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Characteristics of Saccades and Vergence in Two Kinds of Sequential Looking Tasks

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Characteristics of Saccades and Vergence in Two Kinds of Sequential Looking Tasks Vision Research 40 (2000) 2083–2090 www.elsevier.com/locate/visres Rapid communication Characteristics of saccades and vergence in two kinds of sequential looking tasks Ilya V. Malinov a,*, Julie Epelboim b, Andrew N. Herst a, Robert M. Steinman a a Department of Psychology, Uni6ersity of Maryland, College Park, MD 20742-4411, USA b Center for Study of Language and Information, Stanford Uni6ersity, Stanford, CA 94305-4415, USA Received 23 August 1999; received in revised form 29 December 1999 Abstract We determined how saccades were used in the experiments described in Epelboim, Steinman, Kowler, Edwards, Pizlo, Erkelens and Collewijn (1995) [Vision Research, 35, 3401–3422], where unrestrained subjects looked at or tapped nearby targets. We report: (i) the size of binocular saccades; (ii) how well saccade size matched in the two eyes; and (iii) saccadic vergence. A representative sample (3375 saccades) was measured: 83% were B15°, 53% were B5°. Only two were ‘microsaccades’. Saccade sizes were very similar in the two eyes. These results imply that subjects prefer avoiding large saccades. They can do this simply by re-orienting the head appropriately. Subjects under-verged by 25–35% and preformed well. None experienced diplopia. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Saccade; Vergence; Saccade symmetry; Vergence distance; Vergence angle 1. Introduction the extent to which head movements were used to shift gaze, eliminating a need to make large saccades. Also, Saccades, the voluntary, rapid eye movements used note that ‘‘microsaccades were omitted since the noise to shift gaze, have a wide dynamic range. They can be inherent in the EOG technique made it difficult to as small as 5% and as large as 100° with peak velocities measure saccades of one degree or less’’ (p. 468). ranging from 3 to 600 deg/s. Even the smallest saccades Interest in the control of gaze during natural tasks (B12%), called ‘microsaccades’, are under voluntary has increased recently, but the contribution of saccades control (Haddad & Steinman, 1973; Steinman, Haddad, to gaze-shifts has not been treated quantitatively in this Skavenski & Wyman, 1973). How are saccades, which work. For example, Land, Mennie and Rusted (1998) can vary by two log units with respect to both size and used a head-mounted video camera to observe the speed, used in everyday life? The first, and only, at- approximate direction of gaze as subjects made a cup of tempt to answer this question known to the authors tea, and when they constructed a peanut butter sand- was made by Bahill, Adler and Stark (1975), who wich (Hayhoe & Land, 1999). Both reports emphasized reported that ‘‘most naturally occurring human sac- the temporal relationship between where one looked cades have magnitudes of 15° or less’’ (p. 468). For and what one did. Steinman, Epelboim, Forofonova these authors ‘naturally occurring saccades’ referred to and Bogacz (1998) used the Maryland Revolving Field saccades human subjects made as they walked around Monitor (MRFM) to make accurate measurements of the campus of the University of California at Berkeley. binocular gaze control while a subject performed tasks Note that they measured saccades with respect to the drawn from the everyday life of human beings and their head, which means that there was no way of knowing ancestors, viz., use a stick to pick honey out of a comb, cut meat with a stone knife, fashion an obsidian arrow- * Corresponding author. head and assemble a Barbie doll from component parts. 0042-6989/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0042-6989(00)00063-8 2084 I.V. Malino6 et al. / Vision Research 40 (2000) 2083–2090 This report emphasized the control of vergence and the duced a spatially homogeneous magnetic field through- accuracy of cyclopean gaze under natural conditions. out a large fraction (1m3) of the cube’s volume. The nature of saccades, as such, was not described. SKALAR-DELFT sensor-coils measured horizontal The present paper begins to fill this gap. It describes: and vertical eye-angles. Head roll-, pitch- and yaw-an- (i) the size of binocular saccades; (ii) how well saccades gles were measured with two orthogonal coils mounted size matched in the two eyes; and (iii) saccadic vergence on the head with velcro straps. (2) A sparker tracking under naturalistic conditions. subsystem measured 3-D translations of the head by detecting the arrival time of acoustic signals generated by a ‘sparker’ mounted on the head: Translation preci- 2. Methods sion=0.2 mm, accuracy 1 mm. (3) A worktable (91 cm wide×67 cm deep×72 cm high) provided a plat- 2.1. Database form for the targets. Its surface contained an 11×14 grid of 154 wells separated by 4.5 cm. Rods, inserted This paper reports the results of additional analyses into these wells, were topped with different colored of data collected with the Maryland Revolving Field LEDs that served as targets. Targets were turned on Monitor (MRFM) that led to three publications, viz., before the trial began, and remained stationary and Epelboim, Steinman, Kowler, Edwards, Pizlo, Erkelens visible throughout the trial in a well illuminated room. et al. (1995), Epelboim, Steinman, Kowler, Edwards, Pizlo, Erkelens et al. (1997) and Epelboim (1998). These 2.4. Calibrations publications dealt with the control of gaze (the line-of- sight in spatial coordinates). They described: (i) visual The phase-detection principle used in the MRFM search, gaze-shift accuracy and the function of gaze- allows absolute calibration of angles. Two calibrations shifts; (ii) gaze-shift dynamics; and (iii) gaze and reti- were performed at the start of each session with the nal-image-stability in two kinds of sequential looking head in a known position on a biteboard to determine tasks performed under naturalistic conditions. Tasks the zero-reference for each sensor coil because their were naturalistic in the sense that the head and torso position varies across sessions. Each sensor-coils’ orien- were free to move. tation, relative to the line-of-sight, was determined when the subject fixated each eye’s pupil (seen monocu- 2.2. Sampling procedure larly) in a mirror parallel to the back edge of the worktable. The head coils’ zero-references were deter- A representative sample of binocular eye/head move- mined when the XYZ head-position was indicated by ment trials described in Epelboim et al. (1995) was the sparker with the head on a biteboard. Two addi- analyzed. Trials were randomly-sampled within subjects tional calibrations were performed once: (i) sparkers of (four), tasks (two), number of targets (three) and repli- different heights, placed in 18 locations on the work- cations (ten). The two tasks were: (i) look at a sequence table, calibrated ‘sparker-space’ and (ii) the sighting- of 2, 4 or 6 targets (LOOK) or (ii) tap a similar centers of each subject’s eyes were measured sequence (TAP). The angular separation of targets was psychophysically with the head on a bite-board. random, varying between about 1.5° and 35° of visual angle. The distance from the subjects’ eyes to the 2.5. Saccade measurement and analysis targets varied from about 50 to 90 cm, depending on where the targets were and how much each seated Saccades were detected manually with a mouse on a subject moved. graphic display of the data. Why? We anticipated mea- suring saccade amplitudes ranging from 5% to 100°. We 2.3. Apparatus had saccade-detecting algorithms, but they could not cover the entire range efficiently. With parameters set The MRFM, which records binocular gaze accu- to detect microsaccades, fast drifts would be flagged. rately with the head and torso free, was described in Set for large saccades, microsaccades would be missed. Edwards, Pizlo, Erkelens, Collewijn, Epelboim, Kowler Once the range of saccade sizes and peak velocities is et al. (1994) and summarized in the papers just cited. large, visual inspection of all of the algorithm’s detec- Only specifications will be provided. The MRFM con- tions becomes necessary. We chose to sample represen- sists of three subsystems: (1) A revolving-magnetic-field tative trials, rather than use the algorithm on all monitor/sensor coil subsystem using phase-detection to records, and then be forced to separate wheat from measure angular eye and head positions (angle preci- chaff by visual inspection. sion=1%, linearity=0.01%). Data were acquired at 976 Saccades were detected by three experienced individ- Hz, successive sample-pairs averaged and stored at 488 uals, all using the same criteria for estimating the onset Hz. Cube-surface field coils (2.14 m on a side), pro- and offset of the saccades. These criteria were estab- I.V. Malino6 et al. / Vision Research 40 (2000) 2083–2090 2085 Fig. 1. Examples of saccades of various sizes as recorded with the MRFM. Top: A representative 3 s interval in an eye movement record. It shows the POSITION of the right eye on the horizontal meridian as a function of TIME. Four large saccades can be seen. The first, 0.2 s into the record was about 18°, the second, one second into the record, was about 6°, the third, 2 s into the record was about 32°, and the last, about 3 s into the record, was about 25°. The smaller saccade circled about 0.5 s into the record is shown magnified on the bottom left. It was about 31%. One of the two microsaccades found in our sample is shown on the bottom right, where the POSITION of one eye on the vertical meridian is plotted against TIME. The vertical component of this microsaccade was about 7%. It is clearly visible against the 91% bit-noise-level of the recording instrument.
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