TLDR version: the jury's still out. There's limited evidence of improvement in narrow tasks, mainly in memory (particularly on stimulants) and in some sub-groups of individuals. There's also a divergence of results on short and long term effects (mostly noted on modafinil)... possibly due to publication bias. Motivation has been offered as a more important mediating mechanism (than direct cognitive enhancement) in the case of stimulants... but some reviews don't buy this is the main explanation.
I am aware of one older study (2008) having some findings seemingly contradicting the one you mention.
d-Amphetamine improved reactions times on the spatial working memory and Stroop tasks for both individuals with schizophrenia and controls, and improved working memory accuracy in schizophrenia. In addition, d-amphetamine improved language production for both individuals with schizophrenia and controls.
I guess it depends whether improved spatial working memory or language production helps or not with learning, so it may be task-specific. Reaction times are probably the least important, unless it involves acquiring some new hand-eye coordination skills (e.g. learning to drive).
Keep in mind that such studies involve giving drugs to healthy people, so... will generally raise ethics issues, that's one reason you why you don't see many studies in this area. In the one I knew about, the reason they gave d-Amphetamine do healthy people was because they needed some controls; the stated group of interest were people with schizophrenia. But that also meant there was no placebo control group in this study, so its results may be biased in that regard, i.e. some placebo effect may have happened for both schizophrenia and healthy controls.
There are is also a study in children with ADHD which found that
Long-term MPH [Methylphenidate] treatment may improve the neurocognitive profiles of the ADHD children, as seen in their performance in several subtests and in the IQ scores on the WISC-III. And this improvement had no correlation with the decrement of ADHD symptoms.
which raises several interesting questions: do ADHD symptoms (as observed for diagnosis) have anything to do with the lower school performance, and may such "stim-improvable" performance occur in some children who don't show those diagnosable ADHD symptoms? If the latter is the case, then there is at least one other group who could benefit from stimulant treatment, but we don't know how to identify it (other than giving stims to everyone, which it not an acceptable method).
And (to my surprise) I found 2010 systematic review and meta-analysis ("somehow"--as in I'm not surprised--omitted by your article):
The term neuroenhancement refers to improvement in the cognitive, emotional and motivational functions of healthy individuals through, inter alia, the use of drugs. Of known interventions, psychopharmacology provides readily available options, such as methylphenidate and modafinil. Both drugs are presumed to be in widespread use as cognitive enhancers for non-medical reasons. Based on a systematic review and meta-analysis we show that expectations regarding the effectiveness of these drugs exceed their actual effects, as has been demonstrated in single- or double-blind randomised controlled trials. Only studies with sufficient extractable data were included in the statistical analyses. For methylphenidate an improvement of memory was found, but no consistent evidence for other enhancing effects was uncovered. Modafinil on the other hand, was found to improve attention for well-rested individuals, while maintaining wakefulness, memory and executive functions to a significantly higher degree in sleep deprived individuals than did a placebo. However, repeated doses of modafinil were unable to prevent deterioration of cognitive performance over a longer period of sleep deprivation though maintaining wakefulness and possibly even inducing overconfidence in a person's own cognitive performance.
Somewhat similarly, there's another more recent (2014) review of methylphenidate "Cognitive effects of methylphenidate in healthy volunteers: a review of single dose studies" (MPH), but its methodology is limited to "effective study percentage" (i.e. not fully meta-analytical):
The studies reviewed here show that single doses of MPH improve cognitive performance in the healthy population in the domains of working memory (65% of included studies) and speed of processing (48%), and to a lesser extent may also improve verbal learning and memory (31%), attention and vigilance (29%) and reasoning and problem solving (18%), but does not have an effect on visual learning and memory. MPH effects are dose-dependent and the dose–response relationship differs between cognitive domains. MPH use is associated with side effects and other adverse consequences, such as potential abuse. Future studies should focus on MPH specifically to adequately asses its benefits in relation to the risks specific to this drug.
Also, the very article you link to, points to a study, whose actual findings were:
In a double-blind placebo-controlled study, the effects of Adderall on the performance of 16 healthy young adults were measured on four tests of creativity from the psychological literature: two tasks requiring divergent thought and two requiring convergent thought.
RESULTS:
Adderall affected performance on the convergent tasks only, in one case enhancing it, particularly for lower-performing individuals, and in the other case enhancing it for the lower-performing and impairing it for higher-performing individuals.
This again suggests task-dependence ("convergent tasks only") and also group-dependence ("particularly for lower-performing individuals") of stim effects.
Also the main study invoked for showing that "it doesn't work" in the article you mentioned, doesn't quite say that. It actually has a pretty uninformative abstract so what it says in its discussion/conclusion section is:
Recent laboratory evidence suggests that amphetamine and methylphenidate enhance motivation-related processes in healthy participants, and the in-depth interviewing conducted by Vrecko (2013) indicates that enhancement users find the motivational effects of these drugs helpful for enhancing schoolwork. Here we add to the laboratory, survey, and interview evidence by performing the first survey directly comparing users' ratings of stimulants' motivation- and cognition-enhancing effects.
The present study found that student users perceive stimulants as beneficial for cognition, despite the weak evidence for objective cognitive enhancing effects (see Introduction). However, student users also perceive stimulants as advantageous for motivation. Motivation, energy, and attention, the functions viewed as most strongly enhanced, did not differ in their susceptibility to stimulants' subjective effects. Not only were motivational functions found by users to be significantly enhanced, they were found to be somewhat more enhanced as a group than a category of cognitive functions, and this difference was statistically significant. This finding extends previous knowledge by demonstrating that stimulants' motivational effects are viewed by healthy users as prominent despite the common assumption that they work chiefly on cognition. Although our study is limited to assessing perceived enhancement and does not speak to actual behavioral effects on motivation, cognitive processing, or task performance, the results document experiential effects relevant to users' decisions to practice enhancement with prescription stimulants. According to students who use stimulants for cognitive enhancement, these drugs may enable better performance of cognitively demanding work at least in part through their effects on motivation.
So the main finding of this study is that motivation may be the primary mediating mechanism, with actual cognitive enhancements secondary (if any). Their introduction mentions cites some reviews of mixed conclusion; the only systematic one cited is the (2010) one I already mentioned; the other ones are mostly narrative reviews of the "alarm-sounding" variety. Also, the same authors mention that their own replication attempt was not successful:
We carried out a double-blind, placebo-controlled study on the effects of mixed amphetamine salts (Adderall), which was adequately powered to find medium effects. Based on our own failure to find a single drug effect across numerous measures of executive functions, memory, creativity, intelligence, and standardized test performance, we concluded that Adderall “has no more than small effects on cognition in healthy young adults” (Ilieva et al., 2013).
The same research team has a newer study; abstract:
A total of 61 users of unprescribed stimulants and 67 controls (no history of prescription stimulant
use) completed tests of objectively measured and subjectively reported attention. Self-reports on study habits, as well as
motivation during laboratory attention testing, were also administered. Results: Our data replicated previous findings
of relatively lower self-reported attention functioning in users. Extending past research, we showed that user-control
differences in attention were still present but less pronounced on objective measures than on self-report. In addition, we
obtained evidence of lower motivation during cognitive testing and less optimal study habits among users, as compared
with their non-using peers. Conclusion: Unprescribed stimulant use is more strongly related to compromised study
habits, low motivation, and a subjective perception of attention problems than to objective attention performance.
And their conclusion section:
non-medical stimulant use is more strongly
related to a subjective perception of attention difficulties
than to objective attention difficulties (although the latter is
also a factor), as well as to inefficient study habits and low
task motivation.
And this again points to a motivation and a narrower sub-group as the primary beneficiary of the enhancement. Of course the question now is: can you find an equally effective, non-drug motivator (for this sub-group)? This question is not answered, as far as I can tell.
Now the issue with learning (in a school setting) is that it's a complex task; it surely depends on cognitive as well as motivational and personality factors. (See my own question here about grit, for instance.) So it's difficult to predict just from laboratory studies what the longer term effect of stimulants would be in a school setting.
If you want to consider modafinil among the "study drugs", then there's a 2015 review, which does point out to a certain divergence between short-term and long-term results/effects. It's the only review I found that explicitly mentioned learning:
In addition to its approved use treating excessive somnolence, modafinil is thought to be used widely off-prescription for cognitive enhancement. [So they reviewed primary studies ....] investigating the cognitive actions of modafinil in healthy non-sleep-deprived humans. We found that whilst most studies employing basic testing paradigms show that modafinil intake enhances executive function, only half show improvements in attention and learning and memory, and a few even report impairments in divergent creative thinking. In contrast, when more complex assessments are used, modafinil appears to consistently engender enhancement of attention, executive functions, and learning. Importantly, we did not observe any preponderances for side effects or mood changes.
The authors of this review do note "methodological discrepancies" etc. between studies, so their conclusions include a number of recommendations for better research.
Finally, the authors of the motivation-effect research have conducted their own (2015) meta-analysis of MPH and amphetamine: "Prescription Stimulants' Effects on Healthy Inhibitory Control, Working Memory, and Episodic Memory: A Meta-analysis.":
The present meta-analysis was
conducted to estimate the magnitude of the effects of methylphenidate and amphetamine on cognitive
functions central to academic and occupational functioning, including inhibitory control, working memory,
short-term episodic memory, and delayed episodic memory. In addition, we examined the evidence for
publication bias. Forty-eight studies (total of 1,409 participants) were included in the analyses. We found
evidence for small but significant stimulant enhancement effects on inhibitory control and short-term
episodic memory. Small effects on working memory reached significance, based on one of our two analytical
approaches. Effects on delayed episodic memory were medium in size. However, because the effects on long-term
and working memory were qualified by evidence for publication bias, we conclude that the effect of
amphetamine and methylphenidate on the examined facets of healthy cognition is probably modest overall. In
some situations, a small advantage may be valuable, although it is also possible that healthy users resort to
stimulants to enhance their energy and motivation more than their cognition.
Its results are mostly in line with previous conclusions (memory being the main aspect improved), however it emphasized that effect size is small (for short-term effects), and it also found publication bias for long-term effects.
I also found a 2015 review which has a somewhat different perspective of aforementioned meta-analyses (emphasizes memory effects but doesn't dig much the motivation explanation):
In meta-analyses, it was found that methylphenidate exhibits large positive effects (d = 1.4; Repantis et al., 2010) on memory performance, that delayed episodic memory is improved by a moderate (d = 0.45; Ilieva et al., 2015), and short-term episodic memory by a smaller effect size (d = 0.20; Ilieva et al., 2015). This suggests that methylphenidate primarily enhances memory consolidation but neither encoding nor retrieval (cf. McGaugh and Roozendaal, 2009). A review concluded that verbal learning appears to be improved by methylphenidate, whereas visual learning remains unaffected (Linssen et al., 2014). Methylphenidate further improves working memory. Small, but robust, positive effects on spatial working memory have been reported in many studies (for reviews, see Repantis et al., 2010; Franke et al., 2014; Linssen et al., 2014; Ilieva et al., 2015). Further, methylphenidate has been shown to improve inhibitory control and speed of processing (Ilieva et al., 2015). The effects of methylphenidate on attention are less consistent. Most studies have reported no significant improvements in attention (cf. Repantis et al., 2010), or even negative effects (e.g., Rogers et al., 1999). However, a few studies have found small improvements in attention and vigilance (cf. Linssen et al., 2014). It has been speculated that methylphenidate might also affect motivational and emotional functions. However, although some data seem to support this hypothesis (Volkow et al., 2014), to date there is too little evidence to draw definite conclusions. [...]
Modafinil consistently improves attention in non-sleep deprived as well as sleep-deprived healthy individuals (for reviews, see Repantis et al., 2010; Franke et al., 2014; Battleday and Brem, 2015). In particular, experiments have shown improvements in sustained attention (Baranski et al., 2004; Randall et al., 2005; Dean et al., 2011) and selective attention (Schmaal et al., 2013). The effects of modafinil on memory are less clear. Some studies report beneficial effects of modafinil on spatial and numeric working memory (Müller et al., 2004). However, a review of 31 randomized controlled studies reported no significant changes in memory (Repantis et al., 2010). It is assumed that the effects of modafinil strongly depend on the individual baseline performance (Randall et al., 2005). Similar to methylphenidate, modafinil appears to positively affect low-performing individuals to a greater extent than high-performing individuals (Finke et al., 2010). Further, the effects of modafinil are strongest for cognitively demanding tasks (Müller et al., 2013). However, it potentially impairs creative and flexible thinking (Müller et al., 2013; Mohamed, 2014) and can increase feelings of overconfidence in judgment (Baranski et al., 2004).
This last review quoted also covers caffeine and non-drug enhancements, so worth for further reading on those, if you're interested (it's also not paywalled).
