ORCID Profile
0000-0001-8155-8847
Current Organisations
University of Tsukuba
,
IPAC NASA Exoplanet Science Institute
,
NASA Ames Research Center
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Publisher: Springer Science and Business Media LLC
Date: 04-10-2022
DOI: 10.1038/S41467-022-33579-0
Abstract: Prospect theory, arguably the most prominent theory of choice, is an obvious candidate for neural valuation models. How the activity of in idual neurons, a possible computational unit, obeys prospect theory remains unknown. Here, we show, with theoretical accuracy equivalent to that of human neuroimaging studies, that single-neuron activity in four core reward-related cortical and subcortical regions represents the subjective valuation of risky gambles in monkeys. The activity of in idual neurons in monkeys passively viewing a lottery reflects the desirability of probabilistic rewards parameterized as a multiplicative combination of utility and probability weighting functions, as in the prospect theory framework. The erse patterns of valuation signals were not localized but distributed throughout most parts of the reward circuitry. A network model aggregating these signals reconstructed the risk preferences and subjective probability weighting revealed by the animals’ choices. Thus, distributed neural coding explains the computation of subjective valuations under risk.
Publisher: American Astronomical Society
Date: 17-11-2021
Publisher: American Astronomical Society
Date: 25-05-2022
Abstract: Multiplanet systems are valuable arenas for investigating exoplanet architectures and comparing planetary siblings. TOI-1246 is one such system, with a moderately bright K dwarf ( V = 11.6, K = 9.9) and four transiting sub-Neptunes identified by TESS with orbital periods of 4.31, 5.90, 18.66, and 37.92 days. We collected 130 radial velocity observations with Keck/HIRES and TNG/HARPS-N to measure planet masses. We refit the 14 sectors of TESS photometry to refine planet radii (2.97 ± 0.06 R ⊕ , 2.47 ± 0.08 R ⊕ , 3.46 ± 0.09 R ⊕ , and 3.72 ± 0.16 R ⊕ ) and confirm the four planets. We find that TOI-1246 e is substantially more massive than the three inner planets (8.1 ± 1.1 M ⊕ , 8.8 ± 1.2 M ⊕ , 5.3 ± 1.7 M ⊕ , and 14.8 ± 2.3 M ⊕ ). The two outer planets, TOI-1246 d and TOI-1246 e, lie near to the 2:1 resonance ( P e / P d = 2.03) and exhibit transit-timing variations. TOI-1246 is one of the brightest four-planet systems, making it amenable for continued observations. It is one of only five systems with measured masses and radii for all four transiting planets. The planet densities range from 0.70 ± 0.24 to 3.21 ± 0.44 g cm −3 , implying a range of bulk and atmospheric compositions. We also report a fifth planet candidate found in the RV data with a minimum mass of 25.6 ± 3.6 M ⊕ . This planet candidate is exterior to TOI-1246 e, with a candidate period of 93.8 days, and we discuss the implications if it is confirmed to be planetary in nature.
Publisher: Proceedings of the National Academy of Sciences
Date: 09-09-2013
Abstract: We show that monkeys display similar risk preferences and rationality to those of humans, suggesting that despite concerns raised by earlier reports, they can serve as a model for human behavior. Standard experimental economic techniques have long allowed us to evaluate human risk attitudes, but we do not know how they relate to wealth levels, a critical variable in economic models. We find thirsty monkeys to be more risk averse and discuss implications for the role of wealth in human decision making.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 19-05-2023
Abstract: Research in the multidisciplinary field of neuroeconomics has mainly been driven by two influential theories regarding human economic choice: prospect theory, which describes decision-making under risk, and reinforcement learning theory, which describes learning for decision-making. We hypothesized that these two distinct theories guide decision-making in a comprehensive manner. Here, we propose and test a decision-making theory under uncertainty that combines these highly influential theories. Collecting many gambling decisions from laboratory monkeys allowed for reliable testing of our model and revealed a systematic violation of prospect theory’s assumption that probability weighting is static. Using the same experimental paradigm in humans, substantial similarities between these species were uncovered by various econometric analyses of our dynamic prospect theory model, which incorporates decision-by-decision learning dynamics of prediction errors into static prospect theory. Our model provides a unified theoretical framework for exploring a neurobiological model of economic choice in human and nonhuman primates.
Publisher: American Astronomical Society
Date: 03-02-2023
Abstract: We report the discovery and characterization of a nearby (∼85 pc), older (27 ± 3 Myr), distributed stellar population near Lower Centaurus Crux (LCC), initially identified by searching for stars comoving with a candidate transiting planet from TESS (HD 109833 TOI 1097). We determine the association membership using Gaia kinematics, color–magnitude information, and rotation periods of candidate members. We measure its age using isochrones, gyrochronology, and Li depletion. While the association is near known populations of LCC, we find that it is older than any previously found LCC subgroup (10–16 Myr), and distinct in both position and velocity. In addition to the candidate planets around HD 109833, the association contains four directly imaged planetary-mass companions around three stars, YSES-1, YSES-2, and HD 95086, all of which were previously assigned membership in the younger LCC. Using the Notch pipeline, we identify a second candidate transiting planet around HD 109833. We use a suite of ground-based follow-up observations to validate the two transit signals as planetary in nature. HD 109833 b and c join the small but growing population of Myr transiting planets from TESS. HD 109833 has a rotation period and Li abundance indicative of a young age (≲100 Myr), but a position and velocity on the outskirts of the new population, lower Li levels than similar members, and a color–magnitude diagram position below model predictions for 27 Myr. So, we cannot reject the possibility that HD 109833 is a young field star coincidentally nearby the population.
Publisher: Cold Spring Harbor Laboratory
Date: 05-04-2021
DOI: 10.1101/2021.04.04.438415
Abstract: Research in behavioral economics and reinforcement learning has given rise to two influential theories describing human economic choice under uncertainty. The first, prospect theory, assumes that decision-makers use static mathematical functions, utility and probability weighting, to calculate the values of alternatives. The second, reinforcement learning theory, posits that dynamic mathematical functions update the values of alternatives based on experience through reward prediction error (RPE). To date, these theories have been examined in isolation without reference to one another. Therefore, it remains unclear whether RPE affects a decision-maker’s utility and/or probability weighting functions, or whether these functions are indeed static as in prospect theory. Here, we propose a dynamic prospect theory model that combines prospect theory and RPE, and test this combined model using choice data on gambling behavior of captive macaques. We found that under standard prospect theory, monkeys, like humans, had a concave utility function. Unlike humans, monkeys exhibited a concave, rather than inverse-S shaped, probability weighting function. Our dynamic prospect theory model revealed that probability distortions, not the utility of rewards, solely and systematically varied with RPE: after a positive RPE, the estimated probability weighting functions became more concave, suggesting more optimistic belief about receiving rewards and over-weighted subjective probabilities at all probability levels. Thus, the probability perceptions in laboratory monkeys are not static even after extensive training, and are governed by a dynamic function well captured by the algorithmic feature of reinforcement learning. This novel evidence supports combining these two major theories to capture choice behavior under uncertainty. We propose and test a new decision theory under uncertainty by combining pre-existing two influential theories in the neuroeconomics: prospect theory from economics and prediction error theory from reinforcement learning. Collecting a large dataset (over 60,000 gambling decisions) from laboratory monkeys enables us to test the hybrid model of these two core decision theories reliably. Our results showed over-weighted subjective probabilities at all probability levels after lucky win, indicating that positive prediction error systematically bias decision-makers more optimistically about receiving rewards. This trial-by-trial prediction-error dynamics in probability perception provides outperformed performance of the model compared to the standard static prospect theory. Thus, both static and dynamic elements coexist in monkey’s risky decision-making, an evidence contradicting the assumption of prospect theory.
Publisher: Research Square Platform LLC
Date: 09-09-2022
DOI: 10.21203/RS.3.RS-2017714/V1
Abstract: Research in the multidisciplinary field of neuroeconomics has been driven by two influential theories regarding human economic choice: prospect theory, which describes decision-making under risk, and reinforcement learning theory, which describes learning for decision-making. We hypothesized that these two distinct theories guide decision-making in a comprehensive manner. Here, we propose and test a new decision-making theory under uncertainty that combines these highly influential theories. Collecting many gambling decisions from laboratory monkeys allowed for reliable testing of our hybrid model and revealed a systematic violation of prospect theory’s assumption that probability weighting is static. Using the same experimental paradigm in humans, substantial similarities between monkey and human behavior were described by our hybrid model, which incorporates decision-by-decision learning dynamics of prediction errors into static prospect theory. Our new model provides a single unified theoretical framework for exploring the neurobiological model of economic choice in human and nonhuman primates.
Publisher: American Astronomical Society
Date: 27-07-2022
Abstract: As an all-sky survey, NASA’s Transiting Exoplanet Survey Satellite (TESS) mission is able to detect the brightest and rarest types of transiting planetary systems, including young planets that enable study of the evolutionary processes that occur within the first billion years. Here we report the discovery of a young, multiplanet system orbiting the bright K4.5V star, TOI-712 ( V = 10.838, M ⋆ = 0.733 − 0.025 + 0.026 M ⊙ , R ⋆ = 0.674 ± 0.016 R ⊙ , T eff = 4622 − 60 + 61 K). From the TESS light curve, we measure a rotation period of 12.48 days and derive an age between about 500 Myr and 1.1 Gyr. The photometric observations reveal three transiting mini-Neptunes ( R b = 2.049 − 0.080 + 0.120 R ⊕ , R c = 2.701 − 0.082 + 0.092 R ⊕ , R d = 2.474 − 0.082 + 0.090 R ⊕ ), with orbital periods of P b = 9.531 days, P c = 51.699 days, and P d = 84.839 days. After modeling the three-planet system, an additional Earth-sized candidate is identified, TOI-712.05 ( P = 4.32 days, R P = 0.81 ± 0.11 R ⊕ ). We calculate that the habitable zone falls between 0.339 and 0.844 au (82.7 and 325.3 days), placing TOI-712 d near its inner edge. Among planetary systems harboring temperate planets, TOI-712 ( T = 9.9) stands out as a relatively young star bright enough to motivate further characterization.
Publisher: American Astronomical Society
Date: 09-02-2023
Abstract: With data from the Transiting Exoplanet Survey Satellite (TESS), we showcase improvements to the MIT Quick Look Pipeline (QLP) through the discovery and validation of a multiplanet system around M dwarf TOI 4342 ( T mag = 11.032, M ⋆ = 0.63 M ⊙ , R ⋆ = 0.60 R ⊙ , T eff = 3900 K, d = 61.54 pc). With updates to QLP, including a new multiplanet search, as well as faster cadence data from TESS’s First Extended Mission, we discovered two sub-Neptunes ( R b = 2.266 − 0.038 + 0.038 R ⊕ and R c = 2.415 − 0.040 + 0.043 R ⊕ P b = 5.538 days and P c = 10.689 days) and validated them with ground-based photometry, spectra, and speckle imaging. Both planets notably have high transmission spectroscopy metrics of 36 and 32, making TOI 4342 one of the best systems for comparative atmospheric studies. This system demonstrates how improvements to QLP, along with faster cadence full-frame images, can lead to the discovery of new multiplanet systems.
Publisher: American Astronomical Society
Date: 22-02-2023
Abstract: We report the discovery of TOI-4562b (TIC-349576261), a Jovian planet orbiting a young F7V-type star, younger than the Praesepe/Hyades clusters ( Myr). This planet stands out because of its unusually long orbital period for transiting planets with known masses ( P orb = 225.11781 − 0.00022 + 0.00025 days) and because it has a substantial eccentricity ( e = 0.76 − 0.02 + 0.02 ). The location of TOI-4562 near the southern continuous viewing zone of TESS allowed observations throughout 25 sectors, enabling an unambiguous period measurement from TESS alone. Alongside the four available TESS transits, we performed follow-up photometry using the South African Astronomical Observatory node of the Las Cumbres Observatory and spectroscopy with the CHIRON spectrograph on the 1.5 m SMARTS telescope. We measure a radius of 1.118 + 0.013 − 0.014 R J and a mass of 2.30 − 0.47 + 0.48 M J for TOI-4562b. The radius of the planet is consistent with contraction models describing the early evolution of the size of giant planets. We detect tentative transit timing variations at the ∼20 minutes level from five transit events, favoring the presence of a companion that could explain the dynamical history of this system if confirmed by future follow-up observations. With its current orbital configuration, tidal timescales are too long for TOI-4562b to become a hot Jupiter via high-eccentricity migration though it is not excluded that interactions with the possible companion could modify TOI-4562b’s eccentricity and trigger circularization. The characterization of more such young systems is essential to set constraints on models describing giant-planet evolution.
Publisher: American Astronomical Society
Date: 23-11-2021
Abstract: We report the discovery of an ultrahot Jupiter with an extremely short orbital period of 0.67247414 ± 0.00000028 days (∼16 hr). The 1.347 ± 0.047 R Jup planet, initially identified by the Transiting Exoplanet Survey Satellite (TESS) mission, orbits TOI-2109 (TIC 392476080)—a T eff ∼ 6500 K F-type star with a mass of 1.447 ± 0.077 M ☉ , a radius of 1.698 ± 0.060 R ☉ , and a rotational velocity of v sin i * = 81.9 ± 1.7 km s −1 . The planetary nature of TOI-2109b was confirmed through radial-velocity measurements, which yielded a planet mass of 5.02 ± 0.75 M Jup . Analysis of the Doppler shadow in spectroscopic transit observations indicates a well-aligned system, with a sky-projected obliquity of λ = 1.°7 ± 1.°7. From the TESS full-orbit light curve, we measured a secondary eclipse depth of 731 ± 46 ppm, as well as phase-curve variations from the planet’s longitudinal brightness modulation and ellipsoidal distortion of the host star. Combining the TESS-band occultation measurement with a K s -band secondary eclipse depth (2012 ± 80 ppm) derived from ground-based observations, we find that the dayside emission of TOI-2109b is consistent with a brightness temperature of 3631 ± 69 K, making it the second hottest exoplanet hitherto discovered. By virtue of its extreme irradiation and strong planet–star gravitational interaction, TOI-2109b is an exceptionally promising target for intensive follow-up studies using current and near-future telescope facilities to probe for orbital decay, detect tidally driven atmospheric escape, and assess the impacts of H 2 dissociation and recombination on the global heat transport.
Publisher: American Astronomical Society
Date: 02-2022
Abstract: Close-in gas giants present a surprising range of stellar obliquity, the angle between a planet’s orbital axis and its host star’s spin axis. It is unclear whether the obliquities reflect the planets’ dynamical history (e.g., aligned for in situ formation or disk migration versus misaligned for high-eccentricity tidal migration) or whether other mechanisms (e.g., primordial misalignment or planet–star interactions) are more important in sculpting the obliquity distribution. Here we present the stellar obliquity measurement of TOI-1268 (TIC-142394656, V mag ∼ 10.9), a young K-type dwarf hosting an 8.2 day period, Saturn-sized planet. TOI-1268’s lithium abundance and rotation period suggest the system age between the ages of the Pleiades cluster (∼120 Myr) and the Prasepe cluster (∼670 Myr). Using the newly commissioned NEID spectrograph, we constrain the stellar obliquity of TOI-1268 via the Rossiter–McLaughlin effect from both radial velocity and Doppler tomography signals. The 3 σ upper bounds of the projected stellar obliquity ∣ λ ∣ from both models are below 60°. The large host star separation ( a / R ⋆ ∼ 17), combined with the system’s young age, makes it unlikely that the planet has realigned its host star. The stellar obliquity measurement of TOI-1268 probes the architecture of a young gas giant beyond the reach of tidal realignment ( a / R ⋆ ≳ 10) and reveals an aligned or slightly misaligned system.
Publisher: American Astronomical Society
Date: 07-12-2022
Abstract: We validate the presence of a two-planet system orbiting the 0.15–1.4 Gyr K4 dwarf TOI 560 (HD 73583). The system consists of an inner moderately eccentric transiting mini-Neptune (TOI 560 b, P = 6.3980661 − 0.0000097 + 0.0000095 days, e = 0.294 − 0.062 + 0.13 , M = 0.94 − 0.23 + 0.31 M Nep ) initially discovered in the Sector 8 Transiting Exoplanet Survey Satellite (TESS) mission observations, and a transiting mini-Neptune (TOI 560 c, P = 18.8805 − 0.0011 + 0.0024 days, M = 1.32 − 0.32 + 0.29 M Nep ) discovered in the Sector 34 observations, in a rare near-1:3 orbital resonance. We utilize photometric data from TESS Spitzer, and ground-based follow-up observations to confirm the ephemerides and period of the transiting planets, vet false-positive scenarios, and detect the photoeccentric effect for TOI 560 b. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with the iSHELL spectrograph at the NASA Infrared Telescope Facility and the HIRES Spectrograph at Keck Observatory to validate the planetary nature of these signals, which we combine with published Planet Finder Spectrograph RVs from the Magellan Observatory. We detect the masses of both planets at σ significance. We apply a Gaussian process (GP) model to the TESS light curves to place priors on a chromatic RV GP model to constrain the stellar activity of the TOI 560 host star, and confirm a strong wavelength dependence for the stellar activity demonstrating the ability of near-IR RVs to mitigate stellar activity for young K dwarfs. TOI 560 is a nearby moderately young multiplanet system with two planets suitable for atmospheric characterization with the James Webb Space Telescope and other upcoming missions. In particular, it will undergo six transit pairs separated by hr before 2027 June.
Publisher: American Astronomical Society
Date: 13-12-2021
Abstract: We report the discovery of two short-period massive giant planets from NASA’s Transiting Exoplanet Survey Satellite (TESS). Both systems, TOI-558 (TIC 207110080) and TOI-559 (TIC 209459275), were identified from the 30 minute cadence full-frame images and confirmed using ground-based photometric and spectroscopic follow-up observations from TESS’s follow-up observing program working group. We find that TOI-558 b, which transits an F-dwarf ( M * = 1.349 − 0.065 + 0.064 M ⊙ , R * = 1.496 − 0.040 + 0.042 R ⊙ , T eff = 6466 − 93 + 95 K, age 1.79 − 0.73 + 0.91 Gyr) with an orbital period of 14.574 days, has a mass of 3.61 ± 0.15 M J , a radius of 1.086 − 0.038 + 0.041 R J , and an eccentric ( e = 0.300 − 0.020 + 0.022 ) orbit. TOI-559 b transits a G dwarf ( M * = 1.026 ± 0.057 M ⊙ , R * = 1.233 − 0.026 + 0.028 R ⊙ , T eff = 5925 − 76 + 85 K, age 6.8 − 2.0 + 2.5 Gyr) in an eccentric ( e = 0.151 ± 0.011) 6.984 days orbit with a mass of 6.01 − 0.23 + 0.24 M J and a radius of 1.091 − 0.025 + 0.028 R J . Our spectroscopic follow up also reveals a long-term radial velocity trend for TOI-559, indicating a long-period companion. The statistically significant orbital eccentricity measured for each system suggests that these planets migrated to their current location through dynamical interactions. Interestingly, both planets are also massive ( M J ), adding to the population of massive giant planets identified by TESS. Prompted by these new detections of high-mass planets, we analyzed the known mass distribution of hot and warm Jupiters but find no significant evidence for multiple populations. TESS should provide a near magnitude-limited s le of transiting hot Jupiters, allowing for future detailed population studies.
Publisher: American Astronomical Society
Date: 25-05-2023
Abstract: From the thousands of known exoplanets, those that transit bright host stars provide the greatest accessibility toward detailed system characterization. The first known such planets were generally discovered using the radial-velocity technique, then later found to transit. HD 17156b is particularly notable among these initial discoveries because it erged from the typical hot-Jupiter population, occupying a 21.2 day eccentric ( e = 0.68) orbit, offering preliminary insights into the evolution of planets in extreme orbits. Here we present new data for this system, including ground- and space-based photometry, radial velocities, and speckle imaging, that further constrain the system properties and stellar lanetary multiplicity. These data include photometry from the Transiting Exoplanet Survey Satellite that cover five transits of the known planet. We show that the system does not harbor any additional giant planets interior to 10 au. The lack of stellar companions and the age of the system indicate that the eccentricity of the known planet may have resulted from a previous planet–planet scattering event. We provide the results from dynamical simulations that suggest possible properties of an additional planet that culminated in ejection from the system, leaving a legacy of the observed high eccentricity for HD 17156b.
Publisher: Springer Science and Business Media LLC
Date: 11-01-2018
DOI: 10.1038/S41467-017-02614-W
Abstract: Normalization is a common cortical computation widely observed in sensory perception, but its importance in perception of reward value and decision making remains largely unknown. We examined (1) whether normalized value signals occur in the orbitofrontal cortex (OFC) and (2) whether changes in behavioral task context influence the normalized representation of value. We record medial OFC (mOFC) single neuron activity in awake-behaving monkeys during a reward-guided lottery task. mOFC neurons signal the relative values of options via a isive normalization function when animals freely choose between alternatives. The normalization model, however, performed poorly in a variant of the task where only one of the two possible choice options yields a reward and the other was certain not to yield a reward (so called: “forced choice”). The existence of such context-specific value normalization may suggest that the mOFC contributes valuation signals critical for economic decision making when meaningful alternative options are available.
Location: United States of America
No related grants have been discovered for Hiroshi Yamada.