laplace distribution (Alvarado, Santalla and Santisteban, 1999; Santisteban, Alvarado and Cortijo, 2000). The standard Eriksen paradigm is one of distractors flanking the target letter within the same horizontal dimension, a scheme since used with frequency (Eriksen and Eriksen, 1974; Eriksen, 1995; Logan, 1996). The incorporation of an extra number of distractor letters upon different axes with respect to the target letter leads to the possibility of extending the application of the flanker paradigm (Alvarado, Santalla and Santisteban, 1998a). We have therefore written a computer program in C++ which has many potential applications within a wide range of experimental conditions. We call this program Design of Experiments for Visual Attention Tasks (DEVAT). It allows the operator to make experiments with visual stimuli where a target letter is presented surrounded by two, four or eight distractor letters (Alvarado, Santalla and Santisteban, 1998b, 1999; Santisteban, Alvarado and Cortijo, 2000). Among its possibilities, the DEVAT program allows the researcher to measure RT whilst avoiding the accumulated errors produced by the hidden controls of the WINDOWS environment. We include a demo program in WINDOWS for illustrative purposes.
Alvarado & Santisteban: DEVAT
2.
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CHARACTERISTICS OF THE PROGRAM The DEVAT program is given in Appendix A. It allows the operator to manipulate
and evaluate the effects of the following variables: 1. The compatibility between the target letter and the distractors (noise letters). This variable evaluates the degree of interference upon the subjects responses of compatible and incompatible distractor letters compared to neutral ones. The responsecompatibility effect can also be evaluated. 2. The distance between the target letter and the distractors (T-N distance). This variable evaluates the effects of the distractor letters according to their distance from the target letter, and when examined alongside the response-compatibility effect leads to a measurement of the dimensions of the so-called attentional window. 3. The number and location of the distractor letters; by manipulating the display size the operator can present the subject with different configurations of stimuli and the effects of these changes upon the reaction times can be compared for each individual or group of subjects (Alvarado, Santalla and Santisteban, 1998a). 4. Stimulus-onset asynchrony (SOA), which represents the time elapsed between the presentation of the target and distractors on the display. In the basic version of the program, which we describe here, the stimuli are letters. The target letter is right type (b or p) or left type (d or q) and it always appears in the centre of the screen, surrounded by two, four or eight distractor letters, giving rise to configurations such as those shown in Figure 1. As far as the form of the distractor letter is concerned, the trial can be classified into the following groups: 1. Compatible response, in which the form of the distractor letters are the same type as the target letter; for example, the distractors for a target letter p or b are either all p or all b. 2. Incompatible response, in which the distractor letters are of the opposite type from the target letter; for example, the distractors for a target letter p or b are all q or all d. 3. Neutral response, in which the distractors bear no resemblance to the target letters; for example, the distractors letters are x or z.
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z b
q
b
z
p
d z
d
p
d
z
d
d
d
d d
Figure 1. Examples of different display sizes Each separate trial begins with the appearance on the screen of a fixation point (+) at the geometrical centre of an imaginary circumference, upon which the subject is asked to focus; it is here that the target letter will subsequently appear. This focal point is shown for a given time (usually 1000 ms), after which the trial configuration is shown. If the subject identifies the target correctly he or she initiates the next trial merely by pressing the space bar. If the response is incorrect or the subject presses a key unrelated to the task an audible signal (500 MHz buzz) indicates that a mistake has been made. The buzz disappears when the subject presses the space bar to begin the next trial. Output. An ASCII data file is opened for each subject and saved as DEVAT.TXT. All data pertaining to the subject performance in each trial are stored in a row. Several examples of such data can be seen in Figure 2. The codes used for the variables appearing in the consecutive columns are: 1. Target letter: 0 = q, 1 = d, 2 = p, 3 = b. 2. Distractors: 0 = q, 1 = d, 2 = p, 3 = b, 4 = x, 5 = z. 3. Display size: 0 ,1 or 2 for display sizes equal to 3, 5 or 9 respectively 4. T-N distance: 1 = 0.97º, 2 = 1.95º, 3 = 2.92º, and 4 = 3.89º (with the subject at 59 cm in front of the display). 5. SOA: 0, 1, 2 or 3 for 0, 17, 33 or 50 ms respectively. 6. Accuracy: 0 = correct response, 1 = incorrect response, 5 = mistake. 7. RT: Reaction times measured in ms.
Alvarado & Santisteban: DEVAT
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1 1 0 1 1 0 423
(trial 1)
0 1 2 4 3 0 430
(trial 2)
3 5 1 2 2 1 200
(trial 3)
2 0 2 3 1 0 390
(trial 4)
2 4 0 2 3 0 410
(trial 5)
Figure 2. Example of a DEVAT output file. Hardware and software requirements. DEVAT runs in an IBM-PC-compatible DOS environment (486 or later processor). This program must be run in native DOS, not in a DOS window because of the likelihood of interference between the system and the WINDOWS interrupts. The keyboard is normally used as response device although a mouse may be used to achieve better accuracy in timing. Both devices have a mean delay, Xd ! its random error. This error is ! 7.5 ms for the keyboard (the precise value of Xd depends on the particular hardware used but in general it is always less than 15 ms and remains constant for any particular computer). Segalowitz and Graves (1990) pointed out that the keyboard is a suitable response device for a reaction-time task when the stimuli are presented on the same machine and the number of trials or subjects is high, because the standard errors are then negligible. It can be seen in Table 1 that the standard RT errors are always greater than errors due to keyboard use. Bearing in mind a reviewer's comment, we have also introduced the alternative of using a mouse instead of the keyboard. The mouse is in fact considerably more precise if the mouse ball is steady (Xd !2 ms) but much less so if it moves (Xd !15 ms). We conclude therefore that if a mouse is used it should remain stationary.
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Table 1. Means (RT) and standard deviations (SD) in the reaction times (ms) and percentage of incorrect responses (IR) obtained experimentally.
T-N
Display
Distances Size
0.97º
1.95º
2.92º
3.89º
3.
Compatibility of noise letters Compatible Incompatible RT (SD) RT (SD)
Neutral RT (SD)
IR(%)
IR(%)
IR(%)
3
440
(48)
0.6
473
(39)
3.1
431
(27)
1.3
5
437
(34)
0.0
467
(32)
1.3
436
(29)
0.6
9
430
(31)
1.3
488
(36)
3.8
435
(24)
1.3
3
427
(40)
1.3
447
(39)
4.4
429
(39)
1.9
5
418
(33)
1.3
432
(35)
1.9
427
(41)
0.0
9
423
(33)
0.6
446
(20)
3.1
434
(38)
1.9
3
431
(40)
2.5
431
(33)
2.5
423
(26)
0.6
5
423
(38)
0.0
431
(40)
1.9
424
(31)
1.3
9
415
(33)
1.3
439
(35)
1.3
427
(34)
0.6
3
434
(36)
0.0
422
(41)
1.3
422
(37)
0.0
5
414
(31)
0.6
427
(33)
0.6
432
(45)
1.3
9
419
(29)
1.3
428
(29)
1.3
420
(30)
1.3
EXAMPLE We describe here an application of the DEVAT program to a particular case. Subjects. Eight trained subjects participated as volunteers. All right-handed and had
normal or corrected-to-normal vision. Apparatus and stimuli. A standard personal computer with a colour monitor and SVGA card was used to present the visual stimuli and collect the subjects' responses. Stimulus presentation and response recording were controlled by the DEVAT program. The subjects performed the task in a soundproof room with their heads resting on a chin-rest. Viewing distance was 59 cm. They responded via the computer keyboard. The
Alvarado & Santisteban: DEVAT
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time of day, illumination and other environmental conditions were kept constant. The subjects were instructed to respond only to the target letter that appeared in the centre of the display and to respond as quickly as possible whilst avoiding errors. They were also required to keep their eyes on the fixation point. The subjects' task was to identify the target letter that appeared in the centre of the screen surrounded by distractor letters. The subject had to respond by pressing the left cursor (") if the target was q or d or the right cursor (#) if it was p or b. These four target letters appeared at random with equal probability. The noise letters were also presented at random. The target and noise letters appeared simultaneously. The letters x and z were used to establish a non-associative condition because they bear neither morphological nor phonetic similarity to the target letters (neutral). Variables and design. Three independent variables were considered: 1. Compatibility of noise and target letters with three levels: compatible, incompatible and neutral conditions. 2. Display sizes with three, five or nine letters, depending upon the number of noise letters displayed. 3. Target-noise (T-N) distances of 0.97º, 1.95º, 2.92º and 3.89º. All distances were defined between the centres of the letters. The dependent variables measured were: RT and accuracy (i.e. correct or incorrect responses, and the mistake of pressing a different key from the two indicated above). A factorial design of repeated measurements was used. Each subject was presented with the 36 displays obtained by combining the level of the three independent variables (3x3x4) considered. Each display was presented 20 times to each subject in one experimental session. The order of presentation of the 720 (20x36) trials was randomized. There were 8 subjects and thus the total number of trials was 5,760. Experimental results. Mean RT and the number of mistakes and incorrect responses were computed separately for each of the 36 displays for each participant. Only correct trials with RT of less than 1000 ms were included in the analysis (1.81% were discarded). The mean data obtained for 8 subjects under all the conditions considered are shown in Table 1.
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ACKNOWLEDGEMENTS This research was supported by the Complutense University of Madrid (Grant No. PR 156/97-7193). We are grateful to Professor M. Cortijo for his helpful comments on the article and Dr. J. Trout for revising our English text.
REFERENCES [1] Alvarado, J.Mª, Santalla, Z., and Santisteban, C. (1998a). Competición entre respuestas bajo distintas configuraciones estimulares en relación con el SOA y la distancia in La Atención: Un enfoque pluridisciplinar.( J. Botella and V. Ponsoda, Eds.), pp. 107-118. Promolibro. Valencia. [2] Alvarado, J.Mª., Santalla, Z., and Santisteban, C. (1998b). Efecto de la segregación sobre el procesamiento de la estimulación visual. Psicologica, 19, 87-105. [3] Alvarado, J.Mª, Santalla, Z., and Santisteban, C. (1999). An evaluation of the CODE Theory of Visual Attention extended to two dimensions. Acta Psychologica, 103, 239-255. [4] Bundesen, C. (1990). A theory of visual attention. Psychological Review, 97, 523547. [5] Eriksen, B.A., and Eriksen, C.W. (1974). Effects of noise letters upon the identification of a target letter in a nonsearch task. Perception & Psychophysics, 16, 143149. [6] Eriksen, C.W. (1995). The flankers task and response competition: A useful tool for investigating a variety of cognitive problems. in: Visual Selective Attention. (C. Bundesen and H. Shibuya, Eds.), pp.101-118. Lawrence Erlbaum. Hillsdale. [7] Logan, G.D. (1996). The CODE theory of visual attention: An integration of space-based and object-based attention. Psychological Review, 103, 603-649. [8] Segalowitz, S.J. y Graves, R.E. (1990). Suitability of the IBM XT, AT, and PS/2 keyboard, mouse, and game port as response devices in reaction time paradigms. Behavior Research Methods, Instruments, & Computers, 22, 283-289. [9] Santisteban, C. Alvarado, J.M. and Cortijo, M. (2000). A quantitative correction in the CODE Theory of Visual Attention in two dimensions improves the fitting to
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experimental results at small target-letter to noise-letter distances (manuscript submitted for publication). [10] Van Oeffelen, M.P., and Vos, P.G. (1982). Configurational effects on the enumeration of dots: Counting by groups. Memory & Cognition, 10, 396-404. [11] Van Oeffelen, M.P., & Vos, P.G. (1983). An algorithm for pattern description on the level of relative proximity. Pattern Recognition, 16, 341-348.
APPENDIX A BASIC VERSION OF DEVAT PROGRAM in C++ #include #include #include #include #include #include #include #include #include
#define MOUSE 0x33 #define pi 3.14159 /*declare the functions */ int initialize(void);/*function to start the graphics mode */ int time(void); /*function to measure RT */ int displays(void); /*function to show the visual displays */ union REGS regs; int mouse = 0;
/*mouse = 0 when keyboard is used and mouse = 1 when mouse is used */ int size = 2; /*size of the letters in the displays */ int target, distractor; /*element in the visual displays */ int distance, display, soa, rt; /*variables */ int key, m_key; int xasp, yasp;
/*keys */ /*aspect ratio: the screen */
to
make
sure
that
circles
are
int main(void) { int trial, error; FILE *fq; fq=fopen("devat.txt","wt"); /*output file */ initialize(); /*to start the graphics mode */ getaspectratio(&xasp, &yasp); setaspectratio(xasp,yasp); settextstyle(0,0,size); /*size of letter = 2: 16x16 pixels */ /* to start the mouse */ if (mouse==1) regs.x.ax=3; int86(MOUSE, ®s, ®s);
OK
on
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MPR-Online 2000, Vol. 5, No. 4 for (trial=0;trial1) {sound(500); error =1;} break; } case 2: /*pressing the right-arrow key */ {if (target
