bluuebunny/crossref_metadata_2025_split huggingface dataset:
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).
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.
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.
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.
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.