Single-photon absorption and emission from a natural photosynthetic complex.

Photosynthesis is generally assumed to be initiated by a single photon1-3 from the Sun, which, as a weak light source, delivers at most a few tens of photons per nanometre squared per second within a chlorophyll absorption band1. Yet much experimental and theoretical work over the past 40 years has explored the events during photosynthesis subsequent to absorption of light from intense, ultrashort laser pulses2-15. Here, we use single photons to excite under ambient conditions the light-harvesting 2 (LH2) complex of the purple bacterium Rhodobacter sphaeroides, comprising B800 and B850 rings that contain 9 and 18 bacteriochlorophyll molecules, respectively. Excitation of the B800 ring leads to electronic energy transfer to the B850 ring in approximately 0.7 ps, followed by rapid B850-to-B850 energy transfer on an approximately 100-fs timescale and light emission at 850-875 nm (refs. 16-19). Using a heralded single-photon source20,21 along with coincidence counting, we establish time correlation functions for B800 excitation and B850 fluorescence emission and demonstrate that both events involve single photons. We also find that the probability distribution of the number of heralds per detected fluorescence photon supports the view that a single photon can upon absorption drive the subsequent energy transfer and fluorescence emission and hence, by extension, the primary charge separation of photosynthesis. An analytical stochastic model and a Monte Carlo numerical model capture the data, further confirming that absorption of single photons is correlated with emission of single photons in a natural light-harvesting complex.

Comorbidity Burden and Health Care Utilization by Substance use Disorder Patterns among People with HIV in Florida.

Substance use disorder (SUD), a common comorbidity among people with HIV (PWH), adversely affects HIV clinical outcomes and HIV-related comorbidities. However, less is known about the incidence of different chronic conditions, changes in overall comorbidity burden, and health care utilization by SUD status and patterns among PWH in Florida, an area disproportionately affected by the HIV epidemic. We used electronic health records (EHR) from a large southeastern US consortium, the OneFlorida + clinical research data network. We identified a cohort of PWH with 3 + years of EHRs after the first visit with HIV diagnosis. International Classification of Diseases (ICD) codes were used to identify SUD and comorbidity conditions listed in the Charlson comorbidity index (CCI). A total of 42,271 PWH were included (mean age 44.5, 52% Black, 45% female). The prevalence SUD among PWH was 45.1%. Having a SUD diagnosis among PWH was associated with a higher incidence for most of the conditions listed on the CCI and faster increase in CCI score overtime (rate ratio = 1.45, 95%CI 1.42, 1.49). SUD in PWH was associated with a higher mean number of any care visits (21.7 vs. 14.8) and more frequent emergency department (ED, 3.5 vs. 2.0) and inpatient (8.5 vs. 24.5) visits compared to those without SUD. SUD among PWH was associated with a higher comorbidity burden and more frequent ED and inpatient visits than PWH without a diagnosis of SUD. The high SUD prevalence and comorbidity burden call for improved SUD screening, treatment, and integrated care among PWH.

Depression and Anxiety Symptoms and Treatment Utilization, and Associated HIV Outcomes among Adults with HIV in Rural Florida.

Persons living with HIV (PLWH) and depression or anxiety in the rural South may have suboptimal HIV outcomes. We sought to examine the proportion of PLWH from rural Florida with symptoms of depression or anxiety, the proportion who received depression or anxiety treatment, and the relationship between untreated and treated symptoms of depression or anxiety and HIV outcomes. Cross-sectional survey data collected between 2014 and 2018 were analyzed. Among 187 PLWH residing in rural Florida (median age 49 years, 61.5%, male 45.5% Black), 127 (67.9%) met criteria for symptoms of depression and/or anxiety. Among these 127 participants, 60 (47.2%) were not on depression or anxiety treatment. Participants with untreated symptoms of depression and anxiety (OR 3.2, 95% CI 1.2-9.2, p = 0.03) and treated depression and anxiety with uncontrolled symptoms (OR 1.4, 95% CI 0.5-4.0, p = 0.52) were more likely to have viral non-suppression compared to those without depression or anxiety in an unadjusted bivariate analysis. Only the association between untreated symptoms of depression and anxiety and viral non-suppression was statistically significant, and when adjusting for social and structural confounders the association was attenuated and was no longer statistically significant. This suggests that social and structural barriers impact both mental health and HIV outcomes. Our findings support the need for increased mental health services and resources that address the social and structural barriers to care for PLWH in the rural South.

Antiretroviral Therapy Concealment Behaviors and their Association with Antiretroviral Therapy Adherence among People with HIV: Findings from the Florida Cohort Study.

Little is known about HIV medication concealment behaviors and the effect of medication concealment on antiretroviral therapy (ART) adherence among people with HIV (PWH). This study aims to (1) to describe medication concealment behaviors and factors associated with these behaviors, and (2) assess the association between medication concealment and suboptimal ART adherence. The Florida Cohort Study enrolled adult PWH from community-based clinics around the state from October 2020 to September 2022 (n = 416, 62% aged 50+, 56% male, 44% non-Hispanic Black, 18% Hispanic). Participants responded to questions about sociodemographics, stigma, ART adherence (≥ 85%), symptoms of depression, social networks and disclosure to their networks, and actions to conceal ART to avoid inadvertent disclosure of their HIV status. Analyses were conducted using multivariable logistic regressions models. The most common concealment behavior was hiding ART while having guests over (32%), followed by removing ART labels (26%), and putting ART into a different bottle (16%). Overall, 43% reported ≥ 1 behavior. In multivariable models, depressive symptoms, incomplete disclosure of HIV to close social networks, and not having a close social network were associated with ART concealment. After adjusting for risk factors for suboptimal ART adherence, endorsing hiding medication while having guests was associated with suboptimal ART adherence (aOR 2.87, 95% CI 1.15-7.55). Taking any action and other individual behaviors were not associated. ART concealment behaviors were common but did not consistently negatively influence adherence when accounting for other factors. PWH may want to receive ART medications in ways that ensure privacy and reduce the risk of inadvertent disclosure.

Intersectional Stigma, Fear of Negative Evaluation, Depression, and ART Adherence Among Women Living with HIV Who Engage in Substance Use: A Latent Class Serial Mediation Analysis.

Women Living with HIV (WLHIV) who use substances face stigma related to HIV and substance use (SU). The relationship between the intersection of these stigmas and adherence to antiretroviral therapy (ART), as well as the underlying mechanisms, remains poorly understood. This study aimed to examine the association between intersectional HIV and SU stigma and ART adherence, while also exploring the potential role of depression and fear of negative evaluation (FNE) by other people in explaining this association. We analyzed data from 409 WLHIV collected between April 2016 and April 2017, Using Multidimensional Latent Class Item Response Theory analysis. We identified five subgroups (i.e., latent classes [C]) of WLHIV with different combinations of experienced SU and HIV stigma levels: (C1) low HIV and SU stigma; (C2) moderate SU stigma; (C3) higher HIV and lower SU stigma; (C4) moderate HIV and high SU stigma; and (C5) high HIV and moderate SU stigma. Medication adherence differed significantly among these classes. Women in the class with moderate HIV and high SU stigma had lower adherence than other classes. A serial mediation analysis suggested that FNE and depression symptoms are mechanisms that contribute to explaining the differences in ART adherence among WLHIV who experience different combinations of intersectional HIV and SU stigma. We suggest that FNE is a key intervention target to attenuate the effect of intersectional stigma on depression symptoms and ART adherence, and ultimately improve health outcomes among WLHIV.

Assessing HIV-related stigma in the clinical setting: are providers in Florida interested?

HIV-related stigma is a key contributor to poor HIV-related health outcomes. The purpose of this study is to explore implementing a stigma measure into routine HIV care focusing on the 10-item Medical Monitoring Project measure as a proposed measure. Healthcare providers engaged in HIV-related care in Florida were recruited. Participants completed an interview about their perceptions of measures to assess stigma during clinical care. The analysis followed a directed content approach. Fifteen participants completed the interviews (87% female, 47% non-Hispanic White, case manager 40%). Most providers thought that talking about stigma would be helpful (89%). Three major themes emerged from the analysis: acceptability, subscales of interest, and utility. In acceptability, participants mentioned that assessing stigma could encourage patient-centered care and serve as a conversation starter, but some mentioned not having enough time. Participants thought that the disclosure concerns and negative self-image subscales were most relevant. Some worried they would not have resources for patients or that some issues were beyond their influence. Participants were generally supportive of routinely addressing HIV-related stigma in clinical care, but were concerned that resources, especially to address concerns about disclosure and negative self-image, were not available.

The association of depression and anxiety symptoms to three different types of activities of daily living in persons with and without HIV.

Mood disorders are highly prevalent in people living with HIV (PLWH) and represent a potential contributor to functional impairment in activities of daily living. We aimed to determine if (1) Anxiety and depression symptoms were independently associated with impairments in basic self-care, role functioning, and social functioning and (2) PLWH differentially experienced impairments due to mood symptoms compared to those without HIV. Data for this study were obtained from 150 individuals (87 PLWH, 61% male, mean age = 44) via a cross-sectional study on alcohol and HIV-associated brain dysfunction. The Beck Anxiety Inventory (BAI) and the Center for Epidemiologic Studies Depression Scale (CES-D) were used to assess anxiety and depressive symptoms. Higher anxiety symptoms were associated with role functioning impairment, while higher depressive and anxiety symptoms were each associated with social functioning impairment. As depressive symptoms increased, PLWH were 3x more likely to have impairments in role functioning compared to those without HIV. HIV status did not interact with mood symptoms to affect basic self-care or social functioning. Overall, mood symptoms are associated with different types of functional impairment, and improved management of mood symptoms could lead to improved role and social functioning.

A Behavioral Economic Examination of Sexual Behaviors in the Era of Pre-exposure Prophylaxis via Explanatory Sequential Mixed Methods.

Pre-exposure prophylaxis (PrEP) use may be associated with condom use decisions. The current investigation examined sexual decision-making in the context of PrEP among young adult men who have sex with men (MSM) between 18 and 30 years old, using an explanatory sequential mixed methods design. For the quantitative aim, 99 MSM currently taking PrEP (i.e., PrEP-experienced) and 140 MSM not currently taking PrEP (i.e., PrEP-naive) completed an online survey, including the Sexual Delay Discounting Task (SDDT), which captures likelihood of condom use. For the qualitative aim, 15 people from each group were interviewed about their (1) conceptualizations of risky sex and (2) ways they manage their sexual risk. Participants were, on average, 25.69 years old (SD = 3.07) and 64% White. Results from the quantitative aim revealed, controlling for covariates, PrEP-experienced participants exhibited significantly lower likelihood of (1) using an immediately available condom and (2) waiting for a delayed condom (i.e., sexual delay discounting) compared to PrEP-naive participants. Qualitative themes explaining what young adult MSM consider to be risky sex included: (1) any sex as risky sex, (2) risky sex as "sex without a conversation," and (3) risky sex as sex with risk for physical harm. Themes on ways young adult MSM manage sexual risk were classified as proactive, reactive, and passive. Results suggest that PrEP use is related to condom use decisions. Taken together, quantitative differences in sexual delay discounting, but qualitatively similar conceptualizations and management of risky sex, suggest that the SDDT may be a useful tool in sex research to capture processes (i.e., delay discounting) underlying sexual decision-making that may be missed by traditional self-reports. Implications of results, including potentially providing (good quality) condoms with every PrEP prescription, and future research topics are discussed.

Spatial and temporal analysis of HIV clinical outcomes in Florida reveals counties with persistent racial and ethnic disparities during 2012-2019.

BACKGROUND: Racial/ethnic disparities in the HIV care continuum have been well documented in the US, with especially striking inequalities in viral suppression rates between White and Black persons with HIV (PWH). The South is considered an epicenter of the HIV epidemic in the US, with the largest population of PWH living in Florida. It is unclear whether any disparities in viral suppression or immune reconstitution-a clinical outcome highly correlated with overall prognosis-have changed over time or are homogenous geographically. In this analysis, we 1) investigate longitudinal trends in viral suppression and immune reconstitution among PWH in Florida, 2) examine the impact of socio-ecological factors on the association between race/ethnicity and clinical outcomes, 3) explore spatial and temporal variations in disparities in clinical outcomes. METHODS: Data were obtained from the Florida Department of Health for 42,369 PWH enrolled in the Ryan White program during 2008-2020. We linked the data to county-level socio-ecological variables available from County Health Rankings. GEE models were fit to assess the effect of race/ethnicity on immune reconstitution and viral suppression longitudinally. Poisson Bayesian hierarchical models were fit to analyze geographic variations in racial/ethnic disparities while adjusting for socio-ecological factors. RESULTS: Proportions of PWH who experienced viral suppression and immune reconstitution rose by 60% and 45%, respectively, from 2008-2020. Odds of immune reconstitution and viral suppression were significantly higher among White [odds ratio =2.34, 95% credible interval=2.14-2.56; 1.95 (1.85-2.05)], and Hispanic [1.70 (1.54-1.87); 2.18(2.07-2.31)] PWH, compared with Black PWH. These findings remained unchanged after accounting for socio-ecological factors. Rural and urban counties in north-central Florida saw the largest racial/ethnic disparities. CONCLUSIONS: There is persistent, spatially heterogeneous, racial/ethnic disparity in HIV clinical outcomes in Florida. This disparity could not be explained by socio-ecological factors, suggesting that further research on modifiable factors that can improve HIV outcomes among Black and Hispanic PWH in Florida is needed.

Trajectories of Pre-Exposure Prophylaxis (PrEP) Adherence Among Commercially Insured Individuals.

We used group-based trajectory models to identify four distinct trajectory patterns of adherence to pre-exposure prophylaxis (PrEP) among 20,696 users. Only 44.5% were consistently PrEP adherent, with younger age, being female, or having substance use disorder or depression associated with early discontinuation. Public health efforts are needed to improve PrEP adherence.

Human Papillomavirus Positivity at 3 Anatomical Sites Among Transgender Women in Central Brazil.

BACKGROUND: Transgender women (TGW) are susceptible to the acquisition of sexually transmitted infections (STIs), including human papillomavirus (HPV). Nonetheless, the exact data for this population are scarce. We estimated HPV positivity at the anal, genital, and oral sites among TGW and also identified the related characteristics and behaviors that could be risk factors for HPV infection in a sample of TGW in Brazil. Furthermore, we characterized the site-specific HPV genotypes among those who were positive for HPV at these 3 sites. METHODS: A cross-sectional study was conducted on TGW in Goiânia City (Central-Midwest region), Brazil, between April 2018 and August 2019. Respondent-driven sampling was applied for recruitment. Next, self-collected anal, genital, and oral samples were examined for HPV DNA using polymerase chain reaction (SPF-10 primer). Human papillomavirus genotypes were identified in 12 TGW. RESULTS: In the TGW included in the study, the anal, genital, and oral HPV positivity values were 77.2% (95% confidence interval [CI], 67.3%-84.6%), 33.5% (95% CI, 26.1%-48.9%), and 10.9% (95% CI, 5.8%-17.0%), respectively. In addition, the majority of 12 participants who tested for HPV had multiple genotypes. HPV-52 was the most prevalent genotype identified at the anal (66.6%) and genital (40.0%) sites, whereas HPV-62 and HPV-66 were the most common at the oral site (25.0%). CONCLUSIONS: A high HPV positivity was observed among TGW. Therefore, additional epidemiological studies on HPV genotypes should generate health intervention information, including the prevention, diagnosis, and treatment of sexually transmitted infections.

Learning and organization of within-session sequences by pigeons (Columba livia).

Most animals engage in complex activities that are the combination of simpler actions expressed over a period of time. The mechanisms organizing such sequential behavior have been of long-standing biological and psychological interest. Previously, we observed pigeons' anticipatory behavior with a within-session sequence involving four choice alternatives suggestive of a potential understanding of the overall order and sequence of the items within a session. In that task, each colored alternative was correct for 24 consecutive trials as presented in a predictable sequence (i.e., A first, then B, then C, then D). To test whether these four already-trained pigeons possessed a sequential and linked representation of the ABCD items, we added a second four-item sequence involving new and distinct colored choice alternatives (i.e., E first for 24 trials, then F, then G, then H) and then alternated these ABCD and EFGH sequences over successive sessions. Over three manipulations, we tested and trained trials composed of combinations of elements from both sequences. We determined that pigeons did not learn any within-sequence associations among the elements. Despite the availability and explicit utility of such sequence cues, the data suggest instead that pigeons learned the discrimination tasks as a series of temporal associations among independent elements. This absence of any sequential linkage is consistent with the hypothesis that such representations are difficult to form in pigeons. This pattern of data suggests that for repeated sequential activities in birds, and potentially other animals including humans, there are highly effective, but underappreciated, clock-like mechanisms that control the ordering of behaviors.

Does the Relationship Between HIV Stigma Subtypes and Viral Suppression Differ by Age?: A Stratified Analysis of Data from the Florida Medical Monitoring Project.

HIV-related stigma is recognized as a top barrier to achieve viral suppression in the United States, but data describing who is most affected by HIV stigma is limited. The study sought to (1) identify the relationships between HIV-related stigma and unsuppressed viral load and (2) examine whether the association between HIV stigma subtypes and unsuppressed viral load differ by age group (i.e., 18-34, 35-49, and 50+ years-old) using surveillance data from the Florida Medical Monitoring Project (n = 1195). Most participants were 50+ years-old (55%), male (71%), and Black (51%). Enacted stigma was significantly associated with unsuppressed viral loads among the 18-34-year-old age group (OR 1.68, CI 1.09-2.60). After adjusting for potential confounders, only enacted stigma was independently associated with unsuppressed viral load in the 18-34-year-old age group. Results highlight the need for targeted interventions to reduce enacted stigma among younger persons with HIV to achieve viral suppression.

Substance Use and Adherence to HIV Pre-Exposure Prophylaxis in Studies Enrolling Men Who Have Sex with Men and Transgender Women: A Systematic Review.

Optimal adherence to pre-exposure prophylaxis (PrEP) is critical, but challenging. Men who have sex with men and transgender women have high rates of HIV incidence and substance use. Substance use is associated with reduced adherence to other medications, but associations between substance use and adherence to PrEP are less clear. Thus, the current review 1) systematically evaluates the measurement of substance use and PrEP adherence in studies examining both and 2) summarizes reported findings. Peer-reviewed articles published between 2010 - April 2021 examining associations between substance use and PrEP adherence were reviewed. Fifty studies met inclusion criteria. Assessment of substance use (i.e., mostly via self-reports at baseline) and PrEP adherence (i.e., often via tenofovir diphosphate [TFV-DP] concentration levels at follow-up) varied considerably across studies. Many studies used categorical variables (e.g., substance use: yes/no). Studies using TFV-DP levels defined adherence consistently (i.e., TFV-DP ≥ 700 fmol/punch), with slight variations. Qualitative studies (n = 10) indicated that substance use (mainly alcohol) is related to poorer PrEP adherence. While quantitative findings to date are equivocal for alcohol, there is a pattern of findings linking stimulant use with poorer PrEP adherence. This review reveals four methodological gaps, which can be addressed in future research by: 1) use of uniform benchmarks for substance use measures, 2) prospective assessment for substance use, 3) use of continuous outcome variables wherever possible, and 4) more extensive consideration of potential confounders. Addressing these methodological gaps may help us reach more definitive conclusions regarding associations between substance use and PrEP adherence.

Comorbidities among persons living with HIV (PLWH) in Florida: a network analysis.

People living with HIV (PLWH) experience a higher rate of age-related comorbidities at younger ages. Understanding common comorbidities among PLWH and their relationship to one another could be significant in improving aging for PLWH. The goal of the present study is to identify the most common comorbidities among PLWH and the relationship between them using network analysis. We used abstracted electronic medical record (EMR) data of PLWH from the Florida Cohort study, a prospective cohort study conducted in eight cities in Florida, USA. We used International Classification of Diseases (10th revision, ICD-10) code to classify comorbidities and organ systems. Network analysis was conducted to determine the degree and betweenness centrality among comorbidities. We included 756 PLWH with an average age of 46.4 years (SD 11.3) in the analysis. Infectious diseases (A00-B99, 50.8%), mental and behavioural (F01-F99, 47.0%), endocrine, nutritional and metabolic (E00-E88, 45.2%), and circulatory (I00-I99, 39%) disorders were the most prevalent system comorbidities among PLWH. Hypertensive disorder (I10-I1635.8%), dyslipidaemia (E78, 25.7%) and major depressive disorder (F32-F33, 23.9%) were the most common non-infectious conditions affecting PLWH. Viral hepatitis (B15-B19, 17.1%) and syphilis (A15-A53, 12%) were the most common coinfections among PLWH. Hypertension, dyslipidaemia and major depressive disorder were the most central of the comorbidities among PLWH. Comorbidities among PLWH were most prevalent for chronic disease and mental illness. Targeting shared disease risk factors in addition to monitoring known pathological pathways may prevent comorbidities among PLWH.

Critical Role of Semiquinones in Reductive Dehalogenation.

Quinones and products of their redox reactions (hydroquinones and semiquinones) have been suggested as important players in the reductive dehalogenation of organohalogens mediated by natural and pyrogenic organic matter, although based on limited direct evidence. This study focused on the reductive dehalogenation of a model organohalogen (triclosan) by 1,4-benzohydroquinone (H2Q). In the presence of H2Q only, degradation of triclosan does not occur within the experimental period (up to 288 h); however, it takes place in the presence of H2Q and FeCl3 under anoxic conditions at pH 5 and 7 (above the pKa of SQ = 4.1) only to be halted in the presence of dissolved oxygen. Kinetic simulation and thermodynamic calculations indicated that benzosemiquinone (SQ-) is responsible for the reductive degradation of triclosan, with the fitted rate constant for the reaction between SQ- and triclosan being 317 M-2 h-1. The critical role of semiquinones in reductive dehalogenation can be relevant to a wide range of quinones in natural and engineering systems based on the reported oxidation-reduction potentials of quinones/semiquinones and semiquinones/hydroquinones and supported by experiments with additional model hydroquinones.

Excited State Dynamics of a Conformationally Fluxional Copper Coordination Complex.

The conversion of solar energy into chemical fuel represents a capstone goal of the 21st century and has the potential to supply terawatts of power in a globally distributed manner. However, the disparate time scales of photodriven charge separation (∼fs) and steps in chemical reactions (∼µs) represent an inherent bottleneck in solar-to-fuels technology. To address this discrepancy, we are developing earth-abundant coordination complexes that undergo light-induced conformational rearrangements such that charge separation (CS) is hastened, while charge recombination (CR) is slowed. To these ends, we report the preparation and characterization of a new series of conformationally fluxional copper coordination complexes that contain a twisted intramolecular charge transfer (TICT) fluorophore as part of their ligand scaffold. Structural and spectroscopic characterization of the Cu(I) and Cu(II) complexes formed with these ligands in their ground states establish oxidation state-dependent conformational dynamicity, while time-resolved emission and transient absorption spectroscopies define the photophysical parameters of photo-induced excited states. Building on initial reports with a related set of molecules, the improved ligand design presented here greatly simplifies the observed photophysics, effectively shutting down unwanted ligand-centered excited states previously observed. Time-dependent density functional theory (TDDFT) analyses reveal an unusual metal-to-TICT electronic transition only reported once before, and though the formation of a CS state is not observed directly through experiments, TDDFT geometry optimizations in the excited states support the formation of transient Cu(II) CS species, lending credence to the potential success of our approach. These studies establish a clear model for the excited state dynamics at play in proof-of-concept systems and clarify key design parameters for future optimizations toward achieving long-lived CS via photoinduced conformational gating.

Global patterns of drought recovery.

Drought, a recurring phenomenon with major impacts on both human and natural systems, is the most widespread climatic extreme that negatively affects the land carbon sink. Although twentieth-century trends in drought regimes are ambiguous, across many regions more frequent and severe droughts are expected in the twenty-first century. Recovery time-how long an ecosystem requires to revert to its pre-drought functional state-is a critical metric of drought impact. Yet the factors influencing drought recovery and its spatiotemporal patterns at the global scale are largely unknown. Here we analyse three independent datasets of gross primary productivity and show that, across diverse ecosystems, drought recovery times are strongly associated with climate and carbon cycle dynamics, with biodiversity and CO2 fertilization as secondary factors. Our analysis also provides two key insights into the spatiotemporal patterns of drought recovery time: first, that recovery is longest in the tropics and high northern latitudes (both vulnerable areas of Earth's climate system) and second, that drought impacts (assessed using the area of ecosystems actively recovering and time to recovery) have increased over the twentieth century. If droughts become more frequent, as expected, the time between droughts may become shorter than drought recovery time, leading to permanently damaged ecosystems and widespread degradation of the land carbon sink.

A quantum trajectory picture of single photon absorption and energy transport in photosystem II.

We use quantum trajectory theory to study the dynamics of the first step in photosynthesis for a single photon interacting with photosystem II (PSII). By considering individual trajectories we are able to look beyond the ensemble average dynamics to compute the PSII system evolution conditioned upon individual photon counting measurements. Measurements of the transmitted photon beam strongly affects the system state, since detection of an outgoing photon confirms that the PSII must be in the electronic ground state, while a null measurement implies it is in an excited electronic state. We show that under ideal conditions, observing the null result transforms a state with a low excited state population to a state with nearly all population contained in the excited states. We study the PSII dynamics conditioned on such photon counting for both a pure excitonic model of PSII and a more realistic model with exciton-phonon coupling to a dissipative phononic environment. In the absence of such coupling, we show that the measured fluorescence rates show oscillations constituting a photon-counting witness of excitonic coherence. Excitonic coupling to the phonon environment has a strong effect on the observed rates of fluorescence, damping the oscillations. Addition of non-radiative decay and incoherent transitions to radical pair states in the reaction center to the phononic model allows extraction of a quantum efficiency of 92.5% from the long-time evolution, consistent with bulk experimental measurements.

Mechanisms of within-session sequential behavior in pigeons.

Correctly and efficiently selecting among options is critical to the organization of behavior across different time scales (minutes, days, seasons). As a result, understanding the mechanisms underlying the sequential behavior of animals has been a long-standing aim. In three experiments, four pigeons were tested in a four-choice simultaneous color discrimination. Across a session, they had to sequentially select a colored stimulus, and the correct color changed over four 24-trial phases (A→B→C→D). After learning this ABCD within-session sequence, tests identified that both timing and outcome feedback mechanisms contributed to the organization of pigeons' behavior. Different representational mechanisms are considered as accounts for the pigeons' observed sequential behavior.