bluuebunny/crossref_metadata_2025 huggingface dataset:
Discrimination of Optic Flow Components: Access to the Third Dimension?
Authors: Freeman T C A , Meese T S , Harris M G
Published: December 2015

A growing body of evidence suggests that optic flow is processed by specialised 2-D motion mechanisms. We asked whether the visual system has parallel, or rapid serial, access to representations of optic flow components in a spatial 4AFC task. Random-dot kinematograms (144 dots per interval) depicting expansion, rotation, deformation (horizontal shear+vertical shear) or one of these components summed with translation, were presented in four spatially abutting circular windows (2.65 deg in diameter), and were temporally modulated by half a cycle of a 300 ms cosine-wave. Within a session, stimuli were of the same type, but the target had opposite sign and was selected with a mouse and cursor with feedback. Systematic local cues were removed by randomising (i) the orientation of the windowing configuration, and (ii) the dot speeds between intervals (speed gradient varied between 2.4% and 6%; translation varied between 16 and 40 min arc s−1). Preliminary results (average SE=3.4%) showed that in the absence of translation, performance was close to chance (25% correct) for rotation (23% correct) and deformation (27% correct), but was good for expansion (61% correct). The addition of translation had no effect on rotation but improved deformation (58% correct) and impaired expansion (22% correct). In experiment 2, unlimited stimulus repetitions were allowed and performance improved (>93%) for all conditions, though, as predicted from experiment 1, reaction times were fastest for expansion and deformation-plus-translation. Importantly, only these two conditions produced unambiguous 3-D perceptions of the stimuli, suggesting that surface slant and motion in depth are coded by mechanisms more rapidly accessible than those subserving general extraction of 2-D motion.
Do Monocular Tilt and Spatial Frequency Aftereffects Induce the Binocular Perception of Inclination and Slant?
Authors: Bradshaw M F , Rogers B J
Published: December 2015

Prolonged viewing of a set of tilted lines can affect the perceived orientation of a second set of lines with a different physical orientation (tilt AE). Similarly, prolonged viewing of a set of lines of a particular spatial periodicity can affect the perceived periodicity of a second set of lines with a different physical periodicity (spatial frequency shift AE). We investigated whether a binocular difference resulting from monocular tilt or spatial-frequency aftereffects could induce the perception of 3-D inclination or slant, respectively. Observers adapted to monocular patterns (5 deg in diameter) arranged in a vertical ‘dumbbell’ configuration in dichoptic alternation. The adapting patterns differed in either orientation (±6.25° or ±11.25°) or in spatial frequency (±0.5 or ±0.75 octaves) from a test surface comprising vertical lines at 4 cycles deg−1. The period of adaptation was 3 mins. Observers judged whether the test surface appeared to (i) form a convex or concave hinge in depth (after adaptation to tilt) or (ii) to slant in opposite directions about a vertical axis (after adaptation to periodicity). Using a relative slant/inclination judgment may be more sensitive than depth matching or nulling of a single surface (eg Sloane and Blake, 1987 Perception & Psychophysics42 569 – 575). Our results suggest that (i) differences in perceived periodicity in separate monocular images do not induce the impression of stereoscopic slant [confirming the results of Sloane and Blake (1987)] and (ii) differences in perceived orientation in separate monocular images do not induce an impression of stereoscopic inclination.
Psychophysical Discrimination of Spatial Structure in Natural Images
Authors: Carlin P , Watt R
Published: December 2015

We report a series of experiments in which subjects were required to make spatial discriminations about naturally obtained images, as follows. Subjects were shown two natural images on a computer screen, side by side and for a period of 500 ms. Subjects were then shown, on a separate part of the computer screen, a small patch of one of the images selected at random. Subjects were required to decide which of the two full images the patch comes from, and whereabouts in that image it is taken from. They indicated their response by clicking the mouse cursor at the appropriate point on the screen.

The proportion of trials on which the subjects selected the correct image and the accuracy of the spatial position of their correct responses were calculated. These two measures independently indicate the general discriminability of the two image sets, and the extent to which the spatial layout of the image has been perceived. By comparing response accuracy for images that are from different sets (such as mountain or town scenes) with response accuracy when the two images are from the same set it is possible to establish the image properties that underlie coarse scene apprehension. By using sets of images that are perhaps best described as being natural textures (such as patterns of foliage or rock surface) it is possible to measure the extent to which texture is processed spatially. Our results are interpreted in the light of statistical differences between the image sets that we have used.
Face Detection in Natural Scenes
Authors: Hill H , Watt R
Published: December 2015

The first task of any face processing system is detection of the face. We studied how the human visual system achieves face detection using a 2AFC task in which subjects were required to detect a face in the image of a natural scene. Luminance noise was added to the stimuli and performance was measured as a function of orientation and orientation bandwidth of the noise.

Sensitivity levels and the effects of orientation bandwidth were similar for horizontally and vertically oriented noise. Performance was reduced for the smallest orientation bandwidth (5.6°) noise but sensitivity did not decrease further with increasing bandwidth until a point between 45° and 90°. The results suggest that important information may be oriented close to the vertical and horizontal. To test whether the results were specific to the task of face detection the same noise was added to the images in a man-made natural decision task. Equivalent levels of noise were found to be more disruptive and the effect of orientation bandwidth was different.

The results are discussed in terms of models of face processing making use of oriented filters (eg Watt and Dakin, 1993 Perception22 Supplement, 12) and local energy models of feature detection (Morrone and Burr, 1988 Proceedings of the Royal Society of London B235 221 – 245).
One Whole or Two? The Flexible Role of the Observer in Perceptual Grouping
Authors: Freeman E D
Published: December 2015

The flexibility of perceptual grouping was examined from a functional perspective. If particular wholes are good for particular tasks, we can bend the rules of Gestalt organisation in order to see them? Evidence was gained by observing the effects of Gestalt factors on task performance, in the context of subjects' experience and expectation.

Ellipsoids were split into two semi-ellipses by a central gap of variable width, which controlled the relative dominance of a one-whole percept (Narrow-Gap) or a two-parts percept (Wide-Gap). In a sequential comparison task, a reference ellipsoid was paired either with another ellipsoid (Whole/Whole), or with a single semi-ellipse (Whole/Part). Subjects responded “different” to a change in the shape of one semi-ellipse. Stimuli appeared for 50 ms and the reference stimulus was masked with stimulus onset asynchronies (SOAs) of 50 to 250 ms. Subjects were first trained either on Whole/Whole or Whole/Part only; both comparisons types were then mixed randomly. A loss of discriminability was predicted for Whole/Whole with Wide-Gap, and for Whole/Part with Narrow-Gap, but only for untrained and unexpected comparisons.

The predicted interaction was significant. The effects were particularly strong with 50 ms SOA, suggesting an early involvement of task-contextual factors in perceptual grouping, and showing no evidence of global priority. Subjects appeared to readily learn flexible grouping strategies, abstracting only the behaviourally relevant relationships, and thus offsetting the effects of stimulus structure on perceptual grouping.
Searching for Objects in the Visual Periphery: Effects of Orientation
Authors: Newell F N
Published: December 2015

Previous studies have found that the recognition of familiar objects is dependent on the orientation of the object in the picture plane. Here the time taken to locate rotated objects in the periphery was examined. Eye movements were also recorded. In all experiments, familiar objects were arranged in a clock face display.

In experiment 1, subjects were instructed to locate a match to a central, upright object from amongst a set of randomly rotated objects. The target object was rotated in the frontoparallel plane. Search performance was dependent on rotation, yielding the classic ‘M’ function found in recognition tasks. When matching a single object in periphery, match times were dependent on the angular deviations between the central and target objects and showed no advantage for the upright (experiment 2). In experiment 3 the central object was shown in either the upright rotation or rotated by 120° from the upright. The target object was similarly rotated given four different match conditions. Distractor objects were aligned with the target object. Search times were faster when the centre and target object were aligned and also when the centre object was rotated and the target was upright. Search times were slower when matching a central upright object to a rotated target object. These results suggest that in simple tasks matching is based on image characteristics. However, in complex search tasks a contribution from the object's representation is made which gives an advantage to the canonical, upright view in peripheral vision.
Border Ownership as a Special Case of Contrast Ownership
Authors: Anderson B L
Published: December 2015

The classification of boundaries as figure and ground is one of the fundamental computations performed by the visual system. This computation requires classifying one side of a boundary, as an occluding or ‘intrinsic’ contour, and the other side of the contour as occluded or ‘extrinsic’ (also described as the problem of border ownership). A recent analysis of stereoscopic contour junctions revealed that the classification of edges as intrinsic or extrinsic to a neighbouring surface depends critically on the relative depth of the contours at the image junction. Specifically, it was argued that near edges are intrinsically one-sided (ie border ownership is uniquely specified by the junction geometry), whereas far contours are inherently two-sided (ie the border ownership of the far contour cannot be determined on the basis of the local junction geometry). Here I present new stimuli that reveal that the asymmetry of near and far contours in specifying border ownership reflects a more general asymmetry in the classification of near and far contrasts.

A variety of stimuli were created. In one set of patterns, an array of five circular discs was embedded on a homogeneous background (either white or black) and was filled with a texture. A uniform disparity was added to the textured region such that it appeared nearer than the outer edges of the disc. When the power spectrum of the texture was systematically varied as functions of 1/ fn, the texture did not appear as a solid surface. Rather, the discs containing the texture gradients were decomposed into two sources: a constant background colour of the discs, and transparent clouds that appeared to float in front of the discs. Remarkably, the apparent colour of the clouds and the discs could be transformed simply by changing the colour of the background of the regions that neighboured the discs; the texture itself was unchanged. Specifically, when the textured discs were placed on white backgrounds, the pattern appeared as white clouds hovering in front of dark discs; but when the textured discs were placed on black backgrounds, the percept was of black ‘smoke’ hovering in front of white discs. It is argued that these results reveal that the problem of border ownership is a special case of contrast ownership. The two-sided qualities of far contrasts determine the colour of the background, which in turn determines which side of a near gradient is considered to be more or less opaque (ie which side is more occlusive).
A Colour Constancy Phenomenon Related to Perceived 3-D Shape
Authors: Bergström S S , Gustafsson K-A , Jakobsson T
Published: December 2015

Flat, rectangular displays consisting of 2, 3, 4, or 6 vertical grey stripes, alternately light and dark gray (NCS 3500 and NCS 6000, respectively) were presented one at a time illuminated by two identical projectors. One projector illuminated the upper half of the display through a yellowish colour filter (Strand Golden Amber), and the other one illuminated the lower half through a bluish colour filter (Kodak Wratten 80 A). The display appeared ambiguous, periodically and spontaneously shifting between two distinct 3-D shape percepts, A and B. Display A appeared vertically folded along the reflectance edges (‘shape from shading’, where the dark fields appeared to be attached shadows). The colours were quite saturated yellow and blue surface colours. There was no colour constancy; Display B appeared horizontally folded along the illumination edge like a roof (‘shape from shading’, where one of the illuminations appeared to be an attached shadow). The display now appeared very desaturated, even achromatic, but in a ‘warm’ illumination. The colour constancy was almost complete. The phenomenon is demonstrated with a slide, and some psychophysical data on the colour desaturation and on the frequency of shifts between the two percepts are reported as well as some observations on combinations of illuminant colours other than yellow and blue. The reported colour constancy phenomenon is discussed in relation to an earlier presented model for the perception of illumination, colour, and depth [S S Bergström, 1994, in Lightness, Brightness, and Transparency Ed. A Gilchrist (Hillsdale, NJ: Lawrence Erlbaum Associates)].
Euclidean Cues versus Affine or Projective Invariants in the Visual Perception of Three-Dimensional Distances
Authors: Wagemans J , Tibau S
Published: December 2015

Lappin and Fuqua (1983 Science221 480 – 482) reported accurate measurement of three-dimensional (3-D) distances and suggested that performance was based on structural invariance of patterns undergoing perspective transformations. We wanted to replicate their basic findings and test specific hypotheses about the use of Euclidean cues versus affine or projective invariants. The displays consisted of orthographic and perspective projections of three collinear dots rotating rigidly around a fixed centre in a plane slanted 45° in depth. Observers were asked to decide whether the middle of the three dots was exactly centred in 3-D space between the other two dots. The visible rotation segments were 120°, 160°, or 200° and the displacements were 2%, 4%, or 6%. Although our untrained observers performed more poorly overall than Lappin's well-practiced observers, two main results were replicated. First, there was no effect of segment size, which suggests that Euclidean cues were not used. Second, there was no difference between orthographic and perspective displays, which suggests that the invariants used must be projective, not affine. Additional experiments will be performed to investigate the relative importance of invariants such as cross ratios, which are based on the positions of the dots within each time frame, versus invariants of the conic sections, which are based on the elliptic traces of the dots formed over time.
Indicating the Singular Point in First-Order Optical Flow Fields
Authors: Kappers A M L , Te Pas S F , Koenderink J J , Dentener J
Published: December 2015

We investigated the accuracy with which subjects can indicate the singular point in a first-order optical flow field. This singular point might be important in navigation and orientation. The stimuli were expanding or rotating sparse random-dot patterns consisting of 80 dark dots on a light background. The stimulus window was circular with a diameter of 20 deg arc. The singular point could be at one of 48 different locations. Subjects had to indicate the location of this singular point with a cursor, while fixating in the centre of the stimulus. Presentation time was unlimited, though each dot had a limited lifetime (114 ms) to avoid density cues. Both veridicality and reproducibility for our subjects increased with increasing values of expansion or rotation in a nonlinear way. We did not find any systematic differences between expansion and rotation. When we blocked either the outer rim or the central part of the stimulus, performance remained the same for singular points that were within the visible part of the stimulus. For singular points outside this visible part, the reproducibility also remained the same, but subjects tended to locate the singular points closer to the rim of the visible part of the stimulus.
The Role of Components in Recognition across Changes of View
Authors: Liter J C , Bülthoff H H
Published: December 2015

We examined the extent to which recognition of rotated objects is based on complex features, such as differently shaped components, when distractor objects may project images that are very similar to the studied images. The stimuli were unfamiliar computer-generated objects composed of four long thin components connected end to end. Each object was matched to a distractor object that had the same components arranged in a different order. The 3-D structure of the matched objects was otherwise the same—the lengths of the components in corresponding positions and the connection angles between them were identical. Subjects studied six target objects and then performed a 2AFC recognition task with each target and its matched distractor. Either the target, the distractor, or both were rotated ±40° about the vertical axis from the studied viewpoint. As expected, subjects chose the target object most often when it was not rotated (77.2% and 69.4% correct when the distractor was and was not rotated). However, subjects chose the target object 64.4% of the time when it was rotated and the distractor was not. In this case, the image of the distractor was more similar to the studied image than was the image of the target. Recognition of these objects was not based on pixel-based image similarity among studied and tested objects, but on a more general view-based similarity that takes into account a multitude of features, some more stable over changes of view than others. We discuss computational models that also rely on multiple view-based features.
Disparity Modulation Sensitivity for Narrow-Band-Filtered Stereograms Viewed out of the Plane of Fixation
Authors: Lee B , Rogers B J
Published: December 2015

Narrow-band-filtered random-dot stereograms were used to determine stereo thresholds for detecting sinusoidal disparity modulations. These stereograms were designed to stimulate selectively channels tuned to luminance and corrugation spatial frequencies (Schumer and Ganz, 1979 Vision Research19 1303 – 1314). Thresholds were determined for corrugation frequencies ranging from 0.125 to 1 cycle deg−1, luminance centre spatial frequencies ranging from 1 to 8 cycles deg−1 and disparity pedestal sizes ranging from −32 to +32 min arc. For small disparity pedestals, lowest modulation thresholds were found around 0.5 cycle deg−1 corrugation frequency and 4 cycles deg−1 luminance centre spatial frequency. For large disparity pedestals (±32 arc min), lowest thresholds were shifted towards the lower corrugation frequencies (0.125 cycle deg−1) and lower luminance frequencies (2 cycles deg−1). There was a significant interaction between luminance spatial frequency and disparity pedestal size. For small pedestals, lowest thresholds were found with the highest luminance frequency pattern (4 cycles deg−1). For large pedestals, best performance shifted towards the low-frequency patterns (1 cycle deg−1). This effect demonstrates a massive reduction in stereo-efficiency for high-frequency patterns in the luminance domain at large disparity pedestals which is consistent with the ‘size-disparity relation’ proposed by previous researchers.
Binocular Coordination, Fixation Disparity, and Ocular Dominance
Authors: Radach R , Heller D , Wiebories P , Jaschinski W
Published: December 2015

In a series of experiments we have quantified the spatial and temporal dynamics of binocular coordination. Tasks studied ranged from simple scanning and letter detection to complex visual processing in text reading. In all of these paradigms we found similar eye movement characteristics: in 70% to 90% of the observations, the saccade of the abducting eye is larger, relative differences being in the order of 5% to 15% of the amplitude. During the subsequent fixation the disparity is typically reduced by a convergence movement (about 1 deg s−1), which sometimes exceeds the initial saccade amplitude asymmetry. Interestingly, the relative vergence contributions of the eyes depend on saccade length. For progressive 2-letter reading saccades, the left (adducting) eye accounts for only 20% of the total movement as compared to about 70% for 14-letter saccades.

Up to now our analysis was limited to relative rather than absolute estimates of fixation disparity. To overcome this restriction, we measured disparity using the psychophysical method of dichoptically presented nonius lines as well as direct infrared pupil-reflection registration of binocular vs monocular fixation. Both measures were independent of target eccentricity (within a range typical for reading) and produced similar subject rank orders (Spearman's \rho=0.75).

When we studied vergence movements in a letter detection task using autostereograms with different levels of virtual depth, it became clear that spatiotemporal vergence parameters can be quite asymmetric for both eyes. This led to the question of whether unequal contributions to vergence may be related to ocular dominance. This hypothesis is currently being investigated with a new procedure that provides a reliable estimate of subjective visual direction (the ‘cyclopean eye’) under static viewing conditions.
The Effect of Environmental Pitch on Perceived Optic Slant and Eye Level: Lines vs Dots
Authors: Stoper A E , Randle J , Cohen M M
Published: December 2015

Visually perceived eye level (VPEL) has been shown to be strongly affected by the pitch of the visible environment (Stoper and Cohen, 1989 Perception & Psychophysics46 469 – 475), even if this environment consists of only two luminous lines pitched from the vertical (Matin and Li, 1992 Journal of Experimental Psychology: Human Perception and Performance18 257 – 289). Here, two luminous vertical lines or 32 randomly distributed luminous dots were mounted on a plane that was viewed monocularly and was pitched (slanted in the pitch dimension) 30° forward or backward from the vertical. In addition to measuring the VPEL, we measured the perceived optic slant (rather than the perceived geographic slant) of this plane by requiring each of our ten subjects to set a target to the visually perceived near point (VPNP) of the plane. We found that, for the lines, VPNP shifted 50% and VPEL shifted 26% of the physical pitch of the plane. For the dots, VPNP shifted 28% but VPEL shifted only 8%. The effect of the dots on VPEL was weaker than might have been predicted by their effect on VPNP, which was used to indicate perceived optic slant. The weakness of the effect of the dots on VPEL implies that changes in VPEL result from a direct effect of the stimuli on VPEL, rather than one mediated by the perceived optic slant of the plane. The non-zero effect of the dots shows that pitched from vertical line segments are not necessary to shift VPEL.
Priming within the ‘Attentional Blink’ in an RSVP Task
Authors: Chua F K , Goh J , Kek G
Published: December 2015

Recent experiments (eg M M Chun and M C Potter, 1995 Journal of Experimental Psychology: Human Perception and Performance21 109 – 127; J E Raymond, K L Shapiro, and K M Arnell, 1992 Journal of Experimental Psychology: Human Perception and Performance18 849 – 860) with RSVP (rapid serial visual presentation) suggest that the attentional blink is caused by local interference. We present data from three RSVP experiments that provide further clues regarding the attentional blink. In experiment 1, subjects detected an ‘X’ and then identified a red letter; in experiment 2, subjects had to say whether the first red target was an ‘X’ and then identify a red letter. In experiment 3, subjects identified two red letters. We systematically varied the lag between the first and second targets. On half the trials, we also primed the second target by placing an identical letter in the lag one position (the position after the first target). In experiment 3, we also examined if the priming effect was semantic with a lower case letter.

The first two experiments suggest that the priming effect is very short-lived and mainly sensory in nature. The priming effect disappears altogether if the first target is not present. More interestingly, we found that when subjects failed to detect the ‘X’, priming could still happen. The third experiment replicates and extends the results of the first two experiments. We also show that priming, albeit in a weak form, may still happen during the time when the attentional blink is supposed to occur. These results suggest that it is not an inhibition that causes the attentional blink and that sensory processing continues during the blink.
Neural Mechanisms of Visual Perception
Authors: Wiesel T
Published: December 2015

It is more than half a century ago since Keffer H Hartline published his classical receptive fields studies of single optic nerve fibres in the frog. World War II intervened and the full impact of his work did not become apparent until the early fifties, when Horace Barlow extended Hartline's analysis in the frog and Stephen W Kuffler showed the on-centre and off-centre type ganglion cells in the cat retina. The next advances were made in the late fifties when Jerome Lettvin and Humberto Maturana described cells in the frog tectum with very complex response properties and when David Hubel and I discovered that cells in the cat striate cortex were sensitive to orientation of contours and binocular stimulation. Vision research has gone a long distance since that time—nonetheless we have just begun the long journey towards a detailed mechanistic understanding of the neural basis of visual perception.

In this lecture I discuss the processing of visual information at the level of the striate cortex in the cat and monkey, and describe technical advances that have greatly facilitated the analysis of the neural mechanisms of visual perception.
Progress and Paradigm Shifts in Spatial Vision during 20 Years of ECVP
Authors: Levi D M
Published: December 2015

In the beginning there was light, and form, and visual mechanisms. This presentation traces developments in research on spatial vision over the 20 years of ECVP, with particular attention to the interplay between physiological mechanisms and psychophysics.
Integration of Sensory Information in the Brain
Authors: Stein B E
Published: December 2015

That sensory cues in one modality affect perception in another has been known for some time, and there are many examples of ‘intersensory’ influences within the broad phenomenon of cross-modal integration. The ability of the CNS to integrate cues from different sensory channels is particularly evident in the facilitated detection and reaction to combinations of concordant cues from different modalities, and in the dramatic perceptual anomalies that can occur when these cues are discordant. A substrate for multisensory integration is provided by the many CNS neurons (eg, in the superior colliculus) which receive convergent input from multiple sensory modalities. Similarities in the principles by which these neurons integrate multisensory information in different species point to a remarkable conservation in the integrative features of the CNS during vertebrate evolution. In general, profound enhancement or depression in neural activity can be induced in the same neuron, depending on the spatial and temporal relationships among the stimuli presented to it. The specific response product obtained in any given multisensory neuron is predictable on the basis of the features of its various receptive fields. Perhaps most striking, however, is the parallel which has been demonstrated between the properties of multisensory integration at the level of the single neuron in the superior colliculus and at the level of overt attentive and orientation behaviour.
Vision with Age-Related Macular Degeneration
Authors: Valberg A , Fosse P
Published: December 2015

Visual Science at the Norwegian University of Science and Technology (NUST) includes activities in visual psychophysics and electrophysiology, clinical tests of vision, and low vision. In recent years the Tambartun National Resource Centre for the Visually Impaired, in cooperation with NUST, has been encouraging applied projects aimed at improving the quality of life of people with low vision. One such project, headed by Per Fosse, is devoted to the study of the visual capacity of elderly with age-related macular degeneration. Systematic assessments are made of acuity, contrast sensitivity (CS), colour vision, visual field, dark adaptation, and optimal illumination levels for reading and orientation. We report on some preliminary results for ten clients, which is about half the number of anticipated participants in the project. The differences in performance of the individuals that we have seen so far are large, particularly concerning their lighting requirements. Some clients preferred a relatively restricted optimum range of illumination, while others required very high levels. All clients profited from these extensive tests in orientation in real-life settings and in reading. Different CS tests and acuity measures have been compared in the course of the study, and some of these data are also presented.
Correspondence Noise Limits to Global Motion Perception
Authors: Tripathy S P , Barlow H B
Published: December 2015

We measured human observers' thresholds for detecting coherent motion in random-dot kinematograms and compared them with theoretical thresholds predicted by the hypothesis that unavoidable correspondence noise in the kinematograms themselves is the limiting factor. In experiments where the stimulus had two fields we varied the dot density, the ratio of the number of dots in the two fields, and the area of stimulus. In further experiments we varied the number of fields and the method of generation of the coherent dots. The observed thresholds varied in the way predicted by the hypothesis over ranges that were in some cases wide, but even within these ranges the absolute levels of performance were much worse than that calculated for the ideal observer.

Objects moving in the real world generate motion signals in their images covering broad ranges of direction and velocity, and when the spread of motion signals within each presentation of our stimulus was increased, the discrepancy between ideal and actual performance was much reduced. We conclude that correspondence noise present in the stimulus is an important limiting factor in the coherent motion paradigm. Since this noise is largely external and inextricably bound with the signal, to attain high signal/noise ratios the input signals have to be pooled over broad ranges that match the range of motion signals in the natural stimuli. This provides an insight into the design of the motion detecting system, and perhaps into the design of other parts of the cortex.
Integration of Impaired Visual as Well as Impaired Phonological Information Can Cause Reading Errors
Authors: Cornelissen P L , Hansen P C , Stein J F
Published: December 2015

Developmental dyslexia is a common problem amongst school children (5% – 10% are afflicted), yet controversy surrounds the explanation for its cause. Fluent reading requires rapid association of visual with phonological information—therefore problems with either visual or phonological processing could cause reading difficulties. It is known that dyslexics’ speech perception is often impaired, giving rise to ‘fuzzy’ or ‘underspecified’ phonological representations. This leads, in turn, to difficulties with letter-to-sound mapping during reading.

Dyslexic individuals also find it unusually difficult to detect flickering or moving visual stimuli, consistent with impaired processing in the magnocellular visual stream. This raises the question of whether dyslexics' reading problems may be caused not only by abnormal phonological processing but also by magnocellular impairment.

We suggest that, when children read, impaired magnocellular function may degrade information about where letters are positioned with respect to each other. We predicted that this might cause reading errors which contain sounds not represented in the printed word. We call these orthographically inconsistent nonsense errors ‘letter’ errors. To test this idea we assessed magnocellular function in 58 children by using a coherent-motion detection task. We then gave these children a single-word reading task and found that the likelihood of them making ‘letter’ errors was best explained by independent contributions from motion detection (ie magnocellular function) and phonological awareness (assessed by a spoonerism task). This result held even when chronological age, reading ability, and IQ were controlled for. These findings suggest that, when visual and phonological information is integrated during reading, impairments in both domains may indeed affect how children read.
Visual Consequences for Children with Early Acquired Lesions to the Optic Radiation—Vision ‘without Seeing’
Authors: Jacobson L
Published: December 2015

The brain lesion caused by perinatal hypoxic-ischemic events at gestational age 24 – 34 weeks has a typical anatomical pattern known as periventricular leukomalacia (PVL). PVL affects the corticospinal tracts, causing spastic diplegia and/or the geniculocalcarine tract, causing visual impairment. Computer tomography or magnetic resonance imaging are the methods of choice for diagnosing PVL. This lesion is the cause of visual impairment in 20% of all visually impaired children in a Swedish population of children born in 1989 – 1995.

The visual deficit in PVL is characterised by decreased vision with crowding (an inability to resolve linear optotypes, while single optotypes of the same size may be identified) and visual field defects, further complicated by perceptual and cognitive problems. A relatively high single optotype acuity may lead to overestimation of visual function. Oculomotor impairments with strabismus and nystagmus are common findings. Colour vision is often preserved and used by the children as one of many strategies to sort out an otherwise chaotic visual world. Instead of looking, these children listen carefully, talk a lot and remember well, and sometimes use tactile information to solve visual tasks. They easily get lost in new surroundings and they recognise known faces among others only by the voice and the colour of the clothes. Reading is often difficult. This group of visually impaired children call for adaptation of education and habilitation to manage daily life, mobility, and reading.
A Neuromagnetic View of the Human Visual Brain
Authors: Vanni S
Published: December 2015

A visual stimulus typically activates several cortical areas, both sequentially and overlapping in time. Characterisation of this temporal activation sequence has significantly improved with the recent development of whole-scalp neuromagnetometers. The magnetoencephalographic (MEG) signals mainly arise from time-locked cortical activity. Although the spatial localisation of several simultaneously active areas is ambiguous because of the non-uniqueness of the inverse problem, the comparison of estimated source regions across observers and utilisation of previous functional knowledge can usually resolve this ambiguity. Visual object naming, for example, generates cortical activation progressing bilaterally from occipital to temporal and frontal lobes. Simultaneously, the parieto-occipital alpha rhythm dampens as a function of task demands. Similarly, this rhythm is at a lower level after objects than non-objects in an object-detection task, which suggests that the parieto-occipital area is active when attending to visual form. In addition, this area generates evoked responses after voluntary blinks, saccades, and luminance increments, which in turn suggests that it participates in the updating of visual percepts.

The sources of extrastriate MEG signals are generally in good agreement with the location of activation found with other imaging methods: visual motion activates the V5 in the ascending limb of the inferior temporal sulcus, faces the ventral temporo-occipital cortex, and objects the lateral occipital (LO) regions. Interestingly, the strength of the right LO activity closely follows the proportion of correctly detected objects. The future neuromagnetic studies will focus not only on functional localisation of the active areas, but also on how the brain processes various stimuli.
Assessing Interactions through Concurrent-Judgment Experiments
Authors: Olzak L A , Wickens T D
Published: December 2015

Many important psychophysical questions concern the interaction or combination of different components of a stimulus. Classical psychophysical methods for assessing whether two stimulus aspects are coded independently (eg, masking and summation) provide limited information about the nature of whatever interactions are discovered. In both older work in detection and recent work in complex pattern discrimination, we have used a double-judgment paradigm in which the observer rates two aspects of a stimulus simultaneously. The paradigm provides a rich source of information about the codes underlying each psychophysical decision. It is unique in permitting us to investigate effects resulting from correlations in noise. We review the theoretical, technological, and methodological results that led us to develop this approach. Procedural antecedents lie in theories of dimensional interaction, in signal detection theory, and in information theory. Analytically, we draw on methods from several branches of statistics, including categorical data analysis and structural equation modeling. Also key to our work are advances in computational power: both our experimental procedures and our data analysis would have been difficult or impossible two decades ago.
Higher-Order Motion Perception in Human Visual Cortex: Evidence from fMRI
Authors: Greenlee M W , Smith A T , Singh K D , Kraemer F M , Hennig J
Published: December 2015

fMRI was used to investigate human visual cortex responses to higher-order motion stimuli. Acquisition was on a Siemens 1.5 T scanner (T2*, gradient-recalled EPI, TR 3000 ms, TE 84 ms, flip angle 90°, 2 mm × 2 mm voxels, 256 mm FOV, 10 4-mm slices, 54 acquisitions per run). The measured volume included occipital and posterior parietal cortex. T1 scouts and, in some subjects, high resolution T1 volume images were also acquired. Visual stimuli were gamma-corrected movies (480 × 480 pixels), presented by a PowerMac via an LCD projector, shown through the rear of the scanner onto an adjustable mirror fixed above the subject's eyes. Three types of stimuli were used: (1) first-order motion, (2) second-order motion (both radial sine waves on random-dot backgrounds), (3) structure-from-motion consisting of two rotating circular patches (5 deg diameter) within which dots moved in a constant (centripetal) direction, superimposed on randomly moving dots. Three interleaved comparisons were made: stimulus vs blank, first-order vs second-order, and random motion vs structure-from-motion (27 s each phase, 3 repeats).

Analysis was based on a correlation coefficient method, after head-motion correction. Initial correlation was with the stimulus profile vector, then with an average BOLD response vector. Voxels with a correlation >0.5 (p<0.0003) were accepted as significant. In all subjects (seven- teen normals), all stimuli evoked bilateral activity in V1/V2 (BA17/18), and in extrastriate area V5/MT (BA37/19). Bilateral activation was also found in areas V3/V3a (BA19) and BA7. A more pronounced activation of area MST/V5a (BA37/39) was found in response to the structure-from-motion stimulus, compared with random motion.[Supported by: Wellcome Trust, Schilling Foundation.]
Speed and Noise of Neural Membranes: Ion Channel Limitations to Visual Information Transmission
Authors: Weckström M
Published: December 2015

In dim light, photoreceptor cells and subsequent neural elements typically show high absolute sensitivity, implying that both phototransduction and synaptic transmission work at a high gain and even a single photon may produce a large electrical response. However, when there is more light, rapid adaptation at several levels of signal processing ensures that the information channel is not congested, but optimally filled with relevant voltage responses. All this is achieved by carefully tuned mechanisms that include several types of ion channels in the cell membrane. These ion-channel mechanisms have been thoroughly investigated in a few species of invertebrates and vertebrates, and some general principles are being revealed.

The membrane capacitance and the resistance of the cell together define the time constant of the membrane, thus the maximum speed for building up a voltage response to light. Both in vertebrate cones and in insect microvillar photoreceptors, phototransduction takes place in an enlarged part of the cell membrane, which implies a large capacitance. This can be counteracted by making the membrane more leaky by opening more ion channels. In insect photoreceptors several types of potassium channels have been identified that perform exactly this kind of function. The types of channels vary according to the required speed of phototransduction, ie depending on the life style of the animal. In diurnal dipteran insects the potassium channels are typically of the slowly inactivating type. This channel type regulates the cell impedance according to the depolarisation caused by light stimulation. In insects active in dim environments, the potassium channels found have been predominantly rapidly inactivating. The function of this type of channels is currently under debate. In vertebrate photoreceptors several potassium channel types, including channels sensitive to intracellular calcium and pH, are expressed in the inner segments and modulate photoresponses.

Opening and closing of the potassium channels also generates neural noise and thus degrades the signal-to-noise ratio (SNR). However, if the gain of phototransduction is high enough, the dominant noise comes from photon fluctuations, or from the biochemical transduction machinery, or—in some situations—from spontaneous photon-like events. Channel noise is then insignificant by comparison. Thus the optimisation of the SNR is a trade-off between bandwidth (ie speed) and amplification of the signal, and here the voltage-gated potassium channels are of prime importance.
Audiovisual Speech Perception
Authors: Sams M
Published: December 2015

Persons with hearing loss use visual information from articulation to improve their speech perception. Even persons with normal hearing utilise visual information, especially when the stimulus-to-noise ratio is poor. A dramatic demonstration of the role of vision in speech perception is the audiovisual fusion called the ‘McGurk effect’. When the auditory syllable /pa/ is presented in synchrony with the face articulating the syllable /ka/, the subject usually perceives /ta/ or /ka/. The illusory perception is clearly auditory in nature.

We recently studied the audiovisual fusion (acoustical /p/, visual /k/) for Finnish (1) syllables, and (2) words. Only 3% of the subjects perceived the syllables according to the acoustical input, ie in 97% of the subjects the perception was influenced by the visual information. For words the percentage of acoustical identifications was 10%. The results demonstrate a very strong influence of visual information of articulation in face-to-face speech perception. Word meaning and sentence context have a negligible influence on the fusion.

We have also recorded neuromagnetic responses of the human cortex when the subjects both heard and saw speech. Some subjects showed a distinct response to a ‘McGurk’ stimulus. The response was rather late, emerging about 200 ms from the onset of the auditory stimulus. We suggest that the perisylvian cortex, close to the source area for the auditory 100 ms response (M100), may be activated by the discordant stimuli. The behavioural and neuromagnetic results suggest a precognitive audiovisual speech integration occurring at a relatively early processing level.
An fMRI Investigation of a Patient with Cerebral Achromatopsia: Evidence for a Role of Chromatic Extrastriate Mechanisms in Motion Encoding
Authors: Troscianko T , Greenlee M W , Brugger P , Freitag P , Kraemer F M , Radue E W
Published: December 2015

We have previously reported psychophysical results of tests on a patient (WM) who suffered from cerebral achromatopsia but nevertheless showed strong signs of unconscious access to chromatic information. This took the form of (a) better performance when chromatic modulation was added to existing luminance modulation in the stimulus, and (b) evidence suggesting that the chromatic information was supplied by a fast, non-opponent set of channels (Troscianko et al, 1996 Current Biology6 200 – 210). The psychophysical findings led us to predict that WM should show strong potentiating effects of chromatic modulation, and that these should be evident in enhanced activity of extrastriate areas thought to respond to fast flicker and motion, particularly V5 (Brodmann area 37/39 in lateral occipito-temporal cortex), when a (luminance + colour) stimulus is presented as compared to a luminance-only stimulus.

WM was tested with susceptibility-based functional magnetic resonance, by applying gradient-recalled echo-planar imaging (EPI) sequences, with and without visual motion stimulation. Anatomical and functional images were recorded with a 1.5 T Siemens Magnetom Vision scanner (TR 3000 ms, TE 70 ms, flip angle 90°, FOV 250 mm, matrix 112 × 128, 12 4-mm slices through visual cortex). The visual stimuli were drifting sinusoidal gratings with either luminance-only, or colour + luminance modulation, and with the addition of either static or 25 Hz dynamic luminance noise. After correction for head-motion artifacts, we found (a) a significantly enhanced extrastriate (V5/V5a) response when chromatic information was added, and (b) that fast dynamic noise affected the same areas.

We conclude that colour information can activate (or modulate the activity of) motion-specific mechanisms usually thought to be colour-blind. This finding has implications both for the understanding of unconscious information processing in vision, and for considerations of neural efficiency when encoding redundant image information.