A dynamic scale-mixture model of motion in natural scenes.

Some of the most important tasks of visual and motor systems involve estimating the motion of objects and tracking them over time. Such systems evolved to meet the behavioral needs of the organism in its natural environment, and may therefore be adapted to the statistics of motion it is likely to encounter. By tracking the movement of individual points in videos of natural scenes, we begin to identify common properties of natural motion across scenes. As expected, objects in natural scenes move in a persistent fashion, with velocity correlations lasting hundreds of milliseconds. More subtly, we find that the observed velocity distributions are heavy-tailed and can be modeled as a Gaussian scale-mixture. Extending this model to the time domain leads to a dynamic scale-mixture model, consisting of a Gaussian process multiplied by a positive scalar quantity with its own independent dynamics. Dynamic scaling of velocity arises naturally as a consequence of changes in object distance from the observer, and may approximate the effects of changes in other parameters governing the motion in a given scene. This modeling and estimation framework has implications for the neurobiology of sensory and motor systems, which need to cope with these fluctuations in scale in order to represent motion efficiently and drive fast and accurate tracking behavior.

The double-drift illusion biases the marmoset oculomotor system.

The double-drift illusion has two unique characteristics: The error between the perceived and physical position of the stimulus grows over time, and saccades to the moving target land much closer to the physical than the perceived location. These results suggest that the perceptual and saccade targeting systems integrate visual information over different time scales. Functional imaging studies in humans have revealed several potential cortical areas of interest, including the prefrontal cortex. However, we currently lack an animal model to study the neural mechanisms of location perception that underlie the double-drift illusion. To fill this gap, we trained two marmoset monkeys to fixate and then saccade to the double-drift stimulus. In line with human observers for radial double-drift trajectories with fast internal motion, we find that saccade endpoints show a significant bias that is, nevertheless, smaller than the bias seen in human perceptual reports. This bias is modulated by changes in the external and internal speeds of the stimulus. These results demonstrate that the saccade targeting system of the marmoset monkey is influenced by the double-drift illusion.

Stimulus invariant aspects of the retinal code drive discriminability of natural scenes.

Everything that the brain sees must first be encoded by the retina, which maintains a reliable representation of the visual world in many different, complex natural scenes while also adapting to stimulus changes. Decomposing the population code into independent and cell-cell interactions reveals how broad scene structure is encoded in the adapted retinal output. By recording from the same retina while presenting many different natural movies, we see that the population structure, characterized by strong interactions, is consistent across both natural and synthetic stimuli. We show that these interactions contribute to encoding scene identity. We also demonstrate that this structure likely arises in part from shared bipolar cell input as well as from gap junctions between retinal ganglion cells and amacrine cells.

Clinical Impact of a Radiologic Quality Initiative Promoting More Timely Communication of Critical Pulmonary Embolus Results.

BACKGROUND: A section quality initiative was implemented beginning 2013 requiring positive pulmonary embolism (PE) results to be documented and communicated within 90 minutes of exam completion. The objective of this study is to evaluate the effect of this quality initiative on different intervals comprising the total patient processing time, namely the time from when the imaging exam was ordered to study completion interval, the time from study completion to positive PE result communication (TAT interval) or treatment initiation (TTT interval), the time from result communication to treatment initiation (TRCTI interval), and the total patient processing time (TPT interval). METHODS: This was a retrospective, single-institution, IRB-approved cohort study that included 830 patients with the diagnosis of acute PE confirmed by CT pulmonary angiography. A maximum of 10 positive exams per month were identified and analyzed over an 84-month period from January 2010 to December 2016. The following data were obtained: time when exam ordered, time of imaging study completion, time of report completion, time of result communication, time of treatment, type of treatment, and reasons for any treatment delay. Analysis was done by determining the mean time spent in various intervals, the cumulative relative frequency of interval completion, and the fraction of the entire patient processing time spent in each interval. RESULTS: Mean analysis demonstrated a decrease in all time intervals in the postpolicy period (ordered to study completion: Δ24.50%, p = 0.004; TAT: Δ23.91%, p < 0.001; TRCTI: Δ16.86%, p = 0.031; TTT: Δ17.40%, p = 0.005; TPT: Δ15.94%, p = 0.002). Cumulative relative frequency analysis demonstrated a higher rate of interval completion in the postpolicy period (TAT: p < 0.001; TRCTI: p = 0.007; TPT: p = 0.025). Interval fraction analysis demonstrated changes in the fraction of processing time spent in varying intervals (TAT: -Δ14.42%, p = 0.002; TRCTI: +Δ17.65%, p = 0.001). CONCLUSION: Total patient processing time decreased after the policy implementation with a more significant decrease in TAT compared to other intervals. Radiologic processing time does not appear to be the rate-limiting step in total patient processing time.

Effective ATPase activity and moderate chaperonin-cochaperonin interaction are important for the functional single-ring chaperonin system.

Escherichia coli chaperonin GroEL and its cochaperonin GroES are essential for cell growth as they assist folding of cellular proteins. The double-ring assembly of GroEL is required for the chaperone function, and a single-ring variant GroEL(SR) is inactive with GroES. Mutations in GroEL(SR) (A92T, D115N, E191G, and A399T) have been shown to render GroEL(SR)-GroES functional, but the molecular mechanism of activation is unclear. Here we examined various biochemical properties of these functional GroEL(SR)-GroES variants, including ATP hydrolysis rate, chaperonin-cochaperonin interaction, and in vitro protein folding activity. We found that, unlike the diminished ATPase activity of the inactive GroEL(SR)-GroES, all four single-ring variants hydrolyzed ATP at a level comparable to that of the double-ring GroEL-GroES. The chaperonin-cochaperonin interaction in these single-ring systems was weaker, by at least a 50-fold reduction, than the highly stable inactive GroEL(SR)-GroES. Strikingly, only GroEL(SR)D115N-GroES and GroEL(SR)A399T-GroES assisted folding of malate dehydrogenase (MDH), a commonly used folding substrate. These in vitro results are interesting considering that all four of the single-ring systems were able to substitute GroEL-GroES to support cell growth, suggesting that the precise action of chaperonin on MDH folding may not represent that on the intrinsic cellular substrates. Our findings that both effective ATP hydrolysis rate and moderate chaperonin-cochaperonin interaction are important factors for functional single-ring GroEL(SR)-GroES are reminiscent of the naturally occurring single-ring human mitochondrial chaperonin mtHsp60-mtHsp10. Differences in biochemical properties between the single- and double-ring chaperonin systems may be exploited in designing molecules for selective targeting.