Showing papers on "Foveal published in 2005"

Peripheral vision can influence eye growth and refractive development in infant monkeys.

Abstract: PURPOSE—Given the prominence of central vision in humans, it has been assumed that visual signals from the fovea dominate emmetropization. The purpose of this study was to examine the impact of peripheral vision on emmetropization. METHODS—Bilateral, peripheral form deprivation was produced in 12 infant monkeys by rearing them with diffusers that had either 4- or 8-mm apertures centered on the pupils of each eye, to allow 24° or 37° of unrestricted central vision, respectively. At the end of the lens-rearing period, an argon laser was used to ablate the fovea in one eye of each of seven monkeys. Subsequently, all the animals were allowed unrestricted vision. Refractive error and axial dimensions were measured along the pupillary axis by retinoscopy and A-scan ultrasonography, respectively. Control data were obtained from 21 normal monkeys and 3 infants reared with binocular plano lenses. RESULTS—Nine of the 12 treated monkeys had refractive errors that fell outside the 10th- and 90th-percentile limits for the age-matched control subjects, and the average refractive error for the treated animals was more variable and significantly less hyperopic/more myopic (+0.03 ± 2.39 D vs. +2.39 ± 0.92 D). The refractive changes were symmetric in the two eyes of a given animal and axial in nature. After lens removal, all the treated monkeys recovered from the induced refractive errors. No interocular differences in the recovery process were observed in the animals with monocular foveal lesions.

A standard model for foveal detection of spatial contrast.

Abstract: The ModelFest data set was created to provide a public source of data to test and calibrate models of foveal spatial contrast detection. It consists of contrast thresholds for 43 foveal achromatic contrast stimuli collected from each of 16 observers. We have fit these data with a variety of simple models that include one of several contrast sensitivity functions, an oblique effect, a spatial sensitivity aperture, spatial frequency channels, and nonlinear Minkowski summation. While we are able to identify one model, with particular parameters, as providing the lowest overall residual error, we also note that the differences among several good-fitting models are small. We find a strong reciprocity between the size of the spatial aperture and the value of the summation exponent: both are effective means of limiting the extent of spatial summation. The results demonstrate the power of simple models to account for the visibility of a wide variety of spatial stimuli and suggest that special mechanisms to deal with special classes of stimuli are not needed. But the results also illustrate the limited power of even this large data set to distinguish among similar competing models. We identify one model as a possible standard, suitable for simple theoretical and applied predictions.

Reorganization of Visual Processing in Macular Degeneration

Abstract: Macular degeneration (MD), the leading cause of visual impairment in the developed world, damages the central retina, often obliterating foveal vision and severely disrupting everyday tasks such as reading, driving, and face recognition. In such cases, the macular damage eliminates the normal retinal input to a large region of visual cortex, comprising tens of square centimeters of surface area in each hemisphere, which is normally responsive only to foveal stimuli. Using functional magnetic resonance imaging, we asked whether this deprived cortex simply becomes inactive in subjects with MD, or whether it takes on new functional properties. In two adult MD subjects with extensive bilateral central retinal lesions, we found that parts of visual cortex (including primary visual cortex) that normally respond only to central visual stimuli are strongly activated by peripheral stimuli. Such activation was not observed (1) with visual stimuli presented to the position of the former fovea and (2) in control subjects with visual stimuli presented to corresponding parts of peripheral retina. These results demonstrate large-scale reorganization of visual processing in MD and will likely prove important in any effort to develop new strategies for rehabilitation of MD subjects.

Eye movements and word skipping during reading revisited

Abstract: The authors examined word skipping in reading in 2 experiments. In Experiment 1, skipping rates were higher for a preview of a predictable word than for a visually similar nonword, indicating there is full recognition in parafoveal vision. In Experiment 2, foveal load was manipulated by varying the frequency of the word preceding either a 3-letter target word or a misspelled preview. There was again a higher skipping rate for a correct preview and a lower skipping rate when there was a high foveal load, but there was no interaction, and the pattern of effects in fixation times was the same as in the skipping data. Experiment 2 also showed significant skipping of nonwords similar to the target word, indicating skipping based on partial information.

Look Here but Ignore What You See: Effects of Distractors at Fixation

Abstract: Distractor interference effects were compared between distractors in the periphery and those placed at fixation. In 6 experiments, the authors show that fixation distractors produce larger interference effects than peripheral distractors. However, the fixation distractor effects are modulated by perceptual load to the same extent as are peripheral distractor effects (Experiments 1 and 2). Experiment 3 showed that fixation distractors are harder to filter out than peripheral distractors. The larger distractor effects at fixation are not due to the cortical magnification of foveal stimuli (Experiments 4 and 5), nor can they be attributed to cuing by the fixation point (Experiment 2), the lower predictability or greater location certainty of fixation distractors (Experiment 5), or their being in a central position (Experiment 6). The authors suggest that preferential access to attention renders fixation distractors harder to ignore than peripheral distractors.

The locus of fixation and the foveal cone mosaic.

Abstract: High-resolution retinal imaging with adaptive optics was used to record the position of a light stimulus on the cone mosaic, with an error at least five times smaller than the diameter of the smallest foveal cones. We discuss the factors that limit the accuracy with which absolute retinal position can be determined. In five subjects, the standard deviation of fixation positions measured in discrete trials ranged from 2.1 to 6.3 arcmin, with an average of 3.4 arcmin (about 17 microm), in agreement with previous studies (R. W. Ditchburn, 1973; R. M. Steinman, G. M. Haddad, A. A. Skavenski, & D. Wyman, 1973). The center of fixation, based on the mean retinal position for each of three subjects, was displaced from the location of highest foveal cone density by an average of about 10 arcmin (about 50 microm), indicating that cone density alone does not drive the location on the retina selected for fixation. This method can be used in psychophysical studies or medical applications requiring submicron registration of stimuli with respect to the retina or in delivering light to retinal features as small as single cells.

Retinal location of the preferred retinal locus relative to the fovea in scanning laser ophthalmoscope images.

Abstract: Purpose It is difficult to determine the position of a preferred retinal locus (PRL) relative to the fovea in scanning laser ophthalmoscope (SLO) images as a result of disease-related retinal morphologic changes that obscure the fovea. To overcome this problem, we developed a method for determining retinal foveal position based on normal fixation position relative to the optic disk. The normal foveal position measurements can then be used to estimate the distance between a PRL and the fovea. Methods Using the SLO, foveal position was determined for 50 normal subjects by measuring the retinal locus of fixation relative to the optic disk in undistorted SLO images. The resulting normal foveal fixation area is described by a bivariate normal ellipse that can be plotted on any undistorted SLO image. Measurement reliability was assessed by repeated measurements. The PRL relative to the normal foveal fixation area was determined for 24 subjects with macular degeneration and bilateral central scotomas. Results The normal foveal fixation area based on all 50 subjects is described by a p = 0.9 bivariate ellipse whose centroid is located 12.6 degrees temporal to the temporal optic disk edge and 1.4 degrees inferior to a horizontal line bisecting the disk. PRL area is shown to increase with distance from the foveal fixation ellipse centroid. The shape of the PRL, characterized by the ratio of PRL ellipse major to minor axis, was found to depend on whether the PRL was vertically or horizontally aligned with the foveal fixation centroid. Conclusions PRL position relative to the fovea can be reliably estimated by plotting the normal foveal fixation bivariate ellipse on undistorted SLO images of retinas in which the fovea is obscured as a result of the disease process.

Unconscious processing of orientation and color without primary visual cortex.

Abstract: In humans, the primary visual cortex (V1) is essential for conscious vision. However, even without V1 and in the absence of awareness, some preserved ability to accurately respond to visual inputs has been demonstrated, a phenomenon referred to as blindsight. We used transcranial magnetic stimulation (TMS) to deactivate V1, producing transient blindness for visual targets presented in a foveal, TMS-induced scotoma. Despite unawareness of these targets, performance on forced choice discrimination tasks for orientation (experiment 1) and color (experiment 2) were both significantly above chance. In addition to demonstrating that TMS can be successfully used to induce blindsight within a normal population, these results suggest a functioning geniculoextrastriate visual pathway that bypasses V1 and can process orientation and color in the absence of conscious awareness.

Eye movements and the modulation of parafoveal processing by foveal processing difficulty: A reexamination.

Abstract: Henderson and Ferreira (1990) found that foveal load (manipulated via word frequency) modulates parafoveal processing, thereby affecting the amount of preview benefit obtained from the word to the right of fixation The present experiment used the eye-contingent boundary paradigm and, consistent with Henderson and Ferreira, showed that foveal load modulated preview benefit for participants who were not aware of the display changes during reading Also, for these participants, foveal load modulated preview benefit regardless of fixation durations on the foveal word For participants who were aware of the display change, preview benefits occurred regardless of foveal processing difficulty These results have important implications for understanding the way in which foveal load influences parafoveal processing during reading

Parafoveal semantic processing of emotional visual scenes.

Abstract: The authors investigated whether emotional pictorial stimuli are especially likely to be processed in parafoveal vision. Pairs of emotional and neutral visual scenes were presented parafoveally (2.1 degrees or 2.5 degrees of visual angle from a central fixation point) for 150-3,000 ms, followed by an immediate recognition test (500-ms delay). Results indicated that (a) the first fixation was more likely to be placed onto the emotional than the neutral scene; (b) recognition sensitivity (A') was generally higher for the emotional than for the neutral scene when the scenes were paired, but there were no differences when presented individually; and (c) the superior sensitivity for emotional scenes survived changes in size, color, and spatial orientation, but not in meaning. The data suggest that semantic analysis of emotional scenes can begin in parafoveal vision in advance of foveal fixation.

The influence of artificial scotomas on eye movements during visual search.

Abstract: Purpose. Fixation durations are normally adapted to the difficulty of the foveal analysis task. We examine to what extent artificial central and peripheral visual field defects interfere with this adaptation process. Methods. Subjects performed a visual search task while their eye movements were registered. The latter were used to drive a real-time gaze-dependent display that was used to create artificial central and peripheral visual field defects. Recorded eye movements were used to determine saccadic amplitude, number of fixations, fixation durations, return saccades, and changes in saccade direction. Results. For central defects, although fixation duration increased with the size of the absolute central scotoma,, this increase was too small to keep recognition performance optimal, evident from an associated increase in the rate of return saccades. Providing a relatively small amount of visual information in the central scotoma did substantially reduce subjects' search times but not their fixation durations. Surprisingly, reducing the size of the tunnel also prolonged fixation duration for peripheral defects. This manipulation also decreased the rate of return saccades, suggesting that the fixations were prolonged beyond the duration required by the foveal task. Conclusions. Although we find that adaptation of fixation duration to task difficulty clearly occurs in the presence of artificial scotomas, we also find that such field defects may render the adaptation suboptimal for the task at hand. Thus, visual field defects may not only hinder vision by limiting what the subject sees of the environment but also by limiting the visual system's ability to program efficient eye movements. We speculate this is because of how visual field defects bias, the balance between saccade generation and fixation stabilization.

Spatial scaling factors explain eccentricity effects on face ERPs.

Abstract: Event-related potential (ERP) studies consistently have described a strong, face-sensitive response termed the N170. This component is maximal at the fovea and decreases strongly with eccentricity, a result that could suggest a foveal bias in the cortical generators responsible for face processing. Here we demonstrate that scaling stimulus size according to V1 cortical magnification factor can virtually eliminate face-related eccentricity effects, indicating that eccentricity effects on face ERPs are largely due to low-level visual factors rather than high-level cortical specialization for foveal stimuli.

Detection of emotional faces: low perceptual threshold and wide attentional span

Abstract: In two experiments, prime face stimuli with an emotional or a neutral expression were presented individually for 25 to 125 ms, either in foveal or parafoveal vision; following a mask, a probe face or a word label appeared for recognition. Accurate detection and sensitivity (A') were higher for angry, happy, and sad faces than for nonemotional (neutral) or novel (scheming) faces at short exposure times (25-75 ms), in both the foveal and the parafoveal field, and with both the probe face and the probe word. These results indicate that there is a low perceptual threshold for unambiguous emotional faces, which are especially likely to be detected both within and outside the focus of attention; and that this facilitated detection involves processing of the affective meaning of faces, not only discrimination of formal visual features.

Attention and Sensory Gain Control: A Peripheral Visual Process?

Abstract: Attention-related sensory gain control in human extrastriate cortex is believed to improve the acuity of visual perception. Yet given wide variance in the spatial resolution of vision across the retina, it remains unclear whether sensory gain operates homogenously between foveal and nonfoveal retinotopic locations. To address this issue, we used event-related potentials (ERPs) in a variant of the canonical spatial attention task. Participants were cued to expect targets at either fixation (foveal targets) or at a location several degrees above fixation (parafoveal targets). At both target locations, manual reaction times were shorter for cued relative to uncued targets, indicating that attention was consistently oriented to the cued location. Nevertheless, attention-related increases in sensory-evoked cortical activity were only observed at the parafoveal target location, as measured by the amplitude of the lateral occipital P1 ERP component. A second experiment replicated this data pattern using targets with lower stimulus contrast, indicating that the absence of a P1 effect for foveal targets could not be attributed to a saturated P1 response under higher-contrast stimulus conditions. When considered in light of retinogeniculate projections to cortex showing systematic changes in their physiological organization beginning within a degree of visual angle of the fovea, our findings support the proposal that the strategic functions of visual attention may vary with the retinotopic location involved.

Gradients of cone differentiation and FGF expression during development of the foveal depression in macaque retina.

Abstract: Cones in the foveola of adult primate retina are narrower and more elongated than cones on the foveal rim, which in turn, are narrower and more elongated than those located more eccentric. This gradient of cone morphology is directly correlated with cone density and acuity. Here we investigate the hypothesis that fibroblast growth factor (FGF) signaling mediates the morphological differentiation of foveal cones—in particular, the mechanism regulating the elongation of foveal cones. We used immunoreactivity to FGF receptor (R) 4, and quantitative analysis to study cone elongation on the horizontal meridian of macaque retinae, aged between foetal day (Fd) 95 and 2.5 years postnatal (P 2.5y). We also used in situ hybridization and immunohistochemistry to investigate the expression patterns of FGF2 and FGFR1–4 at the developing fovea, and three other sample locations on the horizontal meridian. Labeled RNA was detected using the fluorescent marker “Fast Red” (Roche) and levels of expression in cone inner segments and in the ganglion cell layer (GCL) were compared using confocal microscopy, optical densitometry, and tested for statistical significance. Our results show that morphological differentiation of cones begins near the optic disc around Fd 95, progressing toward the developing fovea up until birth, approximately. Levels of FGF2 and FGFR4 mRNAs expression are low in foveal cones, compared with cones closer to the optic disc, during this period. There is no similar gradient of FGF2 mRNA expression in the ganglion cell layer of the same sections. Maturation of foveal cones is delayed until the postnatal period. The results suggest that a wave of cone differentiation spreads from the disc region toward the developing fovea during the second half of gestation in the macaque. A gradient of expression of FGFR4 and FGF2 associated with the wave of differentiation suggests that FGF signalling mediates cone narrowing and elongation.

Fundus findings and longitudinal study of visual acuity loss in patients with X-linked retinoschisis

Abstract: Purpose To determine the presence of fundus findings and natural course of visual acuity change in patients with juvenile X-linked retinoschisis (XLRS). Methods A retrospective longitudinal study of 38 patients with juvenile XLRS (age range, 9-65 years) was conducted. Best-corrected visual acuity, Goldmann visual fields, and results of slit-lamp biomicroscopy of the anterior segment and dilated fundus examination were obtained for all patients. Visual acuity findings at the most recent and initial visits were compared. Follow-up ranged from 1 year to 28 years (mean, 10.2 years). Twenty-five patients were observed for > 5 years, and 11, for > or = 15 years. Results Foveal lesions varied from predominantly radial striations (3%), microcystic lesions (34%), honeycomblike cysts (8%), or their combinations (31%) to non-cystic-appearing foveal changes, such as pigment mottling (8%), loss of the foveal reflex (8%), or an atrophic-appearing lesion (8%). Twelve patients (32%) had situs inversus of their retinal vessels. We observed a superior nasal restriction in the peripheral visual field even in the absence of clinically apparent peripheral retinoschisis. Of the 38 patients who were seen more than once, using logarithm of the minimum angle of resolution (logMAR) comparison, 4 had a decrease in visual acuity of > 0.1 logMAR, equivalent to > 1 line on an ETDRS chart, in their better seeing eye, and 3 had a reduction in visual acuity of > 0.2 logMAR in their better eye. Conclusions A limited change in visual acuity was observed in our cohort of 38 patients with XLRS even over an extended period. However, those patients with non-cystic-appearing changes within the fovea, including pigment mottling or an atrophic-appearing lesion, tended to have a more appreciable degree of visual acuity impairment compared with those patients with a cystic-appearing foveal change.

Anthropomorphic Visual Sensors

Abstract: The tentative of reproducing a biological eye with artificial electronic devices has always had a major drawback. In fact there is an important difference between how a human eye sees the world and how a standard video camera does: while the common visual sensors generally have constant resolution on each part of the image, in their biological counterparts the picture elements are arranged in order to get a very high amount of information in the central part of the field of view (the so called fovea) and a gradually decreasing density of photoreceptors while approaching the borders of the receptive field. There is a practical motivation behind this evolutional choice: a human being needs both a high resolution in order to distinguish between the small details of a particular object for fine movements (the human eye has actually a maximum resolution of about 1/60 degrees) and, at the same time, a large enough field of view (i.e., about 150 degrees horizontally and about 120 vertically for the human eye) so to have a sufficient perception of the surrounding environment. With a constant resolution array of sensors, this two constraints would have increased the total number of photoreceptors to an incredibly high value, and the consequence of this would have been the need of other unrealistic features such as an optic nerve having a diameter of few centimeters (the actual human optic nerve diameter is about 1.5 mm), in order to transfer this amount of data, and a much bigger brain (weighting about 2300 kg, compared to about 1.4 kg of our brain) in order to process all this information, not considering the huge power requirements of such a big brain. Since a zooming capability would have implied a renounce to the simultaneity of the two features, the evolution has answered to the question on how to optimally arrange a given number of photoreceptors over a finite small surface. A lot of different eyes evolved with the disposition of the photoreceptors adapted to the particular niche. Examples of this diversity can be found in the eyes of insects (see, for example, [1] for a review) and in those of some birds that have two foveal regions to allow simultaneous flying and hunting ([2, 3]). In the human eye (Fig. 1.1) we have a very high density of cones (the color sensitive photoreceptors) in the central part of the retina and a decreasing density when we move towards the periphery. The second kind of receptors (the rods, sensitive to luminance), are absent in fovea, but they have a similar spatial distribution. In fact the cone density in the foveola (the central part of the fovea) is estimated at about 150–180,000 cones/mm2 (see Fig. 1.1). Towards the retinal periphery, cone density decreases from 6000 cones/mm2 at a distance of 1.5 mm from the fovea to 2500 cells/mm2 close to the ora serrata (the extremity of the optic part of the retina, marking the limits of the percipient portion of the membrane). Rod density peaks at 150,000 rods/mm2 at a distance of about 3–5 mm from the foveola. Cone diameter increases from the center (3.3 m at a distance of 40 m from the foveola) towards the periphery (about 10 m). Rod diameter increases from 3 m at the area with the highest rod density to 5.5 m in the periphery [4]. Since this sensor arrangement has been proven by the evolution to be an efficient one, we tried to investigate how this higher efficiency could be translated in the world of artificial vision. From the visual processing point of view we asked on one hand whether the morphology of the visual sensor facilitates particular sensorimotor coordination strategies, and on the other, how vision determines and shapes the acquisition of behaviors that are not necessarily purely visual in nature. Also in this case we must note that eyes and motor behaviors coevolved: it does not make sense to have a fovea if the eyes cannot be swiftly moved over possible regions of interest (active vision). Humans developed a sophisticated oculomotor apparatus that includes saccadic movements, smooth tracking, vergence, and various combinations of retinal and extra-retinal signals to maintain

Foveal vs. parafoveal attention-grabbing power of threat-related information.

Abstract: We investigated whether threat words presented in attended (foveal) and in unattended (parafoveal) locations of the visual field are attention grabbing. Neutral (nonemotional) words were presented at fixation as probes in a lexical decision task. Each probe word was preceded by 2 simultaneous prime words (1 foveal, 1 parafoveal), either threatening or neutral, for 150 ms. The stimulus onset asynchrony (SOA) between the primes and the probe was either 300 or 1,000 ms. Results revealed slowed lexical decision times on the probe when primed by an unrelated foveal threat word at the short (300-ms) delay. In contrast, parafoveal threat words did not affect processing of the neutral probe at either delay. Nevertheless, both neutral and threat parafoveal words facilitated lexical decisions for identical probe words at 300-ms SOA. This suggests that threat words appearing outside the focus of attention do not draw or engage cognitive resources to such an extent as to produce interference in the processing of concurrent or subsequent neutral stimuli. An explanation of the lack of parafoveal interference is that semantic content is not extracted in the parafovea.

Visual function: the problem with eccentricity.

Abstract: Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. With an ageing population, the prevalence of such a condition has resulted in a large proportion of the population relying on peripheral vision to undertake activities of daily living. Peripheral vision is not a scaled-down version of the fovea, simply requiring larger print or increased contrast for detection of objects or reading text. Even when print size is scaled and eye movements are minimised, the peripheral retina cannot perform at the level of the foveal region. Understanding how and why reading performance is limited as a function of eccentricity has important implications for how we approach rehabilitation of patients with central visual loss. This brief review of the extensive literature on reading with peripheral vision and the research aimed at better reading rehabilitation for low vision patients focuses on why many of the problems associated with the reduced reading capability of peripheral vision cannot be completely solved with magnification, reducing eye movements or modifying print.

Towards a biologically plausible active visual search model

Abstract: This paper proposes a neuronal-based solution to active visual search, that is, visual search for a given target in displays that are too large in spatial extent to be inspected covertly. Recent experimental data from behaving, fixating monkeys is used as a guide and this is the first model to incorporate such data. The strategy presented here includes novel components such as a representation of saccade history and of peripheral targets that is computed in an entirely separate stream from foveal attention. Although this presentation describes the prototype of this model and much work remains, preliminary results obtained from its implementation seem consistent with the behaviour exhibited in humans and macaque monkeys.