Evidence for two distinct nonlinear components in the human pattern ERG.

We obtained the electroretinogram (PERG) in three observers to a 4.6 c/deg grating pattern with a sinusoidal luminance profile which was modulated in time simultaneously with two sinusoidal temporal frequencies (f1 and f2), or at a single frequency (either f1 or f2). Input temporal frequency ranged from 2.4 to 7.5 Hz. A pattern modulated in the counterphase mode with a single frequency produces a response containing only even harmonics of that frequency. However, when the pattern is counterphase modulated with both f1 and f2, the PERG contains second order response frequencies corresponding to intermodulation components (sums and differences) between the two fundamental frequencies. Such frequencies do not exist in the stimulus--they can only be generated by nonlinear neural interactions. Our results provide evidence for at least two nonsaturating nonlinear response mechanisms in the human retina.

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.

Relaxation spectra of interactive neural systems.

A nonlinear model of spatially localized interactive neural systems is analyzed in the neighborhood of steady state solutions by computing relaxation spectra which govern the long time approach to steady state activity levels.

Evidence for two systems mediating perceived contrast.

We report psychophysical evidence for a categorical dichotomy in the perception of contrast. Observers were required to rate the contrast of sinusoidal gratings (2.3 c/d) with contrast varying over a given range relative to two standards. One standard was designated "high" contrast and the other was designated "low." There was a boundary effect: contrast judgment depended upon whether the tested ranges included 10-15% contrast and discrimination was sharpest at the boundary between 10 and 15% contrast. These results are consistent with the existence of two systems underlying perceived contrast; one primarily sensitive below 10%, and the other primarily sensitive above 15% contrast.

Hidden visual loss in optic neuropathy is revealed using Gabor patch contrast perimetry.

Visual sensitivity was evaluated in the central 16 degrees of the visual field in normal subjects, in patients with glaucomatous optic neuropathy, in glaucoma suspects, and in eyes of patients with multiple sclerosis without evidence of active optic neuritis. A novel method was used to assess sensitivity called contrast perimetry (CP). CP essentially samples every point in the central 16 degrees field, and the normal and deviant responses are relatable to spatial summation of contrast signals. In each visual field quadrant 1 cycles/degree sinusoidal grating stimuli limited in area by a gaussian circular aperture (called a Gabor stimulus) were presented. Contrast sensitivity was measured as a function of stimulus size. The normal curve is nearly S-shaped: For small Gabors contrast sensitivity increases slowly, then accelerates and then flattens again. Patients' results fell into two broad categories: a loss more or only evident for small Gabors and another type of loss for both small and large Gabor size. Glaucoma suspect and most glaucoma eyes showed predominant losses to small Gabors. There were more eyes and more VF quadrants identified by contrast perimetry as abnormal compared to the diagnostic yield of the Humphrey 30-2 (central) visual field even though a Humphrey VF defect was defined liberally. Apparently, contrast perimetry may yield diagnostically useful information of paracentral visual sensitivity. Furthermore, the results suggest that selective losses in POAG and some glaucoma suspects occur to spatially broad-band retinal mechanisms, presumably ganglion cells.

Psychophysical examination of paracentral defects in glaucoma.

More than 50% of ganglion cells may be damaged before visual field loss is measurable by conventional methods in primary open angle glaucoma. There is general agreement on the need to improve early diagnosis of visual field loss in primary open angle glaucoma. In this article, new techniques that enlist measurement of paracentral regions are discussed, and the ability of each method to detect visual field loss prior to perimetric loss is described.

A model for oxygen storage by hemoglobin.

An oxygen store based on the reversible combination of oxygen with hemoglobin or related hemoproteins could be of considerable value to organisms or tissues faced with intermittent hypoxia or anoxia. It would be of interest, therefore, to know the physical and chemical requirements and limitations of useful storage. A model is presented which predicts storage time from physical, chemical, and geometric properties of the store. Storage time is arbitrarily defined as the time for 50% of the stored O2 to diffuse from a fully-loaded store when the store is exposed to a zero O2 environment. Results are presented in the form of simple approximate equations and in graphical form and indicate that: (1) a wide range of storage times can be obtained with reasonable biological parameters; (2) storage times approaching 1,000 s can be achieved even with stores of microscopic dimension; (3) biological O2 storage is predominantly diffusion-limited, although reaction-limited behavior is not impossible; and (4) the major effect of co-operativity appears to be on the form of O2 release: hemoglobins without co-operativity release O2 at a constantly declining rate, whereas those with high co-operativity release O2 at a constant rate. The model is applicable not only to hemoglobin-based O2 storage, but also to any reaction-diffusion system in which one substance can be "stored" by reversible combination with some other substance which acts as the store.

A short latency cortical component of the foveal VEP is revealed by hemifield stimulation.

Transient evoked potentials were recorded simultaneously over 5 electrodes placed in a horizontal row across the occiput. A range of spatial frequencies were presented as either full-field or hemifield stimuli. Subjects were 11 normal observers and 5 patients with lesions causing a homonymous hemianopic field defect. The shortest latency peak response was at approximately 70 msec, a negative potential (N70). For all spatial frequencies, full-field stimuli evoked a lower amplitude N70 at the midline than the sum of N70 amplitudes to two hemifield stimuli, suggesting partial cancellation. The latency and amplitude of N70 increased as spatial frequency increased. N70 and P100 differed in respect to their response to spatial frequency and field size, further suggesting that they may not be subsets of a unitary response. For hemifield stimulation, N70 had an ipsilateral maximum and attenuated or completely reversed in polarity across the midline. Consistent with the data of normals using hemifield stimuli, in 5 patients a full-field stimulus elicited an N70 lateralized contralaterally to the homonymous hemianopia, i.e., the ipsilateral N70 was absent. The absolute amplitude difference between the left and right electrodes was significant for hemifield stimulation in normals and full-field stimulation in the patients, but not for full-field stimulation in normals. Our results imply that the evaluation of N70 hemispheric distribution is useful for the evaluation of paramacular visual field defects.

Transient and sustained processing: effects of varying luminance and wavelength on reading comprehension.

BACKGROUND: Reading disability (RD) is a serious epidemiologic problem and may affect up to 15% to 20% of elementary school children. This study addresses whether the reading comprehension skills of children with RD improve as wavelength and contrast of light are altered. METHODS: Fifty-six children, identified as either normal or reading disabled, were required to read a series of standardized 300-word passages under four conditions: no filter (baseline); light gray filter, dark gray filter, and blue filter. Each reading selection was timed, reading comprehension was measured, and data were processed. RESULTS: Initial standardized reading comprehension test scores significantly differentiated average from poor readers. Using reading selection levels consonant with each subject's ability, no significant differences were measured in baseline comprehension scores between good and poor readers. Comprehension scores of poor compared to good readers were significantly better using blue filters (p < 0.01). CONCLUSIONS: Our study suggests an association between wavelength of light, luminance, and reading performance. Blue filters were found to improve reading comprehension in poor readers. The results support the concept of a transient system deficit involving wavelength of light and reading performance.

Effect of luminance on visual evoked potential amplitudes in normal and disabled readers.

BACKGROUND: Considerable evidence exists that some reading-disabled children have disordered visual processing, specifically in the fast processing magnocellular (M) pathway. METHODS: The extent that varying luminance and temporal frequency affect amplitude and latency of visual evoked potentials (VEPs) in normally achieving and reading-disabled children grades 4 to 6 was measured. Each group consisted of approximately 30 subjects. Monocular and binocular single channel VEPs were recorded using a sinusoidal checkerboard pattern of spatial frequency 14 min arc at 3 different temporal frequencies (1, 4, and 8 Hz), and an 8 Hz flicker fusion stimulus. Stimuli were presented under high and low luminance conditions. The peak of the major positive wave component (P100) of each waveform and the trough of the previous major negative wave component were identified, and the peak to trough amplitude was measured. RESULTS: Statistical analysis of the VEP amplitudes and latencies in response to different experimental conditions was performed using a repeated measure analysis of variance (MANOVA). VEP amplitudes were significantly higher for normal readers across all conditions. Within all subjects, significant effects were found for monocular vs. binocular viewing, temporal frequencies, and high vs. low luminance. Similar analysis of latencies revealed no significant differences. CONCLUSIONS: The presence of a weaker VEP response in reading-disabled children suggests a deficit early in visual processing. The significant difference in VEP amplitudes between the two reading groups provides an objective measure of a deficit in the M pathway that has been implicated in this condition. Whether serial VEP recordings might help to assess the effects of optometric therapy by providing an independent index of therapeutic efficiency is of special interest.

Eye movement efficiency in normal and reading disabled elementary school children: effects of varying luminance and wavelength.

BACKGROUND: This investigation examines the question of whether decreasing wavelength of light and/or reducing luminance benefits oculomotor efficiency in normal and reading disabled (RD) children. METHODS: Two groups of children were identified as normal or disabled readers using standardized reading tests. After suitable practice, eye movements were objectively measured with the Visagraph II as each of the subjects silently read a series of three different selections at their independent reading level with clear (control), gray, and blue filters. Four variables were measured for each subject. Data were analyzed using a repeated measure ANOVA and post hoc tests. RESULTS: The standardized reading tests significantly differentiated average from poor readers using grade scores and percentiles. Initially, with clear filters, eye movement scores of normal readers were superior to disabled readers for fixations regressions, and rate of reading. Among the RDs--but not the normals--the three variables improved with the blue filter when compared with the clear filter, number of fixations and regressions were significantly lower, and rate was significantly higher. Gray filters yielded no significant changes. Improvement with the blue filters normalized the three variables in that there were no significant differences between normal and disabled readers. CONCLUSION: This investigation confirms a link between wavelength of light and eye movement efficiency in reading. Blue filters resulted in a significant improvement in the number of fixations and regressions and rate of reading in RD children. The outcome broadens the concept of transient system deficit established in previous research to include the effect on oculomotor efficiency. The educational implications of this study are of special interest to optometrists.

Study of teaching residents how to teach.

In the study reported here, the authors evaluated the effectiveness of a teaching skills program for residents. Twenty-two residents in obstetrics and gynecology, medicine, and family medicine were randomly assigned to control and experimental groups. The experimental groups received instruction and feedback about teaching skills during their first and second postgraduate years. Both groups were evaluated at three times: in the first year before the instructional program, in the second year during the instructional program, and in the third year six months after instruction. During each study phase, videotapes were made of each resident teaching a student in the context of a case presentation. Trained raters evaluated eight teaching skills exhibited in the videotapes. In general, there was greater increase and less decline in the scores of the experimental than the control groups. These data suggest that teaching skills can be improved by instruction and that without support residents' teaching skills do not improve in relation to their clinical competence.

N70 and P100 can be independently affected in multiple sclerosis.

We have studied the relationship between N70 and P100 of the pattern visual evoked potential in 98 patients with multiple sclerosis and in 59 controls. In patients with multiple sclerosis, P100 was either absent or had prolonged latency in 121 eyes (61.7%), while N70 was absent or prolonged in 97 eyes (49.5%). The total number of eyes with either N70 and/or P100 abnormalities was 137 (69.9%). Eighty eyes (40.8%) had abnormal latency of both P100 and N70, while 41 eyes showed P100 delays without corresponding N70 changes. Seventeen eyes had abnormal N70, but normal P100 latency. N70 and P100 appear to be more often absent in the definite rather than in the possible multiple sclerosis group. These data show that N70 and P100 can be independently affected in patients with MS.