Asymmetrical visual fields distribution of attention in dyslexic

Neuroscience Letters 290 (2000) 216±218
www.elsevier.com/locate/neulet
Asymmetrical visual ®elds distribution of attention in dyslexic
children: a neuropsychological study
Andrea Facoetti a,b,*, Massimo Turatto b
a
Neuropsychology Unit, Scienti®c Institute `E. Medea', Bosisio Parini, Lecco, Italy
b
University of Padova, Padova, Italy
Received 10 April 2000; received in revised form 29 May 2000; accepted 12 July 2000
Abstract
Visual spatial attention was evaluated in dyslexic and normally reading children by using a ¯anker task. When an
irrelevant distractor is presented adjacent to a target stimulus, interference is observed when the two stimuli are
associated with con¯icting responses. In the present study the distractor ¯anked the target either to the right or to the
left. Results showed an asymmetric ¯anker effect in dyslexics, whereas it was symmetrical in normal readers. Dyslexics
exhibited a reduced ¯anker effect in the left visual ®eld, concomitant with a strong ¯anker effect in the right visual ®eld.
These results give further support to the hypothesized left-side minineglect in dyslexics. Data also provide evidence for a
reduced ability in suppressing distracting information in the right visual ®eld. Such visual ®eld asymmetry is thought to
play a crucial role in reading disorders. Right over-distractibility and left inattention suggest an impairment of the right
parietal functions as supported by the magnocellular theory of dyslexia. q 2000 Published by Elsevier Science Ireland
Ltd.
Keywords: Dyslexia; Spatial attention; Flanker effect; Visual ®eld; Asymmetry; Right parietal lobe
Developmental dyslexia is de®ned as a speci®c reading
disorder despite normal intelligence and teaching, regardless of any manifest sensory de®cit. Recently, the hypothesis of an `auditory temporal processing de®cit' has become
of central interest in the study of dyslexia etiology [4].
However, a temporal processing de®cit appears to affect
information processing in different sensory modalities
[15]. Accordingly, it has been suggested that the major
problem of dyslexic children would be a general impairment
in the processing of rapid streams of information, regardless
of the stimulated modalities. The magnocellular-transient
(M) system, which would also include the auditory system,
is thought to be the neural basis underlying such de®cit [10].
The information processed by the M system ends in the
parietal posterior cortex (PPC), which is an important supramodal selective spatial attention area [17]. An impairment
of this area would lead to a reduced ability to focus attention
in the stimulated modality. As far as the visual domain is
concerned, there is evidence of both an impaired focusing of
attention in dyslexic children [3] and a prolonged dwell time
* Corresponding author. Dipartimento di Psicologia Generale,
Via Venezia 8, 35131 Padova, Italy. Tel.: 139-049-8276929, fax:
139-049-8276600.
E-mail address: [email protected] (A. Facoetti).
in dyslexic adults [8]. In addition, much converging
evidence indicated an asymmetric distribution of attention
between the two visual ®elds in dyslexics. Hari and
Koivikko [7] suggested that, as compared to the right visual
®eld (RVF), dyslexics suffer from `mini-neglect' in the left
visual ®eld (LVF). Fowler et al. [5], and Eden et al. [1]
showed that in visual search tasks dyslexic children omitted
a greater number of targets when presented in the LVF than
in the RVF. This left-side de®cit appears to be linked to a
right-side enhancement in the processing of visual information, as demonstrated by an increased ability of dyslexics in
letter recognition in the RVF [6].
Therefore, the present study was aimed at further exploring a possible attentional visual ®eld asymmetry in dyslexia.
To this purpose, we used the response competition procedure [2]. Typically participants are required to react to a
central target ¯anked by either a response-compatible or
response-incompatible distractor. What has been observed
is that compatible distractors produce faster reaction times
(RTs) than the incompatible ones. The RT difference
between the compatible and the incompatible condition is
de®ned as `¯anker effect'. Participants consisted of 14
dyslexic children (mean and SD, age 12.1 ^ 2.3 years, ten
males, four females) and 11 normally reading children
0304-3940/00/$ - see front matter q 2000 Published by Elsevier Science Ireland Ltd.
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A. Facoetti, M. Turatto / Neuroscience Letters 290 (2000) 216±218
(mean and SD, age 11.4 ^ 2.7 years, eight males, three
females). All children were selected by: (1) a full scale
IQ . 85, (2) normal or corrected-to-normal vision and hearing, (3) normal visual ®eld, (4) absence of attention de®cit
disorder with hyperactivity (ADHD), (5) no known gross
behavioral or emotional problems, (6) absence of drug therapy, (7) normal teaching opportunities, and (8) right manual
preference. The children were classi®ed as dyslexic as their
performances in oral reading of text, words and non-words
were 2 SD below the norm on age-standardized Italian tests.
Variables considered are speed and accuracy. The dyslexic
group had signi®cantly lower reading test scores in comparison to control children (reading accuracy, one-sided t-test,
P , 0:0001, and reading speed, P , 0:0001). The two
groups did not differ with regard to IQs (mean IQ of
dyslexics was 99 ^ 14, mean IQ of normal readers was
104 ^ 12).
Tests were carried out in a dimly lit room with a ambient
luminance of 1.5 cd/m 2. Participants sat in the front of a
monitor (15 inches and with a luminance of 0.5 cd/m 2) with
their head positioned on a headrest so that the eyes-screen
distance was 40 cm. Fig. 1 shows the experimental set-up. A
small white dot (0.58 of visual angle) presented in the center
of the screen served as ®xation point. The target stimulus
was either a left-pointing arrow or right-pointing arrow (28)
presented at the center of the screen. A smaller arrow (18)
¯anking the target either to the right to the left side was used
as ¯anker. Target-¯anker distance covered 18 of visual
angle. In the compatible condition both the target and the
¯anker pointed to the same direction, whereas in the incompatible condition the target and the ¯anker pointed to opposite directions. Stimuli luminance was set at 24 cd/m 2. Each
trial began with the onset of the ®xation point (accompanied
by a 1000-Hz warning tone) which lasted for 201 ms,
followed by a 50-ms blank screen. After that the target
and the ¯anker appeared for 201 ms (see Fig. 1). Eye movements were monitored by means of a system composed of
Fig. 1. Schematic representation of the display used in the
experiment.
217
infrared-ray spectacles. Any eye movements larger than 18
was detected by the system and the correspondent trial was
discarded and replaced. Participants were instructed to press
a key on the keyboard as quickly as possible at the onset of
the target (the `Y' key for the left-pointing arrow, and the
`B' key for the right-pointing arrow). RTs were recorded by
the computer with millisecond accuracy, and the maximum
time allowed for response was 1500 ms. The experimental
session consisted of 240 trials divided into two blocks of
120 trials distributed as follows: compatible and incompatible conditions consisted of 60 trials each. In each condition
there were 30 trials in which the target and the ¯anker
pointed to the left (15 trials with the ¯anker in the LVF
and 15 trials with the ¯anker in the RVF), and 30 trials in
which the target and the ¯anker pointed to the right (15 trials
with the ¯anker in the LVF and 15 trials with the ¯anker in
the RVF). Before the experiment began participants made
some practice trials until they felt con®dent with the task.
Fig. 2 shows participants' RTs for target detection as a
function of ¯anker compatibility and location (LVF and
RVF). Mean correct RTs were entered into a three-way
analysis of variance in which the within-subjects factors
were type of ¯anker (compatible and incompatible) and
location (LVF and RVF), whereas the between-subjects
factor was group (normal readers and dyslexics). The
main ¯anker-type effect was signi®cant F…1; 23† ˆ 66:26;
P , 0:0001, the ¯anker effect was 82 ms. Also, the type
of ¯anker £ location £ group interaction was signi®cant
F…1; 23† ˆ 6:23, P , 0:02, indicating that the two groups
processed the ¯anker in the two visual ®elds differentially.
Planned comparisons showed that in normal readers the
¯anker effect was present in both the LVF (M ˆ 70 ms,
P , 0:02) and the RVF (M ˆ 73 ms, P , 0:02), but the
difference between the two visual ®elds was not signi®cant
(3 ms, P . 0:05). By contrast, dyslexics exhibited an asymmetrical ¯anker effect, with the ¯anker effect virtually
absent in the LVF (M ˆ 36 ms, P . 0:05), but deeply
present in the RVF (M ˆ 144 ms, P , 0:0001). On the
basis of the present results it may be hypothesized that
dyslexics display a reduced processing of the ¯anker in
Fig. 2. Mean reaction times as a function of compatible and
incompatible ¯anker condition, visual ®eld and group. Data
show an asymmetrical effect of the ¯anker in dyslexics but not
in normal readers.
218
A. Facoetti, M. Turatto / Neuroscience Letters 290 (2000) 216±218
the LVF, concomitant with an exaggerated ¯anker processing in the RVF. In line with this interpretation we found
that in compatible trials dyslexics showed an increased ¯anker effect in the RVF vs. the LVF (P , 0:03). Likewise, in
incompatible trials a decreased ¯anker effect was observed
in the LVF vs. the RVF (P , 0:02).
The RT pattern emerged from this study can be interpreted in light of recent ®ndings showing an asymmetrical
distribution of spatial attention in dyslexics. Such an asymmetry is re¯ected by a left-side minineglect as suggested by
Hari and Koivikko [7] and by Stein and Fowler [16], and by
an increased ability in letter recognition in the RVF [6].
Thus, the left de®cit exhibited by dyslexic children can
potentially indicate an increased inhibition for information
presented in the LVF, as shown by a reduced ¯anker effect.
By contrast, the profound ¯anker effect in the right side
might be due to an impairment in the ability to suppress
distracting information in the RVF.
How can these de®cits be related to the reading problem
manifested by dyslexic children? Basically, the idea is that
during reading visual information presented in the RVF is
not ®ltered out ef®ciently in order to obtain an ef®cient letter
and/or word processing. As suggested by LaBerge and
Brown [9], the ability to suppress information ¯anking the
attended area would crucial especially when unfamiliar
words have to be identi®ed. It can be suggested that this
suppressive mechanism could be particularly defective in
the RVF in dyslexics, who would be distracted by letters
or words to the right from ®xation. What is more dif®cult to
explain is how the left inattention de®cit might affect the
reading process in dyslexic children. It can be speculated
that this de®cit might play a role during regressive saccades,
which are rapid backward eye movements involved in reading [11].
A right parietal cortex dysfunction is thought to be the
neurological basis underlying dyslexics' attentional de®cits
[7,15]. This interpretation is supported by the fact that a
visual spatial de®cit similar to that emerged in the present
study and in those from Hari and Koivikko [7] has also been
reported by Ro et al. [12] in subjects suffering from a
temporoparietal junction lesion. These patients, like
dyslexics in our study, did not show the interference ¯anker
effect in the contralesional visual ®eld (left side), but
displayed a larger interference effect in the ipsilesional
visual ®eld (right side). This suggests not only that there
was no interference when incongruent distractors were
presented to the controlesional ®eld but also that there
was a `hyperorienting' response towards the ipsilesional
®eld [13]. In addition, a Smania et al. [14] study revealed
that left-hemineglect and extinction patients with a right
parietal cortex damage showed a paradoxical RT decrement
in the RVF as target eccentricity increased.
To conclude, the present study as well as other recent
works provide converging indications that dyslexia is associated with an asymmetrical distribution of spatial attention
in the visual ®eld. However, on the basis of our ®ndings, we
suggest that the major problem of dyslexic children might
be an inability to suppress distracting information in the
RVF.
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