Individuals with autism spectrum disorder (ASD) show abnormalities in communication and social interaction, as well as markedly restricted interests and stereotyped behaviors 
. Several studies have reported abnormalities in basic visual perception in ASD 
. In particular, a detail-oriented perception could be related to the “core” deficits in the social domain, including face processing 
and emotion recognition through observation of body movements 
In the human visual system, the retina transmits information to the lateral geniculate nucleus and then to the primary visual cortex via two main separate pathways: the magnocellular and parvocellular streams 
. In the extra-striate cortical regions the magnocellular cells provide the principal input to the dorsal stream leading to the dorsolateral occipital cortex and posterior parietal lobe regions 
. The magnocellular-dorsal (M-D) stream responds to rapidly changing stimuli such as flicker and motion 
Braddick and colleagues 
reported a great deal of evidence suggesting that later development of the M-D stream provides more opportunity for neurodevelopmental abnormalities (i.e., the dorsal stream vulnerability hypothesis). They suggest that this vulnerability is not specific to one particular condition, but rather, it is characteristic of many developmental disorders (e.g., ASD, Developmental Dyslexia and Williams Syndrome).
The coherent dot motion (CDM) paradigm 
has often been employed to investigate the integrity of M-D stream in several neurodevelopmental disorders (see 
for a review). In the common version of CDM tasks, a variable proportion of dots (i.e. the signal) move coherently, while the remaining dots (i.e., the noise) move in random directions, at the same speed. Participants are required to make a judgment about the direction of the moving dots, and accuracy increases as a function of the dot proportion moving in the same direction.
Several studies have shown that individuals with ASD require about 10% more coherent motion to correctly report the direction in the CDM paradigm (
, but see also 
) or in other similar tasks 
. Some authors, however, consider the M-D stream vulnerability hypothesis inappropriate for ASD 
. For example, Pellicano and colleagues 
found dissociation in individuals with ASD when evaluating different M-D processing stages. They administered both the CDM (testing the high-level stage of the M-D stream) and the flicker contrast task (testing a lower-level stage of the M-D stream) to participants with ASD. Children with ASD were less able to perceive the CDM than typically developing children. However, children with ASD performed no differently from comparison children on the flicker contrast task. Similar findings have been reported by Bertone and colleagues 
. These results support the hypothesis of a general impairment in the signal integration processing, rather than a specific M-D stream deficit.
Another hypothesis that could explain the poor performance of children with ASD on the CDM task is the perceptual noise exclusion deficit. For example, Sperling and colleagues 
found that children with developmental dyslexia performed poorly in the CDM task. These authors suggested that their results could be interpreted as a noise exclusion deficit (see also 
). Signal enhancement and noise exclusion are two important mechanisms to improve perception 
. Signal enhancement involves maintaining signal integrity during processing, while noise exclusion involves optimizing the perceptual filter. A noise exclusion deficit could also be suitable for the decreased CDM performance shown by children with ASD.
Another aspect that deserves consideration as a possible influence on performance in CDM tasks is spatial attention efficiency. Indeed, neuroimaging and neurophysiological studies suggest a possible top-down role for the fronto-parietal attentional mechanisms in the integration of spatio-temporal information 
. For example, Liu and colleagues showed how efficient spatial attention orienting could improve performance in a CDM task 
. However, the attentional focus is not only oriented towards a specific location, but also has to be adjusted in size. This ability, causally linked with the right frontal eye fields 
, allows processing visual stimuli from a narrow (zoom-in) or a broad (zoom-out) visual region 
ASD has been repeatedly associated with different types of dysfunctions in spatial attention 
. A recent study by Ronconi and colleagues 
investigated the efficiency of attentional zooming mechanisms (zoom-out and zoom-in) in children with ASD. Results
support the hypothesis of a specific zoom-out attentional impairment in ASD, suggesting that in observers with ASD attentional resources appear to be rigidly allocated in a narrow region of the visual field 
. Mann and Walker 
reported similar findings, employing a paradigm that required participants to decide which of two pairs of crosshairs was the longest. ASD participants were less able to make this judgment than the comparison group when the previous pair of cross-hairs was smaller than the one to be judged. The authors argued that individuals with ASD experienced difficulty in zooming out the attentional spotlight, in agreement with findings by Ronconi and colleagues 
Thus, if the attentional zoom-out mechanism is specifically impaired, and consequently, the attentional resources are rigidly allocated in a narrow region of the visual field, the global spatio-temporal integration of the coherently moving dots could be inefficient because the outer portion of dot display simply exceeds the size of the attentional focus.
In this study, we modified the classic CDM paradigm in order to test whether children with ASD demonstrate a general M-D stream vulnerability, a perceptual noise exclusion deficit, or inability to integrate visual information because of their problem in zooming out the attentional focus. CDM perception was measured under two different conditions: (i) In the central condition, the moving dots appeared inside a circle in the central (foveal and para-foveal) portion of the visual field, and (ii) in the peripheral condition, the moving dots were included within an annulus (empty central portion); namely, the motion perception was measured only in the periphery (see ). Since the motion integration required for the CDM task is more efficient in the central visual field 
, we expected better performance in the central than in the peripheral condition for the typically developing group.
Design and results of the Coherent Dot Motion (CDM) task.
According to a general M-D stream vulnerability 
, one would expect that children with ASD would perform poorer in both the central and peripheral conditions, since the M-D deficit should not affect the two portions of the visual field differently. The same expectation is valid for the perceptual noise exclusion deficit hypothesis 
, since the signal-noise ratio was equal in central and peripheral conditions.
On the other hand, a zoom-out attentional deficit in children with ASD should lead to impairment mainly in the central condition. During the central condition, the narrow attentional focus characterizing the children with ASD would select a small central portion of the dots display, excluding the outermost part. The presence of task-relevant information inside the attentional focus (i.e. moving dots) should, indeed, amplify the difficult in zooming out the focus of attention 
. The result of this inappropriate attentional processing will diminish available information regarding the coherently moving dots. On the other hand, in the peripheral condition, participants with ASD should be forced to enlarge their attentional focus because of the complete absence of task-relevant information (i.e., moving dots) in the central portion of the visual field, helping the motion integration process. The two experimental conditions, central and peripheral, were performed in separate blocks, to avoid a rapid switch between a narrowed or broadened attentional focus that seems to be impaired in children with ASD 
. One of our interests was to test if this difficulty in zooming out the attentional focus also persisted in the condition where the information was present only in the periphery, and the time taken to adapt the focus size was not an issue.