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At least 1,207 records · Page 67Linked to original sources
PubMed1989

Spatial distribution of the inhibitory effect of peripheral non-informative cues on simple reaction time to non-fixated visual targets.

It is known that reaction time (RT) for the detection of a light target at extrafoveal locations is lengthened by a previous non-informative light cue at the same location. We describe an additional inhibitory effect from cues remote from the target but occurring within the same lateral or altitudinal visual hemifield. Subjects made a speeded key-press response to the second of two successive light flashes in a pair while maintaining fixation. Each of the two flashes could appear at random in one of four positions, two in the right and two in the left visual fields, or two in the upper and two in the lower visual fields. We found an RT prolongation not only for cued over uncued positions, but also for within-field non-coincident cue-target pairs over between-fields cue-target pairs. The within-field inhibitory effect, though smaller than the same-location effect, was fully apparent even when the target occurred at 1 degree of visual angle from the midline and at 29 degrees from the cue. Both effects were seen with cue-target asynchronies ranging from 0.2 to 1.5 sec. The results are relevant to the understanding of the neural mechanisms for covert shifts of attention across the main meridians of the visual field.

Adult↗
PubMedDate not supplied

An algorithm for visual fields.

The authors present an algorithm, or sequential strategy, for the performance and interpretation of visual fields. Based on the principle that particular areas of the visual field are relatively vulnerable and that particular defect configurations are relatively more diagnostic, the strategy proposes certain "core maneuvers" for all initial examinations, followed by selective exploration of the vertical meridian "rectangle" and central "keyhole." This constitutes the "qualitative" portion of perimetry and combines threshold kinetic and suprathreshold static techniques. If time and patience permit, qualitative perimetry is followed by quantiative definition of defect size, depth and slope, using the requisite number of stimuli. This approach may be adapted to all visual field instruments. The interpretation of the field defects is based on assessing their localizing features.

Eye Diseases↗
PubMed1990

TPA-induced inhibition of gap junctional intercellular communication is not mediated through free radicals.

The present investigation was designed to determine whether the inhibition of gap junction-mediated intercellular communication (GJIC) induced by TPA (12-O-tetradecanoylphorbol-13-acetate) in rat liver epithelial (WB-F344) cells in vitro is mediated through free radical production. As assessed by fluorescence redistribution after photobleaching (FRAP) analysis, GJIC was significantly inhibited in cells treated for 1 hr with either 10 ng/ml TPA or 500 microM hydrogen peroxide (H2O2). Addition of 1000 U/ml catalase or 25 microM N',N'-diphenyl-p-phenylenediamine (DPPD) to TPA-treated cells did not alleviate the TPA-induced inhibition of GJIC. However, the concurrent addition of 1000 U/ml catalase to the culture medium prevented the H2O2 inhibition of GJIC. 2'-7'-dichlorofluorescein-mediated fluorescence, a measure of free radical production utilizing the Meridian ACAS 470 interactive laser cytometer, was not significantly increased in WB-F344 cells treated with 10 and 100 ng/ml TPA when compared to control cells. However, polymorphonuclear leukocytes (PMNs) treated for 10 min with 100 ng/ml TPA showed a substantial oxidative burst, as did WB-F344 cells treated for 1 hr with 500 microM H2O2. The concurrent addition of 1000 U/ml catalase to the culture medium attenuated H2O2-mediated free radical production in both PMNs and WB-F344 cells. Data from this study do not support a role for free radicals in the TPA-induced inhibition of GJIC in WB-F344 cells.

Animals↗
PubMed1982

The dependence of the induced effect on orientation and a hypothesis concerning disparity computations in general.

The induced size effect is an apparent rotation about a vertical axis that results from binocularly viewing a target in which one half-image is vertically magnified. A previous paper (Arditi et al., 1981, Vision Res. 21, 755-764) described a theory of this effect in terms of horizontal disparities that exist between vertically magnified images of oblique features and their unmagnified counterparts. The present studies test two aspects of that theory: the requirement of oblique features in stimuli eliciting the induced effect, and the assumption that binocular associations (inputs to disparity computations) are made across horizontal meridians. The former aspect was confirmed in a stereo discrimination experiment in which the direction of rotation (tilt) for crossed line patterns of varying orientation was judged, for a fixed vertical magnification of one half-image. The latter aspect was rejected on the basis of the results of that experiment, and of two experiments in which observers matched the apparent tilt of the lines with a horizontal adjustment line which could be stereoscopically rotated in depth. The data and some associated demonstrations suggest that stereoacuity and apparent depth of oblique lines vertically magnified in one half-image are determined by the horizontal separation between binocular points which are nearest in a fixed binocular coordinate map, rather than by purely horizontal point-matchings. This "nearest neighbor hypothesis" seems to be operative in classic measures of stereoacuity as well as in the induced effect.

Eye Movements↗
PubMed1983

Sensitivity to spatiotemporal colour contrast in the peripheral visual field.

Contrast detection thresholds for spatiotemporal colour modulation were determined at several retinal locations. Colour discrimination is studied for red-green modulation of a yellow field and for yellow-blue modulation of a white field. For constant target size colour vision deteriorates if the stimulus is moved away from the fovea, but if the shift is combined with a suitable enlargement of the target size colour discrimination at the periphery is comparable to that at the fovea. We did not find any retinal location along the horizontal meridian which was red-green or yellow-blue colour blind. Chromatic bars were detected in the nasal peripheral retina at eccentricities of up to at least 50 degrees, temporal colour contrast was perceptible at nasal eccentricities of up to at least 90 degrees.

Color Perception↗
PubMed1983

Effects of practice and the separation of test targets on foveal and peripheral stereoacuity.

The improvement in stereoacuity of two inexperienced, normal subjects was compared at foveal and at 2.5 degrees and 5 degrees peripheral target locations as a function of practice. Outlines of two squares differing only in binocular disparity were used as test stimuli and estimates of stereoacuity were obtained by application of the method of constant stimuli with feedback. The peripheral thresholds of both subjects improved 60-80% over the course of the first 3000-4000 responses at each stimulus location. Foveal improvement followed an identical time-course with a 73% improvement in one subject and only 23% in the other. This difference was reflected in the peripheral/foveal threshold ratios of the two subjects and underlines the necessity of ensuring the stability of thresholds. Stereoacuity measurements were also obtained using several different square separations at the fovea and at 2.5 degrees, 5 degrees and 10 degrees peripheral locations along the horizontal and vertical retinal meridians of two other normal subjects. Practice-stabilized disparity thresholds using optimal target separations revealed a steeper deterioration between the fovea and 2.5-5 degrees eccentricities than did measurements of the same subjects' minimum angles of resolution (MAR). The decrease of optimal stereoacuity at the more peripheral test locations was more gradual than has been previously reported but was not clearly related to that of the MAR.

Fovea Centralis↗
PubMed1984

Characteristics of smooth eye movements with stabilized targets.

Eye movements of two subjects were recorded with a Double Purkinje Image Tracker while they pursued horizontal triangle or ramp stimuli (1, 2, 4 or 8 degrees/sec, p--p 8 degrees). Subjects then attempted to imitate their smooth pursuit eye movement patterns with an electronically-stabilized target or with an afterimage. Next, they attempted to reset the velocity of the stimulus to their memory of the velocity they had previously pursued. The smooth pursuit eye movements of both subjects were very similar. Their attempts to imitate this pattern of eye movements with a stabilized target were only partially successful and subject to large, qualitative individual differences. These differences did not arise from faulty memories of the nature of the pursuit stimuli. Similar results were obtained on the vertical meridian, with a lighted background, and with the stabilized target in eccentric retinal positions. We conclude that stabilization techniques are of dubious value in elucidating properties of the human smooth pursuit subsystem.

Afterimage↗
PubMed1984

Why the cortical magnification factor in rhesus is isotropic.

We examine the arguments advanced recently [by Sakitt (Vision Res. 22, 417-421, 1982), and Tootell et al. (Science 218, 902-904, 1982)] leading to the conclusion that the retino-cortical magnification factor cannot be isotropic. The relevance of the issue is discussed through the use of complex function theory as the appropriate tool to express the geometry of interconnections between neural lamina. A general expression is obtained to compute the length of a curve under a conformal transformation. When this result is applied to circles in the visual field, it is shown that Sakitt's findings imply radial asymmetry of the retino-cortical map, and not local anisotropy, as claimed. In fact, by taking Schwartz's (Vision Res. 20, 645-670, 1980) proposal that the complex log map including a constant eccentricity term be used to represent the retino-cortical mapping, we show that both local isotropy and radial asymmetry are simultaneously valid. Furthermore, the values of cortical length predicted from this approach account exactly for the data shown by Sakitt. A quantitative prediction is offered also for the ratio between the magnification factor of vertical and horizontal meridians at different eccentricities, which is precisely the value reported by Tootell et al. We conclude that these results show that (1) conformal transformations are appropriate tools to describe the geometrical interconnections between neural lamina; (2) for the case of the retino-cortical mapping the appropriate conformal transform is an eccentric complex logarithm.

Animals↗
PubMed1984

Human orientation discrimination tested with long stimuli.

Human orientation discrimination was measured for single long lines presented sequentially. Orientation sensitivity is better for a narrow range of orientations around the principal meridians. Both the orientation sensitivity and its meridional variation increase with stimulus length. Control experiments show that the "oblique effect" can be obtained as well with a signal detection method as with the method of constant stimuli and that the subjects used a visual orientation cue in their judgments. Comparison of these findings with properties of visual cortical cells suggest that cells similar to S cells of cat and monkey striate cortex, carry the neuronal representation used in the decision process. And indeed the meridional variation in orientation sensitivity can be predicted from the number of monkey S cells preferring different orientations.

Adult↗
PubMed1985

The ganglion cell and cone distributions in the monkey's retina: implications for central magnification factors.

The distribution of cones and ganglion cells was determined in whole-mounted monkey retinae. Ganglion cell density along the horizontal meridian was asymmetric, being up to three times greater in nasal retina. A similar but smaller asymmetry occurred with cones. The total number of ganglion cells varied from 1.4 to 1.8 X 10(6), agreeing well with counts of optic nerve axons. The variation of ganglion cell density with eccentricity indicates the magnification factor (MF) of the retina. This was compared with MF at the dorsal lateral geniculate nucleus and at striate cortex, revealing that the relative representation of the fovea increases substantially in both thalamus and cortex.

Animals↗
PubMed1985

Eye position during fixation tasks: comparison of macaque and human.

Two macaques and three humans fixated luminous targets in a dark field. All subjects had greater dispersion of eye position from trial-to-trial (between-trial variability) than would be predicted from sampling error and within-trial variability. Monkeys had greater between-trial dispersion on the vertical meridian than humans because of less precise control of saccades. Mean vertical eye position of the monkeys varied idiosyncratically with the fixation task (spot-dim or line-tilt). Between-trial fixation variability of both monkeys and humans was large enough to affect the interpretation of experiments relating visual performance to retinal anatomy or to neurophysiology.

Adult↗
PubMed1986

Spectral reflectance of the human eye.

Spectral reflectance of the eye was assessed in four young Caucasian subjects with the Utrecht densitometer. Three retinal locations were studied, the fovea, the optic disk and a spot 12 deg temporal on the horizontal meridian. Reflectance was measured with the visual pigments bleached away. The measuring field subtended 2.5 deg. The reflectance factor, relative to an artificial eye with a focal distance of 21.3 mm, was found to be lowest in the blue (0.1% for 419 nm) and highest in the red (10% for 711 nm). A model with six parameters, four densities of (non-photolabile) ocular pigments and two reflectance factors, was proposed to explain the experimental findings. A good fit to the data was obtained and the calculated densities of the ocular pigments came close to data found in the literature.

Adult↗
PubMed1987

The topography of scalp potentials evoked by pattern pulse stimuli.

Pattern pulse stimuli evoke activity in two distinct regions of the human visual cortex in temporal sequence. The earlier responding region (implicit time 93 msec) is localized to a relatively small region near Oz. Its topography is very sensitive to the position of the evoking stimulus in the visual field; it is dominated by a representation of the visual field near the vertical meridian. The later responding cortical region (implicit time 131 msec) is larger than the earlier one. Its medial border touches the lateral border of the earlier responding cortical region and it extends several cm laterally. Its topography is much less sensitive to the position of the evoking stimulus in the visual field. Stimuli presented to the ipsilateral field evoke either weak or no measurable activity in the earlier responding cortical region but often evoke measurable activity near the lateral boundary of the later responding cortical region. These results suggest that the earlier responding cortical region is striate cortex and that the later responding cortical region is composed of visual areas V2, V3, V3A, and V4.

Brain Mapping↗
PubMed1987

Visual receptive field properties in the posterior suprasylvian cortex of the cat: a comparison between the areas PMLS and PLLS.

Receptive field (RF) properties of single units were examined in two visual areas in the cat's postero-medial and postero-lateral suprasylvian cortex (PMLS/PLLS). Electrophysiological recordings were made in the paralyzed and anesthetized preparation in corresponding regions of the medial and lateral banks of the lateral suprasylvian sulcus (LS). In both areas, cell samples were obtained from within the same range of A/P co-ordinates. In both samples, cells responded best to moving stimuli, had large RF's, and did not differ with respect to the distributions of their ocular dominance. Binocularity was equally high, but units in PLLS showed significantly less binocular summation. Cells in both areas preferred high velocities, and a high percentage was direction selective, with directions up and away from the vertical meridian being most common. While in PMLS the preferred direction of a unit usually could be classified into a "radial" vs a "circular" category when RF position was taken into account, this was not possible in PLLS. Units in PLLS also had significantly higher spontaneous activity, higher optimal velocity, and larger RF sizes. In PLLS the investigation of background-foreground interactions revealed a large variety of phase-dependent responses. This is in contrast to the clear preponderance of in-phase inhibition and antiphase facilitation effects in PMLS. The results indicate important differences of RF properties for the two areas, but do not yet suggest a clear functional differentiation. These differences reflect in some respects the RF properties of cells in the structures that provide the main afferents to the two areas.

Action Potentials↗
PubMed1988

Critical flicker frequency as a function of stimulus area and luminance at various eccentricities in human cone vision: a revision of Granit-Harper and Ferry-Porter laws.

When the photopic luminous flux collected by ganglion cells was kept constant at all retinal locations by reducing average stimulus luminance in inverse proportion to photopic Ricco's area (F-scaling), critical flicker frequency to stimuli of 1.2-88 deg2 in area, presented at various eccentricities along the temporal meridian of the visual field, increased as a single logarithmic function of the number of retinal ganglion cells stimulated. Their number was calculated by multiplying stimulus area by the ganglion cell receptive field density of the human retina. When the number of ganglion cells stimulated was kept constant by enlarging the stimulus area in inverse proportion to the ganglion cell density (M-scaling), the logarithm of CFF to green, yellow, orange and red cone-targets increased as parallel linear functions of logarithmic flux, calculated by multiplying retinal illuminance by photopic Ricco's area.

Adult↗
PubMed1988

Contrast discrimination: invariant to spatial parameters.

Contrast discrimination thresholds were measured for 2-dimensional difference-of-Gaussian stimuli using a spatial 2-alternative forced-choice method. The stimulus diameter was varied from 0.4 to 3 deg and the distance between the centres of stimuli from 1 to 12 deg. When the two stimuli were presented on opposite sides of the fixation point on the horizontal meridian the discrimination thresholds were invariant to stimulus size and distance. When the stimulus configuration was vertical and moved to the temporal visual field, the discrimination thresholds were found to depend slightly on the inter-stimulus distance. A high-contrast bar placed between the stimuli did not influence the discrimination sensitivity.

Contrast Sensitivity↗
PubMed1988

Preferences and asymmetries in saccadic responses to delayed bihemifield stimuli.

Saccaadic response preferences and latencies were measured using dual targets presented at 5, 10 and 15 deg along the horizontal meridian in either one or both visual hemifields. In the unihemifield condition, subjects exhibited a strong preference (about 95%) in favor of the target presented closer to the original fixation point. Further, the presence of the second target did not increase the response latency to the preferred target. In the bihemifield condition using spatially symmetric targets, directional preference varied across subjects from 57 to 100% (mean = 83%). There was also a significant increase in response latency as compared with responses to either single targets or dual targets presented in one hemifield. This latency increase was observed for responses in both the preferred and nonpreferred directions and was taken as evidence of mutual interaction between the two hemispheres. The response preferences could be eliminated by delaying the onset of the preferred target by about 100 msec in the unihemifield condition and by about 50 msec in the bihemifield condition. Further, the interactions between the effects of the targets on response time in the bihemifield condition were found to be asymmetrical in that a target presented in the preferred hemifield has a greater effect than a target in the nonpreferred hemifield on responses to targets presented in the counterhemifield. The results suggest an asymmetry in hemispheric interaction which has not previously been demonstrated in the visualoculomotor system.

Adult↗
PubMed1988

Effects of blur and eccentricity on differential spatial displacement discrimination.

Differential spatial displacement discrimination thresholds were determined for stimuli consisting of blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. The tasks were similar to the two-point discrimination acuity task and the three-dot alignment hyperacuity task. Thresholds were determined as a function of eccentricity along the horizontal meridian of the visual field (from 45 degrees nasal to 65 degrees temporal). The spatial spread or blur parameter of the blobs was adopted as a scale parameter. The results show that the performance of the visual system in differential spatial displacement discrimination tasks becomes progressively more homogeneous for a progressive increase in the blur parameter of the stimuli. Scaling (i) the three-blob alignment results with estimates of the cortical magnification factor and (ii) the two-blob separation discrimination results with their corresponding neural blur parameter shows an impressive isotropy and blur scale-invariance for the mechanisms mediating differential spatial displacement discrimination across the visual field. These results are interpreted in terms of a scaled sampling lattice model of the visual system, in combination with an automatic scale-selection mechanism.

Adult↗