The Relationship between Working Memory Storage and Elevated Activity as Measured with Functional Magnetic Resonance Imaging

Departments of Psychology and Psychiatry, University of Wisconsin-Madison, Madison, Wisconsin 53706.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 09/2012; 32(38):12990-8. DOI: 10.1523/JNEUROSCI.1892-12.2012
Source: PubMed


Does the sustained, elevated neural activity observed during working memory tasks reflect the short-term retention of information? Functional magnetic resonance imaging (fMRI) data of delayed recognition of visual motion in human participants were analyzed with two methods: a general linear model (GLM) and multivoxel pattern analysis. Although the GLM identified sustained, elevated delay-period activity in superior and lateral frontal cortex and in intraparietal sulcus, pattern classifiers were unable to recover trial-specific stimulus information from these delay-active regions. The converse-no sustained, elevated delay-period activity but successful classification of trial-specific stimulus information-was true of posterior visual regions, including area MT+ (which contains both middle temporal area and medial superior temporal area) and calcarine and pericalcarine cortex. In contrast to stimulus information, pattern classifiers were able to extract trial-specific task instruction-related information from frontal and parietal areas showing elevated delay-period activity. Thus, the elevated delay-period activity that is measured with fMRI may reflect processes other than the storage, per se, of trial-specific stimulus information. It may be that the short-term storage of stimulus information is represented in patterns of (statistically) "subthreshold" activity distributed across regions of low-level sensory cortex that univariate methods cannot detect.

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Available from: Bradley R Postle, Aug 05, 2014
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    • "Some of the first evidence that contradicted the view that PFC represents stimulus information in working memory came from neuroimaging studies in humans. Researchers showed that delay period activity in PFC did not encode information specific to the stimulus being held in working memory (Curtis and D'Esposito, 2003; Riggall and Postle, 2012), while the converse was true for posterior sensory areas (Ester et al., 2009; Harrison and Tong, 2009; Serences et al., 2009; Emrich et al., 2013). These findings are important because they confirm that PFC is active during the delay period. "
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    ABSTRACT: A prominent account of prefrontal cortex (PFC) function is that single neurons within the PFC maintain representations of task-relevant stimuli in working memory. Evidence for this view comes from studies in which subjects hold a stimulus across a delay lasting up to several seconds. Persistent elevated activity in the PFC has been observed in animal models as well as in humans performing these tasks. This persistent activity has been interpreted as evidence for the encoding of the stimulus itself in working memory. However, recent findings have posed a challenge to this notion. A number of recent studies have examined neural data from the PFC and posterior sensory areas, both at the single neuron level in primates, and at a larger scale in humans, and have failed to find encoding of stimulus information in the PFC during tasks with a substantial working memory component. Strong stimulus related information, however, was seen in posterior sensory areas. These results suggest that delay period activity in the PFC might be better understood not as a signature of memory storage per se, but as a top down signal that influences posterior sensory areas where the actual working memory representations are maintained.
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    • "This view is supported by studies showing overlapping activations for visual and verbal (Cowan et al., 2011; Majerus et al., 2010), verbal and spatial (Chein, Moore, & Conway, 2011), and verbal and tonal WM (Koelsch et al., 2009). For each sensory domain, there appears to be a domain-general frontoparietal network that directs attention to item-specific information stored in posterior sensory regions during WM maintenance (Harrison & Tong, 2009; Lewis-Peacock, Drysdale, Oberauer, & Postle, 2012; Riggall & Postle, 2012). This model too includes the central executive and it remains to be seen if Cowan's focus of attention and Baddeley's episodic buffer are different. "
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    ABSTRACT: This article is a summary of attention and memory as well as their interactions in the context of brain imaging research performed in the last two decades. Attention is introduced under a three-component framework which divides attention into three distinct components: alerting, orienting, and executive control. Memory is introduced under the three-component model: sensory memory, short-term or working memory (WM), and long-term memory (LTM). Emphasis is put on the components showing the closest interactions, such as interactions between orienting and executive control in the area of attention and between WM and LTM.
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    • "The neural signature of the FOA appears to be fronto - parietal modulation of the neural representations in sensory regions engaged in memory maintenance ( Feredoes et al . , 2011 ; Riggall and Postle , 2012 ; Emrich et al . , 2013 ) . "
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    ABSTRACT: What is the role of top-down attentional modulation in consciously accessing working memory (WM) content? In influential WM models, information can exist in different states, determined by allocation of attention; placing the original memory representation in the center of focused attention gives rise to conscious access. Here we discuss various lines of evidence indicating that such attentional modulation is not sufficient for memory content to be phenomenally experienced. We propose that, in addition to attentional modulation of the memory representation, another type of top-down modulation is required: suppression of all incoming visual information, via inhibition of early visual cortex. In this view, there are three distinct memory levels, as a function of the top-down control associated with them: (1) Nonattended, nonconscious associated with no attentional modulation; (2) attended, phenomenally nonconscious memory, associated with attentional enhancement of the actual memory trace; (3) attended, phenomenally conscious memory content, associated with enhancement of the memory trace and top-down suppression of all incoming visual input.
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