Benjamin R. Stephens

The development of basic mechanisms of pattern vision: Spatial frequency channels

The mature visual system possesses mechanisms that analyze visual inputs into bands of spatial frequency. This analysis appears to be important to several visual capabilities. We have investigated the development of these spatial-frequency channels in young infants. Experiment 1 used a masking paradigm to test 6-week-olds, 12-week-olds, and adults. The detectability of sine wave gratings of different spatial frequencies was measured in the presence and the absence of a narrowband noise masker. The 12-week data showed that at least two spatial-frequency channels with adultlike specificity are present at 12 weeks. The 6-week data did not reveal the presence of narrowband spatial-frequency channels. Experiment 2 used a different paradigm to investigate the same issue. The detectability of gratings composed of two sine wave components was measured in 6-week-olds and adults. The results were entirely consistent with those of experiment 1. The 12-week and adult data indicated the presence of narrowband spatial-frequency channels. The 6-week data did not. The results of these experiments suggest that the manner in which pattern information is processed changes fundamentally between 6 and 12 weeks of age.

Asymmetries in contrast polarity processing in young human infants.

Luminance increments and decrements of equal magnitude are processed asymmetrically in the adult visual system. At detection threshold, decrements are slightly easier to detect than increments. At suprathreshold contrast levels decrements appear to have more contrast than increments when both differ from the background by the same absolute amount. Two experiments are reported with 3.5-month-old human infants examining the processing of luminance increments and decrements. Using two different methods to measure the relative salience of positive and negative polarity high contrast bars, we found consistent evidence that dark bars appeared more salient to infants than light bars when both differed from the background by the same absolute amount. The asymmetry may be explained by noting that when luminance increments and decrements have the same Weber contrast, the decrements will have greater Michelson contrast. Perceived contrast in adults follows Michelson contrast more closely than Weber contrast, and a similar metric may characterize the relations between negative and positive contrasts in young human infants.

Contrast gain control in psychophysical contrast discrimination

Contrast masking may in part reflect the operation of contrast gain control mechanisms in the visual system. Four experiments were conducted in order to examine contrast discrimination under conditions in which the base contrast was interrupted with various maskers. Interrupting the base contrast grating at 8 Hz—with a uniform field, with a randomly phase-shifted version of the same grating, or with a higher contrast version of the same grating—interfered with contrast discrimination. These effects occurred only when the background contrast was above its own threshold. These results may be explained by assuming that the networks responsible for contrast gain control operate by temporally averaging contrast over a period greater than 62.5 msec. When this time-averaged contrast is different from the base contrast, contrast discrimination suffers.

Observers’ Judgments of the Effects of Glare on Their Visual Acuity for High and Low Contrast Stimuli:

Disability glare refers to a reduction in the ability to discern a stimulus that is positioned near another stimulus that has a much higher luminance. While it is common for drivers to report that they have been “blinded” by oncoming headlights, it is unclear whether observers can accurately judge when they are visually disabled by glare. This experiment sought to quantify the accuracy with which observers can judge when a glare source reduces their visual acuity. Seventeen observers estimated their disability glare threshold (DGT)—the luminance of a glare source that would be just sufficient to impair their ability to discern the orientation of a Landolt C that was surrounded by the glare source. These estimated DGTs were compared to the participant’s actual DGTs. Participants consistently underestimated the intensity of glare that was required to impair their acuity. On average, estimates of glare threshold were 88% lower than actual glare threshold intensities. Participants’ judgments were affected by stimulus size but not stimulus contrast. These results suggest that observers can exaggerate the debilitating effects of glare and that they can fail to appreciate that high contrast stimuli are more robust to glare. A driver who believes that even the lowest intensities of headlight glare can visually impair an oncoming driver may be reluctant to use high beam headlamps, despite their significant visibility advantages.

Contrast discrimination under temporally varying contrast conditions

Psychophysical contrast discrimination of a 0.8-cpd vertical grating was tested using a paradigm that alternated test and masking gratings at 8 Hz. Masking contrasts were lower than, equal to, or higher than the test contrasts. Six test contrasts were combined factorially with six masking contrasts to generate a series of six contrast increment threshold versus test contrast curves (tvc curves). A particularly simple relationship existed between these curves. The curves could be brought into alignment by shifting them diagonally by the ratio of their masking contrasts. It is shown that this behavior is predicted by a model in which contrast gain is set by the average of the test and masking contrasts coupled with a simple model of contrast discrimination. Contrast gain control integrates contrast over a period of at least 125 mseC., and contrast discrimination is a function of this time-averaged contrast.

An Efficient Method to Evaluate Skylight Hazard Conspicuity

We evaluated a skylight fall hazard using an “attention conspicuity” measure, in which observers detect and identify hazards in brief presentations of photographs taken from a skylight accident site. Hazard detection and object identification varied with the scene and indicated that the flat-panel warehouse skylights were not conspicuous. A second experiment replicated these results and demonstrated that longer viewing time (3 sec vs. 15 sec) did not improve detection of the skylight, but did increase detection of other hazards. These results suggest that assessment of conspicuity using a quick hazard detection technique may be valid and efficient.

Structural Measures of Undergraduate ePortfolios in Three Educational Contexts

Undergraduate-constructed eportfolios from three programs were used to promote learning goals and aid assessment. We developed objective structural measures of these eportfolios to quantify their descriptions as a function of their context as well as their connections to learning outcomes. ePortfolio objective measures included number of pages, internal links, external links, crosslinks, and document links at hierarchical levels within the website. Results showed strong and significant hierarchical organization, high variability, and high productivity in two educational contexts (a summer research program and a “capstone course”), compared to a third context (a general education eportfolio). These results indicate promise for objective measures of eportfolios as accurate and comprehensive descriptions that may differentiate program learning and assessment goals.

Effect of feedback on estimates of visual sensitivity.

Estimates of visual sensitivity are often difficult to interpret because nonsensory factors may influence performance on a psychophysical task. We examined for 19 naive students, ages 10 to 40 yr., the effect of feedback on estimates of contrast sensitivity and found no evidence that feedback improves performance when compared to no feedback conditions.