Interactions of Memory and Attention: Goal Maintenance Failure and Biased Competition

What differentiates short-term memory and working memory? According to one perspective, working memory tasks require the dynamic online manipulation of information, whereas in short-term memory tasks, information must merely be kept active for later recall. For example, digit span might be considered a short-term memory task, in which the proper string of numbers can merely be repeated moments later. In contrast, operation span would be considered a working memory task, in which some to-be-remembered information must be kept active while other distracting information is used by memory for simple arithmetic operations.

What functions are used to manipulate information in working memory (WM) tasks, but not in short-term memory (STM) tasks? In their 2003 JEP:G article, Kane & Engle argue that the difference is controlled attention, but we might as well call this executive function: processes like goal maintenance, updating, inhibition, set-shifting, and selection might be involved in working memory, but not short-term memory tasks.

As Kane & Engle point out, the idea that "WM = STM + controlled attention" is supported both by latent factor analyses as well as the high correlation between working memory tasks and nonverbal tests of general fluid intelligence, which is unaffected by partialling out variance in WM tasks shared by STM tasks. Recent work with the operation span (OSPAN) working memory measure also supports the idea that controlled attention that is shared by simpler short-term memory tasks does not affect WM's correlation with several high-level measures of cognition, including nonverbal tests of general fluid intelligence. And other work shows that those with low-span are more likely to succumb to the "cocktail party effect," in which a participant notices their name in an unattended stream of speach. Those with high-span are also more able to succeed at the anti-saccade task, in which subjects must quickly look away from a sudden-onset visual stimulus. As a whole, this evidence suggests that domain-general attentional control processes contribute to WM, but not STM span measures.

At a high-level, one can interpret these attention control processes as helping to resolve interference between bottom-up perceptual information and the intended action or goal. This might happen through enhanced activation of the intended goal, or through inhibition of irrelevant stimuli (in fact, these are incredibly hard to distinguish empirically and may ultimately be two sides of the same coin).

How can we measure such goal maintenance? One pillar of cognitive psychology research is the Stroop task, in which subjects must repeatedly suppress their urge to read a word (such as "red") but must instead name the ink color of the word ("yellow"). Failures of goal maintenance are particularly likely when a congruent stimulus ("red") preceedes an incongruent stimulus ("yellow"). In this case, subjects may momentarily lapse in their maintenance of the goal ("read the ink color") since the bottom-up perceptual input leads to the same response as the intended goal. In this case, strengthening the intended goal may take longer (due to competition) and so the subject may be slowed (or less accurate) in responding to the subsequent incongruent stimulus.

How might this goal maintenance ability interact with measures of WM span (which require attentional control)? This is the question investigated by Kane & Engle, in a series of experiments with the Stroop task. Critically, they view Stroop performance as driven by two processes, attention AND memory: in their own words, "resolution of the response competition between color and word dimensions in the Stroop task, an attentional process, will only be engaged when the goal to do so is sufficiently maintained in active memory."

Here are the results of their experiments:

1. In a first experiment, high-OSPAN subjects were more accurate than low-OSPAN subjects in a Stroop task with 75% congruent trials, suggesting that low-OSPAN subjects were more likely to lapse in their active goal maintenance. Additionally, low-OSPAN subjects showed a greater benefit for congruent stimuli above neutral stimuli (i.e., responding to "red" was faster than "xxx") than did high-OSPAN subjects, again suggesting that they were not as successful at limiting their attention to ink color alone.
2. In a second experiment, Kane & Engle had each subject first complete the 75% congruent condition followed by a 0% congruent condition, with feedback following every trial. The results replicated those in the first experiment, but low-spans made slightly less errors than they had previously (suggesting that the error feedback helped them correctly maintain the task goal).
3. The third experiment differed from the second only in that the 0% congruent condition was presented first, followed by the 75% congruent condition. However, the results were quite different: now low-OSPANs were less accurate than high-OSPANs in the 0% condition, and were far slower than high-OSPANs in both the 0% and 75% congruent conditions, but were not less accurate in the 75% condition. The authors interpret this to suggest that goal maintenance was made much more likely throughout the experiment by requiring subjects to undergo a condition where the goal needed to be maintained on every trial, and so instead of accuracy differences, only reaction time showed OSPAN differences (reflecting the attentional resolving of competition between representations).
4. By combining the data from Experiments 1 & 2, Kane & Engle showed that low-OSPANs reaction time difference between incongruent/neutral trials (i.e., "red" vs "xxx") is even larger than the difference for high-OSPANs regardless of task order for the 0% condition. However, task order does make a difference for the 75% condition, in that high OSPANS have lower errors than low OSPANs for the incongruent/neutral comparison when the harder condition is presented first (75% first, experiment 2), whereas high OSPANs merely have shorter reaction time differences between incongruent/neutral trials than low OSPANs when the easier condition is presented first (experiment 3).
5. In a fourth experiment, Kane & Engle replicated the first task-order effect described above with different congruency percentages: similar to the 0% condition, a 20% congruent condition revealed a smaller RT difference between incongruent/congruent trials among high-OSPANs than among low-OSPANs, regardless of when this condition was completed. However, an 80% congruent condition showed span differences between incongruent/congruent in terms of both errors and reaction times regardless of task order, whereas previously the 75% condition had shown such differences in errors only when presented first, and in reaction times only when presented second. These discrepancies might be explained by the fact that previous interference calculations subtracted neutral from incongruent trials, while those in experiment 4 subtracted congruent from incongruent trials. It's also possible that the 20% congruent condition did not reinforce the need for goal maintenance as strongly as the 0% condition had previously, and therefore low-OSPANs were more likely to fail to properly maintain the goal if next presented with the harder condition.
   

The authors interpret these results in a framework where Stroop performance is determined by two factors: attention and memory. Specifically, errors are thought to result from memory failure - failure to actively maintain a goal in mind. On the other hand, reaction time slowing is thought to result from attentional processes - a failure to quickly bias competition towards the correct representation rather than the incorrect representation. Kane & Engle interpret low-OSPAN subjects have a consistent problem with resolving competition between representations, but also show a tendency to fail to maintain the task-relevant goals in some circumstances.

Of course, this dual-component interpretation rests on the idea that goal maintenance cannot directly bias other competing representations, but relies on an additional attentional process to resolve this competition. In contrast, many computational models instantiate these as "one and the same." So, what other data support the distinction made by Kane & Engle?

• Across all subjects, the amount of RT facilitation (i.e., how much faster congruent trials are than neutral trials) correlates with error interference (i.e., how much more accurate neutral trials are than incongruent trials), suggesting that goal maintenance failure is behind both of these phenomena. In contrast, there is no correlation between the RT facilitation effect and RT interference, as would be expected if goal maintenance failure actually gives rise to all of these measures, nor is there a correlation between error & latency interference.
  
• On high-congruency Stroop tasks, schizophrenics show increased errors on incongruent relative to congruent trials, and increased facilitation on congruent relative to neutral trials.
• The distribution of response times on Stroop tasks indicate that incongruent RTs are not only shifted positively by a specific amount (reflective of the increased competition between representations), but also are positively skewed due to a few trials that take much longer than other trials. These are thought to reflect momentary failures of goal maintenance
• This positive skew is exaggerated among older adults, and in young adults when trials are presently very slowly (providing more opportunity for mind-wandering)
• ERP studies of Stroop tasks have identified a wave that may originate from anterior cingulate (ACC) and appears to correspond to response selection and competition processes; in contrast, the activity of a different wave up to 800 ms before stimulus presentation predicts correct performance on the next stimulus (and appears to originate from polar or dorsolateral frontal cortex [dlPFC])
• Event-related fMRI shows a strong negative correlation between delay-period dlPFC activity and Stroop interference, whereas ACC activity is tied to the presentation of incongruent stimuli

Although it may seem more parsimonious to suggest that a single mechanism - active maintenance of goal-relevant information - is responsible for Stroop performance, Kane & Engle have presented an abundance of evidence suggesting that two pieces of active maintenance may be dissociable: momentary failures to maintain the goal, and the time-consuming process of resolving competition between representations by biasing. Many computational models provide a clear way of visualizing the process of biased competition, but only the most recent (see, for example, this one) include a possible mechanism for stochastic goal maintenance failure.