[The Mechanisms of the Visual System Behind Visual Illusions: From Eye to Brain].

Visual illusion is a psychological phenomenon characterized by perception that appears to differ from physical reality. Illusory perception persists even though the sufferers are aware of the physical properties of what they are observing. Thus, studying visual illusions has led to an improved understanding of the neural mechanism underlying visual information processing. Visual illusions are important tools in neuroscience. Some brightness illusions, such as the Hermann grid illusion or Chevreul illusion, can be explained by the function of the center-surround antagonistic receptive field of retinal ganglion cells. Additionally, color aftereffects were found to be produced in the retina. In addition, neurons in the primary visual cortex are responsible for the orientation-contingent color aftereffect, known as the McCollough effect. ISI reversal, a visual motion illusion, is known to be caused by a biphasic temporal mechanism located in the retinal or lower visual areas. Higher visual areas are responsible for the production of visual illusions such as the Ponzo illusion, size constancy illusion, or tabletop illusion. These illusions are perceived through the process of achieving size constancy from a two-dimensional retinal image. Thus, both low-level and high-level vision are involved in the perception of visual illusions.

Visual discomfort from flicker: Effects of mean light level and contrast.

Uncomfortable images generally have a particular spatial structure, which deviates from a reciprocal relationship between amplitude and spatial frequency (f) in the Fourier domain (1/f). Although flickering patterns with similar temporal structure also appear uncomfortable, the discomfort is affected by not only the amplitude spectrum but also the phase spectrum. Here we examined how discomfort from flicker with differing temporal profiles also varies as a function of the mean light level and luminance contrast of the stimulus. Participants were asked to rate discomfort for a 17° flickering uniform field at different light levels from scotopic to photopic. The flicker waveform was varied with a square wave or random phase spectrum and filtered by modulating the slope of the amplitude spectrum relative to 1/f. At photopic levels, the 1/f square wave flicker appeared most comfortable, whereas the discomfort from the random flicker increased monotonically as the slope of the amplitude spectrum decreased. This special status for the 1/f square wave condition was limited to photopic light levels. At the lower mesopic or scotopic levels, the effect of phase spectrum on the discomfort was diminished, with both phase spectra showing a monotonic change with the slope of the amplitude spectrum. We show that these changes cannot be accounted for by changes in the effective luminance contrast of the stimuli or by the responses from a linear model based on the temporal impulse responses under different light levels. However, discomfort from flicker is robustly correlated with judgments of the perceived naturalness of flicker across different contrasts and mean luminance levels.

Spinal Subdural Abscess Following Food Intoxication: A Case Report.

INTRODUCTION: Spinal subdural abscess (SSA) or empyema is a rare pathology and its exact incidence is unknown. Staphylococcus aureus (S aureus) is the most frequently responsible organism. The patients with SSA may have one or more predisposing immunosuppressive conditions. However, here we report a rare case of SSA following food intoxication without any significant comorbidities. CASE REPORT: A 42-year-old healthy man presenting with fever, severe low back pain (LBP), and trunk motion restriction was transferred to our hospital. He had been treated for an unknown fever after food intoxication in another hospital. Eighteen days earlier, he and his colleagues together ate raw horse meat and briefly boiled chicken breast. They all had food intoxication on the following day. Subsequently, our patient began to have a high fever and severe LBP. Laboratory data showed leukocytosis of 16,000/mm3. Also, the C-reactive protein was elevated to 26 mg/dL. The blood culture result was consistent with S aureus. Magnetic resonance imaging (MRI) showed focal epidural fluid collection that appeared contiguous with the subdural fluid collection through a dural defect in the axial plane on T2-weighted (T2W) images. An emergent surgery was performed. Frank pus was expressed from the epidural space as well as from the subdural space through the defect. The pus later grew S aureus. The patient was started on antibiotic therapy postoperatively. The patient completely recovered 1 month after surgery. CONCLUSIONS: SSA following food intoxication is a very rare case. SSA can be identified with a small dural defect and the intrathecal fluid collection compressing the cauda equina in the axial plane on T2W magnetic resonance images. Having suspicion of epidural abscess and likewise subdural abscess and making an early diagnosis using MRI and an emergent surgery are important when the clinician notices a febrile patient with severe LBP and trunk motion stiffness.

Adaptation and visual discomfort from flicker.

Spatial images with unnatural amplitude spectra tend to appear uncomfortable. Analogous effects are found in the temporal domain, yet discomfort in flickering patterns is also strongly dependent on the phase spectrum. Here we examined how discomfort in temporal flicker is affected by adaptation to different amplitude and phase spectra. Adapting and test flicker were square wave or random phase transitions in a uniform field filtered by increasing (blurred) or decreasing (sharpened) the slope of the amplitude spectrum. Participants rated the level of discomfort or sharpness/blur for the test flicker. Before adaptation, square wave transitions were rated as most comfortable when they had "focused" edges, which were defined as characterized by 1/f amplitude spectra, while random phase transitions instead appeared more comfortable the more blurred they were. After adapting to blurred or sharpened transitions, both square wave and random phase flicker appeared more sharpened or blurred, respectively, and these effects were consistent with renormalization of perceived temporal focus. In comparison, adaptation affected discomfort in the two waveforms in qualitatively different ways, and exposure to the adapting stimulus tended to increase rather than decreased its perceived discomfort. These results point to a dissociation between the perceived amplitude spectrum and perceived discomfort, suggesting they in part depend on distinct processes. The results further illustrate the importance of the phase spectrum in determining visual discomfort from flickering patterns.

Effect of spatial attention on spatiotopic visual motion perception.

We almost never experience visual instability, despite retinal image instability induced by eye movements. How the stability of visual perception is maintained through spatiotopic representation remains a matter of debate. The discrepancies observed in the findings of existing neuroscience studies regarding spatiotopic representation partly originate from differences in regard to how attention is deployed to stimuli. In this study, we psychophysically examined whether spatial attention is needed to perceive spatiotopic visual motion. For this purpose, we used visual motion priming, which is a phenomenon in which a preceding priming stimulus modulates the perceived moving direction of an ambiguous test stimulus, such as a drifting grating that phase shifts by 180°. To examine the priming effect in different coordinates, participants performed a saccade soon after the offset of a primer. The participants were tasked with judging the direction of a subsequently presented test stimulus. To control the effect of spatial attention, the participants were asked to conduct a concurrent dot contrast-change detection task after the saccade. Positive priming was prominent in spatiotopic conditions, whereas negative priming was dominant in retinotopic conditions. At least a 600-ms interval between the priming and test stimuli was needed to observe positive priming in spatiotopic coordinates. When spatial attention was directed away from the location of the test stimulus, spatiotopic positive motion priming completely disappeared; meanwhile, the spatiotopic positive motion priming at shorter interstimulus intervals was enhanced when spatial attention was directed to the location of the test stimulus. These results provide evidence that an attentional resource is requisite for developing spatiotopic representation more quickly.

Visual discomfort and flicker.

Flickering lights can be uncomfortable to look at and can induce seizures in observers with photosensitive epilepsy. However, the temporal characteristics contributing to these effects are not fully known. In the spatial domain, one identified source of visual discomfort is when images have Fourier amplitude spectra that deviate from the natural (∼1/frequency, 1/f) statistical characteristics of natural scenes, especially if they contain excess energy at the medium frequencies at which the visual system is most sensitive. We tested for analogous effects in the temporal domain, manipulating both the amplitude and phase spectra of the flicker. Participants judged the relative discomfort of temporal luminance variations in a pair of uniform 17° fields with different temporal modulations. In general, discomfort increased with deviations from natural amplitude spectra, particularly those with excess energy at medium frequencies or biased toward sharper spectra. These ratings of discomfort were also consistent with ratings of how natural the modulations appeared. However, the temporal discomfort judgments were also strongly affected by the phase spectra of the flicker, with fixed vs. random spectra producing very different responses. This was not due to the perceived regularity or predictability of the flicker, but could arise from a number of other potential factors. Our findings suggest that, like spatial patterns, visual discomfort in time-varying patterns depends in part on how similar they are to the amplitude spectra of temporal variations in the natural visual environment, but also point to the critical role of the phase spectrum in the perceived discomfort of flicker.

Individual differences in visual motion perception and neurotransmitter concentrations in the human brain.

Recent studies have shown that interindividual variability can be a rich source of information regarding the mechanism of human visual perception. In this study, we examined the mechanisms underlying interindividual variability in the perception of visual motion, one of the fundamental components of visual scene analysis, by measuring neurotransmitter concentrations using magnetic resonance spectroscopy. First, by psychophysically examining two types of motion phenomena-motion assimilation and contrast-we found that, following the presentation of the same stimulus, some participants perceived motion assimilation, while others perceived motion contrast. Furthermore, we found that the concentration of the excitatory neurotransmitter glutamate-glutamine (Glx) in the dorsolateral prefrontal cortex (Brodmann area 46) was positively correlated with the participant's tendency to motion assimilation over motion contrast; however, this effect was not observed in the visual areas. The concentration of the inhibitory neurotransmitter γ-aminobutyric acid had only a weak effect compared with that of Glx. We conclude that excitatory process in the suprasensory area is important for an individual's tendency to determine antagonistically perceived visual motion phenomena.This article is part of the themed issue 'Auditory and visual scene analysis'.

Motion perception under mesopic vision.

Mesopic and scotopic vision extend over an illuminance range of 106. The goal of the present study was to determine the effect of decreasing light level on the underlying motion mechanism that integrates spatiotemporally separated motion signals. To accomplish this, we took advantage of the phenomenon of visual motion priming, in which the perceived direction of a directionally ambiguous test stimulus is influenced by the directional movement of a preceding priming stimulus. After terminating a drifting priming stimulus, a 180° phase-shifted grating was presented as a test stimulus. The priming and test stimuli were separately presented to the central and peripheral retinas, respectively. The participants judged the perceived direction of this test stimulus at various light levels from photopic to scotopic levels. We found that the effects of motion priming disappeared over 1 log unit of mesopic light levels. When the test stimulus was presented before the offset of the priming stimulus to compensate for the temporal delay in the rod pathway or when both stimuli were presented at the same location in the periphery, a motion-priming effect appeared at mesopic light levels. These results suggest that different temporal characteristics between the cone pathway and rod pathway disturb the function of the putative motion mechanism responsible for the spatiotemporal integration of motion signals, which leads to specific modulation of motion perception over a wide range of mesopic vision.

Pupillometric evidence for the locus coeruleus-noradrenaline system facilitating attentional processing of action-triggered visual stimuli.

It has been argued that attentional processing of visual stimuli is facilitated by a voluntary action that triggers the stimulus onset. However, the relationship between action-induced facilitation of attention and the neural substrates has not been well established. The present study investigated whether the locus coeruleus-noradrenaline (LC-NA) system is involved in this facilitation effect. A rapid serial visual presentation paradigm was used to assess the dynamics of transient attention in humans. Participants were instructed to change a digit stream to a letter stream by pressing a button and specifying successive targets of four letters. Pupil dilation was measured as an index of LC-NA function. Accuracy of target identification was better when the temporal delay between participants' key press and target onset was 800 ms than when targets appeared just after the key press or when targets appeared without key press. Accuracy of target identification was positively correlated with both the peak amplitude of pupil dilation and the pupil size at the time of the key press. These results indicate that target identification in the visual task is closely linked to pupil dilation. We conclude that the LC-NA system plays an important role in the facilitation of transient attention driven by voluntary action.

Effect of light level on the reference frames of visual motion processing.

It is empirically known that some action-related visual tasks, which may rely on the construction of spatiotopic coordinates, are not well conducted under mesopic vision. The aim of this study was to clarify the effect of light level on the reference frame, such as retinotopic and spatiotopic coordinate bases, associated with visual motion processing. For this purpose, we used a phenomenon called visual motion priming in which the perceived direction of a directionally ambiguous test stimulus is influenced by the moving direction of a priming stimulus. Previous studies have shown that negative and positive motion priming are conspicuously observed in retinotopic and spatiotopic coordinates, respectively. In the experiments, participants made a saccade after the termination of the priming stimulus and judged the perceived direction of the test stimulus presented subsequently in retinotopic or spatiotopic coordinates at different light levels. We found that in retinotopic coordinates, negative motion priming was observed at all light levels. In spatiotopic coordinates, positive motion priming was observed at photopic and scotopic light levels, whereas the strength of motion priming was greatly reduced at mesopic light levels. These results were robust to the change in the luminance contrast or the saccadic eye movement per se. Different spatiotemporal properties of cones and rods at mesopic light levels may disturb the construction of a spatiotopic representation of motion, which leads to the disappearance of visual motion priming in spatiotopic coordinates during mesopic vision.

Pupil response and the subliminal mere exposure effect.

The subliminal mere exposure effect (SMEE) is the phenomenon wherein people tend to prefer patterns they have repeatedly observed without consciously identifying them. One popular explanation for the SMEE is that perceptual fluency within exposed patterns is misattributed to a feeling of preference for those patterns. Assuming that perceptual fluency is negatively correlated with the amount of mental effort needed to analyze perceptual aspects of incoming stimuli, pupil diameter should associate with SMEE strength since the former is known to reflect mental effort. To examine this hypothesis, we measured participants' pupil diameter during exposure to subthreshold stimuli. Following exposure, a preference test was administered. Average pupil diameter throughout exposure was smaller when the SMEE was induced than when the SMEE was not induced. This supports the hypothesis that increasing perceptual fluency during mere exposure modulates autonomic nervous responses, such as pupil diameter, and eventually leads to preference.

The reference frame of visual motion priming depends on underlying motion mechanisms.

Several different types of motion mechanisms function in the human visual system. The purpose of this study was to clarify the type of reference frame, such as retinotopic and spatiotopic frames of reference, at which those different motion mechanisms function. To achieve this, we used a phenomenon called visual motion priming, in which the perceived direction of a directionally ambiguous test stimulus is influenced by the moving direction of a preceding stimulus. Previous studies have indicated that negative motion priming is induced by a low-level motion mechanism, such as a first-order motion sensor, whereas positive motion priming is induced by a high-level motion mechanism, such as a feature-tracking system. In the experiments, subjects made a saccade after the termination of a smoothly drifting priming stimulus and judged the perceived direction of a 180° phase-shifted sine-wave grating presented subsequently in retinotopic or screen-based spatiotopic coordinates. By manipulating the stimulus parameters, such as primer duration, velocity, and contrast, both positive and negative priming were observed. We found that positive priming was observed in spatiotopic coordinates, whereas negative priming was observed in retinotopic coordinates. Prominent positive priming in spatiotopic coordinates was observed only when the interval between the priming and test stimuli was longer than around 600 ms. This delayed priming effect was not caused by saccadic eye movements. These results suggest that a low-level motion mechanism functions in retinotopic coordinates, whereas a high-level motion mechanism functions in spatiotopic coordinates, in which the representation builds up slowly.

Visual motion priming reveals why motion perception deteriorates during mesopic vision.

We know empirically that the perception of moving objects deteriorates under mesopic vision, in which both rods and cones operate. The purpose of this study was to examine the cause of this degradation. We utilized a phenomenon called visual motion priming, in which the perceived direction of a directionally ambiguous test stimulus is influenced by the moving direction of a preceding stimulus. The spatial distances between the priming and the test stimuli were varied. At mesopic light levels, a stimulus that is presented at the central retina is presumably processed by the cone system, while a stimulus that is presented at the peripheral retina is processed by the rod system (Raphael & MacLeod, 2011). Subjects judged the perceived direction of 180° phase-shifted sine-wave grating that was followed by a smoothly drifting priming stimulus under different retinal illuminances. We found that, under mesopic conditions, the strength of motion priming was greatly reduced when the priming and test stimuli were presented separately at the center and the periphery, respectively. In contrast, motion priming was perceived in most of the trials under photopic and scotopic conditions or when both the priming and test stimuli were presented at the central retina under mesopic conditions. When the priming and test stimuli temporally overlapped, motion priming was conspicuous irrespective of the retinal illuminance. These results suggest that the incompleteness in the integration of signals that was induced by the temporal delay of rod pathways caused the degradation of motion perception under mesopic vision.

[Reversal of visual motion priming under mesopic vision].

It is known that a directionally ambiguous test stimulus is perceived to move in the same direction as a brief preceding priming stimulus when both stimuli are presented at the same retinal location (visual motion priming). To examine the spatial properties of visual motion priming under different retinal illuminance, we manipulated the distance between the priming and test stimuli. Participants judged the perceived direction of 180 deg phase-shifted, thus directionally ambiguous, sine-wave gratings (test stimulus) displayed immediately after the offset of a smoothly drifting priming stimulus. The distance between priming and test stimuli was varied from 0 to 10 deg in visual angle. Since the spatial summation area broadens under low retinal illuminance, we predicted that visual motion priming would be more conspicuous under mesopic vision than under photopic vision. Contrary to this prediction, as the retinal illuminance decreased and the distance between the primer and the test stimulus increased, the test stimulus was perceived to move in the direction opposite to the priming stimulus. We speculate that different motion integration systems are functioning depending on the retinal illuminance.

Estimation of mental effort in learning visual search by measuring pupil response.

Perceptual learning refers to the improvement of perceptual sensitivity and performance with training. In this study, we examined whether learning is accompanied by a release from mental effort on the task, leading to automatization of the learned task. For this purpose, we had subjects conduct a visual search for a target, defined by a combination of orientation and spatial frequency, while we monitored their pupil size. It is well known that pupil size reflects the strength of mental effort invested in a task. We found that pupil size increased rapidly as the learning proceeded in the early phase of training and decreased at the later phase to a level half of its maximum value. This result does not support the simple automatization hypothesis. Instead, it suggests that the mental effort and behavioral performance reflect different aspects of perceptual learning. Further, mental effort would be continued to be invested to maintain good performance at a later stage of training.

The effect of retinal illuminance on visual motion priming.

The perceived direction of a directionally ambiguous stimulus is influenced by the moving direction of a preceding priming stimulus. Previous studies have shown that a brief priming stimulus induces positive motion priming, in which a subsequent directionally ambiguous stimulus is perceived to move in the same direction as the primer, while a longer priming stimulus induces negative priming, in which the following ambiguous stimulus is perceived to move in the opposite direction of the primer. The purpose of this study was to elucidate the underlying mechanism of motion priming by examining how retinal illuminance and velocity of the primer influences the perception of priming. Subjects judged the perceived direction of 180-deg phase-shifted (thus directionally ambiguous) sine-wave gratings displayed immediately after the offset of a primer stimulus. We found that perception of motion priming was greatly modulated by the retinal illuminance and velocity of the primer. Under low retinal illuminance, positive priming nearly disappeared even when the effective luminance contrast was equated between different conditions. Positive priming was prominent when the velocity of the primer was low, while only negative priming was observed when the velocity was high. These results suggest that the positive motion priming is induced by a higher-order mechanism that tracks prominent features of the visual stimulus, while a directionally selective motion mechanism induces negative motion priming.

Visual motion mechanisms under low retinal illuminance revealed by motion reversal.

The aim of this study is to determine what kinds of motion mechanisms operate at low luminance levels. We used a motion reversal phenomenon in which the perceived direction of motion is reversed when a blank inter-stimulus interval (ISI) frame is inserted between two image frames of similar mean luminance. At low luminance levels, we found that motion reversal was perceived when the moving pattern was presented in the retinal periphery, but no motion reversal was observed when the stimulus was presented in the central retina. When a large stimulus that covers both central and peripheral visual fields was presented, motion reversal did not occur. We conclude that as retinal illuminance decreases, the relative contribution of a feature-tracking mechanism in the central retina becomes larger, while motion perception in the peripheral retina continues to depend on a biphasic, first-order motion mechanism. When both central and peripheral visual fields are stimulated simultaneously, the motion mechanism that dominates in the central retina determines the perceived direction of motion at low luminance levels.

Importance of preserving the C7 spinous process and attached nuchal ligament in French-door laminoplasty to reduce postoperative axial symptoms.

A comparative clinical trial was conducted to clarify the importance of preserving the C7 spinous process and attached nuchal ligament for the reduction of the axial symptoms after French-door laminoplasty in cervical spondylotic myelopathy patients. Forty-one cervical spondylotic myelopathy patients were enrolled. French-door laminoplasty from C3 to C7 in 22 patients (group 1), and from C3 to C6 in 19 patients (group 2) was performed. The whole structure of the C7 spinous process and the attached nuchal ligament were preserved in group 2. The pre- and post-operative evaluation regarding severity of clinical symptoms was assessed using the Japanese Orthopaedic Association (JOA) score. Pre-operative and subjective outcome regarding axial symptoms were also assessed using a visual analog pain scale questionnaire (VAS: 10-0, where a higher score indicates greater pain) at 1- and 2-year follow-up. Non-parametric testing (Mann-Whitney's U test) was used to establish differences between the two groups for categorical data (P < 0.05). There was no significant difference between the two groups in pre- and post-operative JOA score. The mean VAS was 5.6 +/- 1.4 in group 1, 5.4 +/- 1.7 in group 2 pre-operatively, and 6.4 +/- 1.7 in group 1 and 2.4 +/- 1.9 in group 2 at 1-year follow-up. The mean VAS score at 2-year follow-up exhibited 6.2 +/- 1.9 in Group 1, 2.3 +/- 1.8 in group 2. There was no significant difference in VAS between the two groups before surgery (P = 0.506), but significant differences were noticed at 1-year and 2-year follow-up (P < 0.05), indicating the presence of significantly fewer post-operative axial symptoms in group 2. Laminoplasty of the entire C7 structure is not necessary to obtain satisfactory recovery based on JOA score. Preservation of the C7 spinous process and the attached nuchal ligamentous structures is important to reduce post-laminoplasty axial symptoms.

Long-term follow up of surgical outcomes in patients with cervical disorders undergoing hemodialysis.

OBJECT: As increasing numbers of patients receive long-term hemodialysis, the number of reports regarding hemodialysis-related cervical spine disorders has also increased. However, there have been few reports summarizing the surgical results in patients with these disorders. The objective of this study was to evaluate the long-term follow up and clinical results after surgical treatment of cervical disorders in patients undergoing hemodialysis. METHODS: Seventeen patients in whom surgery was performed for cervical spine disorders while they received long-term hemodialysis therapy were enrolled in this study. Of these, 15 underwent follow-up review for more than 3 years after surgery, and these represent the study population. The remaining two patients died of postoperative sepsis. The average follow-up period was 120 months. Five patients without spinal instability underwent spinal cord decompression in which bilateral open-door laminoplasty was performed. Ten patients with destructive spondyloarthropathy (DSA) underwent reconstructive surgery involving pedicle screw (PS) fixation. In eight patients in whom posterior instrumentation was placed, anterior strut bone grafting was performed with autologous iliac bone to treat anterior-column destruction. Marked neurological recovery was obtained in all patients after the initial surgery. In the mobile segments adjacent to the site of previous spinal fusion, the authors observed progressive destructive changes with significant instability in four patients (40%) who underwent circumferential spinal fusion. No patients required a second surgery after laminoplasty for spinal canal stenosis without DSA changes. CONCLUSIONS: Cervical PS-assisted reconstruction provided an excellent fusion rate and good spinal alignment. During the long-term follow-up period, however, some cases required extension of the spinal fusion due to the destructive changes in the adjacent vertebral levels. Guidelines or recommendations to overcome these problems should be produced to further increase the survival rates of patients undergoing hemodialysis.

The effect of eccentricity and the adapting level on the café wall illusion.

The café wall pattern is composed of rows of alternating light and dark tiles, and alternate rows are shifted by one fourth of a cycle. The rows of tiles are separated by narrow horizontal mortar lines whose luminance is between those of the dark and the light tiles. Although the mortar lines are physically parallel, they are perceived to be tilted, which is known as the café wall illusion. In this study, an energy-based model for encoding orientation is implemented in order to estimate the strength of the café wall illusion, and it is shown that the estimated orientation depends on the spatial frequency to which each orientation-encoding unit is tuned. The estimation of mortar line orientation from an orientation-encoding unit tuned to a lower spatial frequency was greater than that from a unit tuned to a higher spatial frequency. It is assumed that the perceived mortar line orientation is the result of an integration of responses from the orientation-encoding units tuned to various spatial frequencies. This leads to the prediction that under viewing conditions in which responses from orientation-encoding units tuned to a higher spatial frequency are presumably weakened, the strength of the café wall illusion increases. In agreement with this prediction, it is shown that the café wall illusion is stronger when the café wall image is presented at the periphery or is observed under low luminance levels. On the other hand, the weighted averaging of the estimated mortar orientations across spatial frequencies overestimates the perceived orientation of the mortar lines. This suggests that the final percept of the café wall illusion could be determined by some kind of nonlinear interaction, such as an inhibitory interaction, between orientation-encoding units.