Journal Club #1

One of the reasons for starting a blog was that I wanted to occasionally write brief summaries of papers that have influenced me along the way. In particular, I wanted to write about papers that either might not have received a great deal of attention when published, or older papers that I think still have a great deal to offer.  I’m starting with a paper I hold dear, as it was the rock that I built my ramshackle PhD upon.

Logan, G. (1990) Repetition priming and automaticity: common underlying mechanisms?, Cognitive Psychology, 22, 1-35

Repetition priming is simply a change in behaviour to a stimulus you have previously experienced relative to one you have not. For example, if I showed you an object and asked you to classify it according to whether it was man-made you would respond more quickly and accurately on the second compared to the first presentation. So what? Firstly, repetition priming can be a big effect. You can be as much as 200msecs faster on the second presentation of an object, a potential 20% increase in speed [1]. Secondly, amnesics show intact priming, despite being impaired at consciously remembering specific events (such as what they had for breakfast that morning; referred to as ‘episodic’ memory)[2]. Thirdly, priming is long lasting – effects have been shown despite a gap of several years between the first and second presentation of a stimulus [3]. So priming seemingly taps into a mechanism(s) that learns quickly (following a single presentation of a stimulus), lasts a long time (several years) and seems to be distinct from ‘conscious’ or ‘explicit’ forms of memory.

What causes repetition priming? There is plenty of healthy debate about the underlying cause(s) of priming. Perhaps the most common idea though relates to the learning of perceptual information when a stimulus is first encountered [4]. For example, when we see an object for the first time we construct a perceptual representation of that object. On second presentation the existence of the previously learned perceptual representation allows us to recognise the object more quickly, speeding up any subsequent task performed on the object. Although many argue about the specifics of such theories [5-6], most would agree that one locus of repetition priming is perceptual in nature. If this is the case, it shouldn’t necessarily matter what particular task we perform on the object each time we see it; faster recognition = faster reaction times = repetition priming.

Logan proposed a completely different idea [7]. He suggested that when we see an object for the first time and respond to the object, we learn a direct mapping between the stimulus and response (an S-R association). When the object is seen for a second time, we simply retrieve the S-R association learned on the first presentation and quickly make the same response. Put simply, we don’t need to figure out what response to make anymore as we just remember what we did last time. This idea makes a clear prediction that ‘perceptual’ theories of repetition priming do not. It predicts that if you have to respond differently on the second presentation of an object relative to the first presentation, repetition priming should decrease. In other words, you should only see repetition priming when you are able to make the same response on the second presentation. Across a series of 4 elegant experiments Logan clearly showed this to be the case. When participants had to perform a different response between stimulus presentations, or compute a new response following a switch in task, repetition priming significantly decreased. He therefore provided strong evidence for his view of repetition priming. 

Again, why should we care? Well, the two theories of priming outlined above are fundamentally distinct learning mechanisms. One relates to perceptual learning, one to associative stimulus-response learning. A phenomenon as robust and ubiquitous as repetition priming deserves to be fully understood. This debate is particularly important for fMRI studies of “neural priming”, where people use an effect called repetition suppression or fMR-adaptation (a decrease in the fMRI BOLD response to a repeated relative to a novel stimulus). Many studies have presumed “neural priming” relates to perceptual learning within visual regions. If Logan is correct, however, the effect they are measuring may actually relate to the learning/retrieval of S-R associations and therefore may not be telling us anything about perceptual learning per se [8]. Without being aware of this experimental psychology research, it would be easy to confound perceptual learning with S-R learning by not including a response manipulation within your experimental design. This is, therefore, a classic example of how cognitive psychology and clever experimental design can help inform more ‘neuroscientific’ studies of the same underlying phenomena.

Finally, just a quick note to say Logan wasn’t completely correct. It seems, as is often the case, that repetition priming results from multiple learning mechanisms. One does relate to the learning of perceptual (and conceptual) representations and another relates to the learning of S-R associations [9]. As a result of Logan’s original work we now have much clearer understanding of the multiple mechanisms underlying repetition priming, as well as having much tighter experimental control over these different contributions.

References:

1. Horner, A. J., & Henson, R. N. (2009). Bindings between stimuli and multiple response codes dominate long-lag repetition priming in speeded classification tasks. Journal of Experimental Psychology: Learning, Memory, and Cognition, 35(3), 757–779.
2. Warrington, E. K., & Weiskrantz, L. (1968). New Method of Testing Long-term Retention with Special Reference to Amnesic Patients. Nature, 217, 972–972.
3. Mitchell, D. B. (2006). Nonconscious priming after 17 years: invulnerable implicit memory? Psychological Science, 17(11), 925–929. 
4. Schacter, D. L. (1990). Perceptual representation systems and implicit memory. Annals of the New York Academy of Sciences, 608, 543–571. 
5. Tenpenny, P. L. (1995). Abstractionist versus episodic theories of repetition priming and word identification. Psychonomic Bulletin & Review, 2, 339–363.
6. Bowers, J. S. (2000). In defense of abstractionist theories of repetition priming and word identification. Psychonomic Bulletin & Review, 7(1), 83–99. 
7. Logan, G. D. (1990). Repetition priming and automaticity: common underlying mechanisms? Cognitive Psychology, 22, 1–35.
8. Dobbins, I. G., Schnyer, D. M., Verfaellie, M., & Schacter, D. L. (2004). Cortical activity reductions during repetition priming can result from rapid response learning. Nature, 428(6980), 316–319.
9. Horner, A. J., & Henson, R. N. (2008). Priming, response learning and repetition suppression. Neuropsychologia, 46(7), 1979–1991.