Two numerical systems--the analogue magnitude system and verbal retrieval--were investigated in Williams syndrome (WS) with three numerical tasks: simple addition, simple multiplication, and number comparison. A new matching technique was introduced in selecting the proper control groups. The WS group was relatively fast in the addition and multiplication tasks, but was slow in number comparison. No reverse numerical effect was observed in the comparison task, and the distance effect was stronger than that in the control groups. The findings indicate a profile with an impaired analogue magnitude system and less impaired verbal retrieval in Williams syndrome.
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