Most people know that amphetamines and related drugs have been prescribed over the years to increase alertness and to fight fatigue (although caffeine apparently works about as well and is safer), to improve morale (although during WWII the Germans restricted its use because of addiction problems), as a diet drug, and for medical conditions from "idiopathic anhedonia" in the 1950s to ADHD today. Those who don't know this history can learn about it from Nicholas Rasmussen's "Life in the Fast Lane", The Chronicle Review, 7/4/2008.
Even more people know that amphetamines have long been used for recreational purposes, among subcultures as diverse as beats, hippies, and bikers; and that non-prescription uses have recently been spreading in the U.S. among several paradoxically unrelated groups, including rural whites, homosexuals, and students at elite colleges.
But few people seem to have picked up on the fact that improved alertness, focus and mood may not be the only reasons that amphetamines are popular as a "study drug".
For more than a decade, evidence has been accumulating that low doses of amphetamines can improve certain kinds of learning and retention, independent of any other physiological effects. The mechanism is not clear, though a plausible candidate is the fact that amphetamine increases concentrations of the neurotransmitter dopamine, which is "a physiological correlate of the reward prediction error signal required by current models of reinforcement learning". (See Hannah Bayer and Paul Glimcher, "Midbrain Dopamine Neurons Encode a Quantitative Reward Prediction Error Signal", Neuron 47(1): 129-141, 2005.)
In the case of word learning in particular, the effects of amphetamine can be quite large. Here's a graph of some results from the most recent study that I've read — Emma Whiting, Helen Chenery, Jonathan Chalk, Ross Darnell and David Copland, "The explicit learning of new names for known objects is improved by dexamphetamine", Brain and Language 104(3) 254-261 2008:
Fig. 1. Proportion of correct responses on the recall task according to frequency and participant group.
There were 37 subjects, 11 males and 26 females, between 18 and 34 years old. Their task was to learn 50 new non-word names (e.g. flane, neech) for familiar objects, represented as line drawings. The critical stimuli were embedded among a larger set of stimuli (including drawings of "novel objects", for which the results were described in a different paper, cited below). The 50 critical objects were divided into two groups — 25 whose original names were high-frequency words (i.e. words that occur often), and 25 whose original names were low-frequency words.
There were five learning sessions on five consecutive mornings. One group of 19 participants (the "DEX" group) got two 5-milligram tablets of dexamphetamine about two hours before the learning session, while the other group of 18 subjects (the "PLC" group) got two placebo tablets. During a learning session, each participant saw a sequence of pairs of drawings and names. After each learning, the participants were tested on recall (ability to produce the name for an object) and recognition (ability to determine whether an object/name pairing is correct). Additional recall and recognition tests (without any drug administration) were given one week and one month later.
In the legend for the graph above, "DEX LF" refers to performance of the DEX group on the objects whose original names were low-frequency words, whereas "DEX HF" refers to that group's performance on the objects whose original names were high-frequency words. The two "PLC" lines represent the performance of the group that got the placebo. (If you're having trouble with the rather tiny plotting characters, the two upper lines represent the performance of the DEX group.)
The name recognition task showed a smaller percentage effect (both absolute and relative):
Still, a difference of about 15 percentage points in recognition scores, tested a month after the training stops, is plenty big enough to be recognized by those who (ab)use such drugs as study aids, and to motivate them to persevere in such use despite the considerable dangers.
Whiting et al. used a variety of other physiological and psychological measures to support their conclusion that "new word learning success was not related to baseline neuropsychological performance, or changes in mood, cardiovascular arousal, or sustained attention". There's an unavoidable problem in such studies — as the authors explain, "due to the nature of dexamphetamine, participants may not have been performing in a fully blinded manner, despite the use of a placebo control". I'd be curious to know what would happen if caffeine tablets were used instead of placebo tablets.
The earlier literature on this subject goes back to 1993. A sample: E. Soetens et al., "Amphetamine enhances human-memory consolidation", Neuroscience Letters 161(1): 9-12, 1993; Caterina Breitenstein et al., "D-Amphetamine Boosts Language Learning Independent of its Cardiovascular and Motor Arousing Effects", Neuropsychopharmacology 29(9): 1704:1714 2004; Emma Whiting et al., "Dexamphetamine enhances explicit new word learning for novel objects", The International Journal of Neuropsychopharmacology, 10:805-816 2007.
The explicit motivation of recent research on this topic has been to improve the treatment of aphasia and other clinical disorders of naming (despite obvious issues with possible hypertension side-effects). This was explicitly tested by Walker-Batson et al., "A double-blind, placebo-controlled study of the use of amphetamine in the treatment of aphasia", Stroke 32(9), 2093–2098, 2001; and Emma Whiting et al., "Dexamphetamine boosts naming treatment effects in chronic aphasia", Journal of the International Neuropsychological Society 13: 972-979, 2007; and surveyed in Friedemann Pulvermmuller and Marcelo Berthier, "Aphasia therapy on a neuroscience basis", Aphasiology 22(6): 563-599, 2008.
I've seen very little explicit discussion of the role of amphetamine in general,, non-clinical cognitive enhancement, although it's mentioned in Nick Bostrom and Anders Sandberg, "Cognitive Enhancement: Methods, Ethics, Regulatory Challenges", Science and Engineering Ethics, 2007. This might be because of concerns that any positive evaluation of these drugs will contribute to their abuse — though the widespread and increasing diagnosis of ADHD, with associated prescription of Ritalin and Adderall, doesn't seem to be inhibited by such considerations. In particular, there's no mention of this issue in Nicholas Rasmussen's interesting new (2008) book On Speed: The Many Lives of Amphetamine. And college students who use prescription Ritalin or Adderall as a study aid tell me that they believe that they get general cognitive enhancement from these drugs, based on their own experience rather than from reading the literature in neuropsychopharmacology.
I should add that I don't recommend amphetamines as an aid to language learning. I've never taken any drugs in this family myself, and don't intend to start, since the same dopaminergic reward system that may perhaps facilitate learning also clearly leads to easy addiction and serious withdrawal effects. You don't have to take the word of anti-drug crusaders for this — you can listen to generally pro-drug-legalization people like Andrew Sullivan, or frankly pro-drug people like Allen Ginsberg (from an interview with Art Kunkin, Los Angeles Free Press, December 1965):
"Let's issue a general declaration to all the underground community, contra speedamos ex cathedra. Speed is antisocial, paranoid making, it's a drag, bad for your body, bad for your mind, generally speaking, in the long run uncreative and it's a plague in the whole dope industry."
But it doesn't help prevent drug abuse to pretend that the positive effects don't exist.
And it would be nice if the psychopharmacologists could figure out how to get the cognitive enhancement without the addiction — though if both effects are mediated by the brain's "reward prediction error signal", that may be hard to do.