Introduction: Adults with amblyopia have been shown to experience monocular and dichoptic distortions of their visual perception. The current study aims to measure for the first time dichoptic distortion in children with amblyopia undergoing routine amblyopia treatment, and relate measured perceptual visual distortions to amblyopia features and clinical outcomes of amblyopia treatment. Methods: Children undergoing standard amblyopia treatment had perceptual visual distortion measured within the central 5° of the visual field. The task was mouse-based target-clicking on a stereoscopic LCD monitor, viewed dichoptically through active shutter glasses. The amblyopic eye viewed the cursor and fellow eye the target dot (presented in 16 locations). Global distortion index (mean vector displacement in degrees of mouse-click location from target dot location) was compared to age-matched control children without amblyopia. Results: Amblyopic subjects (n = 13, mean age 6.19 ± 0.99 years) had a significantly greater global distortion index than age-matched control children (n = 140) (0.76° ± 0.56°, vs. 0.41° ± 0.12°, p = 0.001). Global distortion index appears affected by amblyopia type: strabismic/mixed amblyopia was associated with the highest global distortion index (1.00° ± 0.73°), followed by microtropic amblyopia (0.68° ± 0.27) and anisometropic amblyopia (0.41° ± 0.26°). It was not significantly correlated with interocular acuity difference at treatment commencement, number of lines VA improvement, treatment duration, or current VA/refractive error in either eye. Conclusions: Children who have undergone amblyopia treatment experience distorted perception under dichoptic viewing conditions, independent of the VA improvement gained from standard amblyopia treatment practices. These findings suggest that current amblyopia treatment regimens aimed at improving VA/contrast sensitivity do not address the behavioural consequences of utilising amblyopic vision. Meeting abstract presented at VSS 2014

Psychophysical experiments typically use very simple stimuli, such as isolated dots and gratings on uniform backgrounds, and allow no or only very stereotyped eye movements. While these viewing conditions are highly controllable, they are not representative of real-world vision, which is characterized by a complex, broadband input and several eye movements per second. We performed a series of experiments in which subjects freely watched high-resolution nature documentaries and TV shows on a gaze-contingent display. Eye-tracking at 1000 Hz and fast video processing routines allowed us to precisely modulate the stimulus in real time and in retinal coordinates. The task then was to locate either bandpass contrast changes or geometric distortions that briefly appeared in one of four locations relative to the fovea every few seconds. We confirm a well-known loss of sensitivity when video modulations took place around the time of eye movements, i.e. around episodes of high-speed retinal motion. However, we found that replicating the same retinal input in a passive condition, where subjects maintained central fixation and the video was shifted on the screen, led to a comparable loss in sensitivity. We conclude that no process of active, extra-retinal suppression is needed to explain peri-saccadic visual sensitivity under naturalistic conditions. We further find that the detection of spatial modifications depends on the spatio-temporal structure of the underlying scene, such that distortions are harder to detect in areas that vary rapidly across space or time. These results highlight the importance of naturalistic assessment for understanding visual processing. Meeting abstract presented at VSS 2014

In order to maintain accurate control of eye movements, the oculomotor system rapidly adapts to visual error based on foveal feedback. The short term plasticity of the saccadic system can be tested using intrasaccadic target displacements which induce visual motor error. The saccadic system rapidly adjusts to the perceived error by modifying the amplitude of saccadic eye movements. We tested in four participants whether it is possible to induce disconjugate saccadic adaptation by presenting intrasaccadic displacement to only one eye. We presented stimuli dichoptically using a stereo shutter-glass system. At the beginning of each trial, subjects fixated a central binocular fixation target. After a delay, the target was displaced by 10 degrees. During the preadaptation and postadaptation phases, the target was presented in the same location to both eyes without intrasaccadic displacement. In the adaptation phase, the target for the eye moving in the temporal direction was displaced one degree outward during the saccade. This induced uncrossed disparity which required divergent eye movements and induced a stereoscopic percept. We found that, in the preadaptation phase, the eye moving in the nasal direction systematically undershot the target by a greater distance than the eye moving in the temporal direction. These errors required subsequent vergence movements to correctly fixate the target. In the adaptation phase, the saccade amplitude changed for both eyes. However, the eye in which the intrasaccadic target displacement occurred had a greater change in amplitude, and this was the case for both leftward and rightward eye movements. During the postadaptation phase, saccade amplitudes returned to baseline within just a few trials, but the data suggest that the adapted eye takes longer to return to baseline. Therefore, we found evidence of both conjugate and disconjugate changes in saccadic adaptation, which supports the possibility of dissociable spatial maps for each eye. Meeting abstract presented at VSS 2014

Contrast sensitivity for detecting a Gabor pattern is facilitated by the presence of collinear flanking Gabor elements (Polat and Sagi, Vision Research, 33, 993–999, 1993). We examined the spatial and temporal properties of such facilitation using a psychophysical reverse correlation paradigm. This involved subjects performing a 2AFC task- the detection of a target (2 c/deg Gabor ) embedded in two dimensional noise. A staircase procedure varied target contrast and attempted to converge on a level that maintained detection performance at a constant (75% correct) level. Subjects performed the detection task in the presence and absence of 30% contrast flanking Gabors, spatially arranged to optimize facilitation (Woods et al., Vision Research, 42, 733–745, 2002). The orientation and phase of the flankers relative to the target were varied independently. In order to probe the dynamic properties of facilitation, the noise was either static (duration = 250ms) or dynamic (520 ms). True 14 bit grey scale resolution was obtained using a Bits++ system (Cambridge Research Systems). Classification images (CI) were obtained by conventional means and were then fit with two dimensional Gabor functions to determine the parameters of the perceptive field supporting detection of the target. We report that in both the static and dynamic conditions, the presence of flankers led to facilitation only when they were of the same orientation and phase as the target. The CIs from static noise conditions demonstrate that observers' perceptive fields were well-matched to the target stimulus. The CIs from the dynamic noise condition reveal that subjects rely mainly on the initial frames of the sequence to perform the task, suggesting that the facilitation effects are both rapid and are temporally lowpass tuned.

Objective: Contrast sensitivity for grating stimuli is approximately invariant of the relative phase of components at other spatial scales (Graham & Nachmias, 1971, Vision Research, p251). In natural images however, the relative phases across scales define the distribution of structure and contrast. We examined how this property determines contrast gain control. Methods: Contrast increments were applied exclusively to one octave spatial frequency bands within natural images. The relative alignment of the target octave was left intact or was de-correlated from the rest of the image by rotation, mirror reversal or spatial displacement, leaving the amplitude spectrum unchanged. RMS contrast increment thresholds were measured across the full contrast range as a function of spatial frequency. Results: TvC functions were characteristically ‘dipper’ shaped and were used to infer sigmoidal contrast response functions. Contrast increment sensitivity was higher to decorrelated images by up to 4 dB and implied divisive gain control with no change in sensitivity. The magnitude of enhanced contrast sensitivity increased with the spatial displacement of the target band up to about 2 wavelengths. Conclusions: Contrast gain control operates across spatial scales in broad band natural images, but only over a relatively limited spatial area. This property attenuates contrast responses in regions of high local contrast energy and leaves a greater dynamic range at other points in the image.

There is a dissociation between the orientation of contours and their contrast: suddenly changing the orientation of a grating during brief presentations has no effect on its apparent contrast at suprathreshold levels even though its threshold contrast increases (Fiser et al, Vis. Res. 2003). We call this supra-threshold phenomenon contrast conservation. Here we ask whether there is any limit on the differences between stimuli over which contrast is conserved. Two naïve subjects matched the apparent contrast of a succession of 2 images that were spatially coextensive but different in Fourier spectra (a plaid, a natural scene, or a random noise pattern) to a composite formed by superimposing the two images; or they matched a succession of spectrally similar stimuli that differed orientation or spatial position (a ring of 8 evenly spaced Gabor patches whose elements were either rotated by 90 deg in place or moved 22.5 deg along the ring). All stimuli were presented for 53 msec at 25% RMS contrast, 2 deg from fixation. The spectral change had no effect: the curve describing the growth of apparent contrast was the same whether the stimulus was a simple plaid, a natural scene, a random noise pattern, or one of these followed by a different one. The same was true when the orientation but not the position of the ring of Gabor elements was switched. However, when the positions of the Gabor elements changed, but not their orientation, contrast conservation was abolished. As the spectra of a plaid and random noise are as different as possible (maximally concentrated and evenly distributed), we find that the shape of the Fourier spectra of images places no limit on contrast conservation. However, conservation occurs only over a limited area, even for stimuli that are spectrally similar. Natural scenes are no different from either of these artificial stimuli in this respect.

Contrast sensitivity is routinely measured with sine wave grating stimuli presented on homogenous grey backgrounds, whereas natural images are composed of a broad range of spatial and temporal structure. I examine how contrast sensitivity varies with the context in which it is measured in an effort to extend our band-pass channel-based models of visual processing for application in natural conditions.

Purpose: It is well known that visual acuity and contrast sensitivity in amblyopia are attenuated at high spatial frequencies. Amblyopes, however do not report that images appear blurred or lower in contrast as would be expected from these sensory deficits. Instead amblyopes report severe perceptual distortions, which extend beyond the restricted spatial range of the amblyopic eye. The purpose of this study is to identify and quantify such perceptual distortions in amblyopia. Methods: Perceptual distortions were measured with monocular forced choice discrimination tasks and inter-ocular matching tasks. Intrinsic blur was measured with blur increment and matching as a function of the standard deviation of Gaussian edges. Orientation, position and numerosity global distortions were measured in the same way with psuedo-random arrays of highly visible and resolvable Gabor patches whose local orientation and position were systemically varied. Results: Discrimination thresholds in the amblyopic eye were elevated for blur, orientation and spatial position but were within the normal range for numerosity discrimination. Interocular matching thresholds were also elevated for orientation and positional uncertainty, but were within the normal range for blur and numerosity discrimination. Conclusions: Blur and numerosity are veridically represented within the amblyopic visual system, but the representation of local orientation and spatial position shows greater variability compared to normal. It is this increased local spatial uncertainty that underlies the spatial deficit in amblyopia.

Purpose: Recent data challenges the assumption that observers use optic flow at walking speeds in real environments (Rushton et al, 1998, Current Biology 1191). We compared mobility and eye movement behaviour in low vision observers with central (age-related macular disease) or peripheral (glaucoma) field loss. In the former, wide-field motion (optic flow) cues are spared, but central high resolution acuity is impoverished, in the latter, the opposite is true. This is a preliminary study investigating 2 normal observers and 5 patients. Methods: Mobility, fixation behaviour and direction of gaze were measured with a video-based mobile head-mounted eye-tracker (ASL 501) in patients and age-matched controls while walking a 132m course. The course was divided into six sections, including corridors, stairs and road-crossings. Instructions were given at the beginning of each section. Gaze position was classified into scene categories that were analysed manually and computationally by cross-correlation of the image at fixation over time. Results: Patients with central and peripheral field loss showed impaired mobility and fixation behaviour compared to age-matched controls. Performance was not dependent on the location of the visual field loss. Age-matched controls primarily fixated the pre-determined goal points. Patients had larger and more variable fixations and saccades, viewed more scene categories and took longer to reach the goal. This was confirmed with the objective cross-correlation analysis. Conclusions: Loss of visual field impairs mobility in patients when walking an unfamiliar real environment, independent of the location of the scotoma. Age-matched controls directed their fixations mainly towards the goals, while patients fixated many other objects in the scene indicating that they spent longer searching for the goal or potential obstacles. Both high resolution central vision and wide field sensitivity are critical components of walking behaviour in real environments.

Main point: Threshold experiments have led to the view that visual information passes through channels selective to orientation, spatial frequency, and spatial location. We present here observations that are difficult to understand in that view. Reasoning: As the duration of a flashed grating increases, its contrast threshold decreases and its apparent contrast increases, putatively reflecting the growth of exCitation in the channels sensitive to the grating. Shifting the orientation of the grating by 90í in the middle of that growth should allow exCitation to decay in the previously excited channels and to rise anew in channels sensitive to the new orientation. At any given time, the exCitation should lag behind the levels it would have reached if the grating's orientation had not changed (unless exCitation is asymptotic). The shift should raise contrast thresholds and lower apparent contrast. We show that this is true for thresholds, but not true for the apparent contrast of suprathreshold gratings. We observe the same pattern of results when the phase of the grating is shifted by 180í, and when two different random noise patterns or two different natural images are exchanged during this period of growing exCitation (thereby changing the spatial distribution of energy, the phase spectrum, and, to some extent, the energy distribution in 2D Fourier space). Conclusions: (1) growth of apparent contrast, but not contrast threshold, is independent of changes of orientation, phase, and spatial distribution during short presentations; (2) the concept of channels complicates explanation of these particular suprathreshold phenomena and gives impetus to the search a different conceptual model.