Data Poisoning Attacks in Contextual Bandits

We study offline data poisoning attacks in contextual bandits, a class of reinforcement learning problems with important applications in online recommendation and adaptive medical treatment, among others. We provide a general attack framework based on convex optimization and show that by slightly manipulating rewards in the data, an attacker can force the bandit algorithm to pull a target arm for a target contextual vector. The target arm and target contextual vector are both chosen by the attacker. That is, the attacker can hijack the behavior of a contextual bandit. We also investigate the feasibility and the side effects of such attacks, and identify future directions for defense. Experiments on both synthetic and real-world data demonstrate the efficiency of the attack algorithm.Comment: GameSec 201

El rol de la motivación en los efectos de las relaciones profesor-estudiante en el rendimiento de la lengua extranjera

The present study investigated whether students’ motivational beliefs act as a mediator in the association between teacher-student relationships (TSRs) and foreign language performance with a multiple mediation model. Furthermore, this research examined whether mediating roles of intrinsic and extrinsic motivation differ. A total of 1171 eighth graders (583 male, 588 female) were chosen with purposive sampling in China. Studentreported measures of TSRs, intrinsic and extrinsic motivation, and English as a Foreign Language (EFL) test based on national curriculum were administrated in October 2017. Results showed that the positive link between TSRs and foreign language performance is partially mediated by intrinsic and extrinsic motivation, and the mediation effect of intrinsic motivation is significantly greater than that of extrinsic motivation, controlling for gender and socio-economic status. The results indicated that supportive TSRs can help learners to improve their foreign language proficiency by promoting their motivation, especially intrinsic motivation. The present results may have substantive theoretical and practical implications for teacher education and foreign language learning.El presente estudio investigó si las creencias motivacionales de los estudiantes actúan como mediadores en la asociación entre las relaciones profesor-alumno (TSR) y el rendimiento de un idioma extranjero con un modelo de mediación múltiple. Además, esta investigación examinó si los roles mediadores de la motivación intrínseca y extrínseca difieren. Se eligieron 1171 alumnos de octavo grado (583 hombres, 588 mujeres) con muestreo intencional en China. En octubre de 2017, se aplicaron a los estudiantes las pruebas de TSR, la motivación intrínseca y extrínseca y la prueba de inglés como idioma extranjero (EFL) basadas en el plan de estudios nacional. Los resultados mostraron que el vínculo positivo entre TSR y el desempeño de un idioma extranjero está parcialmente mediado por la motivación intrínseca y extrínseca, y el efecto de mediación de la motivación intrínseca es significativamente mayor que el de la motivación extrínseca, controlando el género y el estatus socioeconómico. Los resultados indicaron que los TSR de apoyo pueden ayudar a los alumnos a mejorar su dominio del idioma extranjero ya que promueven su motivación, especialmente la motivación intrínseca. Los resultados actuales pueden tener importantes implicaciones teóricas y prácticas para la formación del profesorado y el aprendizaje de lenguas extranjeras.&nbsp

El rol de la motivación en los efectos de las relaciones profesor-estudiante en el rendimiento de la lengua extranjera

The present study investigated whether students’ motivational beliefs act as a mediator in the association between teacher-student relationships (TSRs) and foreign language performance with a multiple mediation model. Furthermore, this research examined whether mediating roles of intrinsic and extrinsic motivation differ. A total of 1171 eighth graders (583 male, 588 female) were chosen with purposive sampling in China. Studentreported measures of TSRs, intrinsic and extrinsic motivation, and English as a Foreign Language (EFL) test based on national curriculum were administrated in October 2017. Results showed that the positive link between TSRs and foreign language performance is partially mediated by intrinsic and extrinsic motivation, and the mediation effect of intrinsic motivation is significantly greater than that of extrinsic motivation, controlling for gender and socio-economic status. The results indicated that supportive TSRs can help learners to improve their foreign language proficiency by promoting their motivation, especially intrinsic motivation. The present results may have substantive theoretical and practical implications for teacher education and foreign language learning.El presente estudio investigó si las creencias motivacionales de los estudiantes actúan como mediadores en la asociación entre las relaciones profesor-alumno (TSR) y el rendimiento de un idioma extranjero con un modelo de mediación múltiple. Además, esta investigación examinó si los roles mediadores de la motivación intrínseca y extrínseca difieren. Se eligieron 1171 alumnos de octavo grado (583 hombres, 588 mujeres) con muestreo intencional en China. En octubre de 2017, se aplicaron a los estudiantes las pruebas de TSR, la motivación intrínseca y extrínseca y la prueba de inglés como idioma extranjero (EFL) basadas en el plan de estudios nacional. Los resultados mostraron que el vínculo positivo entre TSR y el desempeño de un idioma extranjero está parcialmente mediado por la motivación intrínseca y extrínseca, y el efecto de mediación de la motivación intrínseca es significativamente mayor que el de la motivación extrínseca, controlando el género y el estatus socioeco-nómico. Los resultados indicaron que los TSR de apoyo pueden ayudar a los alumnos a mejorar su dominio del idioma extranjero ya que promueven su motivación, especialmente la motivación intrínseca. Los resultados actuales pueden tener importantes implicaciones teóricas y prácticas para la formación del profesorado y el aprendizaje de lenguas extranjera

Focusing light through scattering media by full-polarization digital optical phase conjugation

Digital optical phase conjugation (DOPC) is an emerging technique for focusing light through or within scattering media such as biological tissue. Since DOPC systems are based on time reversal, they benefit from collecting as much information about the scattered light as possible. However, existing DOPC techniques record and subsequently phase-conjugate the scattered light in only a single-polarization state, limited by the operating principle of spatial light modulators. Here, we develop the first, to the best of our knowledge, full-polarization DOPC system that records and phase-conjugates scattered light along two orthogonal polarizations. When focusing light through thick scattering media, such as 2 mm and 4 mm-thick chicken breast tissue, our full-polarization DOPC system on average doubles the focal peak-to-background ratio achieved by single-polarization DOPC systems and improves the phase-conjugation fidelity

Sub-Nyquist sampling boosts targeted light transport through opaque scattering media

Optical time-reversal techniques are being actively developed to focus light through or inside opaque scattering media. When applied to biological tissue, these techniques promise to revolutionize biophotonics by enabling deep-tissue non-invasive optical imaging, optogenetics, optical tweezing, and phototherapy. In all previous optical time-reversal experiments, the scattered light field was well-sampled during wavefront measurement and wavefront reconstruction, following the Nyquist sampling criterion. Here, we overturn this conventional practice by demonstrating that even when the scattered field is under-sampled, light can still be focused through or inside scattering media. Even more surprisingly, we show both theoretically and experimentally that the focus achieved by under-sampling can be one order of magnitude brighter than that achieved under the well-sampling conditions used in previous works, where 3×3 to 5×5 pixels were used to sample one speckle grain on average. Moreover, sub-Nyquist sampling improves the signal-to-noise ratio and the collection efficiency of the scattered light. We anticipate that this newly explored under-sampling scheme will transform the understanding of optical time reversal and boost the performance of optical imaging, manipulation, and communication through opaque scattering media

Bit-efficient, sub-millisecond wavefront measurement using a lock-in camera for time-reversal based optical focusing inside scattering media

Time-reversed ultrasonically encoded optical focusing measures the wavefront of ultrasonically tagged light, and then phase conjugates the tagged light back to the ultrasonic focus, thus focusing light deep inside the scattering media. In previous works, the speed of wavefront measurement was limited by the low frame rates of conventional cameras. In addition, these cameras used most of their bits to represent an informationless background when the signal-to-background ratio was low, resulting in extremely low efficiencies in the use of bits. Here, using a lock-in camera, we increase the bit efficiency and reduce the data transfer load by digitizing only the signal after rejecting the background. With this camera, we obtained the wavefront of ultrasonically tagged light after a single frame of measurement taken within 0.3 ms, and focused light in between two diffusers. The phase sensitivity has reached 0.51 rad even when the SBR is 6×10^(−4)

Single-exposure optical focusing inside scattering media using binarized time-reversed adapted perturbation

Light scattering inhibits high-resolution optical imaging, manipulation, and therapy deep inside biological tissue by preventing focusing. To form deep foci, wavefront-shaping techniques that break the optical diffusion limit have been developed. For in vivo applications, such focusing must provide a high gain, high speed, and a high focal peak-to-background ratio. However, none of the previous techniques meet these requirements simultaneously. Here, we overcome this challenge by rapidly measuring the perturbed optical field within a single camera exposure followed by adaptively time-reversing the phase-binarized perturbation. Consequently, a phase-conjugated wavefront is synthesized within a millisecond, two orders of magnitude shorter than the digitally achieved record. We demonstrate real-time focusing in dynamic scattering media and extend laser speckle contrast imaging to new depths. The unprecedented combination of a fast response, high gain, and high focusing contrast makes this work a major stride toward in vivo deep-tissue optical imaging, manipulation, and therapy