Alpha-Band Activity Reveals Spontaneous Representations of Spatial Position in Visual Working Memory

Abstract

Summary An emerging view suggests that spatial position is an integral component of working memory (WM), such that non-spatial features are bound to locations regardless of whether space is relevant [1,2]. For instance, past work has shown that stimulus position is spontaneously remembered when non-spatial features are stored. Item recognition is enhanced when memoranda appear at the same location where they were encoded [3–5], and accessing non-spatial information elicits shifts of spatial attention to the original position of the stimulus [6,7]. However, these findings do not establish that a persistent, active representation of stimulus position is maintained in WM because similar effect have also been documented following storage in long-term memory [8,9]. Here, we show that the spatial position of the memorandum is actively coded by persistent neural activity during a non-spatial WM task. We used a spatial encoding model in conjunction with EEG measurements of oscillatory alpha-band (8–12 Hz) activity to track active representations of spatial position. The position of the stimulus varied trial-to-trial but was wholly irrelevant to the tasks. We nevertheless observed active neural representations of the original stimulus position that persisted throughout the retention interval. Further experiments established that these spatial representations are dependent on the volitional storage of non-spatial features rather than being a lingering effect of sensory energy or initial encoding demands. These findings provide strong evidence that online spatial representations are spontaneously maintained in WM – regardless of task relevance – during the storage of non-spatial features.

Spatially Selective Alpha Oscillations Reveal Moment-by-Moment Trade-offs between Working Memory and Attention

“…This location specificity of alpha-band dynamics is not limited to the hemifield level but also tracks the specific location that is attended (Foster, Sutterer, Serences, Vogel, & Awh, 2017; Rihs, Michel, & Thut, 2007). Indeed, recent studies have used inverted encoding models (IEMs) to reconstruct population-level channel tuning functions (CTFs) from the topographic distribution of alpha-band activity that reveal location-specific information during both covert attention tasks (Foster et al, 2017; Samaha, Sprague, & Postle, 2016) and the maintenance of spatial working memories (Foster, Bsales, Jaffe, & Awh, 2017; Foster, Sutterer, Serences, Vogel, & Awh, 2016). Together, these findings suggest a tight link between alpha-band dynamics and the focus of spatial attention, regardless of whether attention is directed toward external stimuli or toward remembered locations.…”

Build-up of serial dependence in color working memory

“…This method effectively removes all systematic biases introduced in simulated trials (Figure 1c, right). For illustration purposes, we show the application of this method in one dataset (Foster et al I 32 ) with similar systematic biases (Figure 1d, left), that led to a shifted serial bias function (Figure 1d, middle) and finally a folded version, without systematic biases (Figure 1e, right). See Figure S1 for same analyses on each experimental dataset.…”

“…Right, folded version of serial dependence removes all systematic biases without any additional preprocessing. d), same as c) for trials of Foster et al I 32 .…”

“…To further characterize this serial dependence build-up, we used the folded error on each trial as a scalar measuring the evolution of serial dependence in the course of the session (see Methods). Figure 2b illustrates this analysis using a sliding window of 75 trials for Cam-Can 34,35 dataset and of 200 trials for the Foster et al I dataset 32 , both showing a clear increase of serial dependence as the session progressed. To test this effect across subjects and experiments, we obtained the regression slope of the folded error as a function of trial number.…”

“…Instead, our result suggests the involvement of slowly accumulating plastic mechanisms in serial dependence of color delayed-estimations. 32 . c) For each subject, we computed the slope of serial dependence over the course of the session (without averaging).…”