Efficiency Of Human Observers Research Articles

Theoretical feasibility of dual-energy radiography for structural and functional imaging of chronic obstructive pulmonary disease.

Chronic obstructive pulmonary disease (COPD) affects ∼200 million people worldwide. We propose two-dimensional (2D) dual-energy (DE) x-ray imaging of lung structure and function for the assessment of COPD, and investigate the resulting image quality theoretically. We investigated xenon-enhanced DE (XeDE) radiography for functional imaging of COPD and unenhanced DE radiography for structural imaging of COPD. We modeled the ability of human observers to detect ventilation defects in XeDE images and emphysema in (unenhanced) DE images using the detectability index ( ) as a figure of merit. We accounted for the extent of emphysematous destruction and functional impairment as a function of disease severity, defect/lesion contrast, spatial resolution, x-ray scatter, quantum noise, anatomic noise, and the efficiency of human observers. Whether or not disease was detectable was determined based on a detectability threshold of two. For (unenhanced) DE imaging of emphysema, we compared detectability with that of single-energy (SE) imaging. Models of signal and noise were compared to published data. Models of signal and noise agreed well with published data, and model predictions of the detectability of emphysema by SE radiography were consistent with poor sensitivity (i.e., ) to mild to moderate COPD but moderate sensitivity (i.e., ) to severe COPD. The detectability of emphysema by DE radiography was greater than that of SE radiography, but did not cross the threshold of detectability for mild to moderate COPD. The detectability index for XeDE imaging exceeded the detectability threshold for mild, moderate, and severe COPD. Dual-energy radiography may offer modest improvements in the detection of emphysema relative to SE imaging, but will unlikely enable detecting mild and moderate COPD. However, XeDE radiography may enable detection of functional abnormalities associated with mild, moderate and severe COPD at x-ray exposures typical of those used in conventional chest radiography, thus warranting further investigation as a low-dose, low-cost alternative to CT- and MRI-based approaches for functional imaging of COPD.

Perception of temporally filtered X-ray fluoroscopy images.

For noisy X-ray fluoroscopy image sequences we quantitatively evaluated image quality after digital temporal filtering to reduce noise. Using an experimental paradigm called a reference/test adaptive forced-choice method we compared detectability of stationary low-contrast disks in filtered and unfiltered, computer-generated image sequences. In the first experiment, a low-pass first-order recursive filter used in X-ray fluoroscopy was found to be much less effective at enhancing detectability than predicted from the reduction of display noise variance, a common measurement of filter effectiveness. Detectability was reasonably predicted by a nonprewhitening human-observer model (NPW-HVS) that included an independently determined human temporal-contrast-sensitivity function. In another experiment, designed to test models over a range of temporal frequencies, we used paired high-pass and low-pass temporal filters that both reduced noise variance by 25%. The high-pass filter was artificially applied to the noise only and greatly improved detectability, while the low-pass filter had little effect. The human-observer model quantitatively described the measurements, but classical prewhitening and nonprewhitening signal detectors did not. As compared to the nonprewhitening, spatio-temporal matched filter, human-observer efficiency was low and variable at 2.1%, 2.9%, and 0.06% for 60 frames of unfiltered low-pass and high-pass noise, respectively. As compared to this detector, humans were not very effective at combining information across frames. On the other hand, signal to noise ratios (SNR's) from the human-observer model were comparable to human performance, and efficiencies were reasonably constant at 40%, 52%, and 32%, respectively. We conclude that it is imperative to include human-observer models and experiments in the analysis of noise-reduction filtering of noisy image sequences, such as X-ray fluoroscopy.

Insights into motion perception by observer modeling.

The statistical efficiency of human observers performing a simplified version of the motion detection task of Salzman and Newsome [Science 264, 231 (1994)] is high but not perfect. This reduced efficiency may be caused by noise internal to the observers or by the observers' using strategies that are different from that used by an ideal machine. We therefore investigated which of three simple models best accounts for the observers' performance. The models compared were a motion detector that uses the proportion of dots in the first frame that move coherently (as would an ideal machine), a model that bases its decision on the number of dots that move, and a model that differentially weights motions that occur at different locations in the visual field (for instance, differentially weights the point of fixation and the periphery). We compared these models by explicitly modeling the human observers' performance. We recorded the exact stimulus configuration on each trial together with the observer's response, and, for the different models, we found the parameters that best predicted the observer's performance in a least-squares sense. We then used N-fold cross validation to compare the models and hence the associated hypotheses. Our results show that the performance of observers is based on the proportion, not the absolute number, of dots that are moving and that there was no evidence of any differential spatial weighting. Whereas this method of modeling the observers' response is demonstrated only for one simple psychophysical paradigm, it is general and can be applied to any psychophysical framework in which the entire stimulus can be recorded.

Operant control of occipital theta rhythm affects performance in a radar monitoring task.

Detection efficiency of human observers deteriorates rapidly in monotonous monitoring tasks; this effect (the vigilance decrement) has been associated with increased theta band activity in the electroencephalogram. Suppression of theta activity by operant methods enhances monitoring efficiency, whereas theta augmentation further degrades task performance. These results demonstrate a lawful relationship between operantly regulated cortical activity and behavior in man.

Experimental Study of “Tone Deafness”

In a series of experiments designed to compare the efficiency of human observers in amplitude-discrimination tasks to their efficiency in frequency-discrimination tasks, the behavior of one of the four observers suggested that he was completely insensitive to frequency differences. The procedure of the experiments was then diverted to conduct an experimental case study comparing the “tone deaf” observer to three observers with normal frequency discrimination under a variety of experimental conditions. In amplitude-discrimination experiments in which he is detecting pulses of sine waves, he compares favorably. He far surpasses the other three observers when the signal is a sample of “white Gaussian” noise. An interesting aspect in the experiments reported is that the “tone deafness” of the participating observers suggests the possibility that these particular experiments may reflect culture-bound conditions. A further program is planned that may yield data on the question of the environmental influences on sensory capabilities.

Detection of Complex Signals as a Function of Signal Bandwidth and Duration

A study is reported which examined the efficiency of human observers in the detection of a stimulus waveform which is a train of damped sinusoids. The signal duration and degree of damping (or spectral bandwidth) were varied, with the energy of the signal held constant. Increased bandwidth is shown to decrease detection of relatively long duration signals. A negligible effect of bandwidth was found for short durations. Detection of an amplitude difference, in contrast to detection of the presence of weak signal in noise, is shown to be relatively unaffected either by signal duration or bandwidth over the range of values studied.