The universe of activity that is bioengineering research.

What is bioengineering? A concise and meaningful answer to this question is important for pedagogy. This paper demonstrates that research activity within 'bioengineering' resides in a multidimensional space. A specific study can be characterized by an area into which it falls, a problem that it attacks, a specific level of analysis, a species of focus, pertinent disciplines, relevant tools that are employed, and temporal features. Each represents a dimension of the universe and each dimension may be shown to be largely orthogonal to every other dimension. The universe thus defined is sparsely populated. Accordingly, bioengineering is a field of rich opportunity within which frontiers lie almost everywhere.

Noise and its effects on photoreceptor temporal contrast sensitivity at low light levels.

We studied photoreceptors in the locust (Schistocerca americanus) visual system to determine the extent to which quantal noise and intrinsic neural noise limit temporal sensitivity. Typical computational models of the temporal contrast sensitivity function are deterministic, reflect only filter characteristics, and lack explicit noise sources [J. Opt. Soc. Am. 58, 1133 (1968); Vision Res. 32, 1373 (1992)]. We report here that the temporal contrast sensitivity function, at low light levels, is not simply the reflection of a filter function. Our evidence suggests that, at low backgrounds, noise, in conjunction with temporal filtering, plays a role in shaping the temporal contrast sensitivity function. At a given low adaptation level, quantal noise limits sensitivity at low temporal frequencies, while intrinsic noise limits sensitivity at relatively higher temporal frequencies.

Noise location and the slope of the psychometric function for simple motion stimuli.

We measured subject performance as a function of luminance for both detection and discrimination of increment stimuli; some were static, and some were arranged to give two-step apparent motion. Our aim was to examine a prediction for the shape of the psychometric function for motion: an accelerating function due to the presence of a multiplicative nonlinearity contained in many low-level motion models. For the tasks with static stimuli we found psychometric function slopes (of log d' versus log luminance plots) between 1.9 and 2.4 in two subjects, as previously reported. For the tasks with apparent motion stimuli in the same range of detectability, however, the slopes are between 1.2 and 1.7. The lower slopes indicate that many low-level motion models are either incorrect or incomplete as currently specified, and changes in nonlinearities and noise placement are discussed.

Flash masking and facilitation by nearby luminance perturbations.

Light-adapted foveal luminance increment thresholds were measured for white photopic targets of 1.5-arc min diameter and 220-ms duration. We aimed to learn about the properties of mechanisms that subserve the detection of these targets. To study this subject we developed a noise probe technique that inserts noise close to the site of the stimulus. Threshold is more than doubled when zero-mean luminance noise is placed at a pair of flanking spots in the horizontal meridian centered on the test spot and 1.5 arc min distant. The detection mechanisms thus has a broad field, since noise effects persist at 5-arc min separation. The masking effect increases when the noise is in antiphase at the two flanking spots. Neither even- nor odd-symmetric mechanisms are able to explain these findings, regardless of whether linear or nonlinear processing is employed. The target detection may be mediated in part by a motion-sensitive mechanism.

Relative efficiency for the detection of apparent motion.

We measured the relative efficiency for motion and position discriminations of brief, localized spot stimuli with a technique that makes no assumptions about sites of noise or information loss in the visual system. In one task, the observer had to discriminate whether an increment was located at one (left) or another (right) closely spaced spots. In the other task, the observer had to discriminate two successive brief increments of the left spot from a left spot increment followed by a right spot increment. Ideal observer theory predicts identical performance on the two tasks. Observers' thresholds, however, were significantly lower in the motion task at all intervals between flashes (ISIs) less than 60 msec in one observer and all ISIs less than 150 msec in two other observers (P < 0.01, t-test). We conclude that this apparent motion stimulus is seen more efficiently than a non-moving stimulus, and that the higher efficiency may be due to use of a motion sensitive channel in addition to independent position sensitive channels.

Postural stability and stereo-ambiguity in man-designed visual environments.

Our modern rectilinear visual environment contains visual stimuli for which evolution has not had time to optimally shape visual processing. One such stimulus, periodic stripes, is known to lead to visual depth ambiguity. In this paper we show that postural instability, as measured by the variance of fore and aft sway, is increased by viewing such stimuli. This instability may be the precursor of falls. Designers must evaluate the visual impressions conveyed by their systems in order to avoid postural instability due to visual ambiguity.

Saccadic suppression and stimulus uncertainty.

Saccadic suppression is a decline in detectability of a weak flash presented during a saccadic eye movement. We examined the hypothesis of Matin [Psychol. Bull. 81, 899 (1974)] that saccadic suppression may be due to increased stimulus uncertainty during the saccade. Uncertainty could arise from variability and inhomogeneities in the visual frame of reference translation that must accompany a saccade. We measured an average 0.6-log-unit suppression for a brief foveal 1 degree flash in a light-adapted detection task. Receiver-operating-characteristic (ROC) slopes for flash detection during saccades, compared with those when fixating, were reduced, indicating the presence of increased uncertainty. The magnitude of this uncertainty change was estimated and found to be consistent with that required to account for the measured detectability decline. When a flashed pedestal was employed to reduce the effect of uncertainty, there was no saccadic suppression and no ROC slope change. Also, spatially separate flashed markers, intended to reduce uncertainty, led to a significant reduction in saccadic suppression for one of two subjects. Our results are consistent with the hypothesis that a saccade leaves the observer with increased uncertainty as to which subjective visual direction to attend for a stimulus of fixed retinal locus. The magnitude of this uncertainty change can account fully for the saccadic suppression measured.

Wallpaper illusion: cause of disorientation and falls on escalators.

The wallpaper illusion, first described over a century ago, can occur when a person with normal binocular vision views a pattern that is periodic in the horizontal meridian of the visual field. Escalator trends present such a pattern. Evidence is presented favoring the view that disorientation experienced by escalator riders is caused by this illusion. Possibly some of the estimated 60,000 escalator falls occurring in the United States each year are linked to it.

Are rods and cones of dyslexics anomalous?

Grosser and Spafford (in this journal, 1989) have advanced an hypothesis and presented measurements which they believe support the idea of an excess of cones in the peripheral retinae of dyslexics. This note points out that their hypothesis is based on the erroneous assumption that normals have no peripheral cones. Further, their data can be explained by at least two alternative, though uninteresting, methodological hypotheses, that uncontrolled eye movements or experimenter suggestion (or both) could have produced their results. Finally, the requisite methods for assessing color vision, and the cones, were not met in the study.

Analysis of human color mechanisms using sinusoidal spectral power distributions.

We examined the effects of probing human color mechanisms using sinusoidal spectral power distributions (SPD's) varying in frequency (i.e., from 0.1 to 5.0 cycles/300 nm for a constant starting phase) and phase (i.e., from 0 to 360 deg for a fixed frequency of 1 cycle/300 nm) through computer simulation using several color models. Predicted modulation sensitivity functions (MSF's) in spectral frequency and phase differ among the models and indicate that measurements of the minimum amplitudes necessary to detect sinusoidal SPD's would be useful for distinguishing among theories of color vision. MSF's obtained from similar analyses of dichromats' color mechanisms reveal characteristic patterns of modulation sensitivities and suggest that such measures could serve to distinguish type and degree of color-vision defect. Some implications based on sinusoidal approximations to illuminant and reflectance spectra are discussed along with more general considerations regarding sine-wave SPD's as a probe for mechanisms of color vision.

Detection and recognition of visual targets.

To aid the development of detection-based interpretations of visual resolution, we evaluated theorems that (1) relate observers' performance in detection of a single target to that in 1-of-m signal detection and (2) predict recognition performance from measured performance at 1-of-m detection. These theorems require that the sensory effects of the stimuli be continuous and that the m signals be equally detectable and mutually orthogonal. To evaluate the theorems, we tested observers in simple detection, 1-of-m detection, and recognition of Landolt C targets and compared predicted and observed performance. Predictions of 1-of-m detection performance from that for simple detection and predictions of recognition from the 1-of-m receiver operating characteristic (ROC) were both accurate to within 0.03 in P(A). In addition, predictions of recognition based on the 1-of-m ROC predicted from simple detection were generally accurate. Thus, under restricted experimental conditions, recognition is determined completely by that for simple detection.

Visual depth illusion and falls in the elderly.

This article investigates the relation of vision and the effects of age on the maintenance of posture. This relationship in the elderly is explored within the context of visual depth illusions induced by repeating patterns that occur on escalator treads and elsewhere in the environment. Age does not appear to reduce the susceptibility of the elderly to visual depth illusions. However, if age is coupled with declines in motor control and strength, the elderly are probably more susceptible to falls.

Effect of large spatial uncertainty on foveal luminance increment detectability.

In principle, the ability to detect a luminance increment is lowered when there is uncertainty for its spatial location. Frequency-of-seeing curves were generated for small foveal targets. When fixed in space the target's detectability was more than 10 times higher than when it could occur at one of 140 locations.

Discrimination of luminance increments and decrements.

Discrimination of pulsed increments from decrements improves relative to detection when the adaptation level is decreased or stimulus duration is increased. Using Tanner's theory of recognition [J. Acoust. Soc. Am. 28, 882 (1956)], we show that the subject's internal decision variables corresponding to the increment and decrement stimuli also show increased statistical independence as adaptation is decreased or stimulus duration increases. We infer from this result that at high adaptation levels detection is based predominantly on information in one channel, whereas at low adaptation levels there is information from two channels. Similarly, for long durations, we infer two independent channels, which contribute little information about the sign of the stimulus at short durations.

Site of the accelerating nonlinearity underlying luminance-change detection.

The visual psychometric function for luminance-increment detectability (d') is known to be an accelerating function of stimulus energy. Two different models have been suggested to explain this fact and also why a luminance-increment pedestal linearizes the psychometric function. In the present experiment it is shown that a dichoptically presented increment linearizes the psychometric function and facilitates detection of weak signals. Since the dichoptic pedestal combines with the signal centrally, the nonlinearity must then originate more centrally. This result is compatible with the uncertainty model of the nonlinear psychometric function but not with a model that requires a nonlinear transducer.

Performance of cat retinal ganglion cells at low light levels.

Responses of brisk-sustained cat retinal ganglion cells were examined using receiver operating characteristic (ROC) analysis. Stimuli were brief luminance changes superimposed upon a weak steady pedestal ranging from 27 to 47,000 quanta (507 nm) per second at the cornea. Overall quantum efficiencies of cells ranged up to approximately 13% and were compatible with previous estimates at absolute threshold. The main work was done on on-center cells, but a small sample of off-center units behaved similarly. Experimental ROC curves verified a set of qualitative predictions based on a theoretical treatment of performance, assuming that response variability resulted solely from quantum fluctuations. However, quantitative predictions were not fulfilled. The discrepancy could be resolved by postulating a source of added internal variance, R, the value of which could then be deduced from the experimental measurements. A ganglion cell model limited by a fixed amount of added variance from physiological sources and having access to a fixed fraction of incident quanta can account quantitatively for (a) slopes of ROC curves, (b) variation of detectability with magnitude of both increments and decrements, and (c) performance over a range of pedestal intensities. Estimates of the proportion of incident quanta used ranged up to 29% under some conditions, a figure approximately matching estimates of the fraction of corneal quanta that isomerize rhodopsin in the cat.