Slope Of Response Research Articles

MAPPING DISTURBANCE DYNAMICS IN WET SCLEROPHYLL FORESTS USING TIME SERIES LANDSAT

In this study, we characterised the temporal-spectral patterns associated with identifying acute-severity disturbances and low-severity disturbances between 1985 and 2011 with the objective to test whether different disturbance agents within these categories can be identified with annual Landsat time series data. We analysed a representative State forest within the Central Highlands which has been exposed to a range of disturbances over the last 30 years, including timber harvesting (clearfell, selective and thinning) and fire (wildfire and prescribed burning). We fitted spectral time series models to annual normal burn ratio (NBR) and Tasseled Cap Indices (TCI), from which we extracted a range of disturbance and recovery metrics. With these metrics, three hierarchical random forest models were trained to 1) distinguish acute-severity disturbances from low-severity disturbances; 2a) attribute the disturbance agents most likely within the acute-severity class; 2b) and attribute the disturbance agents most likely within the low-severity class. Disturbance types (acute severity and low-severity) were successfully mapped with an overall accuracy of 72.9 %, and the individual disturbance types were successfully attributed with overall accuracies ranging from 53.2 % to 64.3 %. Low-severity disturbance agents were successfully mapped with an overall accuracy of 80.2 %, and individual agents were successfully attributed with overall accuracies ranging from 25.5 % to 95.1. Acute-severity disturbance agents were successfully mapped with an overall accuracy of 95.4 %, and individual agents were successfully attributed with overall accuracies ranging from 94.2 % to 95.2 %. Spectral metrics describing the disturbance magnitude were more important for distinguishing the disturbance agents than the post-disturbance response slope. Spectral changes associated with planned burning disturbances had generally lower magnitudes than selective harvesting. This study demonstrates the potential of landsat time series mapping for fire and timber harvesting disturbances at the agent level and highlights the need for distinguishing between agents to fully capture their impacts on ecosystem processes.

Mesoscale variability of the summer bloom over the northern Ross Sea shelf: A tale of two banks

Multi-year satellite records indicate an asymmetric spatial pattern in the summer bloom in the Northern Ross Sea, with the largest blooms over the shallows of Pennell Bank compared to Mawson Bank. In 2010–2011, high-resolution spatiotemporal in situ sampling focused on these two banks to better understand factors contributing to this pattern. Dissolved and particulate Fe profiles suggested similar surface water depletion of dissolved Fe on both banks. The surface sediments and velocity observations indicate a more energetic water column over Mawson Bank. Consequently, the surface mixed layer over Pennell Bank was more homogeneous and shallower. Over Mawson Bank we observed a thicker more homogeneous bottom boundary layer resulting from stronger tidal and sub-tidal currents. These stronger currents scour the seafloor resulting in sediments less likely to release additional sedimentary iron. Estimates of the quantum yield of photosynthesis and the initial slope of the photosynthesis-irradiance response were lower over Mawson Bank, indicating higher iron stress over Mawson Bank. Overall, the apparent additional sedimentary source of iron to, and longer surface residence time over Pennell Bank, as well as the reduced fluxes from the more isolated bottom mixed layer over Mawson Bank, sustain the observed asymmetric pattern across both banks.

Structural sensitivity and resilience in a predator–prey model with density-dependent mortality

Numerous formulations with the same mathematical properties can be relevant to model a biological process. Different formulations can predict different model dynamics like equilibrium vs. oscillations even if they are quantitatively close (structural sensitivity). The question we address in this paper is: does the choice of a formulation affect predictions on the number of stable states? We focus on a predator–prey model with predator competition that exhibits multiple stable states. A bifurcation analysis is realized with respect to prey carrying capacity and species body mass ratio within range of values found in food web models. Bifurcation diagrams built for two type-II functional responses are different in two ways. First, the kind of stable state (equilibrium vs. oscillations) is different for 26.0–49.4% of the parameter values, depending on the parameter space investigated. Using generalized modelling, we highlight the role of functional response slope in this difference. Secondly, the number of stable states is higher with Ivlev's functional response for 0.1–14.3% of the parameter values. These two changes interact to create different model predictions if a parameter value or a state variable is altered. In these two examples of disturbance, Holling's disc equation predicts a higher system resilience. Indeed, Ivlev's functional response predicts that disturbance may trap the system into an alternative stable state that can be escaped from only by a larger alteration (hysteresis phenomena). Two questions arise from this work: (i) how much complex ecological models can be affected by this sensitivity to model formulation? and (ii) how to deal with these uncertainties in model predictions?

Adaptive Prediction Error Coding in the Human Midbrain and Striatum Facilitates Behavioral Adaptation and Learning Efficiency

SummaryEffective error-driven learning benefits from scaling of prediction errors to reward variability. Such behavioral adaptation may be facilitated by neurons coding prediction errors relative to the standard deviation (SD) of reward distributions. To investigate this hypothesis, we required participants to predict the magnitude of upcoming reward drawn from distributions with different SDs. After each prediction, participants received a reward, yielding trial-by-trial prediction errors. In line with the notion of adaptive coding, BOLD response slopes in the Substantia Nigra/Ventral Tegmental Area (SN/VTA) and ventral striatum were steeper for prediction errors occurring in distributions with smaller SDs. SN/VTA adaptation was not instantaneous but developed across trials. Adaptive prediction error coding was paralleled by behavioral adaptation, as reflected by SD-dependent changes in learning rate. Crucially, increased SN/VTA and ventral striatal adaptation was related to improved task performance. These results suggest that adaptive coding facilitates behavioral adaptation and supports efficient learning.

Design of cascaded all pass network with monotonous group delay response for broadband radio frequency applications

In this study, an iterative procedure is developed for designing radio frequency circuits with a specified group delay dispersion (GDD) characteristics over a broad bandwidth. Second order all pass networks (APNs) are cascaded systematically over multiple stages to achieve the same. The approach is flexible enough to design circuits for various group delay responses. This is demonstrated using two-stage APN with both positive and negative slopes of linear and non-linear group delay responses for a frequency band of 500 MHz-1 GHz. The proposed multi-stage design procedure is shown to help extend the bandwidth over which a uniform resolution of frequency discrimination is possible. Practical limitations in implementing this circuit are addressed while developing the prototype. The circuit is realised as a surface mount device-based implementation of modified two-stage APN. The realised circuit has a GDD of -6 ns/GHz and an insertion loss of 1.2 dB for 500 MHz-1 GHz band. The performance parameters such as GDD, insertion loss and device footprint of the proposed approach are significantly better than previously reported.

Resistance to imidacloprid in different field populations of Aphis gossypii Glover (Hem.: Aphididae) in South of Iran

The cotton aphid, <em>Aphis gossypii</em> Glover (Hemiptera: Aphididae), is a key cucurbits pest in Iran and is managed with repeated insecticide applications. Reports of insecticide control failures have recently increased, particularly with imidacloprid. To quantify resistance to imidacloprid in cotton aphid, seven populations were collected from 7 different places in South of Iran (Shiraz, Jahrom, Saadatshahr, Marvdasht, Kavar, Sadra1 and Sadra2, all in Fars province). To estimate the response of 5 days old <em>A. gossypii</em> populations to imidacloprid, leaf dip bioassays were performed in the laboratory. Lethal concentrations at 50% (LC<sub>50</sub>) values were estimated by probit analysis and used to calculate the resistance ratios (RR). The bioassay results showed significant discrepancy in susceptibility to imidacloprid among the populations. The lowest and highest LD<sub>50</sub> were estimated for Shiraz population with 37.09 and Sadra1 with 636.80 <span>µ</span>g mL<sup>–1</sup> respectively. The highest levels of resistance to imidacloprid were detected for Sadra1 (RR=17.17 fold). In the other populations some levels of resistance were detected. In Jahrom, Kavar, Marvdasht and SaadatShahr populations the RRs were from 3.85 to 7.11. As emphasized by the slope of responses and after a comparison of RRs with other studies it is supposed that resistant populations appear in first stages of development and have the ability to become more resistant with age.

Intimate Partner Violence, Mental Health, and HPA Axis Functioning

Research results are mixed as to whether stress exerts its damaging effects via underor over-production of diurnal cortisol. Facets of the stressor itself as well as the mental health sequelae that follow have been put forward as important considerations in determining levels of cortisol secretion. We hypothesized that the contradictory findings in the literature were the result of variable-oriented methods masking the presence of distinctive subgroups of individuals. Using person-oriented methods, we explored whether there were classes of women who exhibited unique profiles of cortisol secretion, stress, and mental health by assessing 182 community women, many of whom had experienced intimate partner violence. The best fitting model in a latent profile analysis had 5 groups, each with distinct profiles of intimate partner violence stress (pregnancy and postpartum), cortisol secretion [cortisol awakening response (CAR) and diurnal slope], and mental health (posttraumatic stress, depressive, and anxiety symptoms). These were a Physiologically Under-Responsive group, a Healthy group, a Problematic CAR group, a Highest Stress/Normal Diurnal Slope group, and a Moderate Psychopathology/Normal Diurnal Slope group. Except for the Healthy group, the specific patterns of stress, mental health symptoms, and cortisol secretion identified in the literature were not found. The profiles were validated using variables that, in prior research, had shown relationships with the variables used to constitute the profiles—three types of parenting (neglectful, sensitive, and harsh), antisocial behavior, and physical health. We concluded that there is heterogeneity in women’s responses to stress. Current theories focused on the underor over-production of diurnal cortisol in relation to stress and mental health symptoms are simplistic and fail to account for the significant subgroups of women who show unique biological and psychological responses.

Bronchodilator response following methacholine-induced bronchoconstriction predicts acute asthma exacerbations.

Methacholine bronchial provocation test provides the concentration of methacholine causing a 20% decrease in forced expiratory volume in 1 s (FEV1) from baseline (PC20). The dose-response slope (DRS), and other continuous indices of responsiveness (CIR; the percentage decline from the post-diluent baseline FEV1 after the last dose of methacholine), and per cent recovery index (PRI; the percentage increase from the maximally reduced FEV1 after bronchodilator inhalation) are alternative measures. The clinical relevance of these indices in predicting acute asthma exacerbations has not been fully evaluated.In two prospective cohorts of childhood and elderly asthmatics, baseline PC20, DRS, CIR and PRI were measured and evaluated as predictors of acute asthma exacerbations.We found that PRI was significantly related to the presence of asthma exacerbations during the first year of follow-up in both cohorts of childhood (p=0.025) and elderly asthmatics (p=0.003). In addition, PRI showed a significant association with the total number of steroid bursts during 4.3 years of follow-up in the cohort of childhood asthmatics (p=0.04).We demonstrated that PRI, an index of reversibility following methacholine-induced bronchoconstriction, was a good clinical predictor of acute exacerbations of asthma in both childhood and elderly asthmatics.

Shelfbreak current over the Canadian Beaufort Sea continental slope: Wind‐driven events in January 2005

AbstractThe shelfbreak current over the Beaufort Sea continental slope is known to be one of the most energetic features of the Beaufort Sea hydrography. In January 2005, three oceanographic moorings deployed over the Canadian (eastern) Beaufort Sea continental slope simultaneously recorded two consecutive shelfbreak current events with along‐slope eastward bottom‐intensified flow up to 120 cm s−1. Both events were generated by the local wind forcing associated with two Pacific‐born cyclones passing north of the Beaufort Sea continental slope toward the Canadian Archipelago. Over the mooring array, the associated westerly wind exceeded 15 m s−1. These two cyclones generated storm surges along the Beaufort Sea coast with sea surface height (SSH) rising up to 1.4 m following the two westerly wind maxima. We suggest that the westerly along‐slope wind generated a surface Ekman onshore transport. The associated SSH increase over the shelf produced a cross‐slope pressure gradient that drove an along‐slope eastward geostrophic current, in the same direction as the wind. This wind‐driven barotropic flow was superimposed on the background baroclinic bottom‐intensified shelfbreak current that consequently amplified. Summer‐fall satellite altimetry data for 1992–2013 show that the SSH gradient in the southeastern Beaufort Sea is enhanced over the upper continental slope in response to frequent storm surge events. Because the local wind forcing and/or sea‐ice drift could not explain the reduction of sea‐ice concentration over the Beaufort Sea continental slope in January 2005, we speculate that wind‐driven sea level fluctuations may impact the sea‐ice cover in winter.

A High-Sensitivity Potentiometric Mercuric Ion Sensor Based on m-Toluidine Films

A sensor of $m$ -toluidine polymer film coated platinum electrode was fabricated by the electropolymerization using cyclic voltammetry technique for the detection of mercury ions (Hg2+) in aqueous solution. This paper was carried out using the simple potentiometric method and confirmed by a cyclic voltammetry technique. The effects of the polymer film thickness and pH of Hg2+ solutions on the response of the sensor were studied. Moreover, the stability, sensitivity, and selectivity of the $m$ -toluidine sensor were investigated. The optimum thickness of the polymer film was obtained after ten cyclic voltammetric runs. This film has a Nernstian response slope of 29.19 mV/decade with a detection limit of $3.54\times 10^{\mathrm {\mathbf {-5}}}$ M at 293 K by the simple potentiometric method. In addition, it has a sensitivity of $4\times 10^{\mathrm {\mathbf {-7}}}$ AM $^{\mathrm {\mathbf {-1}}}$ with a detection limit of $1.33\times 10^{\mathrm {\mathbf {-7}}}$ M by the cyclic voltammetry method. Moreover, the sensor is specific to Hg2+ ions in the presence of other ions, such as Na+, K+, Mg2+, Ca2+, Co2+, Ni2+, Zn2+, and Pb2+. Furthermore, the most stable response of the sensor to Hg2+ ions in the solution of pH ranged from 4 to 6.4 for a lifetime of about eleven weeks. Moreover, the sensor was applied for detection of four natural samples: 1) tap water; 2) underground water; 3) first distillate water; and 4) another sample wasted with Hg2+ ions.

Impact of physical fitness on salivary stress markers in sedentary to low-active young to middle-aged men

IntroductionSalivary cortisol (sCort) and alpha-amylase (sAA) are popular measurement tools for investigating hypothalamic-pituitary-adrenal and autonomic functioning, respectively. Despite their widespread use, only little is known about whether lifestyle factors such as regular exercise and physical fitness impact these two salivary stress markers. The lack of an objectively assessed fitness status is a major drawback of nearly all previous studies in this area. Our main objective was therefore to examine the impact of fitness status on sCort and sAA responses to a validated stressor. MethodSedentary to low-active men (N=115, 45.7±10.1 years, 26.7±3.4kg/m2) were exposed to the Trier Social Stress Test for Groups and provided saliva samples before and repeatedly after the stressor. An ergometer lactate exercise test provided information on physical fitness, that is, power achieved at the individual anaerobic threshold relative to body weight. From age-specific percentiles, a 2- (n=65 low fitness, n=50 high fitness) and a 3-group (n=37 low fitness, n=49 moderate fitness, n=29 high fitness) solution were calculated to elucidate the impact of different fitness levels. ResultsPrior to stress testing, sCort and sAA levels were similar in all groups. While both markers increased significantly due to stress exposure, fitness effects appeared rather small. When applying the 3-group solution, we observed attenuated sCort slopes in high- and moderate-fit men. Although sAA response slopes were pronounced in high-fit men, they failed to reach statistical significance. Mean stress reactivity (area under the curve with respect to ground, baseline, and response maximum) did not differ between groups and no association with fitness level was found. DiscussionOur findings suggest only a weak effect of objectively assessed physical fitness on biological stress responses with attenuated hypothalamic-pituitary-adrenal but (non-significantly) higher autonomic responses in fitter men. Physical fitness is a factor that should not be neglected in future studies employing salivary stress markers, especially in studies interested in acute stress responses.

Silicon nanodisk array with a fin field-effect transistor for time-domain weighted sum calculation toward massively parallel spiking neural networks

A nanodisk array connected with a fin field-effect transistor is fabricated and analyzed for spiking neural network applications. This nanodevice performs weighted sums in the time domain using rising slopes of responses triggered by input spike pulses. The nanodisk arrays, which act as a resistance of several giga-ohms, are fabricated using a self-assembly bio-nano-template technique. Weighted sums are achieved with an energy dissipation on the order of 1 fJ, where the number of inputs can be more than one hundred. This amount of energy is several orders of magnitude lower than that of conventional digital processors.

Understanding the Intermodel Spread in Global-Mean Hydrological Sensitivity*

Abstract This paper assesses intermodel spread in the slope of global-mean precipitation change ΔP with respect to surface temperature change. The ambiguous estimates in the literature for this slope are reconciled by analyzing four experiments from phase 5 of CMIP (CMIP5) and considering different definitions of the slope. The smallest intermodel spread (a factor of 1.5 between the highest and lowest estimate) is found when using a definition that disentangles temperature-independent precipitation changes (the adjustments) from the slope of the temperature-dependent precipitation response; here this slope is referred to as the hydrological sensitivity parameter η. The estimates herein show that η is more robust than stated in most previous work. The authors demonstrate that adjustments and η estimated from a steplike quadrupling CO2 experiment serve well to predict ΔP in a transient CO2 experiment. The magnitude of η is smaller in the coupled ocean–atmosphere quadrupling CO2 experiment than in the noncoupled atmosphere-only experiment. The offset in magnitude due to coupling suggests that intermodel spread may undersample uncertainty. Also assessed are the relative contribution of η, the surface warming, and the adjustment on the spread in ΔP on different time scales. Intermodel variation of both η and the adjustment govern the spread in ΔP in the years immediately after the abrupt forcing change. In equilibrium, the uncertainty in ΔP is dominated by uncertainty in the equilibrium surface temperature response. A kernel analysis reveals that intermodel spread in η is dominated by intermodel spread in tropical lower tropospheric temperature and humidity changes and cloud changes.

Assessing the Vulnerability of Irish Rail Network Earthworks

The Irish Rail network was largely constructed in the mid-1800s. As a result of this, a significant proportion of the network is comprised of aged cuttings and embankments the construction of which predate modern design standards. Although most of the networks have remained stable for over a hundred and fifty years, a significant proportion of the network has slope angles in excess of those recommended in current design standards. Climate change predictions expect increased rainfall levels across Europe that will deteriorate these slopes further and increase incidence of failure.Current practice when populating earthwork asset databases is to conduct a technical walkover survey. Data obtained in this way is susceptible to bias errors and involve subjective approximations. This is particularly evident in slope geometry attributes such as slope height and angle. Remote sensing data is able to improve precision while reducing bias substantially, while being a much faster alternative than visual assessments on a network scale. Having precise and reliable data over the entire network is a fundamental prerequisite when conducting relative risk assessments of assets. In this paper, post-processed findings from an airborne LiDAR survey of the entire Irish Rail network are presented and compared to walkover assessment data. The current state of assets will also be discussed in light of modern design codes, together with the implications on infrastructure performance.Slope vulnerability to shallow planar type failures is expected to increase with predicted changes in climate such as increased environmental loading (rainfall events are predicted to be more intense and of longer duration, with longer dry periods in between). This type of failure is already the dominant failure mode across Irish Rail network. Typically these failures are instigated by rainwater percolating into the slope to a given depth, filling available pore space thus reducing in-situ soil suctions. This in turn reduces the shear strength of the soil. When the percolating water reaches some critical depth failure occurs.Fragility curves, a particular type of asset vulnerability assessment, provide a connection between triggering actions (such as rainfall) and expected damage to infrastructure assets. They can therefore be potentially useful in estimating a slope's response to predicted future climate loading. An example of a fragility curve applied to a typical slope on the Irish Rail network is presented in this paper.

The Effect of Cell Wall Content of Wheat and Rice Brans on Biological Relative Bioavailability Value of Minerals Binding from the Broilers Diet

An experiment was conducted to study the effect of dietary fiber (DF) consist of NSP and lignin content of wheat bran and rice bran the relative biological availability of minerals supplementation in the broilers diet. A total of 120 broiler chicks (Ross 308) arranged with 4 repetitions (6 birds per each replicated) on the 5 treatments diet in metabolic cages. The experimental diets contain one control diet and 4 diets were formulated, basal diet containing 25% wheat bran and rice bran that those supplemented with multi-enzyme. Data shown that enzyme supplementation had effect on increase biological apparent Ileal digestibility values minerals of diets content wheat bran and rice bran were relatively high P digestibility. Thus, those cell wall were effected on bon quality characteristics and seedor index tibiae respectively (P<0.05). The present study shown that relative bioavailability values (biological availability), of minerals concentrations of wheat bran and rice bran as cell wall consist of nonstarch polysaccharide and lignin source sources based on slope of regression response to control diet was set equal to 100%, on them on daily dietary minerals intake were estimated. Relative bioavailability values, based on daily dietary minerals intake, when response to control diet was set at 100%, were 30.78, 71.43, 0.64, 91.67, 96.15 and 138.46 for rice bran, as well; values of wheat bran are 50.00, 28.57, 1.82, 42.50, 46.15, and 30.77, respectively. The result of experiment indicated that Zn, Fe, Mn and P from rice bran, as well as, Ca, Mn, and Fe for wheat bran were have highly relative bioavailability values, with respect to amount of cell wall consist of non-starch polysaccharide and lignin sources.

Evaluation of Skin Reactivity during (Immuno-) Therapy. Validation of Methods for Estimation of Changes in Skin Reactivity and Correlation to Shock Organ Sensitivity

Background: Parallel line bioassay (PLBA) has been acknowledged to be the gold standard for estimation of changes in reactivity, e.g., in RAST and ELISA inhibition tests. Objective: To study correlations between two simple methods for evaluation of changes in skin prick test (δSPT), using the slope of the allergen dose response (drra) in relation to PLBA. Methods: Skin prick test data from two published immunotherapy trials were used. In a D. farinae trial we used duplicate tests with three fixed ten-fold concentrations and in a P. judaica trial three tenfold individually chosen allergen concentrations causing wheals of similar size to that of histamine dihydrochloride 10 mg/mL, tenfold lower and tenfold higher concentration. Evaluation of the δSPT by PLBA, and two simple methods were correlated. In the D. farinae trial δSPT was compared to the change of conjunctival threshold concentration. Results: The δSPT as measured by both the simple methods gave similar results to that of PLBA (p<0.001). The δSPT was around 30-fold, i.e., about 3% of the pre-treatment reactivity. The δSPT correlated with the δCPT threshold concentration. Conclusions: Estimation of the δSPT during therapy expressed as change in concentration using simple methods based on the slope of the drra correlated well to changes by PLBA and CPT and should therefore be used both in clinical research and in practice.

Broadband Reflectarray Antenna Using Subwavelength Elements Based on Double Square Meander-Line Rings

A linearly polarized broadband reflectarray is presented employing a novel single layer subwavelength phase shifting element. The size of the element is a fifth of a wavelength at the center frequency of 10 GHz and the element consists of double concentric square rings of meander lines. By changing the length of the meander line, a 420° phase variation range is achieved at the center frequency. This characteristic makes the proposed configuration unique, as most of the reported subwavelength reflectarray elements can only realize a phase range far less than 360°. In addition, the slope of the phase response remains almost constant from 9 to 11 GHz, demonstrating a broadband property. A $48 \times 48$ -element reflectarray antenna is simulated, fabricated, and measured. Good agreement is obtained between simulated and measured results. A measured 1.5-dB gain bandwidth of 18% and 56.5% aperture efficiency is achieved.

Polydopamine / Cobalt Hexacyanoferrate Composite Films and Their Electrochemical Behavior in the Presence of Dpamine

Polydopamine/cobalt hexacyanoferrate (PDA-CoHCF) composite films on a glassy carbon electrode (GCE) have been electropolymerized by using cyclic voltammetry. The composite films were electrochemically activated in a potassium hydroxide solution and were characterized by scanning electron microscope (SEM) measurements. The electrochemical activation could cause some changes of composite membrane interface structure and improve the electrochemical performance of the corresponding modified electrode in a phosphate buffer solution containing dopamine. The PDA-CoHCF film modified glassy carbon electrode could response dopamine sensitively in cyclic voltammetry measurements for electrochemical oxidation and reduction. The linear response slope and regression coefficient is 1.546 μA/μM and 0.985, respectively, in the concentration range from 2.0 μM to 10 μM dopamine.

Achievement of linear response for competitive bioaffinity assays of ligands: criteria of optimized interaction systems

For a linear response, an optimized competitive bioaffinity assay of a ligand requiresCRT&gt; 3 ×CPT,CPT&gt; 50 ×KdR, andKdR&gt; 260 ×KdX(CRTandCPTare concentrations of the probe and protein whileKdXandKdRareKdfor the ligand and probe, respectively).

Hypothalamic–pituitary–adrenal axis response to acute psychosocial stress: Effects of biological sex and circulating sex hormones

Dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis influences the risk for developing stress-related disorders. Sex-dependent differences in the HPA axis stress response are believed to contribute to the different prevalence rates of stress-related disorders found in men and women. However, studies examining the HPA axis stress response have shown mixed support for sex differences, and the role of endogenous sex hormones on HPA axis response has not been adequately examined in humans. This study utilized the largest sample size to date to analyze the effects of biological sex and sex hormones on HPA axis social stress responses. Healthy, 18- to 30- year-old community volunteers (N=282) completed the Trier Social Stress Test (TSST), a widely used and well-validated stress-induction laboratory procedure. All women (n=135) were tested during the follicular phase of their menstrual cycle (when progesterone levels are most similar to men). Adrenocorticotropic hormone (ACTH) and cortisol measures were collected at multiple points throughout pre- and post-TSST. Testosterone and progesterone (in men) and progesterone and estradiol (in women) were determined pre-TSST. Following the TSST, men had greater ACTH and cortisol levels than women. Men had steeper baseline-to-peak and peak-to-end ACTH and cortisol response slopes than women; there was a trend for more cortisol responders among men than women. Testosterone negatively correlated with salivary cortisol response in men, while progesterone negatively correlated with ACTH and cortisol responses in women. These data confirm that men show more robust activation of the HPA axis response to the TSST than do women in the follicular phase of the menstrual cycle. Testosterone results suggest an inhibitory effect on HPA axis reactivity in men. Progesterone results suggest an inhibitory effect on HPA axis reactivity in women. Future work is needed to explain why men mount a greater ACTH and cortisol response to the TSST than do women during the follicular phase of the menstrual cycle.