Increment and decrement probe thresholds were measured during the presentation of two types of temporal masking stimuli. In Experiment 1, we measured thresholds for increment or decrement rectangular probes presented during the presentation of an increment or decrement Gaussian masking stimulus. We find that thresholds are higher when the probe and the Gaussian mask are of the same sign (e. g. both increments). However, both types of Gaussian mask raised increment and decrement probe thresholds above steady state conditions. In Experiment 2, we presented increment or decrement probes at one of eight possible phases of a 1 Hz luminance-modulated sine wave. For both increment and decrement probes, threshold variation with phase is non-sinusoidal in shape, but increment and decrement probe thresholds vary as a function of the sinusoid phase. These experiments show that increment and decrement thresholds vary as a function of the adaptation state of the visual system, and as a function of the direction of change in the adaptation state. Data from both experiments are discussed in terms of a recent neurophysiological model [Hood & Graham (1998). Threshold fluctuations on temporally modulated backgrounds: a possible physiological explanation based upon a recent computational model. Visual Neuroscience, 15 (5), 957-967]. We find that the predicted ON- and OFF-pathway responses do not correlate in a straightforward manner with the psychophysical thresholds, suggesting that detection of increment and decrement probes may not be performed exclusively by one pathway. Our data have implications for modeling visual performance under conditions where visual adaptation is dynamic, such as when scanning complex images or natural scenes.

We investigated the type of spatial structure present in nighttime imagery that is perceptually relevant for human observers to be able to perform texture-based segmentation of real world scenes. Three psychophysical tasks were developed to evaluate performance of the nighttime imagery. The test imagery consisted of scenes obtained via an image-intensified low-light CCD, a long-wave infrared sensor and monochrome sensor-fusion. For one task, performance was best with the fused imagery, but for two tasks, performance with fused imagery was not improved (compared to performance with ir imagery). Spatial filtering of the scenes and further testing revealed that the mid spatial frequencies (1-4 cpd) were more critical in determining performance than either the low or high frequencies. Fourier analysis of the scenes revealed a strong relationship between power and performance, where scenes with more power (especially at the middle frequencies) supported better performance. Implications of this research are that performance depends on power at the middle frequencies for these low-level visual tasks and that fusion algorithms may be improved if this is taken under consideration.

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Based on this spectrum of findings, the diagnosis of Rosai-Dorfman disease was made. To date, the patient has been followed-up for 3 years without medical intervention and without visual deterioration. Careful follow-up is a reasonable management if patients are asymptomatic.

Sawtooth modulation has been used in the past to examine visual sensitivity to luminance increments and decrements. The threshold elevation caused by adaptation depends on the spatial profile of the stimulus field and the polarities of the adaptation and test stimuli. We hypothesized that the adaptation effects reflect a change in the sensitivity of the spatiotemporal channels that detect the stimuli. We used a 2-deg disk centered in a larger surround field. Five levels of contrast between the test field and surround were investigated: equiluminant, three intermediate levels, and dark. At each contrast, observers adapted for 5 s to 2-Hz sawtooth modulation (rapid-on or rapid-off). Immediately after adaptation, thresholds were measured for detection of a single cycle of either a rapid-on or a rapid-off waveform. Varying the contrast of the surround affected observers' sensitivity to the polarity of the sawtooth stimulus to the extent that the pattern of sensitivity with the equiluminant surround was the opposite of that with the dark surround. To examine temporal factors, we measured thresholds for slow (500-ms ramps) and fast (8.3-ms pulses) test stimuli. The adaptation effect was preserved with the ramp stimuli but not with the pulse stimuli. Blurring the edge between the test and surround fields in the equiluminant surround condition raised thresholds for all sawtooth test stimuli, suggesting that spatiotemporal channels sensitive to high spatial frequencies and low temporal frequencies facilitate detection in that condition. These findings suggest that adaptation to sawtooth modulation can differentially effect the sensitivity of ON and OFF pathways, but the relative desensitization of each pathway depends on an interaction with the adaptation state of spatiotemporal channels that are involved in detection.

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While primary infection may manifest as infectious mononucleosis, like other viruses in the herpes virus family, there may be reactivation of the virus later in life.