In the figure, the test stimulus on trial n (penguins) was the sample stimulus from trial n-1. The repetition of a stimulus on the immediately following test could potentially produce confusion—and thereby proactive interference—as to whether the test stimulus matches the sample stimulus on the current trial or the sample stimulus on the previous trial.
If there is such interference, then at least two things could be controlling the effect, which Devkar and Wright sought to disentangle: One possibility is the number of events (or stimuli) between the interfering stimulus (the “interferent”) and the test (in the above figure that would be just 1 event) or the amount of time between the two, or both.
Devkar and Wright found a strong effect of the number of events between the presentation of the interferent and the test trial, with performance being poorer when the interferents occurred immediately before the test trial. Interestingly, no corresponding effect of time delay was observed. This was surprising as there is much evidence to suggest that time delay is likely to matter, especially if one assumes that memory for earlier information decays with the passage of time.
To investigate this possibility further, Devkar and Wright ran the same experiment a second time but included longer delays between stimuli. Again, they observed no effect of time. This suggests that the number of events in between the interferent and the test trials were controlling performance, rather than time.
To make doubly sure, Devkar and Wright ran a third experiment in which they compared the impact of using both familiar and completely novel stimuli as the interferent and on the test trials. Again, they found an effect of the intervening number of events—however, they also found that the type of intervening event mattered, with the impact of proactive interference being greater with the novel stimuli. This suggest that the encounters with novel events were better remembered than familiar events (on trial n-1) and therefore caused more proactive interference (on trial n).
The event-based finding is in strong contrast to an analogous experiment in pigeons
which found that time was an important factor in predicting the likelihood of proactive interference.
Author Tony Wright says “monkeys showed less memory interference when the interference and test were separated by more intervening trials (events), but this was independent of time manipulations. By contrast, in a previous companion study, pigeons had shown memory depended upon time, characterized by the ratio between delay time and time since interference.”
“Primates, unlike birds, perhaps explicitly remembered the intervening event” he continues.
This difference is “astounding” (that is a direct quote from the paper and I agree with it). So what does this tell us? Well, it suggests that there are fundamental differences between monkeys and pigeons in how memories are processed and the way in which they interact and interfere with each other. The authors hesitantly suggest that the primate brain might be better at using existing knowledge to make predictions about novel events than the avian brain. Any such conclusion would, of course, require much more work, however, this piece opens up exciting new possibilities for comparative cognition research.
And in case you are wondering, unlike pigeons, humans are also not affected by the passage of time
in this task—just like our Simian relatives.
Focus article for this post:
Devkar, D. T., & Wright, A. A. (2016). Event-based proactive interference in rhesus monkeys. Psychonomic Bulletin & Review.