Article at io9. The paper is available here.

The researchers showed monkeys specific images and then trained them to select those images out of a larger set after a time delay. They recorded the monkeys' brain function to determine which signals were important. The experiment tests the monkey's performance on this task in different cases, as described by io9:

Once they were satisfied that the correct mapping had been done, they administered cocaine to the monkeys to impair their performance on the match-to-sample task (seems like a rather severe drug to administer, but there you have it). Immediately, the monkeys' performance fell by a factor of 20%.

It was at this point that the researchers engaged the neural device. Specifically, they deployed a "multi-input multi-output nonlinear" (MIMO) model to stimulate the neurons that the monkeys needed to complete the task. The inputs of this device monitored such things as blood flow, temperature, and the electrical activity of other neurons, while the outputs triggered the individual neurons required for decision making. Taken together, the i/o model was able to predict the output of the cortical neurons — and in turn deliver electrical stimulation to the right neurons at the right time.

And incredibly, it worked. The researchers successfully restored the monkeys' decision-making skills even though they were still dealing with the effects of the cocaine. Moreover, when duplicating the experiment under normal conditions, the monkeys' performance improved beyond the 75% proficiency level shown earlier. In other words, a kind of cognitive enhancement had happened.

This research is a remarkable followup to research that was done in rodents last year.

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Only tangentially related: apes (their tail-less cousins) surpass humans in many visual memory tasks, similar to the ones employed in the OP study:

"The chimps further show an aptitude for photographic memory, demonstrated in experiments in which the jumbled digits are flashed onto a computer screen for less than a quarter of a second, after which the chimp, Ayumu, is able to correctly and quickly point to the positions where they appeared in ascending order. The same experiment was failed by world memory champion Ben Pridmore on most attempts."

Source, includes video.

It would be interesting to escalate that same experimental setup to such chimps, i.e. to ascertain whether that same method can be used to upgrade even super-human abilities.

That study is extremely interesting, but its central claim is disputed. Here it is claimed that when humans get to practice as much as Ayumu, they can reach his level:

There was a request for these full texts, so I'm providing them here: Silberberg & Kearns (2009) and Cook & Wilson (2010).

Cook & Wilson's abstract:

Do chimpanzees have better spatial working memory than humans? In a previous report, a juvenile chimpanzee outperformed 3 university students on memory for briefly displayed digits in a spatial array (Inoue & Matsuzawa, 2007). The authors described these abilities as extraordinary and likened the chimpanzee's performance to eidetic memory. However, the chimpanzee received extensive practice on a non-time-pressured version of the task; the human subjects received none. Here we report that, after adequate practice, 2 university students substantially outperformed the chimpanzee. There is no evidence for a superior or qualitatively different spatial memory system in chimpanzees.

blarg, reading comprehension. Never mind.