Fixation disparity, i.e. the vergence error within Panum's area, can be measured psychophysically with two nonius (vernier) lines that are presented dichoptically, i.e. one to each eye. The observer adjusts these nonius lines to subjective alignment; the resulting physical nonius offset indicates the amount of fixation disparity. The present experiments investigate the relation between fixation disparity and the nonius bias, which is the physical offset of the nonius lines that is adjusted by the observer in order to perceive them as aligned when both nonius lines are presented to both eyes (binocular nonius bias) or both to the left or both to the right eye (monocular nonius bias). It was found that (1) the fixation disparity is correlated with the binocular nonius bias in the horizontal and vertical meridian and (2) the binocular nonius bias can be predicted from the average of the right eye and left eye monocular nonius bias. To remove the influence of the nonius bias on measured fixation disparity it is possible to calculate the fixation disparity relative to the individual binocular nonius bias, rather than to the physical coincidence of the nonius lines. This procedure tends to increase the correlation between fixation disparity and the tonic resting position of vergence. We discuss the clinical relevance of the dichoptic nonius method for measuring fixation disparity and its limitations as compared to physical recordings of eye position. (+info)
Dominant frequency content of ocular microtremor from normal subjects.
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Ocular microtremor (OMT) is a high frequency tremor of the eyes present during fixation and probably related to brainstem activity (Coakley, D. (1983). Minute eye movement and brain stem function. CRC Press, FL.). Published observations on the frequency of OMT have varied widely. Ocular microtremor was recorded in 105 normal healthy subjects using the Piezoelectric strain gauge technique. The dominant frequency content of a signal was determined using the peak counting method. Values recorded ranged from 70 to 103 Hz, the mean frequency being 83.68 Hz (S.D. +/- 5.78 Hz). (+info)
The role of optical defocus in regulating refractive development in infant monkeys.
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Early in life, the two eyes of infant primates normally grow in a coordinated manner toward the ideal refractive state. We investigated the extent to which lens-induced changes in the effective focus of the eye affected refractive development in infant rhesus monkeys. The main finding was that spectacle lenses could predictably alter the growth of one or both eyes resulting in appropriate compensating refractive changes in both the hyperopic and myopic directions. Although the effective operating range of the emmetropization process in young monkeys is somewhat limited, the results demonstrate that emmetropization in this higher primate, as in a number of other species, is an active process that is regulated by optical defocus associated with the eye's effective refractive state. (+info)
Stereoscopic depth but not shape perception from second-order stimuli.
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Depth can be seen using either linear (first-order) or non-linear (second-order) stereo micropatterns when, in the latter, contrast envelopes contain the disparity information. We examined whether a second-order mechanism can contribute to the perception of 3-D surface shape. Using a variety of different stimulus types, we show that for each, shape is easy to see with linear stimuli. Over a wide range of parameters however, none of our observers perceived shape, however faintly, from the non-linear stimuli. To explore why these elements failed, we simplified our stimulus to a step-edge in depth and measured performance while varying the number of elements. We show how performance declined when more than two non-linear elements were used. We discuss reasons for the limitation found for non-matching elements, including a dissociation for stereopsis between seeing surface shape and depth. (+info)
Temporal constraints on the grouping of contour segments into spatially extended objects.
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The speed of contour integration was investigated in a task that can be solved by grouping contour segments into elongated curves. Subjects had to detect a continuous curve, which could be intersected by one or two other curves. At locations where these curves came in close proximity, the assignment of contour segments to the different curves could be based on collinearity. Reaction times exhibited a strong dependence on (1) the presence of intersections among curves; and (2) the context provided by the stimulus set from which individual stimuli were selected. Reaction times were shortest when grouping of contour segments depended on information at a single location in the visual field. In this condition, responses to stimuli containing an intersection were faster than responses to stimuli that did not. When responses were determined by information at spatially separate locations, responses were delayed, and every intersection increased the reaction time considerably. This result contrasts with earlier investigations which have suggested that contour integration on the basis of collinearity is performed pre-attentively but is in accordance with studies on curve tracing. We propose that the assignment of contour segments to equally coherent curves, a process which may be called figure-figure segregation, is a function of object-based attention. Moreover, the protracted reaction times for some of the stimuli indicate that spread of attention within an object costs time. This implies that object recognition is not always as fast as is sometimes assumed. (+info)
Visual attention modifies spectral sensitivity of nystagmic eye movements.
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If we look out of the window of a travelling train our eyes move rapidly back and forth (saccadic movement). With no attention to individual objects, gaze velocity is low but nystagmic frequency is high (stare nystagmus). If we are interested in individual objects, the angular velocity of gaze is high and the nystagmic frequency low (look nystagmus) (Ter Braak, J.W.G. (1936). Untersuchungen ueber optokinetischen Nystagmus. Archives Neerlandaises de Physiologie de L'homme et des Animaux, 21, 309-376). We show that the spectral sensitivities of the two types of nystagmus differ and that the short-wavelength-sensitive cones significantly contribute only to look nystagmus. (+info)
Peripheral vision and oculomotor control during visual search.
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The present study concerns the dynamics of multiple fixation search. We tried to gain insight into: (1) how the peripheral and foveal stimulus affect fixation duration; and (2) how fixation duration affects the peripheral target selection for saccades. We replicated the non-corroborating results of Luria and Strauss (1975) ('Eye movements during search for coded and uncoded targets', Perception and Psychophysics 17, 303-308) (saccades were selective), and Zelinsky (1996) (Using eye movements to assess the selectivity of search movements. Vision research 36(14), 2177-2187) (saccades were not selective), by manipulating the critical features for peripheral selection and discrimination separately. We found search to be more selective and efficient when the selection task was easy or when fixations were long-lasting. Remarkably, subjects did not increase their fixation durations when the peripheral selection task was more difficult. Only the discrimination task affected the fixation duration. This implies that the time available for peripheral target selection is determined mainly by the discrimination task. The results of the present experiment suggest that, besides the difficulty of the peripheral selection task, fixation duration is an important factor determining the selection of potential targets for eye movements. (+info)
Search for feature and for relative position: measurement of capacity limitations.
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Palmer and his associates (Palmer, Ames & Lindsey (1993). Journal of Experimental Psychology: Human Perception and Performance, 19, 108-130; Palmer (1994). Vision Research, 34, 1703-1721) have confirmed that searches for simple feature targets are not limited by perceptual processing capacity and the effect of set size on performance can be accounted for by integration stage processes only. In this study I used a similar difference threshold method with target and distractor stimuli defined by the relative position of their elements (line drawings of bisected squares) and found clear capacity limitations. Feature search condition, however, with nearly comparable bisected square stimuli did replicate the results of Palmer and associates. This experiment demonstrates that a search for targets defined by relative position in the set of line drawing type of stimuli is fundamentally different from a search for more simple (feature) stimuli and may conform to a strict capacity limited model. (+info)