Ординатура / Офтальмология / Английские материалы / Eye Movements A Window on Mind and Brain_Van Gompel_2007

requiring manipulation of objects, such as 500 ms while making a pot of tea (Land, Mennie, & Rusted, 1999) and 450 ms while manipulating small parts to construct a plastic model (Pelz et al., 2000), though in both cases, the mean duration was affected by a small number of very long fixations, some lasting several seconds.

The mean extent of saccades is also task dependent. Rayner (1984) reported a mean saccade size of 1 5 during oral reading, increasing to 2 for silent reading (though measures scale with the font size). While free-viewing images on a small display, Henderson & Hollingworth (1998) reported average saccade sizes of 2 4 . Pelz et al. (2000) reported that the mean scaled with display size, up to an average of 10 for a 50” display subtending about 50 . Task-dependence extends to active tasks as well. Turano and colleagues reported mean sizes ranging from 3.1 to 5 6 for subjects walking down a hallway with the goal of locating a specific doorway (Turano, Geruschat, & Baker, 2003); Pelz & Canosa (2001) reported a mean saccade size of 11 for subjects walking down a hallway without such a specific target. Mean saccade sizes as large as 19 have been reported for complex tasks such as making a pot of tea (Land et al., 1999), though again the mean was affected by a relatively small number of very large gaze changes.

Andrews & Coppola (1999) examined the degree to which fixation duration and saccade size varied by observer and task while viewing five “visual environments”. Subjects’ eye movements were recorded in a range of conditions; in the drak, viewing repetitive textures, viewing photographs, during visual search, and while reading. They found idiosyncratic differences between individuals that covaried significantly within two groupings; 1) viewing photographs, simple patterns, and in the dark, and 2) visual search and reading. There was no significant covariance between the two groupings.

The distributions of fixation durations and the saccade magnitude represent low-level characteristics of subjects’ oculomotor behavior; monitoring the direction of gaze during a task can reveal higher-level, though subconscious, strategies adopted by subjects. Examining eye movements while walking provides an opportunity to examine how vision guides action under variable environmental and task demands. One would expect that the degree to which subjects fixate the surface on which they plan to walk will vary based on its physical characteristics, its predictability, and its visibility. In a study of how patients with retinitis pigmentosa navigate with significant visual field loss, Turano et al. (2001) had normal subjects navigate a hallway wearing a head-mounted display with an integrated eyetracker. The head-mounted display provided the same view to both eyes from a single head-mounted camera. The display’s limited field-of-view required that subjects actively acquire information that would normally be available from the periphery. Even with the reduced cues available for moving in the hallway, however, fewer than 25% of normal subjects’ fixations were directed toward the floor or boundaries between the floor and walls. Patla & Vickers (2003) studied the gaze behavior of normal subjects as they walked a predetermined path. They measured the gaze of subjects with normal vision as they walked a 10 m path under three conditions; 1) stepping on regularly spaced footprints, 2) stepping on irregularly spaced footprints, and 3) a control condition with no markings on the floor. Under all three conditions the predominant gaze behavior was what Patla and Vickers termed “traveling gaze”, in which subjects’ gaze moved at the

same rate as, and at a fixed distance ahead of, the subjects. The travel gaze accounted for over 60% of the total gaze duration, even when there were no specific targets for foot placement. Given the predictability of the surface, and lack of explicit instructions on foot placement in the control condition, it is surprising that Patla and Vickers’ subjects focused on the floor immediately before them 60% of the time.

In the experiment presented here, we extend the investigation of task-dependent oculomotor behavior to consider the effect of environment. Incorporating the environment in the experimental design allows us to probe two issues. We can (1) examine the effect of task and environment on low-level fixation duration and saccade-amplitude metrics, and

(2) address Patla and Vicker’s (2003) surprising findings regarding gaze direction while walking without explicit directions on foot placement. By monitoring gaze as subjects walk in man-made and natural environments, we seek to understand how the varying physical characteristics and the predictability of the path affect oculomotor behavior. A paved surface should require less active monitoring because the surface is more predictable and because peripheral acuity is sufficient to detect typical variations. Walking on an uneven dirt path, however, is expected to require more active guidance, and therefore more frequent foveation.

1. Methods

Subjects performed two different tasks within two environments. The two Environments we tested were outside of an apartment complex (Man-made Environment) and in the dense woods (Wooded Environment). Within each Environment subjects were required to perform two Tasks: Free-viewing (standing in one place and looking about the scene) and

Walking. The Walking Task in the Man-made Environment was performed on a paved path, and the Walking Task in the Wooded Environment was performed on a dirt trail.

Three adult volunteers from the student population at the Rochester Institute of Technology participated and received an honorarium for their participation. Subjects had normal or corrected-to-normal vision. The experimental protocol was approved by the Institutional Review Board at the Rochester Institute of Technology. Participants provided their informed consent.

Eye-movement records were collected using a custom-built, wearable eyetracker that allowed subjects’ eye movements to be monitored without limiting head or trunk movement. The eyetracker has a scene camera placed directly above the participant’s eye. The scene camera has a field-of-view of approximately 40 × 30 . Figure 1 shows the system consisting of lightweight headgear and backpack (see Babcock & Pelz, 2004 for a detailed description of the system). The image from the scene camera was used for analysis of subjects’ gaze records, and to extract head movements (see below). Rather than perform eyetracking in real-time, the system uses a video multiplexer to record eye and scene images onto a single videotape. The video is later de-multiplexed and processed in a laboratory eyetracking system. Figure 2a shows the raw anamorphic eye and scene images; Figures 2b and 2c show the resultant scene image with overlay cursor indicating gaze

Figure 1. Wearable eyetracker records eye and scene camera images for offline analysis.

Figure 2. a) Multiplexed scene and eye images b) Wooded Environment c) Man-made Environment.

position for the Wooded and Man-made environments respectively. A semitransparent eye image is superimposed on the scene image to aid in analysis. The eye image allows blinks and track loss to be identified and be excluded from analysis.

Because gaze position was calculated off-line after completion of the experiments, the field calibration required only that the participant follow a target moved about the scene in front of the subject. The target was moved to cover a range of approximately ±20 horizontal and ±10 vertical.

The eye and scene images were demultiplexed and fed into a laboratory computer equipped with ISCAN Model PCI-726/PCI-636 processing cards. Calibration was

completed by correlating the recorded eye and scene images collected at five positions in the field. The wearable eyetracker provides a video record of gaze superimposed over the scene image and a data stream consisting of 60 Hz horizontal and vertical eye-in-head position. The statistics of saccade amplitudes and duration and inter-saccade periods were obtained with software written to extract saccades from the gaze angle data. Because the eye movements were generated as part of ongoing natural behavior, it was rare for the eye-in-head signal to be stable due to nearly constant linear and/or rotational Vestibular- Ocular-Reflex (VOR) movements. Parsing the eye-movement patterns into “‘fixations” and “saccades” was accomplished by parallel analysis using three algorithms with different classification criteria: velocity threshold, hidden Markov model of eye velocity distributions, and an adaptive velocity algorithm (Salvucci & Goldberg, 2000; Sicuranza & Mitra, 2000). Note that in this context “‘fixation” represents periods during which a portion of the visual scene is stabilized on the retina, and not necessarily the case where the eye is stable in the orbit. Fixation duration histograms were created by counting fixations within 100 ms wide bins based on the 60-Hz eyetracker data.

Head motion was estimated from the video recording of the scene camera. A sparsemotion field was obtained by tracking specified points in successive video frames. The points were selected according to Tomasi and Kanade’s algorithm (1991) and tracked across frames using a pyramidal implementation of the basic feature tracker described by Lucas and Kanade (1981). The egomotion of the scene camera was calculated using methods described in Tian, Tomasi, & Heeger (1996). The estimated rotational head motion was aligned with the eyetracking recording via the timestamp from the video track (see Rothkopf & Pelz, 2004 for a description).

Because the subjects were free to make unconstrained head and trunk movements, a “‘fixation” in this context refers to a period during which a given point of regard is stabilized on the retina. In fact it was relatively rare for the eye to actually be still in the orbit. Only when making small eye movements during the Free-view task were traditionally defined fixations observed. The majority of the time the head and body were undergoing linear and/or rotational motion resulting in a “base” of linear and rotational vestibular eye movements upon which saccades were superimposed. For the most part, there were no objects in motion within the scene that would illicit smoothpursuit or optokinetic eye movements (although when a person did come into view s/he was invariably fixated).

Figure 3 shows horizontal and vertical eye-in-head movements for a 5-second Freeview segment in the Man-made environment. The eye moves through 60 . Except for the 300 ms fixation at 1700 ms, the eye is in constant motion. The saccades move gaze left and right as head movements and VOR return eye-in-head toward the midline. Figure 4 illustrates the horizontal and vertical head movements over the same period. Figure 5 shows the integrated horizontal and vertical gaze position over the 5-second period as the integrated eye and head movements form a series of gaze fixations and saccadic movements, despite the near-constant motion of eye and head. Comparing Figures 3 and 5 illustrates subjects’ ability to make very large gaze changes (a range of 120 is evident in Figure 5), while limiting eye-in-head movements to 60 as seen in Figure 3.