In this study we investigated self-heading direction discrimination using OF stimuli in ASD and TD adults. Direction discrimination thresholds were measured with OF stimuli that varied in dot speed and density. We found that thresholds in both groups were affected significantly by dot density but not dot speed, which agrees with previous findings.147,205,291 However, we found
specific patterns of abnormality related to individuals with autism which are summarized as follows:
1- In the eccentricity task, the autism group showed similar performance to the control group. However, their sensitivity was lower when the number of dots was high, while the speed parameter had no significant effect.
2- In the contrast sensitivity task autism participants showed decreased contrast sensitivity only when the number of dots was low; thresholds in both groups were similar in the high dot density condition. Finally in both groups the effect of dot speed on threshold was not significant.
These results provide new evidence of selective impairments to OF motion perception in autism that might be related to ASD-related changes in area visual MST, which has been suggested to be a selective response to the complex patterns of OF.280,294 Receptive fields in of V5/MT cells
may relatively be small to cover the gamut of OF motion parameters (patterns, speeds, and directions, which can extend up to 360°). However, MST neurons were found to be directionally selective with receptive fields that cover large parts of the visual field.295,296 Consequently, our
results here show that performance impairments to radial motion may require strong pattern activation in the MST area in the autism group.291 This ASD-related impairments depends,
however, on dot density, more than dot speed, which agrees with previous findings related to OF motion.297,298 Our results are the first to show that dot density affects OF motion in autism,
particularly for high dot density. High dot density should evoke strong activation at the MST area.296 This strong activation would correspond to the integration between V5/MT area that is
directionally selective to local motion information, and MST area that processes global heading direction.280
We hypothesize that reduced functional and structural connectivity between and within distributed cortical networks of MT and MST might be the underlying cause of reduce sensitivity
to OF responses in adults with ASD.
Indeed, studies utilizing visual evoked potentials (VEPs), event-related potentials (ERPs), and diffusion tensor imaging (DTI) in autism show that neuro-integrative processing at higher cortical levels in both the ventral and dorsal pathways might be impaired in ASD,282 while lower-
level processing is spared when both levels are activated at the same time. Interestingly, our findings show that performance at low dot density was intact in autism, which may be related to intact MT in autism, particularly when the dots move in a coherent direction (zero noise
level).232,237,275 Altered connectivity within V1 in autism, as posited by the underconnectivity
theory,298 also appears to the perception of self-direction of heading.204 Yamasaki et al282
suggested that deficits in the P-pathway (with intact M-pathway) in V1 might underlie perceptual impairment found in ASD. However, our results show that sensitivity of individuals with ASD declined only when number of dots was high. Neurons in V1 responds better to high spatial frequency, slow speed, and contrast variation which suggest that our stimuli would evoke stronger activation in areas beyond V1. Moreover, comparing perceptual thresholds in our data across dot density conditions for individuals with ASD and TD show reciprocal changes of speed trends with large dot density, and low sensitivity at both speeds for ASD when compared to TD which might result due to reduction in processing complex information related to deficit in the connectivity in the hMT+ area.301 Recently, Zeng et al302 proposed that the whole brain
functional connectivity in children with autism is characterized by significantly reduced network activation that may affect multiple cognitive domains and brain systems in ASD.
Our findings lead to a new insight into the neural circuit mechanism underlying the deficits to local /global perception in ASD. These deficits might be result from rather complex functional alterations in visual networks rather than enhanced processing of local details and reduced processing of global structure.
The speed parameter provides a different criteria in defining OF motion.296 Our results
show that heading direction was strongly affected by dot density varied but not dot speed. Hence, it is possible that the connectivity between MST and occipital channels would be closely coupled to neural dysfunctional activity in ASD. This is based on the assumption that OF motion processing of high spatio-temporal frequencies requires integration of neurons in the v-d (IPL) stream beyond V5/MT. Therefore, impaired OF perception found in our study in ASD adults
may result from the dysfunctional integration of the higher level(s) of MST area rather than V5/ MT area. Of course, these ideas are speculative? because very little is known about how self- motion perception in autism is affected by dot speed and density. Hence, we consider this a novel finding and further neuroimaging, electrophysiological, and psychophysical studies are required to verify this hypothesis.