Dwight J. Peterson

Hits and misses: leveraging tDCS to advance cognitive research

The popularity of non-invasive brain stimulation techniques in basic, commercial, and applied settings grew tremendously over the last decade. Here, we focus on one popular neurostimulation method: transcranial direct current stimulation (tDCS). Many assumptions regarding the outcomes of tDCS are based on the results of stimulating motor cortex. For instance, the primary motor cortex is predictably suppressed by cathodal tDCS or made more excitable by anodal tDCS. However, wide-ranging studies testing cognition provide more complex and sometimes paradoxical results that challenge this heuristic. Here, we first summarize successful efforts in applying tDCS to cognitive questions, with a focus on working memory (WM). These recent findings indicate that tDCS can result in cognitive task improvement or impairment regardless of stimulation site or direction of current flow. We then report WM and response inhibition studies that failed to replicate and/or extend previously reported effects. From these opposing outcomes, we present a series of factors to consider that are intended to facilitate future use of tDCS when applied to cognitive questions. In short, common pitfalls include testing too few participants, using insufficiently challenging tasks, using heterogeneous participant populations, and including poorly motivated participants. Furthermore, the poorly understood underlying mechanism for long-lasting tDCS effects make it likely that other important factors predict responses. In conclusion, we argue that although tDCS can be used experimentally to understand brain function its greatest potential may be in applied or translational research.

The Gestalt principle of similarity benefits visual working memory

Visual working memory (VWM) is essential for many cognitive processes, yet it is notably limited in capacity. Visual perception processing is facilitated by Gestalt principles of grouping, such as connectedness, similarity, and proximity. This introduces the question, do these perceptual benefits extend to VWM? If so, can this be an approach to enhance VWM function by optimizing the processing of information? Previous findings have demonstrated that several Gestalt principles (connectedness, common region, and spatial proximity) do facilitate VWM performance in change detection tasks (Jiang, Olson, & Chun, 2000; Woodman, Vecera, & Luck, 2003; Xu, 2002, 2006; Xu & Chun, 2007). However, one prevalent Gestalt principle, similarity, has not been examined with regard to facilitating VWM. Here, we investigated whether grouping by similarity benefits VWM. Experiment 1 established the basic finding that VWM performance could benefit from grouping. Experiment 2 replicated and extended this finding by showing that similarity was only effective when the similar stimuli were proximal. In short, the VWM performance benefit derived from similarity was constrained by spatial proximity, such that similar items need to be near each other. Thus, the Gestalt principle of similarity benefits visual perception, but it can provide benefits to VWM as well.

Differential Frontal Involvement in Shifts of Internal and Perceptual Attention

BACKGROUND Perceptual attention enhances the processing of items in the environment, whereas internal attention enhances processing of items encoded in visual working memory. In perceptual and internal attention cueing paradigms, cues indicate the to-be-probed item before (pre-cueing) or after (retro-cueing) the memory display, respectively. Pre- and retro-cues confer similar behavioral accuracy benefits (pre-: 14-19%, retro-: 11-17%) and neuroimaging data show that they activate overlapping frontoparietal networks. Yet reports of behavioral and neuroimaging differences suggest that pre- and retro-cueing differentially recruit frontal and parietal cortices (Lepsien and Nobre, 2006). OBJECTIVE/HYPOTHESIS This study examined whether perceptual and internal attention are equally disrupted by neurostimulation to frontal and parietal cortices. We hypothesized that neurostimulation applied to frontal cortex would disrupt internal attention to a greater extent than perceptual attention. METHODS Cathodal transcranial direct current stimulation (tDCS) was applied to frontal or parietal cortices. After stimulation, participants completed a change detection task coupled with either pre- or retro-cues. RESULTS Cathodal tDCS across site (frontal, parietal) hindered performance. However, frontal tDCS had a greater negative impact on the retro-cued trials demonstrating greater frontal involvement during shifts of internal attention. CONCLUSIONS These results complement the neuroimaging data and provide further evidence suggesting that perceptual and internal attention are not identical processes. We conclude that although internal and perceptual attention are mediated by similar frontoparietal networks, the weight of contribution of these structures differs, with internal attention relying more heavily on the frontal cortex.

Contralateral delay activity tracks the influence of Gestalt grouping principles on active visual working memory representations

Recent studies have demonstrated that factors influencing perception, such as Gestalt grouping cues, can influence the storage of information in visual working memory (VWM). In some cases, stationary cues, such as stimulus similarity, lead to superior VWM performance. However, the neural correlates underlying these benefits to VWM performance remain unclear. One neural index, the contralateral delay activity (CDA), is an event-related potential that shows increased amplitude according to the number of items held in VWM and asymptotes at an individual’s VWM capacity limit. Here, we applied the CDA to determine whether previously reported behavioral benefits supplied by similarity, proximity, and uniform connectedness were reflected as a neural savings such that the CDA amplitude was reduced when these cues were present. We implemented VWM change-detection tasks with arrays including similarity and proximity (Experiment 1); uniform connectedness (Experiments 2a and 2b); and similarity/proximity and uniform connectedness (Experiment 3). The results indicated that when there was a behavioral benefit to VWM, this was echoed by a reduction in CDA amplitude, which suggests more efficient processing. However, not all perceptual grouping cues provided a VWM benefit in the same measure (e.g., accuracy) or of the same magnitude. We also found unexpected interactions between cues. We observed a mixed bag of effects, suggesting that these powerful perceptual grouping benefits are not as predictable in VWM. The current findings indicate that when grouping cues produce behavioral benefits, there is a parallel reduction in the neural resources required to maintain grouped items within VWM.

The locus of color sensation: Cortical color loss and the chromatic visual evoked potential

Color losses of central origin (cerebral achromatopsia and dyschromatopsia) can result from cortical damage and are most commonly associated with stroke. Such cases have the potential to provide useful information regarding the loci of the generation of the percept of color. One available tool to examine this issue is the chromatic visual evoked potential (cVEP). The cVEP has been used successfully to objectively quantify losses in color vision capacity in both congenital and acquired deficiencies of retinal origin but has not yet been applied to cases of color losses of cortical origin. In addition, it is not known with certainty which cortical sites are responsible for the generation of the cVEP waveform components. Here we report psychophysical and electrophysiological examination of a patient with color deficits resulting from a bilateral cerebral infarct in the ventral occipitotemporal region. Although this patient demonstrated pronounced color losses of a general nature, the waveform of the cVEP remains unaffected. Contrast response functions of the cVEP are also normal for this patient. The results suggest that the percept of color arises after the origin of the cVEP and that normal activity in those areas that give rise to the characteristic negative wave of the cVEP are not sufficient to provide for the normal sensation of color.

The steady-state visual evoked potential reveals neural correlates of the items encoded into visual working memory

Visual working memory (VWM) capacity limitations are estimated to be ~4 items. Yet, it remains unclear why certain items from a given memory array may be successfully retrieved from VWM and others are lost. Existing measures of the neural correlates of VWM cannot address this question because they measure the aggregate processing of the entire stimulus array rather than neural signatures of individual items. Moreover, this cumulative processing is usually measured during the delay period, thereby reflecting the allocation of neural resources during VWM maintenance. Here, we use the steady-state visual evoked potential (SSVEP) to identify the neural correlates of individual stimuli at VWM encoding and test two distinct hypotheses: the focused-resource hypothesis and the diffuse-resource hypothesis, for how the allocation of neural resources during VWM encoding may contribute to VWM capacity limitations. First, we found that SSVEP amplitudes were larger for stimuli that were later remembered than for items that were subsequently forgotten. Second, this pattern generalized so that the SSVEP amplitudes were also larger for the unprobed stimuli in correct compared to incorrect trials. These data are consistent with the diffuse-resource view in which attentional resources are broadly allocated across the whole stimulus array. These results illustrate the important role encoding mechanisms play in limiting the capacity of VWM.

The Role of Attention in Item-Item Binding in Visual Working Memory.

An important yet unresolved question regarding visual working memory (VWM) relates to whether or not binding processes within VWM require additional attentional resources compared with processing solely the individual components comprising these bindings. Previous findings indicate that binding of surface features (e.g., colored shapes) within VWM is not demanding of resources beyond what is required for single features. However, it is possible that other types of binding, such as the binding of complex, distinct items (e.g., faces and scenes), in VWM may require additional resources. In 3 experiments, we examined VWM item-item binding performance under no load, articulatory suppression, and backward counting using a modified change detection task. Binding performance declined to a greater extent than single-item performance under higher compared with lower levels of concurrent load. The findings from each of these experiments indicate that processing item-item bindings within VWM requires a greater amount of attentional resources compared with single items. These findings also highlight an important distinction between the role of attention in item-item binding within VWM and previous studies of long-term memory (LTM) where declines in single-item and binding test performance are similar under divided attention. The current findings provide novel evidence that the specific type of binding is an important determining factor regarding whether or not VWM binding processes require attention.

The neural fate of individual item representations in visual working memory

Problem/Research Questions Visual working memory (VWM) allows us to temporarily store relevant information from the visual world despite frequent interruptions such as saccades. Despite the importance of VWM in a variety of cognitive tasks, this process is capacity limited. Behavioral estimates of capacity converge on a storage limit of ~3-4 items (Cowan, 2001). Converging neural evidence from neuroimaging techniques supports these behavioral estimates. For example, in functional magnetic resonance imaging (fMRI) experiments, delay-related activity in regions of posterior parietal cortex increases according to the number of items held within VWM (Todd & Marois, 2004; Xu & Chun, 2006). When capacity is reached the signal asymptotes, indicating that no additional neural resources are available to store any remaining items. Additionally, an event-related potential (ERP) known as the contralateral delay activity (CDA) has been used to measure storage capacity by recording from posterior scalp sites during the delay period during VWM tasks. The CDA amplitude increases as additional items are added, reaching asymptote when individual item limits are reached (Vogel & Machizawa, 2004; Vogel, McCollough, & Machizawa, 2005). Importantly, in these previous studies the neural correlates of VWM capacity reflect the aggregate processing of all of the presented stimuli. As such, the neural-correlate signal associated with each individual item is obscured within this cumulative activity. Additionally, the majority of these studies have focused almost exclusively on maintenance processes, creating uncertainty regarding the influence of encoding processes on capacity limitations. This leaves a fundamental but important question regarding basic VWM processes unanswered. Can cumulative neural activity during encoding be used to understand the neural fate of individual items presented in VWM tasks? Here we present evidence that cumulative activity during VWM encoding can be used to identify and quantify the neural-correlate signals associated with individual stimuli. Additionally, we describe novel frequency tagging, steady-state visual evoked potential (SSVEP) techniques used to isolate and examine these neural-correlate signals.

Further studies on the role of attention and stimulus repetition in item–item binding processes in visual working memory.

A fundamental question for human memory research relates to the role of attention during the binding of distinct components into an integrated representation. A number of important differences exist between the working memory and episodic memory literature in terms of methodological implementation and empirical outcomes. For instance, episodic memory studies indicate that, although divided attention reduces performance, the magnitude of this reduction is similar regardless of whether distinct item components or the associative binding between these components is tested (e.g., Naveh-Benjamin, Guez, & Marom, 2003). In contrast, recent examinations of working memory indicate that reductions in performance under divided attention are larger during tests of item–item binding compared with item tests (Peterson & Naveh-Benjamin, 2017). The current study used methods typical of both episodic and working memory paradigms to further examine the role of attention in item–item binding in visual working memory. Faces and scenes used to create face–scene pairs were either sampled with replacement (i.e., repeated across trials as is typical in working memory experiments) or without replacement (i.e., nonrepeated across trials as is typical in episodic memory experiments) to examine visual working memory performance under parametric variation of concurrent load. Results from Experiment 1 (no load, articulatory suppression) and Experiment 2 (articulatory suppression, backward counting by two) revealed greater reductions in item–item binding relative to single item performance under divided attention regardless of whether item components were repeated or not repeated across trials of each experiment. These results provide further evidence that visual working memory binding requires attention.

Childhood Memory: An Update from the Cognitive Neuroscience Perspective

Children are not simply miniature adults. As such, the memory of a child is significantly different from the memory of an adult. Furthermore, the ability to form memories is not innate and instead develops over the first nearly two decades of life. Consequently, memory researchers working in the developmental domain must carefully design studies to probe memory function using age-appropriate paradigms. This is especially true given the growing range of experimental approaches that can be leveraged to understand the neural underpinnings of memory and its development. Techniques such as functional near-infrared spectroscopy (fNIRS), functional magnetic resonance imaging (fMRI), and high-density electroencephalography (HD-EEG) join workhorse behavioral and neuropsychological methodologies to monitor many aspects of brain function and behavior during memory formation and retrieval.