Biomedical subjects

J Epelboim

Publications and source records attributed to J Epelboim.

13 recordsLinked to original sources

PubMed2001

A model of eye movements and visual working memory during problem solving in geometry.

The Oculomotor Geometry Reasoning Engine (OGRE) was proposed to model eye movements and visual working memory during problem solving in geometry. OGRE postulates that geometrical elements from diagrams are added to visual working memory when they are scanned. Newly-added elements overwrite elements already in memory. The model was applied to eye-movement patterns of three subjects: two geometry experts and one non-expert. Their eye movements and verbal protocols were recorded as they solved geometry problems posed with diagrams. Subjects used highly redundant eye-movement patterns with multiple rescans of the same geometrical elements. OGRE's model of visual memory provided a good fit for the distribution of times between rescans. The model was used to estimate the size of visual working memory used in geometry. The estimates varied as a function of both problems and subjects, with means and standard deviations for each subject being: 5.3+/-1.4, 4.0+/-0.9 and 4.7+/-1.6.

Eye Movements↗

PubMed2001

Temporal coordination of the human head and eye during a natural sequential tapping task.

The 'natural' temporal coordination of head and eye was examined as four subjects tapped a sequence of targets arranged in 3D on a worktable in front of them. The head started to move before the eye 48% of the time. Both the head and eye started to move 'simultaneously' (within 8 ms of each other) 37% of the time. The eye started to move before the eye only 15% of the time. Gaze-shifts required to perform the tapping task were relatively large, 68% of them were between 27 degrees and 57 degrees. Gaze-shifts were symmetrical. There were almost as many lefts as rights. Very little inter- or intra-subject variability was observed. These results were not expected on the basis of prior studies of head/eye coordination performed under less natural conditions. They also were not expected given the results of two rather similar, relatively natural, prior experiments. We conclude that more observations under natural conditions will have to be made before we understand why, when and how human beings coordinate head and eyes as they perform everyday tasks in the work-a-day world.

Eye Movements↗

PubMed2001

The role of vergence in the perception of distance: a fair test of Bishop Berkeley's claim.

Binocular eye movements were measured while subjects perceived the wallpaper illusion in order to test the claim made by Bishop Berkeley in 1709 that we perceive the distance of nearby objects by evaluating the vergence angles of our eyes. Four subjects looked through a nearby fronto-parallel array of vertical rods (28-35 cm away) as they binocularly fixated a point about 1 meter away. The wallpaper illusion was perceived under these conditions, i.e. the rods appeared farther away than their physical location. We found that although binocular fixation at an appropriate distance was needed to begin perceiving the wallpaper illusion (at least for naive observers), once established, the illusion was quite robust in the sense that it was not affected by changing vergence. No connection between the apparent localization of the rods and vergence was observed. We conclude that it is unlikely that vergence, itself, is responsible for the perceived distance shift in the wallpaper illusion, making it unlikely that vergence contributes to the perception of distance as Bishop Berkeley suggested. We found this to be true even when vergence angles were relatively large (more than 2 deg), the region in which the control of vergence eye movements has been shown to be both fast and effective.

Adult↗

PubMed2000

Characteristics of saccades and vergence in two kinds of sequential looking tasks.

We determined how saccades were used in the experiments described in Epelboim, Steinman, Kowler, Edwards, Pizlo, Erkelens and Collewijn (1995) [Vision Research, 35, 3401-3422], where unrestrained subjects looked at or tapped nearby targets. We report: (i) the size of binocular saccades; (ii) how well saccade size matched in the two eyes; and (iii) saccadic vergence. A representative sample (3375 saccades) was measured: 83% were <15 degrees, 53% were <5 degrees. Only two were 'microsaccades'. Saccade sizes were very similar in the two eyes. These results imply that subjects prefer avoiding large saccades. They can do this simply by re-orienting the head appropriately. Subjects under-verged by 25-35% and preformed well. None experienced diplopia.

Convergence, Ocular↗

PubMed1999

Invariance of brain-wave representations of simple visual images and their names.

In two experiments, electric brain waves of 14 subjects were recorded under several different conditions to study the invariance of brain-wave representations of simple patches of colors and simple visual shapes and their names, the words blue, circle, etc. As in our earlier work, the analysis consisted of averaging over trials to create prototypes and test samples, to both of which Fourier transforms were applied, followed by filtering and an inverse transformation to the time domain. A least-squares criterion of fit between prototypes and test samples was used for classification. The most significant results were these. By averaging over different subjects, as well as trials, we created prototypes from brain waves evoked by simple visual images and test samples from brain waves evoked by auditory or visual words naming the visual images. We correctly recognized from 60% to 75% of the test-sample brain waves. The general conclusion is that simple shapes such as circles and single-color displays generate brain waves surprisingly similar to those generated by their verbal names. These results, taken together with extensive psychological studies of auditory and visual memory, strongly support the solution proposed for visual shapes, by Bishop Berkeley and David Hume in the 18th century, to the long-standing problem of how the mind represents simple abstract ideas.

Adult↗

PubMed1999

Invariance between subjects of brain wave representations of language.

In three experiments, electric brain waves of 19 subjects were recorded under several different experimental conditions for two purposes. One was to test how well we could recognize which sentence, from a set of 24 or 48 sentences, was being processed in the cortex. The other was to study the invariance of brain waves between subjects. As in our earlier work, the analysis consisted of averaging over trials to create prototypes and test samples, to both of which Fourier transforms were applied, followed by filtering and an inverse transformation to the time domain. A least-squares criterion of fit between prototypes and test samples was used for classification. In all three experiments, averaging over subjects improved the recognition rates. The most significant finding was the following. When brain waves were averaged separately for two nonoverlapping groups of subjects, one for prototypes and the other for test samples, we were able to recognize correctly 90% of the brain waves generated by 48 different sentences about European geography.

Brain↗

PubMed1998

Gaze and retinal-image-stability in two kinds of sequential looking tasks.

UNLABELLED: Eye and head movements used to keep the gaze on target were examined as unrestrained seated subjects performed two tasks: (1) tapping sequences of 3-D targets; and (2) only looking at sequences of 3-D targets. Large differences were observed in the head/eye coordination patterns used in each task. During tapping, the head moved quickly and continuously. Retinal-image speeds were relatively high (up to 5 degrees/s). During looking-only, the head moved more slowly and almost came to rest between gaze-shifts. Retinal image velocities between gaze-shifts were 60-77% lower during looking-only than during tapping. CONCLUSION: coordinated eye/head patterns are adjusted to satisfy the demands of a particular task. These adjustments affect low-level oculomotor mechanisms between as well as during the gaze shifts, themselves (reported previously).

Algorithms↗

PubMed1997

Gaze-shift dynamics in two kinds of sequential looking tasks.

Gaze-shift dynamics of unrestrained seated subjects were examined. The subjects participated in two tasks. In the first task, they tapped sequences of 3-D targets located on a table in front of them. In the second task, they only looked at similar sequences of targets. The purpose of the task (tapping vs only looking) affected the dynamics of gaze-shifts. Gaze and eye-in-head peak velocities were higher and gaze-shift durations were shorter during tapping than during looking-only. We conclude that task variables affect gaze-shift dynamics, altering characteristics of the so-called saccadic "main sequence".

Fixation, Ocular↗

PubMed1997

Fillers and spaces in text: the importance of word recognition during reading.

Current theories of reading eye movements claim that reading saccades are programmed primarily on the basis of information about the length of the upcoming word, determined by low-level visual processes that detect spaces to the right of fixation. Many studies attempted to test this claim by filling spaces between words with various non-space symbols (fillers). This manipulation, however, confounds the effect of inserting extraneous characters into text with the effect of obscuring word boundaries by filling spaces. We performed the control conditions necessary to unconfound these effects. Skilled readers read continuous stories aloud and silently. Three factors were varied: (i) position of the fillers in the text (at the beginning, the end, or surrounding each word); (ii) the presence or absence of spaces in the text; and (iii) the effect of the type of filler on word recognition (from greatest effect to least effect: Latin letters, Greek letters, digits and shaded boxes). The effect of fillers on reading depended more on the type of filler than on the presence of spaces. The greater effect the fillers had on word recognition, the more they showed reading. Surrounding each word with digits or Greek letters slowed reading as much as filling spaces with these symbols. Surrounding each word with randomly chosen letters, while preserving spaces, slowed reading by 44-75%--as much as, or more than, removing spaces from normal text. Removing spaces from text with Latin-letter fillers slowed reading by only 10-20% more. We conclude that fillers in text disrupt reading by affecting word recognition directly, without necessarily affecting the eye movement pattern.

Humans↗

PubMed1995

An exponential pyramid model of the time-course of size processing.

Two prior approaches to size processing are discussed in this paper. The first approach is based on measurements of mental size transformations, the second on measurements of thresholds for size and separation. We first analyze these prior approaches and point out differences among prior models and similarities among prior results. This analysis led to new psychophysical experiments that tested the effect of size, relative precision, and eccentricity on the speed of perceptual processing. Speed was not affected by size, but was affected by relative precision and eccentricity. These new results, along with prior results, are then used to formulate a new model based on an exponential pyramid algorithm. This new model, which uses elements of both traditional approaches, can better account for prior, as well as our new results, on the time-course of size processing.

Algorithms↗

PubMed1994

Reading unspaced text: implications for theories of reading eye movements.

According to current theories of reading, the reader's saccades are guided primarily by spaces between words, clearly the most prominent visual feature in most modern texts. This belief was investigated by recording eye movements with unprecedented accuracy and precision while subjects read spaced and unspaced passages both silently and aloud. Modest increases in fixation durations and decreases in overall reading speed were observed when unspaced texts were read. However, subjects read unspaced texts with the same level of comprehension and percentage of regressions as they read spaced texts. The only global eye movement parameter that changed appreciably when spaces were removed was progressive (rightward) saccade length. Progressive saccades were shorter in unspaced texts. However, unspaced texts were denser and narrower because they were constructed so as to contain the same number of words/line as the spaced texts. This meant that unspaced texts contained more informational characters/degree of visual angle. The observed decrease in progressive saccade lengh tended to be proportional to this increase in text density. Therefore, the number of saccades/line of text remained approximately the same in both spaced and unspaced texts. Furthermore, a detailed examination of local eye movement properties, i.e. where within words the subjects fixated and how many times they fixated words of different lengths, suggested that the same oculomotor strategy was used for reading spaced and unspaced texts. This was true for both silent reading and reading aloud. Thus, a model that could explain reading spaced texts could also explain reading unspaced texts with only a change of a single global parameter, namely, saccade length. We conclude that the current tendency to emphasize spaces as guides to reading eye movements must be reconsidered. Words, not spaces, may serve as the perceptual units that guide the line of sight through the text.

Adult↗

PubMed1993

Slow control with eccentric targets: evidence against a position-corrective model.

Does slow control work by keeping the image at a particular retinal location (position-correction), or by keeping the image relatively stable on whatever position it occupies on the retina (velocity-correction)? The best prior evidence that slow control is not position-corrective was that a stable line of sight can be maintained anywhere on or within a small figure. This result, however, does not preclude position-correction with respect to an imagined reference position selected relative to the figure's contour. To test the importance of an imagined reference position, we compared slow control with targets for which selection of a stable reference position is easy (two points arranged symmetrically about the line of sight) and difficult (one or two points located on one side of the line of sight). We found that the stability, velocity and direction of slow control were the same with both kinds of targets. Slow drifts were in idiosyncratic directions, and not toward the eccentric target. Drift speed increased with eccentricity, but drifts did not exceed 10'/sec even at the largest eccentricity tested (4.1 degrees). The independence of slow control from the spatial configuration of the target shows that slow control does not work by bringing either a visible or an imagined reference position to the optimal fixation locus on the retina. The gradual loss of stability with increasing eccentricity is consistent with prior reports of a reduction in the number of motion detectors tuned to low velocities as eccentricity increases. We conclude that motion signals, not position signals, provide the sole sensory input to slow control.

Eye Movements↗

PubMedDate not supplied

When push comes to shove: compensation for passive perturbation of the head during natural gaze shifts.

The effects of passive displacements to the head delivered by an abrupt push to the upper body were studied in human subjects during gaze shifts to nearby targets while the head was completely unrestrained. Accurate measurements of gaze were obtained via the Maryland Revolving Field Monitor, used to measure head and eye rotations unconfounded with translations, and by an acoustic ranging system, used to measure head translations. Compensation for head perturbations was quite good, with gaze errors much the same as gaze errors in the absence of the push. Compensation along one or both meridians was achieved by means of the vestibulo-ocular response in many of the gaze shifts. The results suggest an impressive ability to coordinate head and eye movements during natural gaze shifts, carried out by one or more different kinds of compensatory systems that the subject can access at will or according to task demands.

Eye Movements↗