Impaired Prefrontal Cortical Response by Switching Stimuli in Autism Spectrum Disorders Assessed by near-Infrared Spectroscopy

Background: Impairments in various kinds and degrees of higher-order cognitive processing related to prefrontal cortex (PFC) are reported in autism spectrum disorder (ASD) patients. We have previously reported the lack or delayed oxygenation of prefrontal cortex (PFC) when task-switch paradigm was conducted to ASD subjects using near-infrared spectroscopy (NIRS), suggesting a failure in timely switching of signal processing in the PFC upon stimulation in ASD (IMFAR 2008).

Objectives: We aim to further investigate the reactive pattern of PFC in ASD upon consecutive exhibit of cognitive and less-cognitive stimulation, by using geometrical figure-memorization paradigm in which working memory requiring task (WM) and non-working memory requiring task (NWM) are alternately appeared.PFC oxygenation level was measured by NIRS during the paradigm. 

Methods: Preliminary 11 ASD subjects (IQ>65 by WISC III, ages of 14-46, mean 29.5 y.o., 3 males and 8 females) and 14 healthy controls (ages of 19-51, mean 27.3 y.o, 1 male and 13 females) were studied. The task-switching paradigm was designed using randomized geometrical figures in combination of three shapes (circle, triangle, and square) and four colors (red, yellow, blue, and green).  For WM, subjects were required to memorize figures which appear one by one every 3 seconds on a PC screen, and subsequently were ordered to touch the figures in consecutive order as they memorized, from a multiple choices appeared on the screen. For NWM, all test figures are already appeared with the multiple choices so that the subjects can touch the figures while watching them.  The number of figures was gradually increased up to 6.
PFC oxygenation was continuously measured by NIRS during the task performance, and the oxygenated Hb level in PFC during each task was examined between control and ASD subjects.

Results: The NIRS parameters during each task were normalized and averaged.  In the controls, oxygenated Hb level was increased during WM and decreased during NWM, and was gradually elevated when the number of figures is increased, suggesting that in the control PFC, shifting of signal processing was sensitively occurred according to the task switch.   Additionally, laterality of right PFC was consistently observed in the controls, which is predictable from the nature of the present paradigm that requires spatial cognition.
On the contrary, ASD subjects lack the clear switching tendency of PFC oxygenation according to the WM/NWM switch, and obvious right PFC laterality was not observed.  However, the task performance of ASDs was not necessarily worse than the controls.  The comparison of oxy-Hb transition curve during the whole tasks were significantly different between controls and ASDs (Rt p<0.001, Lt p<0.05, controls vs ASDs respectively by two-way ANOVA )  

Conclusions: Our present study revealed that a rapid and obvious response of PFC oxygenation to the incoming switching stimuli was not observed in ASD subjects, which may explain their difficulties in the daily life, the school, or work.  Several ASD subjects showed rather superior performance rate in harder task, which might be an important clue to understand their superiority in particular brain functions.