Low Spatial Acuity Research Articles

Vibrotactile spatial acuity on the back.

Vibrotactile feedback can be built into clothing such as vests. This means that often vibrotactile information is presented to the back. It is known that the back has a relatively low spatial acuity. Spatial acuity varies across different limbs and sometimes with different locations on a limb. These known anisotropies suggest that there might be systematic variations in vibrotactile spatial acuity for different areas of the back and also for different orientations (i.e. horizontal vs. vertical). Here we systematically measured spatial acuity in four areas of the back for both horizontal and vertical orientations. The results show no significant differences in spatial acuity for the back areas that were tested. Spatial acuity was, however, higher in the horizontal direction than in the vertical direction by roughly a factor of two. This means that when designing vibrotactile displays for the back the tactor density can be lower in the vertical direction than in the horizontal direction and density should be constant for different areas of the back.

A 2D Synthesized Image Improves the 3D Search for Foveated Visual Systems.

Current medical imaging increasingly relies on 3D volumetric data making it difficult for radiologists to thoroughly search all regions of the volume. In some applications (e.g., Digital Breast Tomosynthesis), the volumetric data is typically paired with a synthesized 2D image (2D-S) generated from the corresponding 3D volume. We investigate how this image pairing affects the search for spatially large and small signals. Observers searched for these signals in 3D volumes, 2D-S images, and while viewing both. We hypothesize that lower spatial acuity in the observers' visual periphery hinders the search for the small signals in the 3D images. However, the inclusion of the 2D-S guides eye movements to suspicious locations, improving the observer's ability to find the signals in 3D. Behavioral results show that the 2D-S, used as an adjunct to the volumetric data, improves the localization and detection of the small (but not large) signal compared to 3D alone. There is a concomitant reduction in search errors as well. To understand this process at a computational level, we implement a Foveated Search Model (FSM) that executes human eye movements and then processes points in the image with varying spatial detail based on their eccentricity from fixations. The FSM predicts human performance for both signals and captures the reduction in search errors when the 2D-S supplements the 3D search. Our experimental and modeling results delineate the utility of 2D-S in 3D search-reduce the detrimental impact of low-resolution peripheral processing by guiding attention to regions of interest, effectively reducing errors.

Neoptile feathers contribute to outline concealment of precocial chicks

Camouflage is a widespread strategy to increase survival. The cryptic plumage colouration of precocial chicks improves camouflage often through disruptive colouration. Here, we examine whether and how fringed neoptile feathers conceal the outline of chicks. We first conducted a digital experiment to test two potential mechanisms for outline concealment through appendages: (1) reduction of edge intensity and (2) luminance transition. Local Edge Intensity Analysis showed that appendages decreased edge intensity whereas a mean luminance comparison revealed that the appendages created an intermediate transition zone to conceal the object’s outline. For edge intensity, the outline diffusion was strongest for a vision system with low spatial acuity, which is characteristic of many mammalian chick predators. We then analysed photographs of young snowy plover (Charadrius nivosus) chicks to examine whether feathers increase outline concealment in a natural setting. Consistent with better camouflage, the outline of digitally cropped chicks with protruding feathers showed lower edge intensities than the outline of chicks without those feathers. However, the observed mean luminance changes did not indicate better concealment. Taken together, our results suggest that thin skin appendages such as neoptile feathers improve camouflage. As skin appendages are widespread, this mechanism may apply to many organisms.

Visual acuity in the flying snake, Chrysopelea paradisi.

Visual control during high-speed aerial locomotion requires a visual system adapted for such behaviors. Flying snakes (genus: Chrysopelea) are capable of gliding at speeds up to 11 m s-1 and perform visual assessments before take-off. Determining nuances of control requires a closed-loop experimental system, such as immersive virtual arenas. To characterize vision in the flying snake Chrysopelea paradisi, we used digitally reconstructed models of the head to determine a 3D field of vision. We also used optokinetic drum experiments and compared slowphase optokinetic nystagmus (OKN) speeds to calculate visual acuity and conducted preliminary experiments to determine whether snakes would respond to closed-loop virtual stimuli. Visual characterization showed that C. paradisi likely has a large field of view (308.5 ± 6.5° azimuthal range), with a considerable binocular region (33.0 ± 11.0° azimuthal width) that extends overhead. Their visual systems are broadly tuned and motion-sensitive, with peak OKN response gains of 0.50 ± 0.11 seen at 46.06 ± 11.08 Hz, and a low spatial acuity, with peak gain of 0.92 ± 0.41 seen at 2.89 ± 0.16 cpd (cycles per degree). These characteristics were used to inform settings in an immersive virtual arena, including framerate, brightness, and stimulus size. In turn, the immersive virtual arena was used to reproduce the optokinetic drum experiments. We elicited OKN in open-loop experiments, with a mean gain of 0.21 ± 0.9 seen at 0.019 ± 6x10-5 cpd and 1.79 ± 0.01 Hz. In closed-loop experiments, snakes did not exhibit OKN, but held the image fixed, indicating visual stabilization. These results demonstrate for that C. paradisi responds to visual stimuli in a digital virtual arena. The accessibility and adaptability of the virtual setup make it suitable for future studies of visual control in snakes and other animals in an unconstrained setting.

Extra Visual Function of the Human Eye.

Abstract The eye is part of the sensory nervous system. However, there are a number of organ systems that also work with the eye. The retina is the only tissue in mammals that regulates photoreception due to the presence of photoreceptors, the rods and cones and performs both visual and non-visual functions Light plays a fundamental role in the behavior of almost all organisms. In addition to visual processes, light also induces important physiological responses. People with mild vascular disease that causes damage to the retina in the eye are more likely to have problems with thinking and memory skills. Everyone has a natural body clock that they are born with and all organs in the body operate according to biological rhythms. Our experiments with ophthalmic mutant rats also showed that the loss of vision also hampered their physiological activities and their rhythmicity was also disturbed. The menstrual cycle disturbances and age of menarche are regulated by many factors; nevertheless, blindness is one of the most impotent factors in regulating biological clock dependent functions. The human eyes are the only organs in the body capable of “seeing”- wavelengths of light and turning it into visual images. We can't “see” or get a visual image to the brain without eyes. The eye-like ability of skin to sense light by using a receptor (Cryptochrome) but failed to form image. Photoreceptors contain chemicals that change when they are hit by light. This causes an electrical signal, which is then sent to the brain along the optic nerve. Different types of photoreceptor allow us to see an enormous range of light and colours. There are two types of photoreceptors in the human retina, rods and cones. Rods are responsible for vision at low light levels (scotopic vision). They do not mediate colour vision and have a low spatial acuity. The blind: People who have lost their sight have different experiences. Some describe seeing complete darkness, like being in a cave. Some people see sparks or experience vivid visual hallucinations that may take the form of recognizable shapes, random shapes and colours, or flashes of light. An afterimage is an image that continues to appear in the eyes after a period of exposure to the original image. Afterimages occur because photochemical activity in the retina continues even when the eyes are no longer experiencing the original stimulus.

Aerial and aquatic visual acuity of the grey bichir Polypterus senegalus, as estimated by optokinetic response.

The present study assessed the aerial and aquatic visual abilities of juvenile grey bichir Polypterus senegalus, fish capable of terrestrial locomotion, by measuring the optokinetic response to stimuli of varying speed and spatial frequency. In water, fish tracked slow-moving (2° s-1 ) stimuli moderately well and fast-moving stimuli very poorly. Spatial acuity was very low compared with many other species, with maximum response observed at 0.05-0.075 stimulus cycles per degree of visual arc; however, it should be noted that adult fish, with their larger eyes, are likely to have somewhat improved spatial acuity. Low spatial acuity and limited stimulus tracking ability might be expected in a nocturnal ambush predator such as P. senegalus, where gaze stabilization may be less crucial and other sensory inputs may have greater importance in perception of the environment. In air, spatial and temporal acuity were both poorer by every measure, but some visual ability persisted. As the eye shows no anatomical specialization for aerial vision, poor vision was expected; however, the large decrease in saccade velocity observed in air trials was unexpected. Stimulus parameters typically have little effect on the characteristics of the saccade, so this finding may suggest that the function of the reflex system itself could be compromised in the aerial vision of some fishes capable of terrestrial locomotion.

Resolving the Trade-off Between Visual Sensitivity and Spatial Acuity-Lessons from Hawkmoths.

The visual systems of many animals, particularly those active during the day, are optimized for high spatial acuity. However, at night, when photons are sparse and the visual signal competes with increased noise levels, fine spatial resolution cannot be sustained and is traded-off for the greater sensitivity required to see in dim light. High spatial acuity demands detectors and successive visual processing units whose receptive fields each cover only a small area of visual space, in order to reassemble a finely sampled and well resolved image. However, the smaller the sampled area, the fewer the photons that can be collected, and thus the worse the visual sensitivity becomes-leading to the classical trade-off between sensitivity and resolution. Nocturnal animals usually resolve this trade-off in favour of sensitivity, and thus have lower spatial acuity than their diurnal counterparts. Here we review results highlighting how hawkmoths, a highly visual group of insects with species active at different light intensities, resolve the trade-off between sensitivity and spatial resolution. We compare adaptations both in the optics and retina, as well as at higher levels of neural processing in a nocturnal and a diurnal hawkmoth species, and also give a perspective on the behavioral consequences. We broaden the scope of our review by drawing comparisons with the adaptive strategies used by other nocturnal and diurnal insects.

Space–time interdependence: Evidence against asymmetric mapping between time and space

Time and space are intimately related, but what is the real nature of this relationship? Is time mapped metaphorically onto space such that effects are always asymmetric (i.e., space affects time more than time affects space)? Or do the two domains share a common representational format and have the ability to influence each other in a flexible manner (i.e., time can sometimes affect space more than vice versa)? In three experiments, we examined whether spatial representations from haptic perception, a modality of relatively low spatial acuity, would lead the effect of time on space to be substantially stronger than the effect of space on time. Participants touched (but could not see) physical sticks while listening to an auditory note, and then reproduced either the length of the stick or the duration of the note. Judgements of length were affected by concurrent stimulus duration, but not vice versa. When participants were allowed to see as well as touch the sticks, however, the higher acuity of visuohaptic perception caused the effects to converge so length and duration influenced each other to a similar extent. These findings run counter to the spatial metaphor account of time, and rather support the spatial representation account in which time and space share a common representational format and the directionality of space–time interaction depends on the perceptual acuity of the modality used to perceive space.

Parallel Input Channels to Mouse Primary Visual Cortex

It is generally accepted that in mammals visual information is sent to the brain along functionally specialized parallel pathways, but whether the mouse visual system uses similar processing strategies is not known. It is important to resolve this issue because the mouse brain provides a tractable system for developing a cellular and molecular understanding of disorders affecting spatiotemporal visual processing. We have used single-unit recordings in mouse primary visual cortex to study whether individual neurons are more sensitive to one set of sensory cues than another. Our quantitative analyses show that neurons with short response latencies have low spatial acuity and high sensitivity to contrast, temporal frequency, and speed, whereas neurons with long latencies have high spatial acuity, low sensitivities to contrast, temporal frequency, and speed. These correlations suggest that neurons in mouse V1 receive inputs from a weighted combination of parallel afferent pathways with distinct spatiotemporal sensitivities.

(C)overt attention and visual speller design in an ERP-based brain-computer interface.

BackgroundIn a visual oddball paradigm, attention to an event usually modulates the event-related potential (ERP). An ERP-based brain-computer interface (BCI) exploits this neural mechanism for communication. Hitherto, it was unclear to what extent the accuracy of such a BCI requires eye movements (overt attention) or whether it is also feasible for targets in the visual periphery (covert attention). Also unclear was how the visual design of the BCI can be improved to meet peculiarities of peripheral vision such as low spatial acuity and crowding.MethodHealthy participants (N = 13) performed a copy-spelling task wherein they had to count target intensifications. EEG and eye movements were recorded concurrently. First, (c)overt attention was investigated by way of a target fixation condition and a central fixation condition. In the latter, participants had to fixate a dot in the center of the screen and allocate their attention to a target in the visual periphery. Second, the effect of visual speller layout was investigated by comparing the symbol Matrix to an ERP-based Hex-o-Spell, a two-levels speller consisting of six discs arranged on an invisible hexagon.ResultsWe assessed counting errors, ERP amplitudes, and offline classification performance. There is an advantage (i.e., less errors, larger ERP amplitude modulation, better classification) of overt attention over covert attention, and there is also an advantage of the Hex-o-Spell over the Matrix. Using overt attention, P1, N1, P2, N2, and P3 components are enhanced by attention. Using covert attention, only N2 and P3 are enhanced for both spellers, and N1 and P2 are modulated when using the Hex-o-Spell but not when using the Matrix. Consequently, classifiers rely mainly on early evoked potentials in overt attention and on later cognitive components in covert attention.ConclusionsBoth overt and covert attention can be used to drive an ERP-based BCI, but performance is markedly lower for covert attention. The Hex-o-Spell outperforms the Matrix, especially when eye movements are not permitted, illustrating that performance can be increased if one accounts for peculiarities of peripheral vision.

Temporal Factors in Tactile Spatial Acuity: Evidence for RA Interference in Fine Spatial Processing

We investigated the extent to which subjects' ability to perceive the fine spatial structure of a stimulus depends on its temporal properties (namely the frequency at which it vibrates). Subjects were presented with static or vibrating gratings that varied in spatial period (1-8 mm) and vibratory frequency (5-80 Hz) and judged the orientation of the gratings, presented either parallel or perpendicular to the long axis of the finger. We found that the grating orientation threshold (GOT)-the spatial period at which subjects can reliably discriminate the orientation of the grating-increased as the vibratory frequency of the gratings increased. As the spatial modulation of SA1 and RA afferent fibers has been found to be independent of vibratory frequency, the frequency dependence of spatial acuity cannot be attributed to changes in the quality of the peripheral signal. Furthermore, we found GOTs to be relatively independent of stimulus amplitude, so the low spatial acuity at high flutter frequencies does not appear to be due to an inadequacy in the strength of the afferent response at those frequencies. We hypothesized that the RA signal, the strength of which increases with vibratory frequency, interfered with the spatially modulated signal conveyed by SA1 fibers. Consistent with this hypothesis, we found that adapting RA afferent fibers improved spatial acuity, as gauged by GOTs, at the high flutter frequencies.

Neural mechanisms of tactile sensation: Implications for haptic interfaces

Data from a variety of sources suggest complementary roles for the SAI, RA, and PC afferent systems. The following is a working hypotheses: The SAI system is the primary spatial system and is responsible for tactual form and rough perception when the fingers contact a surface directly and for the perception of external events through the distribution of forces across the skin surface. The PC system is responsible for the perception of external events that are manifested through transmitted high-frequency vibrations. The RA system has a lower spatial acuity than the SAI system but a higher sensitivity to local vibration and is responsible for the detection and representation of localized movement between skin and a surface as well as for surface form and texture when surface variation is too small to engage the SAI system. The evidence for these hypotheses will be reviewed. The issue for haptic interfaces is how to drive these systems in order to simulate real contact between the hand and objects in the environment. That will be the subject of the remainder of the talk [Johnson and Hsiao, Ann. Rev. Neurosci. 15, 227–250 (1992)]. [Work supported by NIH.]

Spatial acuity of honeybee vision and its spectral properties

1. In a series of behavioural experiments designed to measure spatial acuity, freely-flying honeybees were trained to discriminate between a horizontal and a vertical grating in a Y-shaped, dual-tunnel apparatus (Fig. 1). Each grating was placed at the entrance to a tunnel, and one of the gratings carried a reward of sugar solution. After training, the spatial frequency of the two gratings, as seen from the tunnel entrances, was varied by varying, symmetrically, their distances from the tunnel entrances. At each spatial frequency, the bees' response (percentage correct discriminations) was calculated from the number of entrances that they made into the tunnels associated with the rewarded and unrewarded gratings, to obtain Response-versus-spatial-frequency (RSF) curves (Figs. 3–5, 10–12). 2. In general, response decreases with increasing spatial frequency. With black-and-white gratings, the RSF curve exhibits a corner spatial frequency (corresponding to a response level of 65%) of ca. 0.25 cycles per degree of visual angle (c/deg), and the response is statistically indistinguishable from the 50% level (corresponding to random choice) at a spatial frequency of ca. 0.34 c/deg (Figs. 3, 4). 3. The shape of the RSF curve does not depend upon which grating is rewarded (horizontal or vertical). Similar RSF curves are also obtained when bees are trained to discriminate a horizontal or a vertical grating from a uniform grey field (Figs. 4, 5). These results imply that the acuities in the horizontal and vertical plane at the front of the eye are essentially equal for freely-flying bees. 4. An independent estimate of spatial acuity was derived by analysing video-films of the bees' flight trajectories when they approached the incorrect stimulus (Figs. 6–9). This analysis estimates acuity to be slightly poorer (corner spatial frequency: ca. 0.18 c/deg) than that inferred from the RSF curves. The films also confirm that the bees use the frontal regions of their eyes in making the visual discrimination (Fig. 10). 5. Detailed observation of the bees' decision behaviour indicates that, when the stimuli in the two tunnels cannot be distinguished at the tunnel entrances, each individual choice that a bee makes between the two tunnels tends to be independent of the outcome of the previous choice. That is, the success or failure of a choice is not memorized. 6. Comparison of acuity measured behaviourally with that predicted from the optical characteristics of the compound eye, reveals that the bees' capacity to discriminate orientation of linear gratings is limited primarily by the size of the visual fields of individual photoreceptors. 7. Experiments conducted using grey gratings, and dual-colour gratings which provide contrast exclusively to a single spectral class of receptors (blue or green) reveal that orientation discrimination of vertical and horizontal high-spatial-frequency gratings is mediated chiefly by the greenreceptor channel, and is therefore colour-blind (Figs. 10–13). 8. Experiments investigating orientation discrimination of dual-colourradial gratings reveal an acuity that is poorer andnot colour-blind (Fig. 14). We suggest that, besides the high-acuity colour-blind mechanism, there is another mechanism that is not colour blind, works on the basis of a memorized, colour-coded spatial template, and has lower spatial acuity.

The distribution of displaced ganglion cells in the retina of the pigeon.

Displaced ganglion cells in the pigeon's retina, at the inner margin of the inner nuclear layer, were labelled by retrograde axonal transport of horseradish peroxidase (HRP). Large HRP injections were made in order to fill all the retinal projection sites in the thalamus and midbrain. The distribution of labelled cells was studied in retinal whole mounts incubated with tetramethyl benzidine (TMB) substrate for HRP. A maximum of 5,300 HRP labelled displaced ganglion cells was found. They were concentrated in a band of retina centred on the horizontal meridian, with high density areas (of about 110 cells/mm2) near the area centralis and in the mid-temporal retina. This is a different distribution to that of ganglion and inner nuclear layer cells; these are concentrated in the area centralis and red field. The orientation of retinal maps was checked by ophthalmoscopic measurements of the angle of the pecten to the horizontal in alert pigeons; this was found to be approximately 70 degrees. The array of displaced ganglion cells, studied by nearest neighbour distributions, was irregular and nearly random, which is consistent with a system of low spatial acuity. In the central retina only the cell bodies and not the dendrites of small displaced ganglion cells (7.5 microns diameter) were labelled; towards the periphery large displaced ganglion cells (16 microns diameter) with 2-5 radially arranged primary dendrites were found.