Atsuhiko Iijima

Target spatial frequency determines the response to conflicting defocus- and convergence-driven accommodative stimuli

Asthenopia, or visual fatigue, is a frequent complaint from observers of stereoscopic three-dimensional displays. It has been proposed that asthenopia is a consequence of anomalous oculomotor responses generated by conflict between accommodative and convergence stimuli. The hypothesis was examined by measuring accommodation and convergence continuously with a Shin-Nippon SRW5000 infrared autorefractor and a limbus tracking device. Subjects viewed a high contrast Maltese Cross target at three levels of Gaussian filter target blur under conditions of relatively low- and high-conflict between accommodation and convergence stimuli, the latter inducing the sensation of stereopsis. Under the low-conflict conditions accommodation was stable, but convergence-driven accommodation was dominant when the target was extremely blurred. Under the high-conflict conditions the role of convergence-driven accommodation increased systematically with the degree of target blur. It is proposed that defocus-driven accommodation becomes weak when the target comprises low spatial frequency components. Large accommodative overshoots to step stimuli that are not blurred or only mildly blurred were consistently observed and are attributed to the initial accommodative response being convergence-driven. Whereas the possibility that high-conflict conditions are a cause of asthenopia has been previously reported, this is the first evidence that they specifically affect accommodative responses while viewing stereoscopic displays.

Visual fatigue caused by stereoscopic images and the search for the requirement to prevent them: A review

Abstract Recent literatures related to the evaluations of visual fatigue induced by stereoscopic images (VFSI) are reviewed with the short introduction of cortical mechanisms related to early visual processing, vergence eye movement and lens accommodation. Based on this knowledge, the requirements to prevent visual fatigue and discomfort induced by viewing stereoscopic images were sought. Firstly, the careful alignment in the right/left eye images is required for the stereoscopic vision without discomfort. Secondly, the conflict between the demands for vergence eye movement and lens accommodation in the near response should be avoided, by using modest binocular disparity. Thirdly, the frequency of changes in binocular disparity should be restricted. Finally, the appropriate viewing distance is also recommended to avoid visual fatigue.

Intrasulcal electrocorticography in macaque monkeys with minimally invasive neurosurgical protocols.

Electrocorticography (ECoG), multichannel brain-surface recording and stimulation with probe electrode arrays, has become a potent methodology not only for clinical neurosurgery but also for basic neuroscience using animal models. The highly evolved primates brain has deep cerebral sulci, and both gyral and intrasulcal cortical regions have been implicated in important functional processes. However, direct experimental access is typically limited to gyral regions, since placing probes into sulci is difficult without damaging the surrounding tissues. Here we describe a novel methodology for intrasulcal ECoG in macaque monkeys. We designed and fabricated ultra-thin flexible probes for macaques with micro-electro-mechanical systems technology. We developed minimally invasive operative protocols to implant the probes by introducing cutting-edge devices for human neurosurgery. To evaluate the feasibility of intrasulcal ECoG, we conducted electrophysiological recording and stimulation experiments. First, we inserted parts of the Parylene-C-based probe into the superior temporal sulcus to compare visually evoked ECoG responses from the ventral bank of the sulcus with those from the surface of the inferior temporal cortex. Analyses of power spectral density and signal-to-noise ratio revealed that the quality of the ECoG signal was comparable inside and outside of the sulcus. Histological examination revealed no obvious physical damage in the implanted areas. Second, we placed a modified silicone ECoG probe into the central sulcus and also on the surface of the precentral gyrus for stimulation. Thresholds for muscle twitching were significantly lower during intrasulcal stimulation compared to gyral stimulation. These results demonstrate the feasibility of intrasulcal ECoG in macaques. The novel methodology proposed here opens up a new frontier in neuroscience research, enabling the direct measurement and manipulation of electrical activity in the whole brain.

Re-evaluation of tropicamide in the pupillary response test for Alzheimer’s disease

In 1994, a pupillary response test using very dilute (0.01%) tropicamide, a cholinergic antagonist, evoked remarkable pupil dilation in subjects with Alzheimers disease (AD) and has since been considered a diagnostic tool for AD. However, since this test was first reported, there have been studies suggesting it cannot provide a differential diagnosis of AD. Thus, the present study re-evaluated the pupillary dilation of AD (n=17) and non-AD (n=20) subjects at a 0.01% concentration of tropicamide and found that non-AD subjects, including young subjects, showed the same extent of pupil dilation as AD subjects. Furthermore, there was no significant difference between the average dilation rate of the two groups (P>0.05). When the tropicamide concentration was diluted to half of the initial concentration and performed for AD (n=14), vascular dementia (VD) (n=14), and young (n=16) subjects, the AD subjects showed a large dilation (mean pupil dilation rate: 133.8+/-15%) while the non-AD subjects did not show hypersensitivity to the new concentration of eye drops (105.4+/-9%). These differences in pupillary dilation between AD and non-AD patients were statistically significant (P<0.001). Based on these results, the most effective cutoff point of 0.005% tropicamide for differential diagnosis was 114.5% of the average pupil dilation rate for 60 min.

A pilot study on pupillary and cardiovascular changes induced by stereoscopic video movies.

BackgroundTaking advantage of developed image technology, it is expected that image presentation would be utilized to promote health in the field of medical care and public health. To accumulate knowledge on biomedical effects induced by image presentation, an essential prerequisite for these purposes, studies on autonomic responses in more than one physiological system would be necessary. In this study, changes in parameters of the pupillary light reflex and cardiovascular reflex evoked by motion pictures were examined, which would be utilized to evaluate the effects of images, and to avoid side effects.MethodsThree stereoscopic video movies with different properties were field-sequentially rear-projected through two LCD projectors on an 80-inch screen. Seven healthy young subjects watched movies in a dark room. Pupillary parameters were measured before and after presentation of movies by an infrared pupillometer. ECG and radial blood pressure were continuously monitored. The maximum cross-correlation coefficient between heart rate and blood pressure, ρmax, was used as an index to evaluate changes in the cardiovascular reflex.ResultsParameters of pupillary and cardiovascular reflexes changed differently after subjects watched three different video movies. Amplitudes of the pupillary light reflex, CR, increased when subjects watched two CG movies (movies A and D), while they did not change after watching a movie with the real scenery (movie R). The ρmax was significantly larger after presentation of the movie D. Scores of the questionnaire for subjective evaluation of physical condition increased after presentation of all movies, but their relationship with changes in CR and ρmax was different in three movies. Possible causes of these biomedical differences are discussed.ConclusionThe autonomic responses were effective to monitor biomedical effects induced by image presentation. Further accumulation of data on multiple autonomic functions would contribute to develop the tools which evaluate the effects of image presentation to select applicable procedures and to avoid side effects in the medical care and rehabilitation.

Time-varying factors model with different time-scales for studying cybersickness

We have investigated cybersickness in terms of image motion vectors, visual characteristics, and the autonomic nervous regulation. We obtained the RR interval, respiration, and blood pressure time-series and estimated the low-frequency (LF) and high-frequency (HF) power components to determine the some sensation intervals. Then, we traced the time-series of the LF component backwards to find out the local minimum as the onset. An experiment consisted of five consecutive exposure sessions of the same first-person-view video image. In the unpleasant group from fifteen healthy young subjects, the LF/HF increased with respect to the number of trials and a significant difference was confined between two groups. The trigger points concentrated around the specific segments. Within the unpleasant group, eyes did not follow the camera motion around the trigger points. Accordingly, it recommends to monitor image motion vectors as a trigger factor and autonomic nervous regulation as an accumulation factor for studying cybersickness.

Image analysis of quick phase eye movements in nystagmus with high-speed video system

There are several methods of measuring horizontal, vertical and torsional nystagmus for the diagnosis of vertigo. Using Frentzel glasses, electronystagmography (ENG) and video-oculography (VOG) are common methods for observing nystagmus. In this study, a high-speed video camera is used to analyse high-speed eye movements, including three components of nystagmus, in order to monitor precisely the amplitude and velocity of the quick phase of nystagmus. To confirm the accuracy of the analysis, images of optokinetic nystagmus (OKN) evoked by a stimulating device were analysed. We obtained a high linearity with regard to the relation between the horizontal nystagmus amplitude detected with the high-speed video system and ENG (R2=0.99). On comparison of the maximum velocities detected with 30 Hz and with 250 Hz at each amplitude, the velocities calculated at 30 Hz were smaller than those calculated at 250 Hz. At an amplitude of 5°, the 30 Hz velocity decreased by 50%, while a decrease of 35% was observed at 15°. The vertical nystagmus data obtained using high-speed VOG was different in appearance to that obtained using ENG. The torsional component of nystagmus was also measured, and was recorded in the form of a large number of sampling points.

Associative-memory representations emerge as shared spatial patterns of theta activity spanning the primate temporal cortex

Highly localized neuronal spikes in primate temporal cortex can encode associative memory; however, whether memory formation involves area-wide reorganization of ensemble activity, which often accompanies rhythmicity, or just local microcircuit-level plasticity, remains elusive. Using high-density electrocorticography, we capture local-field potentials spanning the monkey temporal lobes, and show that the visual pair-association (PA) memory is encoded in spatial patterns of theta activity in areas TE, 36, and, partially, in the parahippocampal cortex, but not in the entorhinal cortex. The theta patterns elicited by learned paired associates are distinct between pairs, but similar within pairs. This pattern similarity, emerging through novel PA learning, allows a machine-learning decoder trained on theta patterns elicited by a particular visual item to correctly predict the identity of those elicited by its paired associate. Our results suggest that the formation and sharing of widespread cortical theta patterns via learning-induced reorganization are involved in the mechanisms of associative memory representation.

Pupillography of automated swinging flashlight test in amblyopia

Relative afferent pupillary defects (RAPDs) in amblyopia have been reported, and it is widely accepted that amblyopes can have an RAPD. We investigated whether or not this could be confirmed by the use of binocular pupillography. We examined twelve patients (6 males and 6 females, aged 7–57 years) with unilateral amblyopia associated with anisometropia and/or strabismus, using binocular infrared video pupillography (Newopto, Kawasaki, Japan). Eight normal subjects were also tested in the same manner. Two patients’ data had to be excluded because of poor recording quality. Only one patient with moderate anisometropic amblyopia was found to have reduced contraction amplitude in the amblyopic eye, and one patient with a borderline pupillary defect. The other amblyopes, some of whom showed even denser amblyopia, did not have a pupillary defect. This study has confirmed that only a small proportion of amblyopes have a reduced pupillary contraction amplitude in the affected eye, as established by pupillographic recordings, and even these amblyopes are not necessarily associated with dense amblyopia.

Relationships between sensory stimuli and autonomic nervous regulation during real and virtual exercises

BackgroundApplication of virtual environment (VE) technology to motor rehabilitation increases the number of possible rehabilitation tasks and/or exercises. However, enhancing a specific sensory stimulus sometimes causes unpleasant sensations or fatigue, which would in turn decrease motivation for continuous rehabilitation. To select appropriate tasks and/or exercises for individuals, evaluation of physical activity during recovery is necessary, particularly the changes in the relationship between autonomic nervous activity (ANA) and sensory stimuli.MethodsWe estimated the ANA from the R-R interval time series of electrocardiogram and incoming sensory stimuli that would activate the ANA. For experiments in real exercise, we measured vehicle data and electromyogram signals during cycling exercise. For experiments in virtual exercise, we measured eye movement in relation to image motion vectors while the subject was viewing a mountain-bike video image from a first-person viewpoint.ResultsFor the real cycling exercise, the results were categorized into four groups by evaluating muscle fatigue in relation to the ANA. They suggested that fatigue should be evaluated on the basis of not only muscle activity but also autonomic nervous regulation after exercise. For the virtual exercise, the ANA-related conditions revealed a remarkable time distribution of trigger points that would change eye movement and evoke unpleasant sensations.ConclusionFor expanding the options of motor rehabilitation using VE technology, approaches need to be developed for simultaneously monitoring and separately evaluating the activation of autonomic nervous regulation in relation to neuromuscular and sensory systems with different time scales.