Gennadiy Gurariy

The neural representation of objects formed through the spatiotemporal integration of visual transients

Oftentimes, objects are only partially and transiently visible as parts of them become occluded during observer or object motion. The visual system can integrate such object fragments across space and time into perceptual wholes or spatiotemporal objects. This integrative and dynamic process may involve both ventral and dorsal visual processing pathways, along which shape and spatial representations are thought to arise. We measured fMRI BOLD response to spatiotemporal objects and used multi-voxel pattern analysis (MVPA) to decode shape information across 20 topographic regions of visual cortex. Object identity could be decoded throughout visual cortex, including intermediate (V3A, V3B, hV4, LO1-2,) and dorsal (TO1-2, and IPS0-1) visual areas. Shape-specific information, therefore, may not be limited to early and ventral visual areas, particularly when it is dynamic and must be integrated. Contrary to the classic view that the representation of objects is the purview of the ventral stream, intermediate and dorsal areas may play a distinct and critical role in the construction of object representations across space and time.

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 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.

Towards a unified perspective of object shape and motion processing in human dorsal cortex

Although object-related areas were discovered in human parietal cortex a decade ago, surprisingly little is known about the nature and purpose of these representations, and how they differ from those in the ventral processing stream. In this article, we review evidence for the unique contribution of object areas of dorsal cortex to three-dimensional (3-D) shape representation, the localization of objects in space, and in guiding reaching and grasping actions. We also highlight the role of dorsal cortex in form-motion interaction and spatiotemporal integration, possible functional relationships between 3-D shape and motion processing, and how these processes operate together in the service of supporting goal-directed actions with objects. Fundamental differences between the nature of object representations in the dorsal versus ventral processing streams are considered, with an emphasis on how and why dorsal cortex supports veridical (rather than invariant) representations of objects to guide goal-directed hand actions in dynamic visual environments.

Electrophysiological correlates of encoding processes in a full-report visual working memory paradigm

Why are some visual stimuli remembered, whereas others are forgotten? A limitation of recognition paradigms is that they measure aggregate behavioral performance and/or neural responses to all stimuli presented in a visual working memory (VWM) array. To address this limitation, we paired an electroencephalography (EEG) frequency-tagging technique with two full-report VWM paradigms. This permitted the tracking of individual stimuli as well as the aggregate response. We recorded high-density EEG (256 channel) while participants viewed four shape stimuli, each flickering at a different frequency. At retrieval, participants either recalled the location of all stimuli in any order (simultaneous full report) or were cued to report the item in a particular location over multiple screen displays (sequential full report). The individual frequency tag amplitudes evoked for correctly recalled items were significantly larger than the amplitudes of subsequently forgotten stimuli, regardless of retrieval task. An induced-power analysis examined the aggregate neural correlates of VWM encoding as a function of items correctly recalled. We found increased induced power across a large number of electrodes in the theta, alpha, and beta frequency bands when more items were successfully recalled. This effect was more robust for sequential full report, suggesting that retrieval demands can influence encoding processes. These data are consistent with a model in which encoding-related resources are directed to a subset of items, rather than a model in which resources are allocated evenly across the array. These data extend previous work using recognition paradigms and stress the importance of encoding in determining later VWM retrieval success.