The separable effects of feature precision and item load in visual short-term memory

Working memory performance is tied to stimulus complexity

Working memory is the cognitive capability to maintain and process information over short periods. Behavioral and computational studies have shown that visual information is associated with working memory performance. However, the underlying neural correlates remain unknown. To identify how visual information affects working memory performance, we conducted behavioral experiments in pigeons (Columba livia) and single unit recordings in the avian prefrontal analog, the nidopallium caudolaterale (NCL). Complex pictures featuring luminance, spatial and color information, were associated with higher working memory performance compared to uniform gray pictures in conjunction with distinct neural coding patterns. For complex pictures, we found a multiplexed neuronal code displaying visual and value-related features that switched to a representation of the upcoming choice during a delay period. When processing gray stimuli, NCL neurons did not multiplex and exclusively represented the choice already during stimulus presentation and throughout the delay period. The prolonged representation possibly resulted in a decay of the memory trace ultimately leading to a decrease in performance. In conclusion, we found that high stimulus complexity is associated with neuronal multiplexing of the working memory representation possibly allowing a facilitated read-out of the neural code resulting in enhancement of working memory performance.

Disentangling Processing and Storage Accounts of Working Memory Development in Childhood

Researchers have been asking the question of what drives the development of working memory (WM) during childhood for decades. This question is particularly challenging because so many aspects of cognition develop with age that it is difficult to disentangle them and find out which factors are causal or fundamental. In this review, we first prepare to discuss this issue by inquiring whether increases in storage, processing, or both are the fundamental driving factor(s) of the age-related increase in WM capability in childhood. We contend that by experimentally manipulating either factor and observing changes in the other, it is possible to learn about causal roles in WM development. We discuss research on school-aged children that seems to suggest, by means of such an approach, that the growth of storage is causal for some phases or steps in WM tasks, but that the growth of processing is causal for other steps. In our theoretical proposal, storage capacity of the focus of attention determines earlier steps of information processing by constraining the selective encoding of information into WM, whereas processing dependent on the focus of attention determines later steps, like the detection of patterns that can simplify the effective memory load and adoption of a proactive stance of maintenance in dual-task settings. Future directions for research are discussed.

A major role for retrieval and/or comparison in the set-size effects of change detection

Set-size effects in change detection have been attributed to capacity limits in a variety of processes, including perception, memory encoding, memory storage, memory retrieval, comparison, and decision. In this study, we investigated the locus of the effect of increasing set size from 1 to 2. The task was to detect a 90 degree change in the orientation of 1 or 2 briefly presented Gabor patterns in noise. To measure purely attentional effects and not another phenomena, such as crowding, a precue was used to manipulate relevant set size while keeping the display constant. The locus of the capacity limit was determined by varying when observers were cued to a single relevant stimulus. To begin, we measured the baseline set-size effect for change detection. Next, a dual-task procedure and a 100% valid postcue was added to test for an effect of decision: This modification did not reliably change the set-size effects. In the critical experiments, a 100% valid cue was provided during the retention interval between displays, or only one stimulus was presented in the second display (local recognition). For both of these conditions, there was only a relatively small set-size effect. These results are consistent with the bulk of capacity limits being in memory retrieval or comparison and not in perception, memory encoding, or memory storage.

A circular diffusion model of continuous-outcome source memory retrieval: Contrasting continuous and threshold accounts

A circular analogue of the diffusion model adapted for continuous response tasks is applied to a continuous-outcome source memory task. In contrast to existing models of source retrieval that attribute all of the variability in responding to memory, the circular diffusion model decomposes noise into variability arising from memory and from decision processes. We compared three models: (1) a single diffusion process with trial-to-trial variability in drift rate, (2) a mixture of two diffusion processes, one with positive drift that does not vary from trial-to-trial, and a second zero-drift process that represents discrete guessing, and (3) a hybrid model that mixed positive and zero-drift processes with trial-to-trial variability in the positive drift process. Comparison of model fits to joint response error and response-time (RT) data suggest that a memory strength threshold under which no information is retrieved appears to underlie responding in a continuous-report source memory task. Additionally, we also conditioned participants' source responding on their confidence in an old/new recognition task, ruling out the possibility that participant guessing was only due to unrecognized items. Overall, our findings support an all-or-none or some-or none view of source memory retrieval and pose a challenge to continuous models of source memory.

A single, simple, statistical mechanism explains resource distribution and temporal updating in visual short-term memory

Investigations into the way that information is held and integrated within the visual system provides some basis for understanding how visual information is represented and processed. Just over sixty years ago, Swets, Shipley, McKey, and Green (1959) demonstrated that performance within an auditory detection task increases as a function of the square root of the number of stimulus observation intervals, following the predictions of basic sampling theory, indicating the efficient perceptual integration of stimulus information. This principle of observer performance contingent on a constant rate of stimulus sampling also forms the basis of the sample-size model (Palmer, 1990; Sewell, Lilburn, & Smith, 2014) which seeks to provide an account of how memory resources might be divided among item representations in visual short-term memory (VSTM). In this article, we combine the multiple observations paradigm of Swets and colleagues with the VSTM paradigm of Sewell and colleagues and show that the sample-size relationship accounts for both the increase in performance with the number of presentation intervals and the way that performance changes as a function of the number of items in memory. The model provides an account of both the overall information limit of VSTM and an account of the dynamics of that limit, demonstrating not only that observers can selectively update specific representations in memory but that performance in this task is accounted for by a simple statistical constraint. We discuss the implications for models of VSTM capacity and architecture generally, focusing on the implications for objecthood and the characteristics of encoding to and retrieval from memory.