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Visual working memory (VWM) is the ability to
maintain visual information in a readily available and easily updated state.
Converging evidence has revealed that VWM capacity is limited by the number of
maintained objects, which is about 3 - 4 for the average human. Recent work
suggests that VWM capacity is also limited by the resolution required to
maintain objects, which is tied to the objects’ inherent complexity.
Electroencephalogram (EEG) studies using the Contralateral Delay Activity (CDA)
paradigm have revealed that cortical representations of VWM are at a minimum
loosely organized like the primary visual system, such that the left side of
space is represented in the right hemisphere, and vice versa. Recent functional
magnetic resonance imaging (fMRI) work shows that the number of objects is
maintained by representations in the inferior intraparietal sulcus (IPS) along
dorsal parietal cortex, whereas the resolution of these maintained objects is
subserved by the superior IPS and the lateral occipital complex (LOC). These
areas overlap with recently-discovered, retinotopically-organized visual field
maps (VFMs) spanning the IPS (IPS-0/1/2/3/4/5), and potentially maps in lateral
occipital cortex, such as LO-1/2, and/or TO-1/2 (hMT+). Other fMRI studies have
implicated early VFMs in posterior occipital cortex, suggesting that visual
areas V1-hV4 are recruited to represent information in VWM. Insight into
whether and how these VFMs subserve VWM may illuminate the nature of VWM. In
addition, understanding the nature of these maps may allow a greater
investigation into individual differences among subjects and even between
hemispheres within subjects.
As determined by transmission electron microscopy (TEM), the reduction of selenate and selenite by Desulfovibrio desulfuricans, a sulfate-reducing bacterium, produces spherical (Se, S) sub-micro particles outside the cell. The particles are crystalline or amorphous, depending on medium composition. Amorphous-like Se-rich spherical particles may also occur inside the bacterial cells. The bacteria are more active in the reduction of selenite than selenate. The Desulfovibrio desulfuricans bacterium is able to extract S in the (S, Se) solid solution particles and transform S-rich particles into Se-rich and Se crystals. Photoautotrophs, such as Chromatium spp., are able to oxidize sulfide (S2-). When the bacteria grow in sulfide- and selenide-bearing environments, they produce amorphous-like (S, Se) globules inside the cells. TEM results show that compositional zonation in the (S, Se) globules occur in Chromatium spp. collected from a top sediment layer of a Se-contaminated pond. S2- may be from the products of sulfate-reducing bacteria. Both the sulfate-reducing bacteria and photosynthetic Chromatium metabolize S preferentially over Se. It is proposed that the S-rich zones are formed during photosynthesis (day) period, and the Se-rich zones are formed during respiration active (night) period. The results indicate that both Desulfovibrio desulfuricans and Chromatium spp. are able to immobilize the oxidized selenium (selenate and/or selenite) in the forms of elemental selenium and (Se, S) solid solutions. The bacteria reduce S in the (Se, S) particles and further enrich Se in the crystalline particles. The reduced S combines with Fe2+ to form amorphous FeS.