Showing papers on "Foveal published in 1981"

Magnification factor and receptive field size in foveal striate cortex of the monkey.

Abstract: Receptive field size and magnification have been studied in striate cortex of awake, behaving rhesus monkeys at visual eccentricities in the range of 5–160 min. The major findings that emerge are (1) magnification in the foveola achieves values in the range of 30 mm/deg, (2) mean field size is not proportional to inverse magnification in contrast with previous reports, and (3) the product, magnification X aggregate field size, is greater in central vision than in peripheral vision. Thus, a point of light projected onto foveal retina is “seen” by larger numbers of striate cortical cells than a point of light projected onto peripheral retina. Implications of these findings for visual localization and two-point discrimination are discussed.

Masking of foveal and parafoveal vision during eye fixations in reading.

Abstract: A window or visual mask as moved across text in synchrony with the reader's eye movements. The size of the window or mask was varied so that either information in foveal or parafoveal vision was masked on each fixation. In another experiment, the onset of the mask was delayed for a certain amount of time following the end of the saccade. The results of the experiments point out the relative importance of foveal and parafoveal vision for reading and further indicate that most of the visual information necessary for reading can be acquired during the first 50 msec that information is available during an eye fixation.

The functional properties of the light-sensitive neurons of the posterior parietal cortex studied in waking monkeys: foveal sparing and opponent vector organization.

Abstract: We describe in this paper the results of a new study of the inferior parietal lobule in 10 waking monkeys combining the methods of behavioral control, visual stimulation, and single neuron analysis. In this study, 1682 neurons were identified; 804 were studied in detail. Neurons insensitive to visual stimuli comprise the fixation, oculomotor, and projection-manipulation classes thought to be involved in initiatives toward action. The largest group of the light-sensitive (LS) neurons were activated from large and frequently bilateral response areas that excluded the foveal region; we term this foveal sparing. The remaining cells subtended areas including the fovea, when tested with large stimuli (6 degrees X 6 degrees), but only 8 of 216 cells studied in detail responded to the small fixation target light. We propose that a dynamic central neural process associated with the acts of fixation and visual attention suppresses responses to foveal stimuli. Parietal LS neurons are sensitive to stimulus movement and direction over a wide range of velocities. The vectors point either inward toward the center or outward toward the perimeter of the visual field, and for neurons with bilateral response areas, the vectors commonly point in opposite directions in the two half-fields; we term this opponent vector organization. The functional properties of area 7 LS neurons are such that they could signal motion in the immediate surround and the apparent motion accompanying head movements and forward locomotion. We surmise that they contribute to a central neural image of immediately surrounding space and to the perceptual constancy of that space obtaining during bodily movement. These properties are suitable for the attraction of gaze and attention to objects and events in the peripheral visual fields. It is this system, together with the classes of parietal neurons concerned with action initiatives, whose destruction is thought to account for the hemi-inattention and neglect of the parietal lobe syndrome in primates.

Saccade size in reading depends upon character spaces and not visual angle

Abstract: Recently there has been a considerable amount of research involving the use of eye movements to study the reading process (Rayner, 1978). This trend is similar to another large-scale research effort undertaken a number of years ago that also dealt with eye movements and reading (Huey, 1908; Tinker, 1958, 1963; Woodworth, 1938). One difference between the earlier research and the more current work is that more sophisticated equipment is now used and display changes contingent upon the position of the eye can be made (McConkie & Rayner, 1975; O'Regan, 1980; Rayner, 1975). Much of the research using dynamic display changes has focused on perceptual aspects of the reading process. Despite this widespread activity, it is still the case that a rather elementary aspect concerning visual factors has not been adequately resolved. That is, it remains unclear as to whether saccades are executed to traverse a certain amount of visual angle or a certain number of letters. For example, eye movements in reading might average around 2 deg of visual angle (Rayner, 1978) because they serve the purpose of bringing text into foveal vision for detailed analysis. If saccades are determined by a critical visual angle, then when viewing distance is decreased (or if the letters are larger) and fewer characters fall within the fovea, the number of characters per saccade will decrease. On the other hand, larger letters or closer viewing distances might allow the letters to be perceived farther out in extrafoveal vision. Is so, readers might execute saccades of a greater visual angle in order to cover a desired number of characters. If either of these alternatives is correct, measures of saccade length in one metric (either visual angle or character spaces) will remain constant as letter size or viewing distance is altered (both change the size of the retinal image, hence the number of letters falling within the fovea), while measures in the other metric will change drastically as retinal image size changes. Huey (1908) and O'Regan (1980) have addressed

Retinal sensitivity and spatial summation in the foveal and parafoveal regions.

Abstract: Retinal sensitivity and the critical area of the spatial summation in the foveal and parafoveal regions were investigated by using a fundus-controlled perimeter to monitor accurately a subject's fundus picture and target position on the retina. The first experiment showed that retinal sensitivity and the critical area change continuously from fovea to periphery and that the diameter of the critical area is a linear function of retinal eccentricity. In the second experiment, this linear relationship was verified by another method.

Eye line-of-sight responsive wide angle visual system

Abstract: Visual system (37) for providing high detail, high resolution imagery anywhere a trainee is looking throughout a wide field of view. An image generation system (38) of visual system (37) accepts inputs from a helmet mounted oculometer system (30) which determines a trainee's instantaneous eye line-of-sight. The image generation system (38) provides a foveal view of high detail, high resolution imagery at the eye line-of-sight. The foveal view is projected on a screen (46) by a foveal projection system (40). The image generation system (38) further provides a peripheral view of low detail, low resolution imagery that is projected on screen (46) by a peripheral projection system (42). The foveal view is merged with the peripheral view by a merge electronics system (44).

Eye-Head Coordination

Abstract: To direct head and eyes toward a target and ultimately fixate it with the fovea, an animal must solve three problems. First, he must compute the angular distance between his foveal lines of sight and the target to be acquired. This angular distance is referred to as retinal error; its absolute magnitude will determine the amplitude of the saccadic eye movement that will be produced. Second, the animal must initiate a head movement that will be compatible in amplitude with the saccadic eye movement. Third, since the eyes usually move first and with higher velocity than the head, their lines of sight will reach and fixate the target while the head is still moving. To stabilize his eyes with respect to a stationary target during head movement, the animal must perform a rotational eye movement that, by being opposite in direction from the movement of the head, but equal to it in amplitude and velocity, allows the fovea to remain constantly on the target. This movement is termed compensatory.

Retinal location and its effect on the processing of target and distractor information.

Abstract: Two experiments were conducted to study the perceptual facilitation and inhibition that occurs between foveal and parafoveal or peripheral regions of the eye. Experiment 1 used a Stroop task to compare color detection latencies of foveal and parafoveal targets. The target and distractor components of the Stroop stimuli were separated and presented with varied stimulus onset asynchronies. Experiment 2 used a Stroop-like task to replicate and extend the findings into the visual periphery. Subjects were found to process foveally presented distractor information while attending to targets presented in parafoveal or peripheral regions of the eye. Distractor information that was incompatible was suppressed while compatible information was used to facilitate target processing. When the targets were presented foveally along with the distractor information, subjects appeared to automatically process the distractor information. The findings are discussed within the framework of past studies that presented subjects with competing tasks across retinal location. The implications of these findings to a two-process theory of attention are also considered. Language: en

Visual evoked potentials (VEP) elicited by checkerboard versus foveal stimulation in multiple sclerosis. A clinical study in 235 patients.

Abstract: In 235 patients with suspected multiple sclerosis (MS) the diagnostic value of visual evoked potentials (VEP) elicited by checkerboard and central foveal stimulation was compared. No significant difference was found. Both methods are supplementary in diagnostic value. Foveal stimulation may provide an additional diagnostic clue. Normal VEPs do not exclude a prior retrobulbar neuritis. Electronystagmography and examination of CSF are at least essential for the diagnosis of MS as VEPs. The combination of these methods increase the accuracy of diagnosis.

Temporal factors in the foveal ERG

Abstract: Peak latencies of the foveal (4° diameter) ERG and the sensitivity of the technique in various diseases have been investigated. The method previously reported (1) uses a rapidly flickering red test light against a white surround of rod saturating luminance. Control ERG's from the same stimulus are also recorded from blind spot stimulation. In senile macular degeneration the sensitivity limit of the test is about 20/60 acuity. New findings include:(1) In senile macular degeneration, the foveal ERG peak latency is usually not abnormally delayed (only 7% of 30 eyes delayed).(2) In contrast, in Stargardt's macular dystrophy, the foveal ERG peak latency is frequently abnormally delayed (47% of 34 eyes). Even at 20/20 to 20/30 acuity, the majority of eyes with Stargardt's have either delayed peak latency, abnormal amplitude or both.(3) In retinitis pigmentosa the sensitivity limit of the foveal ERG (amplitude ratio) is 20/30 to 20/40 (29 eyes). Peak latencies were delayed in 31% of the eyes.

Foveal diplopia thresholds and fixation disparities.

Abstract: Comparison of the magnitude and intrinsic spread of foveal diplopia thresholds with the accuracy of ocular alignment as determined with a subjective alignment method shows that: the accuracy of alignment in the vertical direction (within 1–2 min) is remarkably good and much better than in the horizontal direction; the largest disparities occurring due to restricted alignment accuracy are usually substantially smaller than the foveal diplopia thresholds; interindividual variability in the magnitude of foveal diplopia thresholds is not due only to interindividual variability in the alignment accuracy; and the spread of foveal diplopia thresholds exceeds the spread of ocular alignment, which implies that the noise in the foveal disparity domain is not only due to the restricted alignment accuracy but also to sensory processes. Finally, the data confirm that, unlike the case with diplopia thresholds, the spread of stereoscopic thresholds is not affected by the restricted alignment accuracy.

Backward masking in monkeys after foveal prestriate and inferior temporal cortex lesions

Abstract: In the monkey, foveal prestriate and inferior temporal cortex lesions produce a profound impairment of visual discrimination learning. In this experiment, we examined whether these impairments were associated with a loss of visual sensitivity under conditions of visual masking. Backward masking curves were obtained for two monkeys before and after temporal lobe lesions and for one normal human subject. When tested under the same conditions, human and animal curves were the same. The lesions had no effect on visual masking, although they did impair visual learning.

Spatial Summation in the Foveal and Parafoveal Region

Abstract: Spatial summation effects have been studied in several retinal loci between 0° and 10° from fovea in normal and abnormal subjects using fundus controlled perimetry. Results of spatial summation were as follows: in normal subjects, spatial summation curves have two asymptotes — one shows complete spatial summation and the other shows no summation; in the fovea of amblyopic eye, spatial summation curves represent abnormal gradient with small target; in the eye of optic neuritis patient, spatial summation curves utterly lack the region of complete spatial summation immediately after the visual acuity returned to normal. These results suggest that our new instrument applied to evaluation of visual field in clinical practice is also available for analysis of human abnormal vision function in relation to pathogenesis.

Psychophysical and Electrophysiological Determinants of Motion Detection

Abstract: Displacement thresholds for moving line targets were measured for man and for retinal ganglion cells of cat and monkey. Several studies are summarized and should be of interest for perimetry. A luminous line was stationary at one site and moved at constant velocity to a test site where it again remained stationary. The optimal stimulus parameters for kinetic perimetry were considered to be the range of target luminance, line length and duration of motion which produced minimum and consistent thresholds. For durations of motion below 0.5 seconds, displacement thresholds remained constant for both foveal and peripheral (18 °) vision. Increases in stimulus luminance and line length produced a successive reduction in displacement thresholds in peripheral vision, whereas foveal displacement thresholds were essentially unaffected by these variables. Displacement thresholds were as small or smaller than acuity measures throughout the central 40° of visual field.

Receptive Field-Like Properties Tested with Critical Flicker Fusion Frequency

Abstract: A new psychophysical function reflecting receptive field-like properties has been described. It is based on measurement of the critical flicker fusion frequency (CFF) for a small target centered within a circular background, the size of which is varied as the test parameter. The foveal CFF function has a characteristic V-shape similar to that of the conventional sustained-like function.

Properties of Foveal Pattern Stimuli Which Determine the Morphology and Scalp Distribution of Visual Evoked Potentials

Abstract: A foveal stimulus field with an adapting surround is used to present luminance compensated pattern stimuli. The signals derived from electrodes placed to sample schematic striate and pre-striate foveal projections show noticeable differences in morphology with expected inter-individual variations (Drasdo 1980), but the consistency of form of the signal elicited by any specified pattern for one individual is remarkable. A series of effective patterns which have been selected by empirical observation have been examined to demonstrate their main spatial frequency components. Other patterns generated to include known spatial frequencies in one or many meridians have been compared in an attempt to identify the most effective stimuli for clinical purposes and to relate the observed phenomena to hypothetical mechanisms in the generation of the visual evoked potential.

The effect of a moving foveal target on the subjective sensation of motion

Abstract: In this paper, we report on two experiments concerning the effect of the visual field of fovea on the subjective estimation of angular velocity. Experiment 1 investigates the effect of a slow moving target on the perception of self motion. The result of this experiment can be summarized as follows: a slow moving target seen in the visual field of fovea by a stationary person generates in this person a sensation of self rotation in the same direction as the motion of the target. This phenomenon will be called foveal induced ego motion. Experiment 2 investigates the latency for the detection of a self angular acceleration when the person focusses his fovea on a slowly moving target. From the results of this experiment we conclude that the latency for detection of a small self angular acceleration is shorter if the person sees a small foveal target moving with respect to the person in the direction of self rotation than if that small foveal target is moving (with respect to the person) in the opposite direction. The results of these experiments help us in refining existing models of visual-vestibular interaction, by providing a model which accounts for the phenomenon of oculogyral illusion.