Showing papers on "Foveal published in 1979"

Reading without a fovea

Abstract: A visual mask moving in synchrony with the eye obliterated foveal vision during reading under certain conditions When foveal vision was masked, reading became difficult In another condition, a window of readable text moved in synchrony with the eye, and parafoveal vision was masked on each fixation The results point out the importance of foveal and parafoveal vision in reading

Regional differences in photoreceptor performance in the eye of the praying mantis

Abstract: 1. Photoreceptor spacing, and angular, spectral, absolute and relative sensitivities have been measured across the compound eye of the praying mantisTenodera australasiae using optical and electrophysiological techniques. 2. Together, the two compound eyes cover virtually all spatial directions. The huge binocular fields extend vertically 240° with a maximum horizontal overlap of 35° in the frontal part of the eye (Fig. 2). 3. Interommatidial angles (Δφ) range from 0.6° in the frontal eye region up to 2.5° in edge regions of the eye (Fig. 5; Table 1). The eye region with minimumΔφ-values is defined as a fovea. 4. The acceptance angles (Δρ) of light-adapted photoreceptors are almost equal to the interommatidial angles over the whole eye (Δρ is 0.7° in the fovea and 2.5° in the edge of the eye) (Fig. 6; Table 1). The measured values ofΔρ are close to those predicted by the theories of Snyder (1977) and Horridge and Duelli (1979) from the optical and anatomical dimensions of the eye. In this context, the facet diameters are larger and the crystalline cones are longer in the fovea than elsewhere, whereas the rhabdom diameters are smaller. It is concluded that diffraction limitsΔρ in the fovea, whereas the acceptance function of the rhabdom limitsΔρ in eye regions outside the fovea (Fig. 16). 5. The angular sensitivity depends on the state of light adaptation and the time of day. In a defined foveal region the photoreceptors have mean acceptance angles of 0.74° (S.D. = 0.1°) when light-adapted, 1.1° (S.D. = 0.2°) when dark-adapted in daytime, and 2° (S.D. = 0.4°) when dark-adapted at night. The corresponding angles for a defined dorsal eye region are 2.4° (S.D. = 0.3°), 3.2° (S.D. =0.3°), and 6° (S.D.= 0.5°) (Fig. 10). 6. All units recorded from have similar spectral sensitivities, with a maximum in the wavelength range 500 nm to 520 nm, and a weak secondary peak around 370 nm (Fig. 12). 7. Dark-adapted photoreceptors produce bumps whose mean amplitude varies from cell to cell from 1 to 3 mV. Retinular cells in the defined foveal and dorsal eye regions have almost identical quantum capture sensitivities (defined as number of bumps per incident peak axial photons per cm2). Consequently foveal photoreceptors must have a lower quantum capture efficiency (defined as the number of bumps per incident peak axial photons perfacet), because their ommatidia have larger facet diameters. The quantum capture efficiencies are 0.04 (S.D. = 0.02) in the fovea and 0.10 (S.D. = 0.02) in the dorsal eye (Fig. 13; Table 2). This finding supports theoretical predictions that the acceptance function of the rhabdom of foveal photoreceptors is narrowed to decreaseΔρ. 8. Absolute and relative sensitivities of photoreceptors, defined as the reciprocal of the quantal flux required to generate a voltage response of 50% maximum, were determined in the defined foveal and dorsal eye regions. To a point source dark adapted retinula cells from both eye regions have almost identical sensitivities (Fig. 14b, c; Table 3). However, to a large diffuse source, dark-adapted foveal photoreceptors with their relatively small acceptance angles are less sensitive than receptors in the dorsal eye with their largeΔρ values, and retinula cells of both eye regions are more sensitive at night than during the day because their fields of view are broadened (Fig. 14d, e; Table 4). 9. The primary visual task of the praying mantis is the recognition and localization of prey moving against a disrupted background. Prey capture mainly occurs during the day, while the sexual behaviour takes place at night. Some relations between the eye performance and the visual behaviour are discussed.

Bilateral symmetry of vision disorders in typical retinitis pigmentosa.

Abstract: Bilateral symmetry of disorders of vision is examined in 60 typical patients with retinitis pigmentosa. We observed a very high degree of interocular congruence in the patterns of both kinetic visual field defects and threshold profiles and in abnormalities of foveal colour discrimination and visual acuity. Abnormalities of foveal colour vision are highly correlated with the extent of visual field loss.

Local Foveal Inhibitory Effects of Global Peripheral Excitation

Abstract: Global excitation produced by oscillating a peripheral square-wave grating back and forth through one-half cycle inhibits the visibility of an incremental test flash only when the flash is presented in the foveal region of the visual field. This finding is discussed in the context of the neurophysiological periphery effect and shift-effect and their possible role in saccadic suppression.

A study of the Stiles-Crawford (S-C) function at 35 degrees in the temporal field and the stability of the foveal S-C function peak over time.

Abstract: Directional sensitivity of the retina (the Stiles-Crawford function of the first kind) was measured at a point 35 degrees from fixation and at the point of fixation. Three normal observers were used. The peripheral test point intercepted the retina between the optic nerve head and the ora serrata. At both test points, photopic and scotopic (one subject) Stiles-Crawford function peaks were contained within the pupillary bound and approximated the center of the pupil. Directionality at the two points was rather similar. These findings add strength to the argument that retinal receptors align approximately with the center of the exit pupil of the eye. Evidence for stability in Stiles-Crawford peak location in time is also presented.

Visual loss and foveal lesions in Usher's syndrome.

Abstract: Forty-eight patients with Usher's syndrome were assess for the degree of visual loss, extent of lens opacities, and presence of foveal lesions. Twenty-one patients (44%) showed either atrophic or cystic-appearing bilateral foveal changes. Despite having foveal lesions and variable degrees of lens opacities most patients maintained central visual acuity of 20/60 or better until their middle 30s.

Eye Movements in Reading: Eye Guidance and Integration

Abstract: Eye guidance in reading and control of fixation durations are discussed and relevant data reviewed. It is concluded that the location of fixations in reading is determined in a nonrandom manner and that fixation durations are affected by cognitive activities occurring during the fixation. Recent experiments on the integration of information across successive saccades are described. These experiments suggest that I) eye movements per se are not necessary for integration since similar patterns of results were obtained when subjects made eye movements and when the saccade was simulated; 2) attentional allocation is tied to the direction of an eye movement; and 2) purely visual information obtained from parafoveal vision is not overlapped with visual information in foveal vision after the saccade. On the basis of the data and experiments reviewed, a tentative processmonitoring view of eye movements in reading is proposed. Saccadic eye movements during reading generally extend about 8 character spaces (or 2 deg of visual angle), while the mean duration of the fixational pauses separating each saccade is 200–250 msec. However, there is a great deal of variability in both these eye movement characteristics so that the range of saccades is often 1 to 20 character spaces and the mean fixation duration is from 100 to over 500 msec (Rayner & McConkie, 1976). Recently, there have been a large number of studies utilizing eye movement data as dependent variables in attempts to understand the reading process (see Rayner, 1978a, for a review.)

Foveal cone electroretinograms in strabismic amblyopia: comparison with juvenile macular degeneration, macular scars, and optic atrophy.

Abstract: A hand-held stimulator-ophthalmoscope was used to elicit foveal cone electroretinograms (ERGs) from fifteen patients with strabismic amblyopia. The ERGs were in response to a 4 degrees stimulus visualized on the fundus and centred on the fovea throughout testing. Foveal cone ERGs from amblyopic eyes were normal in amplitude and normal in b-wave implicit time. Interocular differences in ERGs in patients with amblyopia were no greater than those in normal subjects. Patients with comparable visual acuity loss due to macular scars or juvenile hereditary macular degeneration had abnormal foveal cone ERGs, while patients with optic atrophy had normal responses. These findings support the idea that the defect in strabismic amblyopia does not involve a functional abnormality in the fovea distal to the ganglion cell layer.

Latency of peripheral saccades

Abstract: Displaying the point of gaze to the observer in addition to a point target provides a secondary visual feedback (2VFB). Eccentric fixation is achieved using a biased 2VFB to yield an experimentally imposed “eccentric fovea.” The target is suddenly moved to a new position and the task is to regain it, in the “eccentric fovea.” It is found that the pattern of eye-movement response consistently starts with saccadric foveal exploration of the target, but its latency has twice the duration of a regular voluntary saccade. Practice, however, makes for the shortened latency tending asymptotically to the regular saccadic duration.

Optokinetic nystagmus in artificial hemianopsy.

Abstract: Optokinetic nystagmus in cases of artificial hemianopsy was studied. Hemianopsy was produced by using a combination of a projector, an erasing device and DC ENG. The effect of the condition of foveal vision in hemianopsy, whether reserved or involved, was discussed. When foveal vision was reserved, nystagmus during stimulation in cases of hemianopsy was essentially the same as in the normal eye. In cases of hemianopsy in which foveal vision was involved, there was a remarkable difference between the mystagmus induced by foveofugal stimulation and that induced by foveopetal stimulation. In the former case the nystagmus was very fine and irregular, in spite of the fact that the nystagmus was well induced (it was nearly the same as in the normal eye in the latter case). Foveal vision is very important for the formation of the slow component of optokinetic nystagmus. Foveopetal movement of the image on the peripheral retina is very important for the formation of the quick component. Foveofugal movement on the peripheral retinal has, on the contrary, only a small effect in attracting the eye.

Dependence of Foveal Visual Acuity on the Size of the Receptive Fields of Retinal Ganglion Cells.

Abstract: : Visual acuity is analyzed in terms of the retinal ganglion cell response to different stimuluses on the receptor matrix. The antagonism between the central and peripheral responses of the ganglion cell receptive field is shown to effect both the sensitivity profile and the Ricco field (Area X Intensity) response. The difficulties with each method of analyzing the receptive field are discussed and an experimental protocol is formulated which derives information by comparisons between the different approaches. Data is presented to show that the ganglion cell receptive fields comparison many receptors even in the portion of the retina where the visual acuity is highest. The experimental data for a similarity between ganglion cell receptive fields in the central and peripheral portions of the retina in the cat is given and compared with similar data for the monkey. (Author)