Eunsam Shin

Span, crunch, and beyond: Working memory capacity and the aging brain

Neuroimaging data emphasize that older adults often show greater extent of brain activation than younger adults for similar objective levels of difficulty. A possible interpretation of this finding is that older adults need to recruit neuronal resources at lower loads than younger adults, leaving no resources for higher loads, and thus leading to performance decrements [Compensation-Related Utilization of Neural Circuits Hypothesis; e.g., Reuter-Lorenz, P. A., & Cappell, K. A. Neurocognitive aging and the compensation hypothesis. Current Directions in Psychological Science, 17, 177–182, 2008]. The Compensation-Related Utilization of Neural Circuits Hypothesis leads to the prediction that activation differences between younger and older adults should disappear when task difficulty is made subjectively comparable. In a Sternberg memory search task, this can be achieved by assessing brain activity as a function of load relative to the individuals memory span, which declines with age. Specifically, we hypothesized a nonlinear relationship between load and both performance and brain activity and predicted that asymptotes in the brain activation function should correlate with performance asymptotes (corresponding to working memory span). The results suggest that age differences in brain activation can be largely attributed to individual variations in working memory span. Interestingly, the brain activation data show a sigmoid relationship with load. Results are discussed in terms of Cowans [Cowan, N. The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24, 87–114, 2001] model of working memory and theories of impaired inhibitory processes in aging.

Multiple Levels of Stimulus Representation in Visual Working Memory

Object recognition presumably involves activation of multiple levels of representation. Here we use the encoding-related lateralization (ERL) method [Gratton, G. The contralateral organization of visual memory: A theoretical concept and a research tool. Psychophysiology, 35, 638647, 1998] to describe the sequential activation of several of these levels. The ERL uses divided-field encoding to generate contralaterally biased representations in the brain. The presence and nature of these representations can be demonstrated by examining the event-related potentials (ERPs) elicited by centrally presented test probes for lateralized activity corresponding to the encoding side. We recorded ERPs during a memory-search task. Memory sets were composed of two or four uppercase letters displayed half to the left and half to the right of fixation. Probe stimuli were composed of one letter presented foveally in either upper- or lowercase. Letter case was manipulated to differentiate the time course of physical and symbolic levels of letter representation. Memory set size was manipulated to examine a relational level of letter representation. We found multiple ERLs in response to the probes: (1) An early (peak = 170 msec) case-dependent (but set size independent) ERL, most evident at P7/P8, indexing the availability of a physical level of letter representation; (2) a later (200400 msec) more diffusedly distributed ERL, independent of both letter case and set size, indexing a symbolic level of letter representation; (3) a long-latency (400600 msec) ERL occurring at posterior sites, larger for the case match, Set Size 2 condition, indexing competition for neural representation across multiple letters. By assuming that these ERL activities track the progression of letter representation over time, we propose a model of letter processing in the context of visual working memory.

Hemispheric Organization of Visual Memory: Analyzing Visual Working Memory With Brain Measures

We review research investigating the reactivation of memory representations through the combined use of behavioral, event-related brain potential, and optical measures. We assume that (1) early stimulus processing creates memory representations in the same brain regions that participated in that processing; and (2) re-exposure to the same stimuli results in brain and behavioral responses that reflect the initial processing. Hence, if stimuli are presented using a divided field paradigm, the ensuing memory representations should be lateralized to the hemisphere contralateral to the side of initial presentation. By studying differential adaptation in the two hemispheres, aspects of the memory representations can be accessed, supporting the hemispheric organization of visual memories and showing that at least some memory phenomena depend on latent activations triggered by the initial stimulus presentation. This approach can also be used to reveal the relative timing of reactivation of visual memory representations.

Training Improves the Capacity of Visual Working Memory When It Is Adaptive, Individualized, and Targeted

The current study investigated whether training improves the capacity of visual working memory using individualized adaptive training methods. Two groups of participants were trained for two targeted processes, filtering and consolidation. Before and after the training, the participants, including those with no training, performed a lateralized change detection task in which one side of the visual display had to be selected and the other side ignored. Across ten-day training sessions, the participants performed two modified versions of the lateralized change detection task. The number of distractors and duration of the consolidation period were adjusted individually to increase the task difficulty of the filtering and consolidation training, respectively. Results showed that the degree of improvement shown during the training was positively correlated with the increase in memory capacity, and training-induced benefits were most evident for larger set sizes in the filtering training group. These results suggest that visual working memory training is effective, especially when it is adaptive, individualized, and targeted.

When crowding meets binocular rivalry: Challenges for object perception

Both crowding and binocular rivalry impair object perception, but their influence on object perception has so far only been investigated in separate fields. Three experiments investigated the joint influences of crowding and rivalry on object perception (orientation discrimination). Experiment 1 investigated how crowding and rivalry influence orientation discrimination together. Experiment 2 tested whether rivalry between flankers affects crowding using an orientation discrimination task. Experiment 3 tested whether crowding affects the temporal dynamics of the rivalry between a target and a rival stimulus. In Experiment 1, judgments of target orientation were more impaired when crowding and rivalry were simultaneously induced than when they were separately induced and their effects were combined. In Experiment 2, judgments of target orientation were impaired even when flankers were undergoing rivalry, thus highlighting the importance of the presence of flankers. Experiment 3 showed that flankers presented in the neighborhood of a target undergoing rivalry shortened target dominance and prolonged target suppression. The augmented impairments of object perception found in Experiments 1 and 3 suggest that crowding and rivalry interact, presumably through signal suppression. The adverse effect of flankers shown in Experiment 2 suggests that inappropriate feature integration may have additionally contributed to this interaction.

Electrophysiological revelations of trial history effects in a color oddball search task

In visual oddball search tasks, viewing a no-target scene (i.e., no-target selection trial) leads to the facilitation or delay of the search time for a target in a subsequent trial. Presumably, this selection failure leads to biasing attentional set and prioritizing stimulus features unseen in the no-target scene. We observed attention-related ERP components and tracked the course of attentional biasing as a function of trial history. Participants were instructed to identify color oddballs (i.e., targets) shown in varied trial sequences. The number of no-target scenes preceding a target scene was increased from zero to two to reinforce attentional biasing, and colors presented in two successive no-target scenes were repeated or changed to systematically bias attention to specific colors. For the no-target scenes, the presentation of a second no-target scene resulted in an early selection of, and sustained attention to, the changed colors (mirrored in the frontal selection positivity, the anterior N2, and the P3b). For the target scenes, the N2pc indicated an earlier allocation of attention to the targets with unseen or remotely seen colors. Inhibitory control of attention, shown in the anterior N2, was greatest when the target scene was followed by repeated no-target scenes with repeated colors. Finally, search times and the P3b were influenced by both color previewing and its history. The current results demonstrate that attentional biasing can occur on a trial-by-trial basis and be influenced by both feature previewing and its history.