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Both Lexical and Non-Lexical Characters Are Processed

during Saccadic Eye Movements

Hao Zhang

, Hong-Mei Yan

*, Keith M. Kendrick

, Chao-Yi Li

1,2

1 Key Laboratory for NeuroInformation of Ministry of Education, University of Electronic Science and Technology of China, Chengdu, China, 2 Shanghai Institutes of

Biological Sciences, Chinese Academy of Sciences, Shanghai, China

Abstract

On average our eyes make 3–5 saccadic movements per second when we read, although their neural mechanism is still

unclear. It is generally thought that saccades help redirect the retinal fovea to specific characters and words but that actual

discrimination of information only occurs during periods of fixation. Indeed, it has been proposed that there is active and

selective suppression of information processing during saccades to avoid experience of blurring due to the high-speed

movement. Here, using a paradigm where a string of either lexical (Chinese) or non-lexical (alphabetic) characters are

triggered by saccadic eye movements, we show that subjects can discriminate both while making saccadic eye movement.

Moreover, discrimination accuracy is significantly better for characters scanned during the saccadic movement to a fixation

point than those not scanned beyond it. Our results showed that character information can be processed during the

saccade, therefore saccades during reading not only function to redirect the fovea to fixate the next character or word but

allow pre-processing of information from the ones adjacent to the fixation locations to help target the next most salient

one. In this way saccades can not only promote continuity in reading words but also actively facilitate reading

comprehension.

Citation: Zhang H, Yan H-M, Kendrick KM, Li C-Y (2012) Both Lexical and Non-Lexical Characters Are Processed during Saccadic Eye Movements. PLoS ONE 7(9):

e46383. doi:10.1371/journal.pone.0046383

Editor: Markus Lappe, University of Muenster, Germany

Received April 17, 2012; Accepted August 31, 2012; Published September 28, 2012

Copyright: ß 2012 Zhang et al. This is an ope n-access article distributed under the terms of the Creative Commons Attribution License, which permits

unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was supported by the Major State Basic Research Program (2007CB311001), and the Natural Science Foundations of China (90820301,

60972108, 91120013, 61075109). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manu script.

Competing Interests: The authors have declared that no competing interests exist.

* E-mail: hmyan@uestc.edu.cn

Introduction

While reading this paper, you are continually making ballistic,

saccadic eye movements, which serve to bring a new region of text

into foveal vision for detailed analysis. Every saccade is associated

with a transient but high speed displacement of the retinal image.

A large number of studies have shown that visual stimuli presented

just before and during saccades are not perceived [1,2,3]. This

reduced visual performance is attributed to so called saccadic

suppression or saccadic omission, which results in us being

unaware of any blurring of viewed images during perception and

maintaining visual stability. However, saccadic suppression is

proved to be selective, for instance, stimuli of low spatial

frequencies are very difficult to detect if flashed just prior to a

saccade, while stimuli of high spatial frequency and equiluminance

remain equally visible or even more sensitive [4,5]. There may also

be saccadic compression of both spatial and temporal aspects of

visual processing [6,7,8]. The neural mechanism of saccadic

suppression and compression is still not very clear.

A key additional question is whether higher level cognitive

processing might also be suppressed during saccades. To date

researches have shown that saccades disrupt counting [9],

judgments of numerical magnitude [10,11] and spatial direction

[12], mental rotation [13], and some shifts of spatial attention [14]

but not other cognitive processes, such as the integration of visual-

form information [15] and perceptual stability [16].

In the field of cognitive linguistics there is an ongoing and long-

standing debate about whether lexical processing during reading is

also suppressed during saccades. Some studies such as EZ reader

model have concluded that information processing only occurs

during period of eye fixation and not during the execution of

saccades [17]. This is in line with claims that during saccades the

eyes are moving so quickly across characters and words that only a

blur would be perceived [18,19]. This view has led to saccade

duration conventionally being subtracted from reading compre-

hension time in most eye movement experiments. However, other

studies investigating sentence reading and text comprehension

have suggested that higher-level cognitive processing such as

lexical processing might not be completely be suppressed during

saccades [20,21,22]. Overall therefore we still do not have a clear

idea whether linguistic information is being actively processed

during saccades [23,24] and this is clearly an important and

fundamental question in this field which needs to be definitively

resolved.

We have therefore investigated whether both words (lexical -

Chinese) and no intrinsic meaning (non-lexical - alphabetic)

characters could be processed or not during fast saccadic eye

movement. Our results provide the first evidence that discrimina-

tion of both these lexical and non-lexical characters is remarkably

accurate for those scanned during the saccade towards the next

fixation point, but is considerably less so for those not scanned by

the saccade beyond the fixation point. Discrimination perfor-

mance for the characters scanned during the saccade is almost

similar to that during actual fixation, although with the latter

discrimination of characters after the fixation point is considerably

PLOS ONE | www.plosone.org 1 September 2012 | Volume 7 | Issue 9 | e46383

better. The functional significance of these results in guiding the

next fixation position and in facilitating information processing is

also discussed.

Materials and Methods

Subjects

Six subjects (5 male, 1 female) aged 21–24 yr participated in the

experiments. All subjects were native Chinese speakers although

they all had some knowledge of spoken and written English. All

subjects had normal or corrected-to normal vision and provided

written informed consent. All research experiments were approved

by the Ethics and Human Participants in Research Committee,

University of Electronic Sciences and Technology of China,

Chengdu, China. One of the subjects was an author of this study

while the others were naı

ve with respect to its aim. All the subjects

were given an initial training period of about 30 min before the

experiments in order to be familiar with the task. Data from this

training session were not included in the final analysis. Each

subject performed in two sessions for each experiment, and each

session included 10 blocks resulting in a total of 840 individual

trials.

Experimental setup

The subjects were seated in a dark room specially designed for

psychophysical experiments. The visual stimuli were presented on

a210 color monitor (DELL Trinitron) providing a frame

frequency of 100 Hz at a spatial resolution of 128061024 pixels.

The viewing distance was 80 cm, and stimuli appeared on a grey

background which was adjusted to a mean luminance of about

22 cd/m

. Originally the contrast of both Chinese and Alphabetic

characters were 1.0 however preliminary tests showed that at this

level discrimination performance on Alphabetic characters showed

a ceiling effect which would prevent comparisons across viewing

conditions and so task difficulty was increased by decreasing the

contrast to 0.5.

Eye movements were recorded with an infrared eye tracker

(Eyelink2000, SR Research Ltd.) and sampled at 1000 Hz. Head

movements were restricted by a forehead and chin rest. The pupil

of the left eye was tracked at a sample rate of 1000 Hz and a

spatial resolution of 0.1u.

Stimuli

Two sets of stimuli were used in the experiments. The first set

consisted of 30 Chinese characters which were all composed of 4

strokes and had the same usage frequency [25]. The second set

comprised 25 capital alphabetic characters (Q was excluded to

avoid confusion with O). All the characters were displayed on the

screen at the same size (0.5u60.5u). For each trial, fourteen

characters (with no lexical or semantic relationship) were

randomly chosen with seven on each side of the scheduled

fixation point. The one which was chosen as the target for

discrimination was displayed in bold face. The distance between

each character was 0.2u.

Experimental paradigm

The main experiment (Experiment 1) consisted of two

recognition tasks involving: (a) Chinese character discrimination

and (b) Alphabetic character discrimination. The two tasks had the

same experimental protocol. Fig. 1A shows the procedure for the

Chinese character discrimination task. Both the fixation dot and

stimulus characters were presented on a CRT screen against a

homogenous gray background. Each trial started with the

appearance of a black fixation dot (0.3u in diameter) at the center

of the screen, which was extinguished after 1200,1500 ms and

followed by the appearance of a dot occurring randomly at either

7u on the left or 7u on the right of the center fixation point,

indicating the goal of the saccade (0.3u in diameter). At this point,

subjects were required to perform a saccade from the fixation

point (FP) to the goal point (saccadic target, ST). The direction of

the saccadic goal point was varied randomly within each block of

trials. Eye positions were recorded and the velocity of eye

movement was calculated on line.

We used the initiation of eye movement to trigger the

presentation of character stimuli. As soon as the stimulation

program detected the initiation of a saccade (velocity$35u/s) [26],

fourteen characters were presented for 30 ms, seven on each side

of the goal point. As timing is critical in the experiment, we

determined the delay from eye movement to stimulus update to be

about 10 ms in our system. The average duration of saccade

execution is about 40,50 ms, during which the character stimuli

were presented and the recognition task performed. About

1000 ms after the saccade, a panel including all the test characters

was shown on the screen (Fig. 1B), and the subjects were required

to pick out the appointed character using a computer mouse.

When the subjects had made their choice, the next trial started.

The temporal sequences for the experimental protocol are shown

in Fig. 1C.

In order to establish discrimination performance relative to

fixation and immediately following saccadic eye movement, two

further experiments were carried out. In Experiment 2, Chinese

and Alphabetic character discrimination was investigated during

steady fixation. All stimuli were presented on a homogeneous grey

background. A fixation dot (0.3u in diameter) was presented at the

center of the display screen and subjects were instructed to

maintain fixation on this point during the trial. After

1200,1500 ms, 14 Chinese or alphabetic characters were

presented on either side (7 each side) of the fixation point for

30 ms. The character panel was then shown on the screen

1000 ms later and subjects were required to pick out the appointed

one. Experiment 3 was similar to that for character discrimination

during saccadic execution, but the characters were not presented

until 100 ms after saccade initiation, namely, when the eyes

steadily re-fixated the saccadic landing position.

To better understand the function of active saccades in reading,

a control experiment was carried out where the eye was exposed to

a moving word without actually making a saccade (i.e. the

equivalent of a passive saccade)(Experiment 4). The procedure was

similar to that for character discrimination during the steady

fixation condition. During the trials, when a fixation dot was

presented at the center of the display screen, subjects were

instructed to maintain their fixation on this point. After

800,1100 ms, fourteen characters with normal type face were

presented either 7.5u on the left or right of the center fixation point

and moved towards the direction of the fixation point at a speed of

30u/s. This low speed was deliberately chosen so it would make

potential information processing easier than with higher ones.

After the characters had moved for 230 ms, one of them was

displayed in bold face for 30 ms and swept across the fixation

point. The characters were presented in normal type face again

and continued to move until the end point of 7.5u opposite to the

starting position. About 1000 ms later, the subjects were required

to pick out the appointed character using the computer mouse. In

this way we could determine whether visual information process-

ing during reading requires the making of an active saccadic

movement or can also occur when character information passes

across the retina for the same amount of time and speed as for very

slow saccade.

Information Processed during Saccades

PLOS ONE | www.plosone.org 2 September 2012 | Volume 7 | Issue 9 | e46383

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