James G. May

Eye movement indices of mental workload

Four investigations were carried out to assess the feasibility of using eye movement measures as indices of mental workload. In the first experiment, saccadic extent was measured during free viewing while subjects performed low, moderate and high complexity, auditory tone counting as the workload tasks. The range of saccadic extent decreased significantly as tone counting complexity (workload) was increased. In the second experiment the range of spontaneous saccades was measured under three levels of counting complexity with a visual task that did not require fixation or tracking. The results indicated that the extent of saccadic eye movements was significantly restricted as counting complexity increased. In the third experiment, the effects of practice were examined and decreased saccadic range under high tone counting complexity was observed even when significant increases in performance occurred with practice. Finally, in experiment 4, the first experiment was repeated with additional optokinetic stimulation and the saccadic range was again observed to decrease with tone counting complexity. The results indicated that the extent of spontaneous and elicited eye movements was significantly restricted as counting complexity increased. We conclude that this measure may provide a valuable index of mental workload.

Temporal order judgements in good and poor readers

The amount of time required to make accurate (75% correct) temporal order judgements was measured in groups of 3rd and 4th grade children who were selected on the basis of reading ability. The stimuli, two words (Box and Fox), were tachistoscopically presented to the left and right or above and below a fixation point. Subjects were asked to say which word came first under one condition, or which position contained the stimulus that occurred first under another condition. The stimulus onset asynchrony (SOA) was varied in half octave steps until threshold was determined. The results indicate that good readers required significantly longer SOAs to achieve 75% correct than adults, but significantly shorter SOAs than poor readers. There was not a significant difference in thresholds for the word and position conditions for adults or good readers, but poor readers required significantly more time to achieve criterion for the word condition. The word thresholds were highly correlated (-0.77) with reading level, but the correlation between position threshold and reading level was not significant. The implications of a temporal order deficit are discussed.

Loss in Pattern-Elicited Electroretinograms in Optic Nerve Dysfunction

Both flash- and pattern-elicited electroretinograms and visual-evoked potentials were recorded from a patient with well-documented unilateral optic nerve dysfunction. Although the flash-elicited electroretinograms from the left and right eyes did not differ in amplitude or latency, the flash-elicited visual-evoked potentials were greatly attenuated. Prominent pattern-elicited electroretinograms and visual-evoked potentials, were recorded from the better eye, but neither could be obtained from the affected eye. These results supported the contention that pattern-elicited electroretinograms are derived from optic nerve activity and that the absence of such responses may be diagnostic of loss of optic nerve function. This suggests that testing protocols aimed at assessing optic nerve function might benefit from the inclusion of pattern-elicited electroretinographic recordings. We also obtained contrast sensitivity functions from both eyes. Although considerably suppressed, the contrast sensitivity of the affected eye exhibited a 3-octave range, indicating some pattern-processing capability.

Meridional differences in temporal resolution.

Previous investigations of temporal resolution have shown that performance is influenced by a number of stimulus parameters. The interstimulus interval needed for accurate two-pulse discrimination has been shown to (i) decrease monotonically with flash duration, luminance, and contrast; and (ii) increase monotonically with the spatial frequency of the target. A signal-detectability two-alternative forced-choice procedure was employed to reexamine the effects of spatial frequency on temporal resolution. Also assessed was the effect of grating orientation on such performance. The results confirm that temporal resolution declines with increases in spatial frequency. Furthermore, temporal resolution was significantly lower when oblique, as opposed to vertical, grating targets were used. This ‘oblique effect’ in temporal resolution was observed only with the highest-spatial-frequency target (15 cycles deg−1), and not with stimuli of lower spatial frequency (0·9 and 3·8 cycles deg−1). These findings suggest that stimulus parameters which elicit greater transient channel activity, as opposed to sustained channel activity, enhance temporal resolution. When transient activity is at a minimum, meridional differences in temporal resolution are likely to be attributable to sustained channel activity.

Spatial localization discrepancies : a visual deficiency in poor readers

In two studies, we compared the size of the spatial discrepancies made by young, good and poor readers when locating patterns in space. In the first study, each child was asked to point to the location of a briefly displayed pattern in a 7 x 7 matrix, and the discrepancy between the targets location and the childs response was recorded. The pattern was either a shape or a letter, and the target appeared at nine distances from the middle of the display. The discrepancies made by both groups of children increased with eccentricity, but the rate of increase was significantly greater for the poor readers. The second study required that two temporally and spatially separated target patterns be located on each trial. The discrepancies between their positions and the positions specified by the children were recorded for each target as a function of its eccentricity, and the results for both targets were similar to those obtained in the first study. That is, the discrepancies made by both groups of children increased with eccentricity, but the rate of increase was significantly greater for the poor readers. It was argued that the results of both studies are consistent with the hypothesis that poor readers are handicapped by a low-level processing or perceptual deficiency in the visual encoding system.

Teller Acuity Cards Versus Clinical Judgment in the Diagnosis of Amblyopia with Strabismus

Teller acuity card testing, which is a form of the preferential-looking procedure, is a popular way of assessing visual acuity in preverbal patients. The authors suspected that the clinical judgment of a pediatric ophthalmologist is superior to the Teller acuity cards in diagnosing amblyopia when strabismus is present. Acuity card and fixation preference measurements on each eye were compared at the same clinical visit in a group of 108 strabismic patients. The authors found that the acuity cards could be used to detect amblyopia. However, the pediatric ophthalmologist was more sensitive in diagnosing amblyopia than the Teller acuity cards in the presence of strabismus.

Behavioral and biochemical effects of neonatal treatment of rats with 6-hydroxydopa ☆

Rats receiving injection of either 6-hydroxydopa (60 mug/g) or saline on Days 1, 3, and 5 of life were studied in adulthood on a number of behavioral tasks before being sacrificed at 8 or 12 months for NE assay. The treated rats exhibited impaired passive avoidance, less shock-induced aggression, and more locomotor open-field activity than the control rats. There were no differences between the groups in male copulatory behavior, food and water intake, or thermoregulation. In comparison to the saline rats, 6-hydroxydopa rats showed elevated levels of endogenous NE in lower brainstem regions, e.g., midbrain and pons-medulla, as well as cerebellum. Hypothalamic NE level was not affected. Significant depletions of NE were obtained in the hippocampus and neocortex.

Chromatic adaptation of orientation- and size-specific visual processes in man

This study involved two experiments investigating the effects of adaptation to chromatic gratings of (a) different orientation and (b) different spatial frequency upon the threshold for chromatic test gratings of the same or different wavelength, orientation, or spatial frequency. Thresholds were elevated by adaptation to gratings of the same wavelength and orientation, or the same wavelength and spatial frequency as the test grating, while adaptation to gratings of wavelength and/or orientation, or wavelength and/or spatial frequency different from the test grating did not effect thresholds significantly. In addition, homogeneous chromatic adapting fields were used and it was found that the thresholds for chromatic test gratings were not significantly elevated by such adaptation. These results were in general agreement with previous studies of orientation-specific and size-specific visual mechanisms and it was concluded that these findings offer corroboration of the McCollough effect using threshold measures.

The effects of spatial filtering and contrast reduction on visual search times in good and poor readers

Recent experiments with reading disabled children have shown that image blurring (produced with frosted acetate overlays) results in an immediate benefit in search performance, eye movement pattern and reading comprehension. This suggests that the contrast and spatial frequency content of visual stimuli are important factors for these children. In the present experiment, spatial frequency filtering and contrast reduction were employed to determine whether either of these factors contributes to the beneficial effects observed. Letter arrays were spatially filtered to produce low pass (< 3.5 c/deg) and high pass (> 7.0 c/deg) images. In addition, a low contrast control image was generated to match the low contrast of the high pass image. Children classified as good reader controls (CON), specific reading disabled (SRD), attention deficit disordered (ADD) or comorbid SRD/ADD (COM) were asked to perform a visual search task with each type of image. With high contrast, unfiltered arrays, the search times for the CON and ADD groups were much shorter than those of the SRD and COM groups. While both high pass and low pass filter conditions improved the search speed for the COM group, improvement for the SRD group was only obtained with low contrast stimuli. These results support the notion that the beneficial results of image blurring with SRDs derives from the contrast reduction produced by such manipulations.

Metacontrast reveals asymmetries at red–green isoluminance

We examined the spatiotemporal response of the visual system to red–green isoluminant stimuli by using two metacontrast masking paradigms. With both, we found that green stimuli surrounded by red fields, as compared with red stimuli surrounded by green fields, yielded weaker metacontrast masking. These results run counter to those of models of metacontrast and spatiotemporal vision that predict the same magnitude of metacontrast for the two stimulus-surround cases. Possible visual processes associated with the obtained asymmetries at red–green isoluminance are discussed.