Biomedical subjects

Shinichi Koyama

Publications and source records attributed to Shinichi Koyama.

9 recordsLinked to original sources

PubMed2007

[Neural mechanisms for object and color recognition].

We reported double-dissociation between the visual processing of the edges and the surfaces of objects. Patients with lateral occipital damage showed selective impairment in the perception of edges whereas those with medial ventral occipital damage showed selective impairment in the perception of the 3D structure of the surface. Patients with medial ventral occipital damage also exhibited impaired perception of color, which is also a surface property. Those results were consistent with those from neuroimaging studies. Taken together, those studies suggest that objects may be processed in two separate pathways in the ventral occipital cortex: the edges of objects are processed in the lateral pathway and the surface of objects are processed in the medial pathway. Both edges and surfaces play important roles in object recognition, and both types of perception should be evaluated in patients with visual agnosia.

Cerebral Infarction↗

PubMed2006

Evaluation of cerebral dyschromatopsia using color afterimage.

Cerebral dyschromatopsia is traditionally evaluated by color discrimination tests such as the Farnsworth-Maunsell 100 Hue Test. These tests are also used to evaluate color-perception deficits caused by eye disease, and they do not tell us whether color perception is impaired in the eye or in the brain. Here, we used color afterimage to isolate color-perception deficits in the brain from those in the eyes, in a patient with cerebral dyschromatopsia. The results showed that the patient perceived color afterimage of yellow and blue with abnormal duration and chromaticity. On the other hand, the patient performed normally on the Farnsworth-Maunsell 100 Hue Test. We conclude that the color afterimage test would be useful to evaluate color perception in the brain.

Color Perception↗

PubMed2005

Separate processing of different global-motion structures in visual cortex is revealed by FMRI.

The visual system has the remarkable ability to extract several types of meaningful global-motion signals, such as radial motion, translation motion, and rotation, for different visual functions and actions. In the monkey brain, different groups of cells in MST respond best to different types of global motion [1, 2] whereas in lower cortical areas including MT, no such differential responses have been found. Here, we show that an area (or areas) lower than MST in the human brain [3] responds to different types of global motion. A series of human functional magnetic resonance imaging (fMRI) experiments, in which attention was controlled for, indicated that the center of radial motion activates the corresponding location in the V3A representation, whereas translation motion activates mainly in a more peripheral representation of V3A. These results suggest that in the human brain, V3A is an area that differentially responds according to the type of global motion.

Adult↗

PubMed2005

Seeing what is not there shows the costs of perceptual learning.

Perceptual learning is an improvement in one's sensory abilities after training and is thought to help us to better adapt to the sensory environment. Here, we show that perceptual learning also can lead to misperceptions, such that subjects actually perceive stimuli when none are physically presented. After learning, subjects not only showed enhanced performance when tested with the motion direction of the trained stimulus but also often reported seeing dots moving in the trained direction when no stimulus was displayed. We further show that these misperceptions are not attributable to a response bias. These results show that there are costs as well as benefits to perceptual learning and that performance enhancements for a specific feature also can be accompanied by misperceptions of the visual environment.

Adolescent↗

PubMed2005

[A case of complete atrioventricular block after induction of general anesthesia].

A 75-year-old man with a past history of bilateral thalamic hemorrhage was scheduled for cholecystectomy and cholelithotomy under general anesthesia. Although the preoperative ECG showed a complete right bundle branch block, the echocardiogram revealed no abnormality. Anesthesia was induced with thiopental and vecuronium, and maintained with sevoflurane in oxygen. Soon after changing to the left decubitus position for the insertion of an epidural catheter, ECG showed complete atrioventricular block, which did not respond to atropine. Adrenalin was transiently effective, but arrhythmia continued. After administration of dopamine, norepinephrine and isoproterenol, we inserted a temporary transvenous pacemaker catheter, and the hemodynamics became stable by ventricular pacing. The operation was postponed. Subsequent cardiologic examination showed no ischemia. The atrioventricular block disappeared 7 hours after the induction of general anesthesia. We should be very careful with the anesthetic management of a patient with a complete right bundle branch block.

Aged↗

PubMed2004

Task-dependent changes of the psychophysical motion-tuning functions in the course of perceptual learning.

In some cases, perceptual learning is task-specific. However, task-dependent effects of perceptual learning on psychophysical motion-tuning functions have yet to be clarified. In the present study, subjects performed motion detection or discrimination of the same stimulus over the course of four sessions held on separate days. Subjects who performed motion detection showed the most highly improved performance on the trained motion directions. However, after discrimination training, the highest improvement was not observed at the trained directions but shifted away from them. These results can be explained by lateral inhibition. Task demands may differentially modulate excitatory and inhibitory signals to directions in the vicinity of the trained directions.

Adult↗

PubMed2004

[Surgical treatment for bone and soft tissue sarcoma].

There are many kinds of wide excision or wide resection, which are methods to remove the tumor with surrounding tissues. The curability of wide resection depends on the range and characteristics of the normal surrounding tissues. The fascia, periosteum and perivascular sheath act as barriers against invasion of tumors. Therefore, evaluation of the surgical margin is essential in surgery for malignancy. Some sarcomas characteristically show invasive growth patterns while others have a discrete border. Causes of the local recurrence are 1) insufficient surgical margin, 2) skip metastasis, 3) tumor thrombus, and 4) lymph node metastasis. The so-called "Safety surgical margin" is the margin that prevents local recurrence due to insufficient surgical treatment. Local recurrence due to other causes can not be treated by surgery alone as chemotherapy is also required. For example, the infiltrative type of malignant fibrous histiocytoma requires a curative procedure. On the other hand, non-infiltrative types of sarcoma or high-grade sarcoma, which are good responders to preoperative treatment, are locally controlled by an adequate wide margin procedure. When the tumor is close to the bone, main vessels and/or nerves, it is sometimes very difficult to preoperatively decide whether or not to sacrifice these important organs. A new surgical method, "In Situ Preparation (ISP)," is a useful method to solve such problems. ISP makes it possible to evaluate the surgical margin without contamination. And additional procedures including alcohol soaking or pasteurization can be achieved according to the surgical margin. Because ISP can prevent overtreatment, the number of our cases that require resection of the nerves or vessels have decreased. After a wide resection, there have been many possible complications, including infection, deep venous thrombosis, loosening of prosthesis, skin necrosis, and arterial occlusion. Once postoperative complications occur, patients must remain in the hospital for a long time. To prevent this, the preservation of normal tissue, meaning reduction of surgical margin, is important.

Blood Vessel Prosthesis Implantation↗

PubMed2002

Greater plasticity in lower-level than higher-level visual motion processing in a passive perceptual learning task.

Simple exposure is sufficient to sensitize the human visual system to a particular direction of motion, but the underlying mechanisms of this process are unclear. Here, in a passive perceptual learning task, we found that exposure to task-irrelevant motion improved sensitivity to the local motion directions within the stimulus, which are processed at low levels of the visual system. In contrast, task-irrelevant motion had no effect on sensitivity to the global motion direction, which is processed at higher levels. The improvement persisted for at least several months. These results indicate that when attentional influence is limited, lower-level motion processing is more receptive to long-term modification than higher-level motion processing in the visual cortex.

Adult↗

PubMed2002

[Fentanyl administration during sevoflurane anesthesia suppresses spike waves from epileptic focus on electrocorticogram].

A 36-year-old man with intractable epileptic seizures underwent insertion of subdural electrodes on bilateral temporal lobes under air-oxygen-sevoflurane anesthesia. After the completion of the operation, we measured electrocorticogram at end-tidal sevoflurane concentration of 2.5%, 1.5%, and 1.5% with 0.1 mg intravenous fentanyl. When sevoflurane concentration was reduced to 1.5%, the incidence and voltage of the spike waves on electrocorticogram were reduced. When 0.1 mg fentanyl was intravenously administrated during 1.5% sevoflurane anesthesia, the frequency of the spike waves was further reduced. Caution should be taken when using sevoflurane-fentanyl anesthesia because this combination may interrupt identification of epileptic focus on intraoperative electrocorticogram.

Adjuvants, Anesthesia↗