Fredrik Bergström

Neurocognitive Architecture of Working Memory

A crucial role for working memory in temporary information processing and guidance of complex behavior has been recognized for many decades. There is emerging consensus that working-memory maintenance results from the interactions among long-term memory representations and basic processes, including attention, that are instantiated as reentrant loops between frontal and posterior cortical areas, as well as sub-cortical structures. The nature of such interactions can account for capacity limitations, lifespan changes, and restricted transfer after working-memory training. Recent data and models indicate that working memory may also be based on synaptic plasticity and that working memory can operate on non-consciously perceived information.

Electronic-energy migration and molecular rotation within bichromophoric macromolecules. Part 1.—Test of a model using bis(9-anthrylmethylphoshonate) bisteroid

We report a model for determining the rate of energy migration, within pairs of donor (D) molecules from the fluorescence anisotropy. The D molecules within each pair reside in an anisotropic environment, and undergo rotational motions, similar to what could be the case in a protein molecule. To test the model experimentally, we have synthesized mono- and bis-(9-anthryl-methylphosphonate) bisteroid molecules. A procedure is presented for extracting the rate of energy transfer, as well as the D–D distance from the fluorescence anisotropy. The rate of energy migration obtained from experiments, ω≈ 3.5 × 108 s–1, agrees very well with that predicted. The distance 23.7 ± 2 A between the anthracenes and the mutual angle of 131 ± 3° between their orientational distributions, obtained at different temperatures, are in excellent agreement with independently determined values.

Two-photon excitation and time-resolved fluorescence: I. The proper response function for analysing single-photon counting experiments

An accurate instrumental response function is needed to deconvolute fluorescence data obtained by time-correlated single-photon counting (TCSPC) upon multi-photon excitation. Hitherto the response function was obtained by measuring Rayleigh scattering (RS) from colloidal solutions, as is also used in one-photon excited fluorescence. We show that hyper Rayleigh scattering (HRS) provides a better choice for deconvolution of fluorescence decays, as obtained by TCSPC and two-photon excitation (TPE). The one- and two-photon response functions were monitored as RS and HRS from colloidal gold particles at 800 and 400 nm, respectively.

Maintenance of non-consciously presented information engages the prefrontal cortex

Conscious processing is generally seen as required for flexible and willful actions, as well as for tasks that require durable information maintenance. Here we present research that questions the assumption that only consciously perceived information is durable (>500 ms). Using the attentional blink (AB) phenomenon, we rendered otherwise relatively clearly perceived letters non-conscious. In a first experiment we systematically manipulated the delay between stimulus presentation and response, for the purpose of estimating the durability of non-conscious perceptual representations. For items reported not seen, we found that behavioral performance was better than chance across intervals up to 15 s. In a second experiment we used fMRI to investigate the neural correlates underlying the maintenance of non-conscious perceptual representations. Critically, the relatively long delay period demonstrated in experiment 1 enabled isolation of the signal change specifically related to the maintenance period, separate from stimulus presentation and response. We found sustained BOLD signal change in the right mid-lateral prefrontal cortex, orbitofrontal cortex, and crus II of the cerebellum during maintenance of non-consciously perceived information. These findings are consistent with the controversial claim that working-memory mechanisms are involved in the short-term maintenance of non-conscious perceptual representations.

The conjunction of non-consciously perceived object identity and spatial position can be retained during a visual short-term memory task.

Although non-consciously perceived information has previously been assumed to be short-lived (< 500 ms), recent findings show that non-consciously perceived information can be maintained for at least 15 s. Such findings can be explained as working memory without a conscious experience of the information to be retained. However, whether or not working memory can operate on non-consciously perceived information remains controversial, and little is known about the nature of such non-conscious visual short-term memory (VSTM). Here we used continuous flash suppression to render stimuli non-conscious, to investigate the properties of non-consciously perceived representations in delayed match-to-sample (DMS) tasks. In Experiment I we used variable delays (5 or 15 s) and found that performance was significantly better than chance and was unaffected by delay duration, thereby replicating previous findings. In Experiment II the DMS task required participants to combine information of spatial position and object identity on a trial-by-trial basis to successfully solve the task. We found that the conjunction of spatial position and object identity was retained, thereby verifying that non-conscious, trial-specific information can be maintained for prospective use. We conclude that our results are consistent with a working memory interpretation, but that more research is needed to verify this interpretation.

Hyper Rayleigh Scattering Yields Improved Response Function in Analysing 2-Photon Excited Fluorescence

An accurate instrumental response function is needed to conclusively deconvolute fluorescence data based on time-correlated single-photon counting (TCSPC) and multiphoton excitation. Routinely the response function is measured as Rayleigh scattering (RS) from a colloidal solution, even if the excitation is a multiphoton event. Present work demonstrates that a response function obtained as hyper Rayleigh scattering (HRS) provides a better choice for deconvolution of 2-photon excited fluorescence decays. The 1- and 2-photon response functions were monitored as RS and HRS from colloidal gold particles at 800 and 400 nm, respectively.

Extended Förster theory of donor–donor energy migration in bifluorophoric macromolecules. Part II. Method for determining intramolecular distances with experimental validation using mono and bifluorophoric systems

Recently an approximate theory was presented and applied for determining intramolecular distances in proteins. The rate of donor–donor energy migration (DDEM) is extracted and analysed from fluorescence depolarisation experiments by means of the DDEM model (Karolin et al., Biophys. J., 1998, 74, 11; Bergstrom et al., Proc. Natl. Acad. Sci., 1999, 96, 12477). Previously an extended Forster theory (EFT) was derived (Johansson et al., J. Chem. Phys., 1996, 105, 10896), which accounts for DDEM between reorienting molecules. For the first time, this rigorous theory is applied for analysing time-resolved fluorescence depolarisation data, accumulated by using the time-correlated single photon counting (TCSPC) technique. A simulation–deconvolution algorithm is presented which reduces the need of the DDEM model (Edman et al., Phys. Chem. Chem. Phys., 2000, 2, 1789), and other approximate theories (Edman et al., Mol. Phys., submitted). Two bifluorophoric systems were studied, namely; 1,32-dihydroxy-dotriacontane-bis(rhodamine) 101 ester solubilised in lipid vesicles, and bis(9-anthrylmethyl-phosphonate) bisteroid dissolved in propane-1,2-diol. The bis-rhodamine molecules span across lipid bilayers, so that the two rhodamine moieties of the molecule are localised on opposite sides of a bilayer. From the analyses of the fluorescence anisotropy, the donor–donor distances were determined to be 36.5±1 and 21.0±1.5 A, for the membrane spanning molecule and the bisteroid, respectively. The results are in good agreement with independent studies.

Application of donor-donor energy migration (DDEM) for examining protein structure and function

Donor-Donor Energy Migration (DDEM) and fluorescence anisotropy experiments can be utilized as a versatile tool for examining protein structure and function. For this, pairs of identical fluorescent probes (D) are attached to unique residues created by means of site specific mutagenesis. Present work illustrates the applicability of the method on the latent form of Plasminogen Activator Inhibitor-1 (PAI-1). Different DD-pairs of mutated PAI-1 were prepared and studied, namely; V106C-H185C, H185C-M266C and M266C-V106C. The Cys residues were labelled with a sulfhydryl specific derivative of BODIPYR [N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a- diaza-s-indacene-3-yl)methyl iodo-acetamide]. To determine the rate of DDEM within such a pair, intramolecular order and dynamics must be considered (Biophys. J., 74, 11-21, 1997). For analysis of data, additonal information was obtained from experiments with the corresponding D-labelled single Cys mutants, that is, V106C, H185C and M266C. The stability of values determined was tested by generating and re-analyzing synthetic data. The intramolecular distances obtained agree, reasonably well, with those determined from the X-ray structure of latent PAI-1.