An Easily Compatible Eye-tracking System for Freely-moving Small Animals

Measuring eye movement is a fundamental approach in cognitive science as it provides a variety of insightful parameters that reflect brain states such as visual attention and emotions.Combining eye-tracking with multimodal neural recordings or manipulation techniques is beneficial for understanding the neural substrates of cognitive function.Many commercially-available and custom-built systems have been widely applied to awake,head-fixed small animals.However,the existing eyetracking systems used in freely-moving animals are still limited in terms of their compatibility with other devices and of the algorithm used to detect eye movements.Here,we report a novel system that integrates a general-purpose,easily compatible eye-tracking hardware with a robust eye feature-detection algorithm.With ultra-light hardware and a detachable design,the system allows for more implants to be added to the animal's exposed head and has a precise synchronization module to coordinate with other neural implants.Moreover,we systematically compared the performance of existing commonly-used pupil-detection approaches,and demonstrated that the proposed adaptive pupil feature-detection algorithm allows the analysis of more complex and dynamic eye-tracking data in freemoving animals.Synchronized eye-tracking and electroencephalogram recordings,as well as algorithm validation under five noise conditions,suggested that our system is flexibly adaptable and can be combined with a wide range of neural manipulation and recording technologies.

Absolute Internal Energy of the Real Gas

The internal energy U of the real, neutral-gas particles of total mass M in the volume V can have positive and negative values, whose regions are identified in the state chart of the gas. Depending on the relations among gas temperature T, pressure p and mass-specific volume v=V/M, the mass exists as a uniform gas of freely-moving particles having positive values U or as more or less structured matter with negative values U. In the regions U>0?above the critical point [Tc , pc , vc] it holds that p(T,v)>pc and v>vc, and below the critical point it holds that p(T,v)c and v>vv , where vv is the mass-specific volume of saturated vapor. In the adjacent regions with negative internal energy values Uc is the line of equal positive and negative energy contributions and thus represents a line of vanishing internal energy ?U=0. At this level along the critical isochor the ever present microscopic fluctuations in energy and density become macroscopic fluctuations as the pressure decreases on approaching the critical point;these are to be observed in experiments on the critical opalescence. Crossing the isochor vc from U>0 to UΔU>0 happens without any discontinuity. The saturation line vv also separates the regions between U>0 and U , but does not represent a line U=0. The internal-energy values of saturated vapor Uv and condensate Ui can be determined absolutely as functions of vapor pressure p and densities (M/V)v and (M/V)i , repectively, yielding the results Uiv, U=Ui+Uvc and U=Ui=Uv=0 at the critical point. Crossing the line Vv from U=Uv>0 to U=Uv+UiΔU=-Ui>0 to be removed from the particle system. The thermodynamic and quantum-mechanical formulations of the internal energy of a particle system only agree if both the macroscopic and microscopic energy scales have the same absolute energy reference value 0. Arguments for the energy reference value in the state of transition from bound to freely- moving particles in macroscopic classical and microscopic quantum particle systems are discussed.