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Update
Trends in Cognitive Sciences Vol.12 No.7
Letters
Better to lose the anchor than the whole ship
Johannes C. Ziegler
Laboratoire de Psychologie Cognitive, Aix-Marseille Universite´ and Centre National de la Recherche Scientifique, 3 place Victor
Hugo, 13331 Marseille Cedex 3, France
In a recent TICS article, Ahissar [1] proposed an interesting new theory for understanding dyslexia, the ‘perceptual
anchor theory’. The idea is straightforward. Most psychophysical tasks used to investigate perceptual deficits in
dyslexia repeat stimuli, such as tones in frequency discrimination or objects in rapid automatized naming (RAN).
Theoretically, this massed repetition allows unimpaired
participants to form a perceptual anchor. Indeed, Ahissar
and colleagues [2] showed that dyslexics failed to form a
perceptual anchor when the same reference stimulus
(anchor) was repeated across trials or when a small set
of stimuli was used repeatedly. These results were taken as
evidence against the phonological deficit theory of dyslexia
[3].
Here, I present three pieces of evidence that are
inconsistent with the strong claims of the perceptual
anchor theory. The first comes from RAN. According to
Ahissar [1], perceptual anchoring accounts for RAN deficits because a small set of repeated items is used in
RAN. Thus, the theory predicts that anchoring deficits
should not exist in the first trials of RAN because
anchoring needs time to build up. To assess this prediction, I re-analyzed the dyslexics’ naming performance
on the very first appearance of the five repeated
objects in a computerized RAN task [4]. The results of
the task, involving 24 dyslexics and 24 controls and two
sets of objects, clearly showed that naming deficits
already were present on the very first encounters of
the objects (see Figure 1a). Furthermore, the size of
the deficit was unchanged after five or ten repetitions.
The interaction between group and repetition was not
significant (F < 1).
The second piece of evidence comes from speech-perception-in-noise deficits. Anchoring theory predicts that these
deficits can only be found in small-set conditions that give
rise to anchoring. However, we reported speech-perception-in-noise deficits when a large set of 16 pseudo words
was used in a repetition task [5]. As shown in Figure 1b,
the deficit already was present in the very first session
(48 items, three repetitions) and its size remained constant
across repeated sessions (no group by session interaction,
F < 1).
Finally, dyslexics are not poor in all psychophysical
auditory tasks that involve anchors. For example, dyslexics are poor in perceiving rise time and duration, but
not intensity [6,7]. How come dyslexics can form perceptual
Figure 1. Naming and speech perception deficits of children with dyslexia and specific language impairments (SLI). Deficits as a function of item repetition in rapid
automatized naming (a) and speech perception in noise (b) for children with dyslexia and SLI.
Corresponding author: Ziegler, J.C. ([email protected]).
244
Author's personal copy
Update
anchors in some psychophysical tasks but not others?
Together then, at present, the anchor theory does not
provide a satisfactory account of the processing deficits
found in dyslexia.
References
1 Ahissar, M. (2007) Dyslexia and the anchoring-deficit hypothesis.
Trends Cogn. Sci. 11, 458–465
2 Ahissar, M. et al. (2006) Dyslexia and the failure to form a perceptual
anchor. Nat. Neurosci. 9, 1558–1564
3 Vellutino, F.R. et al. (2004) Specific reading disability (dyslexia): what
have we learned in the past four decades? J. Child Psychol. Psychiatry
45, 2–40
Trends in Cognitive Sciences
Vol.12 No.7
4 Ziegler, J.C. et al. (2008) Developmental dyslexia and the dual route
model of reading: simulating individual differences and subtypes.
Cognition 107, 151–178
5 Ziegler, J.C. et al. (2005) Deficits in speech perception predict
language learning impairment. Proc. Natl. Acad. Sci. U. S. A. 102,
14110–14115
6 Richardson, U. et al. (2004) Auditory processing skills and phonological
representation in dyslexic children. Dyslexia 10, 215–233
7 Corriveau, K. et al. (2007) Basic auditory processing skills and specific
language impairment: a new look at an old hypothesis. J. Speech Lang.
Hear. Res. 50, 647–666
1364-6613/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tics.2008.04.001
Letters Response
Response to Ziegler: The anchor is in the details
Merav Ahissar1,2 and Yulia Oganian1
1
2
Department of Psychology, The Hebrew University, Mt. Scopus, Jerusalem 91905, Israel
Institute of Neural Computation, The Hebrew University, Mt. Scopus, Jerusalem 91905, Israel
Assessing dyslexics’ psychoacoustic abilities, we found puzzling results. On the one hand there was no evidence for a
specific low-level impairment [1]. On the other hand their
performance in seemingly simple tasks, such as two-tone
frequency discrimination, was often quite poor [2]. We then
found that dyslexics’ difficulties depend on the assessment
protocol (e.g. [3]). Their performance was impaired only in
paradigms that use the same stimuli repeatedly. These
repetitions improve controls’ performance substantially
more than they do so for dyslexics [4]. Based on these
findings, we proposed that dyslexics’ difficulties stem from
poor ‘anchoring’ to recently presented stimuli rather than
from poor long-term representations. Because we obtained
similar results for speech perception [4] and others found a
similar type of impairment in phonological tasks (e.g. [5]),
we proposed that ‘anchoring deficits’ in a variety of domains
can account for the broad range of dyslexics’ difficulties [6].
Ziegler challenges this hypothesis noting that intensity
discrimination is typically not impaired in dyslexia,
whereas perception of rise time and duration is. Presumably if they can form an effective anchor for one dimension
(e.g. intensity), they should be able to do so for other
stimulus aspects (e.g. rise time). We should first note that
the ability of the general population to anchor to repeated
stimuli is not the same for all dimensions and depends on
how these dimensions are represented in the brain. Behavioral studies show that anchoring to intensity is more
difficult than anchoring to frequency repetitions (e.g. Ref.
[7]). Regarding dyslexics’ difficulties in perceiving rise
time, recent studies indicate that these difficulties are
not consistent and depend on the protocol of assessment
[8], as predicted by the anchoring-deficit hypothesis.
A more direct challenge to the anchoring-deficit hypothesis is based on Ziegler’s re-analysis of his studies of children
with language [9] and reading [10] disabilities. Ziegler aims
Corresponding author: Ahissar, M. ([email protected]).
to assess the prediction that dyslexics’ difficulties will
increase with stimulus repetitions, owing to controls’ success and dyslexics’ failure to anchor to repeated stimuli. He,
thus, analyzed performance in response to the ‘very first
appearance of the five repeated objects in a computerized
RAN task’ [10] and ‘in the very first session’ of speech
perception (pseudo words) in noise [9]. In both cases dyslexics’ performance was already poorer than controls’, and
their relative deficits did not increase in subsequent repetitions.
However, a closer look at the methodologies used in these
two studies shows that in contrast to Ziegler’s claims, in both
cases these were not the first trials to use these stimuli. They
were the first official trials, but they were preceded by
training sessions that allowed specific anchoring to these
stimuli. In the RAN study children first were presented with
the pictures of the five objects they were subsequently tested
with and were asked to name them. They were then given
ten training trials. Only then did the experiment formally
begin and responses to the ‘very first appearance’ were
measured. Similarly, when assessing speech perception in
noise, a block in which participants were asked to identify
the same words in quiet was administered first. We replicated Ziegler’s procedure for assessing speech perception in
our lab and found huge priming effects. Namely, by using a
set of 16 pseudo words, word identification in noise is substantially better when it is preceded by a block of identification of these words in quiet. The magnitude of this priming
effect depends on the set (reaching up to 50% improvement;
P < 0.01) and is particularly high for pseudo words.
The anchoring-deficit hypothesis asserts that the
impact of this priming is larger for controls. Thus, speech
perception in noise was so much poorer in children with
language disability (Ziegler’s results are particularly
strong compared with other studies) because they were
worse than controls in utilizing stimulus specific repetitions. In fact Ziegler’s results are in line with this interpret245