Calendar calculating savants with autism - how do they do it?

Savants with autism are people who exhibit an exceptional ability whilst also having social and cognitive impairments. One such ability is calendar calculating - being able to say, with astounding accuracy and alacrity, what day of the week a given date falls on. Just how some savants with autism are able to achieve this feat has baffled researchers. It's been suggested that they use complex algorithms, but this seems implausible given that the same individuals often struggle with maths.

To help solve the mystery, Anna Dubischar-Krivec and colleagues recruited three savant calendar calculators with autism and pitted their calendrical skills against three neuro-typical calendar calculators recruited through a Swiss science TV show.

The participants were tested with questions that took the following form: "Is it true that 6 November 1974 = Thurs?". The savants with autism beat the neuro-typical calendar experts, in terms of speed and accuracy, for past dates (these went back fifty years) and dates from the current month. By contrast, the performance of the two groups was matched for future dates, which were taken from up to fifty years ahead.

As usual, the savants were unable to say how they achieved their calendar skills. However, the researchers said the pattern of results implies that the savants were using different strategies from the neuro-typicals. Whereas the neuro-typicals relied on algorithms for past, present and future dates, the savants probably relied on rote memory for past and present dates, the researchers said, hence their superior speed and accuracy for these, whilst they probably fell back on some kind of algorithmic system for future dates.

These conclusions were supported by the fact that the savants' answers seemed too quick, at least as regards dates in the current month (their average response time was less than three seconds), for them to have performed algorithmic calculations. Also they appeared to have made use of memory "anchor dates" based around the month of December, as betrayed by their reaction times tending to be quicker for months later and earlier in the year.

However, the mystery remains far from solved. For example, if the savants were relying on memory for their astonishing calendrical feats, you'd think a memory test would reveal their unusual memory ability. Yet a standard psychometric comparison of memory performance between the savant and neuro-typical calendar calculators found no differences, except the neuro-typicals were better on a form of working memory.
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Dubischar-Krivec, A., Neumann, N., Poustka, F., Braun, C., Birbaumer, N., & Bölte, S. (2008). Calendar calculating in savants with autism and healthy calendar calculators. Psychological Medicine, 39 (08) DOI: 10.1017/S0033291708004601

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