Static Aspects of Eye and Head Movements during Reading in a Simulated Computer-Based Environment with Single-Vision and Progressive Lenses

Abstract

purpose. Reading with two different intermediate progressive lens designs was investigated regarding eye and head movement patterns and compared with movement patterns with a conventional single vision lens in a computer-based work environment.

methods. Two-dimensional eye (horizontal, vertical) and three-dimensional head (horizontal, vertical, and torsional) movements were recorded objectively and simultaneously at a rate of 60 Hz during reading of moderate contrast (40%) single- and double-page text formats at 60 cm with binocular viewing. In addition, global reading ability was rated subjectively for each lens. Subjects were 11 visually normal, presbyopic individuals aged 45 to 71 years selected by convenience sampling from a clinic population. Reading was performed with three types of spectacle lenses: a single-vision lens (SVL; 60° horizontal [H] clear field-of-view [FOV]); a progressive addition lens (PAL) with a relatively wide intermediate zone (PAL-I; 7.85 mm, 18°H clear FOV); and a PAL with a relatively narrow intermediate zone (PAL-II; 5.60 mm, 13°H clear FOV).

results. Many reading-related parameters, as well as eye- and head-movement parameters, were adversely affected by the PALs compared with the SVL. One reading-related parameter (i.e., number of regressions) differentiated between PALs. Subjective rating of global reading ability was highest with the SVL and lowest with the PAL-II.

conclusions. The optical design of a spectacle lens had significant impact on reading performance and on the combined eye-head movements initiated during reading. Both horizontal eye and head movements discriminated well between PALs and the SVL, but not between PALs, despite subjective preferences. This suggests that nonoculomotor factors contribute to patients’ nonacceptance of PALs. Vertical eye and head movements and torsional head movements were not as discriminatory as were their horizontal counterparts.

Adaptation to and satisfaction with progressive addition lenses (PALs) have been hampered in part by the restricted zones that may make reading difficult, ^{1 2} including at the intermediate working distance of the typical computer user. The narrow intermediate zones of clear vision may cause an increase in the frequency of compensatory head movements, which is thought to be a major contributor to nonacceptance of PALs. ^{1 2}  

Few studies have been conducted in which both eye and head movements during reading have been objectively recorded simultaneously, ³ especially in multiple spatial dimensions. However, in one study in which only horizontal head movement was recorded, more head movements occurred with PALs than with a conventional bifocal. ⁴  

In the present study, two-dimensional eye movements and three-dimensional head movements were recorded objectively during reading while wearing a conventional single-vision lens (SVL) or either of two PALs with different intermediate zone widths. Single- and double-page text formats of adult-level material were used similar to that which the viewer confronts in the typical contemporary computer-based environment. 

Materials and Methods

Recording of Eye and Head Movements

Eye and head movements during reading were recorded objectively using an ISCAN integrated eye-head movement computer-based system (ISCAN, Burlington, MA). Horizontal and vertical eye position, as well as horizontal, vertical, and torsional head position, were recorded simultaneously during reading under the three lens conditions with two text formats in each subject at a single test session. 

The eye-movement system (ISCAN ETL-400) used a video-based, dark pupil-to-cornea reflection method. It consisted of a head-mounted recording system attached to a stable cap, an eye and visual-field imaging system, and a computer with specialized analysis and graphics software. An infrared light-emitting unit and video camera were mounted on the lightweight cap worn by the subject. The eye was illuminated by the infrared light, and the camera recorded the image of the eye. The video eye image was transmitted simultaneously to the monitor and computer. Differences between the corneal Purkinje image 1 and pupil center were calculated as changes in eye position. The system recorded horizontal and vertical eye movements simultaneously and independently with a sample frequency of 60 Hz and a linear range of ±20° with a resolution of 0.3°. 

The head-movement system (Polhemus Insidetrak; ISCAN) used an electromagnetic field recording method. It consisted of an electromagnetic emitting unit and a sensor. The small sensor was attached to the cap, which was fixed with straps onto the head, and the emitting unit was placed on a table 40 cm away. Three-dimensional (horizontal, vertical, and torsional) head movements were recorded simultaneously and independently with a sample frequency of 60 Hz and a linear range of 360° with a resolution of 0.1°. 

Subjects

Subjects were 11 visually normal, presbyopic individuals aged 45 to 71 years, who were selected by convenience sampling from the patients of the clinic practice. All had undergone a complete vision examination within 6 months of testing. None was taking any drugs or medications that would adversely affect either eye movements or attentional state. Six of the subjects were habitual PAL wearers, and the other five were novice wearers. Best spectacle-corrected distance and near binocular visual acuity in each subject was 20/20 or better. Range of distance spectacle correction was from −5 to +2 D, with only two subjects having astigmatism greater than 2D (−2.75 and −2.25 D); near adds ranged from +1.25 to +2.25 D. The tenets of the Declaration of Helsinki were followed, and written informed consent was obtained. 

Reading Materials

Reading was performed with two hard-copy text formats: a standardized single-page text (horizontal [H]14°, vertical [V]17°) positioned along the subject’s midline at a distance of 60 cm, and standardized double-page text (H37°/V17°) arranged so that the end of line 1 of one page of text continued with line 1 of the other text page, and so forth for successive lines. One page of text was positioned along the subject’s midline, and the center of the other text page was positioned 30° to the right. The first condition simulated standardized normal reading at a computer screen, whereas the second simulated the computer-work situation in which an additional page of text is situated beside the computer screen. Both texts consisted of standardized adult-level paragraphs (Visagraph 2; Taylor Associates, Huntington Station, NY) specially printed at 40% contrast. At this level of contrast and higher, there is little effect on reading eye movements and reading rate in normal observers. ⁵ Reading was conducted under normal room illumination conditions (47 cd/m²) using diffuse fluorescent overhead lighting with binocular viewing. 

Lenses

Reading was performed with three types of moderately large spectacle lenses (A 52 mm, B 46 mm, and ED 59 mm box size). ¹ These commercially available lenses included a conventional single-vision lens (SVL) with a 60° clear field horizontally, a PAL with a relatively wide intermediate zone of 7.85 mm (18° clear field horizontally; PAL-I), and a PAL with a relatively narrow intermediate zone of 5.60 mm (13° clear field horizontally; PAL-II). 

Procedures

Subjects sat in front of the reading material at a distance of 60 cm without a chin rest and were asked to maintain a constant reading distance. During testing, the experimenter carefully observed the subjects to assure constancy of reading distance. Although they were allowed to move their heads, they were not encouraged to do so. A practice trial preceded the test trials in all subjects. During the experiment, subjects read each of the test texts with each pair of spectacles. Subjects were instructed to read for comprehension and to keep each fixated word clear. Type of spectacle lens and text format were counterbalanced across subjects to prevent order effects. 

After completing reading with the three types of lenses, the subjects were asked to evaluate reading ease and ability subjectively for each PAL lens, using the following 7-point rating scale, with the SVL preassigned a score of 50 (satisfactory): 100, excellent; 83, very good; 67, better than satisfactory; 50, satisfactory; 33, less than satisfactory; 17, very poor; and 0, unacceptable. They were not told the type of lens used during the reading of each text. 

Parameter Analysis

A variety of eye- and head-movement parameters were investigated. ⁶ The parameters for horizontal eye and head movements were (1) total amplitude of horizontal head movement per line; and (2) total amplitude of horizontal return-sweep saccadic eye movements per line. Return-sweep saccade eye movement refers to the large right-to-left, slightly oblique saccadic eye movement(s) that shifts the eyes from near the end of one line of text to near the beginning of the next line of text. The amplitude of the horizontal component is approximately equal to the text width, and the amplitude of the vertical component is approximately equal to the distance between successive lines; (3) total amplitude of the horizontal return-sweep saccade in relation to gaze position per line. Gaze position is the eye position in space, which is the sum of eye and head position. For example, if the eyes move to the right 5° and the head moves to the left 5°, the gaze position would be constant (0°); (4) total amplitude of progressive saccadic eye movements per line. Progressive saccadic eye movement refers to the small (1–3°), left-to-right eye movements that occur when the eyes shift from one fixation point to another in the text; (5) total combined horizontal head- and eye-movement amplitude (peak-to-peak) per line; (6) difference in amplitude between the sum of head movement versus the return-sweep saccadic eye movement and return-sweep saccade in gaze position per line. The sum of head movement versus the return-sweep saccadic eye movements refer to the absolute value of the combined movement amplitudes performed by the eye and head; and, return-sweep saccade in gaze position is the eye position in space during the return-sweep saccade. The difference between these two parameters indicates the extent of extra movements (more than needed in pure gaze) that were made by the eye and head; (7) percentage of head movement amplitude embedded in the return-sweep saccade gaze position; and, (8) number of steplike horizontal head movements (1° or greater) per line. 

Parameters for vertical eye and head movements were (1) maximum amplitude of vertical eye movement per line; (2) maximum amplitude of vertical head movement per line; (3) total amplitude of maximum vertical eye and head movements per line; and (4) maximum amplitude of torsional head movement per line. For reading rate, the parameters were (1) fixations per 100 words. Fixations refer to the number of pauses of the eyes during reading; (2) the number of progressive or rightward saccadic eye movements per 100 words; (3) the number of regressive or leftward saccadic eye movements per 100 words; (4) the number of words per fixation (span of recognition); This parameter is inversely related to parameter 1, in that the number of fixations per 100 words equals 100 times the number of words per fixation, and thus parameters 1 and 4 are dependent variables but are traditionally used as though they are different (independent) parameters in the reading eye movement literature ⁶ ; (5) reading rate (number of words per minute); and (6) number of return-sweep saccades per overall return-sweep movement. All data were collected online, and subsequently analyzed either using the computer software (ISCAN) or manually using our own customized software. 

Results

The group mean subjective ratings of the three lenses are presented in Table 1 . The PAL-II lens was rated lower than the PAL-I lens (one-tailed t-test, t ₍₁₀₎ = 2.228, P = 0.021). 

The statistically significant group results were as follows for the horizontal head and eye movements (Table 2) : (1) Head movement was greater with the PALs than with the SVL for both text formats. (2) The sum of the amplitude of progressive saccadic eye movements was greater with the SVL than with the PALs for both text formats. (3) The difference in amplitude between the return-sweep saccades and sum of progressive saccades was greater with the PALs than with the SVL for both text formats. (4) The total amplitude of head movement and return-sweep saccades was greater with the PALs than with the SVL for both text formats, with this parameter being inversely related to parameter 3. (5) The percentage of maximum head movement to return-sweep saccade gaze change was smaller with the SVL than the PALs, and thus head movement was greater with the PALs. 

The statistically significant group results were as follows for vertical eye movement and the vertical and torsional head movements (Table 3) : (1) Vertical head-movement amplitude was greater with the PALs than with the SVL for both text formats. (2) The total amplitude of the combined head and eye movements was greater with the PALs than with the SVL for the double-page format. (3) Torsional head movement was greater with the PALs than the SVL for the single-page text format. (4) There was a greater number of head movements per line with the PALs than with the SVL for both text formats. 

The statistically significant group results for the reading eye-movement parameters for the three lenses and two text formats are summarized in Table 4 : (1) Reading rate was significantly lower with the PALs than with the SVL for both text formats. (2) The number of return-sweep saccades per line was higher with the PALs than with the SVL for the double-page format. (3) The number of fixations per 100 words was higher with the PALs than with the SVL for both text formats. (4) The number of regressions per 100 words was higher with the PALs than with the SVL for both text formats. Furthermore, it was greater with the PAL-II than with the PAL-I for the single-page text format. (5) The number of words per fixation was greater with the SVL than with the PALs for both text formats, and this parameter was inversely related to parameter 3. 

Figure 1 presents typical two-dimensional eye (horizontal and vertical) and three-dimensional head (horizontal, vertical, and torsional) movements during reading of the single-page text with the SVL (Fig. 1A) and PAL-II (Fig. 1B) for comparison between these two extreme lens designs. With the SVL, three-dimensional head position was remarkably stable. Only eye movements were executed during reading of the single-page text format. In contrast, with the PAL-II and its relatively narrow intermediate zone of clear vision, continuous three-dimensional head movement was present along with the continuous two-dimensional reading eye movements. 

The horizontal eye and head movements during reading of the single-page text format with the three lenses are presented in Figure 2 . With the SVL, the head was extremely stable, and essentially only the eyes changed lateral position (Fig. 2A) . The eyes shifted from left to right across the line of text by a series of progressive saccades with relatively few regressions and at the end of one line returned to the beginning of the next line by large return-sweep saccadic movements. The summed amplitude of the progressive saccades was essentially equal to the (total) amplitude of the return-sweep saccade(s). In Figures 2B and 2C , the results of reading of the single page of text with the PALs are presented. Regular and continuous head movement was prominent, especially with the PAL-II. When head movements occurred, the reading-related progressive saccade amplitude was reduced, which is to be expected, because the combined head and eye movements must approximate the lateral extent of the text amplitude. 

Figure 3 presents horizontal eye- and head-movement recordings during reading of the double-page text format. Although large horizontal head movements occurred because of the large text spacing with all three lenses, there were greater head movements and smaller progressive saccadic eye movements with the PALs (Figs. 3B 3C) than with the SVL (Fig. 3A) . 

Horizontal gaze position was calculated by combining the eye and head positions (Table 2) . This represents the absolute eye position in space. Figure 4 presents horizontal gaze and horizontal eye positions during reading of the single-page text format with the three lens types. There was no difference in total amplitude of the return-sweep saccadic components among the three lens test conditions. During reading with the SVL, gaze changes mirrored the eye movement changes. However, these overall reading profiles were modified somewhat during reading with the PALs. The rate of change of gaze position was reduced, and hence it took a longer time to read each line. Furthermore, there appeared to be more irregular gaze adjustments (Fig. 4C) and some longer fixational pauses (Figs. 4B 4C) . 

The sequence of horizontal head and eye movements with the two types of lenses was as follows for the group results. With the SVL, the head was essentially stable. Nearly all the measurable movement was executed by the eye movement system alone. In contrast, with the PALs, there was considerable head movement during the return-sweep saccade (Fig. 5) . First, the eyes shifted from right-to-left to the beginning of the next line of text, with the head stable. This was followed by synchronized and simultaneous head and eye movement. The head shifted to the left to place the clear intermediate lens zone within the text area of interest at the beginning of the line, as the eyes shifted to the right as a result of the compensatory vestibulo-ocular reflex. This was followed by both smooth and step-like head movements to maintain the lens’s intermediate clear region superimposed on the word of interest concurrent with the progressive foveating saccades. 

Last, the subjects were divided into experienced (n = 6) versus novice (n = 5) PAL wearers. When statistical analyses were performed comparing the experienced versus novice PAL wearers across all the eye and head movement parameters for each lens condition, no statistically significant differences were found (t-test, df = 9, P > 0.05). 

Discussion

The present results demonstrated that the amplitude of horizontal head movement differed when subjects read with the various lenses and text formats. With the SVL, the clear field-of-view was approximately ±30°, which far exceeded the page width (14°) in the single-page text format. Head movement was minimal in amplitude (<1.0°), and hence reading was accomplished essentially exclusively by eye movements in 7 of 11 subjects. The remaining four exhibited small amounts of head movement but only intermittently and of small magnitude (<1.0°). This result is similar to that found by Bahill et al. ⁷ under natural viewing conditions in the outdoor environment during ambulation. The eye alone typically (i.e., 85% of the time) executed a saccade to fixate objects of interest less than 15° from the line-of-sight, with relatively little if any correlated head movement present. Thus, this appears to be a relatively general phenomenon in eye-head interaction and strategy. Similarly, with the double-page text format, head movement was still least with the SVL. In contrast, with the PAL-I and -II lenses having intermediate clear zone widths of ±9° and ±6.5°, respectively, considerable head movement was demanded for clear vision. For example, in the worst-case scenario with the PAL-II lens and its half-field clarity of only 6.5°, if a subject fixates on a word at the beginning of a line subtending 14° in horizontal extent, two or three head movements would be predicted and in fact demanded optically, to read across the line of print, assuming the requirement of clear vision all of the test time. 

Gaze position refers to the eye position in space, and it reflects the exact overall combined head and eye movements needed for foveation during the reading task. Thus, gaze position relates directly to the target’s position in space. The percentage of head movement embedded within the mean total gaze change was greater for the PALs than the SVL as expected, because of the greater degree of head movement required and, in fact, imposed by the PALs optical lens design. Thus, there were more combined eye-head movements with the PALs than with the SVL. This may be responsible, in part, for patients’ complaints of transient confusion, shifting to the wrong new line, “fatigue,” and perhaps even some optical “swim” sensation during reading. ¹  

Despite marked differences in horizontal angular extent of the zone of clear vision for each of the three lenses, the (mean total) amplitude of the return-sweep saccades was essentially the same. This suggests that the overall return-sweep movement may be preprogrammed and essentially open loop, thus bringing the subject’s final gaze to the estimated or predicted new position at the beginning of the next line of print, without much regard to the quality of the initial eccentrically placed retinal image. 

The vertical eye movements and the vertical and torsional head movements did not discriminate as well in relation to lens type and text format (Table 3) . None of these oculomotor parameters discriminated between the PALs. They did so only between the SVL and the PALs, as was true for the horizontal movement comparisons. However, the magnitudes of these eye and head movements were remarkably small (i.e., <5°), and hence presence of such small changes probably contributed to the relative lack of discrimination. For example, given the large horizontal extent of head movement executed under the double-page text format (i.e., up to a mean of 16° to 28°; see Table 2 ), mean torsional head movement was less than 2°. 

In contrast, reading eye movement parameters were somewhat more sensitive to the lens types for both text formats. Subjects consistently performed better with the SVL than the PALs. Furthermore, reading ability discriminated between the PALs in regressions; more were made with the PAL-II than with the PAL-I lens. Thus, although PALs have many advantages, such as cosmesis and convenience, there may be some oculomotor and reading-based disadvantages to be considered, as well as those in the psychophysical and perceptual domains alluded to earlier. 

Each subject commented verbally and rated his or her overall impression of reading ease and ability with each of the three test lenses after completion of the reading tasks. All considered the SVL to be the best, in that it was quicker and easier to find the region of clarity with the SVL than with either PAL. Furthermore, for the same reasons, the PAL-I was considered to be better than the PAL-II. 

It is of interest that the subjective rating scale discriminated between the PALs, whereas the extensive objective testing in general did not. This suggests that other non-motor-based factors may be involved in an individual’s perception of global reading ease and ability with these lenses, such as optically and perceptually based phenomena resulting from nonuniform magnification astigmatic and defocus effects produced in the spectacle lens periphery. ¹  

There were no differences found across parameters between the experienced and novice PAL wearers, perhaps due to either the small sample size or very rapid initial oculomotor adaptive aspects in the novice wearers. Future investigations should focus on this important and practical aspect of PAL lenses in a larger population of such wearers. 

Visual performance should improve in the future, with PALs having an even wider intermediate zone and overall improved optics. This improvement was suggested by the superiority of the present PAL-I versus PAL-II in subjective assessment and in one reading parameter (i.e., regressions). Thus, the lens design of the intermediate zone must be carefully considered in new PAL designs. In fact, our goal is to use the objective eye- and head-movement parameters and strategies, in conjunction with related sensory (e.g., contrast sensitivity) and perceptual (e.g., “swim”) test results, as well as quantitative subjective ratings, in the future to create and then test comprehensively and quantitatively new PAL lens designs with the intent of producing more efficient and comfortable reading, approaching that found with the SVL. ⁸  

The present results have implications in the typical computer-based workplace environment. Even in the absence of spectacles, presbyopic computer users have a constellation of visually based complaints and symptoms, including glare, focusing difficulties, and eye fatigue. ¹ Now, with the addition of either SVLs corrected for the intermediate distance (i.e., 60 cm) or different optical designs of PALs, these visually based problems will typically worsen, at least initially, primarily because of lens glare with the SVL and the restricted field of view with the PALs. In the latter case, reading either a single- or double-page text format also demanded more eye and/or head movements, as well as eye-head dynamic adaptive strategies, therefore reducing reading rate and overall oculomotor reading efficiency. ⁹ Furthermore, individuals may also notice the perceptual phenomenon of “swim” due to nonuniform and asymmetric magnification for the optical regions adjacent to the intermediate lens zone. ¹ Thus, individuals may need time to adapt to these motor and perceptual components to optimize use of the PALs and become symptom free. 

In closing, the eye- and head-movement findings were especially insightful, and furthermore provided a multidimensional picture of this complex process. Head-movement aspects were worse with the PALs than with the SVL. This is consistent with the eye-movement findings. Thus, both horizontal head- and eye-movement aspects appear to be sensitive indicators of lens optical design parameters on overall reading performance and are important in our understanding of the patient’s acceptance of new PAL lens designs. Vertical and torsional oculomotor aspects were helpful but not as discriminatory in the evaluative process. Combined head and eye movements were elegantly executed to foveate and read each word clearly through the appropriate intermediate region of the lens at all times, hence optimizing reading rate. The details and dynamic quantitative analyses of the combined eye and head movements will be the topic of a future report. 

 Supported by The Spectacle Lens Group of Johnson & Johnson Vision Care, Inc.

 Submitted for publication September 10, 2001; revised February 22 and June 5, 2002; accepted July 8, 2002.

 Commercial relationships policy: F.

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Table 1.

 

Subjective Ratings

	SVL	PAL-I	PAL-II
Rating	50 ± 0	32.45 ± 2.87	23.55 ± 3.63

Table 2.

 

Mean Values of Horizontal Head and Eye Movement Parameters during Reading

	Text Format	Lens	Mean (deg)	F(2,20)	P	Post hoc LSD Test (P)
												SVL/PAL-I	SVL/PAL II	PAL-I/PAL-II
HM amplitude	SP	SVL	0.7	20.677*	0.000	0.000	0.000	0.179
		PAL-I	4.2
		PAL-II	5.2
	DP	SVL	16.4	16.040*	0.000	0.001	0.000	0.175
		PAL-I	25.2
		PAL-II	28.2
RSS EM amplitude	SP	SVL	11.9	2.157	0.142	0.199	0.054	0.480
		PAL-I	11.3
		PAL-II	11.0
	DP	SVL	24.7	1.866	0.181	0.164	0.082	0.702
		PAL-I	22.2
		PAL-II	21.6
Sum of Sac. EM	SP	SVL	11.3	17.786*	0.000	0.000	0.000	0.240
		PAL-I	7.7
		PAL-II	6.7
	DP	SVL	17.3	10.735*	0.001	0.002	0.000	0.475
		PAL-I	9.0
		PAL-II	7.4
Sum of HM+RSS EM	SP	SVL	12.6	12.751*	0.000	0.001	0.000	0.355
		PAL-I	15.5
		PAL-II	16.2
	DP	SVL	41.1	29.430*	0.000	0.000	0.000	0.055
		PAL-I	47.4
		PAL-II	49.8
RSS Gaze	SP	SVL	12.1	1.641	0.219	0.919	0.145	0.121
		PAL-I	12.2
		PAL-II	11.6
	DP	SVL	32.8	0.271	0.765	0.557	0.510	0.942
		PAL-I	33.1
		PAL-II	33.1
(HM+RSS EM)− (RSS gaze)	SP	SVL	0.5	17.028*	0.000	0.001	0.000	0.091
		PAL-I	3.3
		PAL-II	4.6
	DP	SVL	8.3	39.912*	0.000	0.000	0.000	0.111
		PAL-I	14.3
		PAL-II	16.7
HM/RSS gaze	SP	SVL	6%	19.822*	0.000	0.000	0.000	0.105
		PAL-I	34%
		PAL-II	45%
	DP	SVL	50%	21.964*	0.000	0.000	0.000	0.286
		PAL-I	76%
		PAL-II	85%

Table 3.

 

Mean Values of Vertical and Torsional Head and Eye Movement Parameters during Reading

	Text Format	Lens	Mean (deg)	F(2,20)	P	Post hoc LSD Test (P)
												SVL/PAL-I	SVL/PAL-II	PAL-I/PAL-II
Vertical EM amplitude	SP	SVL	0.62	0.583	0.567	0.364	0.989	0.357
		PAL-I	0.48
		PAL-II	0.51
	DP	SVL	1.23	0.308	0.738	0.918	0.541	0.476
		PAL-I	1.52
		PAL-II	1.57
Vertical HM amplitude	SP	SVL	0.31	6.334*	0.007	0.048	0.002	0.169
		PAL-I	0.65
		PAL-II	0.99
	DP	SVL	0.88	6.545*	0.007	0.019	0.002	0.362
		PAL-I	2.16
		PAL-II	2.63
Vertical EM+HM	SP	SVL	0.93	2.664	0.094	0.435	0.034	0.155
		PAL-I	1.13
		PAL-II	1.50
	DP	SVL	2.11	4.283	0.028	0.047	0.011	0.495
		PAL-I	3.68
		PAL-II	4.20
Torsional HM amplitude	SP	SVL	0.19	15.888*	0.000	0.004	0.000	0.025
		PAL-I	0.64
		PAL-II	0.97
	DP	SVL	1.35	0.835	0.449	0.250	0.311	0.886
		PAL-I	1.86
		PAL-II	1.80
HM	SP	SVL	0.71	24.327*	0.000	0.000	0.000	0.090
		PAL-I	2.98
		PAL-II	3.79
	DP	SVL	3.60	12.992*	0.000	0.000	0.000	0.826
		PAL-I	7.96
		PAL-II	8.19

Table 4.

 

Mean Reading Eye Movement Parameter Values as a Function of Lens Type and Text Format

	Text Format	Lens	Mean	F(2,20)	P	Post hoc LSD Test (P)
												SVL/PAL-I	SVL/PAL-II	PAL-I/PAL-II
WPM	SP	SVL	214	7.321*	0.004	0.022	0.000	0.212
		PAL-I	187
		PAL-II	173
	DP	SVL	182	10.077*	0.001	0.005	0.000	0.249
		PAL-I	156
		PAL-II	146
RSS	SP	SVL	1.5	2.903	0.078	0.251	0.026	0.234
		PAL-I	1.7
		PAL-II	1.9
	DP	SVL	2.5	3.703*	0.043	0.024	0.036	0.846
		PAL-I	3.0
		PAL-II	2.9
Fix/100W	SP	SVL	61	7.235*	0.004	0.009	0.002	0.484
		PAL-I	68
		PAL-II	70
	DP	SVL	61	10.181*	0.001	0.003	0.000	0.345
		PAL-I	68
		PAL-II	70
Reg/100W	SP	SVL	13	8.884*	0.002	0.080	0.000	0.029
		PAL-I	16
		PAL-II	21
	DP	SVL	16	4.613*	0.023	0.015	0.017	0.941
		PAL-I	22
		PAL-II	22
W/Fix	SP	SVL	1.7	6.636*	0.006	0.013	0.002	0.472
		PAL-I	1.5
		PAL-II	1.5
	DP	SVL	1.7	10.469*	0.001	0.002	0.000	0.573
		PAL-I	1.5
		PAL-II	1.5

Figure 1.

 

Eye (horizontal, vertical) and head (horizontal, vertical, and torsional) movement recordings from subject 10. Reading of the single-page text format with (A) SVL and (B) PAL-II. For both eye and head movements, in the horizontal traces, up is rightward; in the vertical traces, up is upward; and in the torsional traces, up is a shift to the right shoulder. RSS, return-sweep saccade; fix, fixation and progressive saccades; reg, regressive saccades; EM, eye movement; HM, head movement.

Figure 2.

 

Horizontal eye- and head-movement recordings from subject 11 during reading of the single-page text format with (A) SVL, (B) PAL-I, and (C) PAL-II. For both eye- and head-movement traces, up is rightward. Dashed vertical rectangle: data to be discussed in Figure 5 . EM, eye movement; HM, head movement.

Figure 3.

 

Horizontal eye- and head-movement recordings from subject 2 during reading of double-page text format with (A) SVL, (B) PAL-I, and (C) PAL-II. For both eye- and head-movement traces, up is rightward. EM, eye movement; HM, head movement.

Figure 4.

 

Horizontal gaze position and eye position from subject 4 during reading of single-page text with (A) SVL, (B) PAL-I, and (C) PAL-II. Conventions as in Figure 2 . EM, eye movement.

Figure 5.

 

Horizontal eye- and head-movement recordings from subject 11 during reading of single- page text format with PAL-II. Conventions as in Figure 2 . (A) Detailed traces taken from Figure 2 PAL-II (area within dashed rectangle). (B) Detailed traces taken from (A) showing return-sweep saccadic movement without coordinated head movement. (C) Detailed traces taken from (A) showing return-sweep saccadic movement with coordinated head movement. (D) Schematic representation of the sequence of head and eye movements in (C). (D1) relates to (B), (D2) to the first half of (C), and (D3) to the second half of (C).