Impedance magnetocardiogram.

We have developed an impedance magnetocardiogram (IMCG) system to detect the change of magnetic field corresponding to changes in blood volume in the heart. A low magnetic field from the electrical activity of the human heart--the so-called magnetocardiogram (MCG)--can be simultaneously detected by using this system. Because the mechanical and electrical functions in the heart can be monitored by non-invasive and non-contact measurements, it is easy to observe the cardiovascular functions from an accurate sensor position. This system uses a technique to demodulate induced current in a subject. A flux-locked circuit of a superconducting quantum interference device has a wide frequency range (above 1 MHz) because a constant current (40 kHz) is fed through the subject. It is shown for the first time that the system could measure IMCG signals at the same time as MCG signals.

Preattentive perception of multiple illusory line-motion: a formal model of parallel independent-detection in visual search.

The phenomenon referred to as illusory line-motion (ILM; O. Hikosaka, S. Miyauchi, & S. Shimojo, 1993a) has been described as a measure of the local facilitation of attention gradient. However, J. Kawahara, K. Yokosawa, S. Nishida, and T. Sato (1996) have demonstrated a spatially parallel search for an "odd man out" in the ILM direction. Apart from showing preattentive ILM perception in terms of an analogy between line-motion and apparent motion, the authors examined whether ILM perception is possible without attention from another point of view. Four experiments revealed that the ILM target can be detected in parallel without invoking attentional facilitation and invalidated the possible contribution of attentional set in parallel ILM search. Participants were able to correctly detect the ILM target among multiple nontargets, even when the line orientation was changed from trial to trial. The authors' independent-detection model predicted ILM search performance well on several occasions. These findings strongly support a preattentive and stimulus-driven explanation of ILM perception.

[Three-dimensional curve tracing].

The time required in judging if two probes are on the same curve increased monotonically with the separation of the probes along that curve. This process is called "curve tracing" (Jolicoeur, Ullman & Mackay, 1986). In this study we examined whether curve tracing would occur on a three-dimensionally presented curve with depth variation. By comparing the performance on depth varying and no depth varying stimuli, we examined the properties of three-dimensional curve tracing. The mean RT on three-dimensional stimuli increased monotonically as the distance between two probes increased, which indicates that curve tracing also occurs on three-dimensional stimuli. The mean RT on three-dimensional stimuli was longer than that on two-dimensional stimuli. Our results suggest that three-dimensional structure of the stimuli caused additional costs on curve tracing.

Intracorporeal imaging and differentiation of living tissue with an ultra-high-frequency ultrasound probe.

Intraoperative diagnostic tissue differentiation is expected to be useful clinically. We have fabricated a 3-mm diameter rod-shaped ultrasound (US) probe mounted with a 120-MHz transducer whose lateral resolution is the same as the cellular size of 13 microm. The probe can image a microscopic structure (i.e., the cellular arrangement inside intracorporeal living tissue). We imaged normal kidney tissue of a living mouse and tumor tissue implanted in another mouse kidney. We anesthetized the mice, exteriorized the kidneys, and punctured the kidneys with the probe. Renal corpuscle-like structures were seen in the healthy kidney, but a wavy spindle-like structure was seen in the tumor. The similarity between the ultrasonic images and histological sections taken from the imaged organs demonstrates the possibility of real-time tissue differentiation by ultra-high-frequency US.

[Does negative priming occur by rotated characters?].

We conducted three experiments in order to investigate the effect of stimulus orientation on negative priming (NP). Using the picture naming task, Murray (1995) reported the occurrence of semantic NP by rotated distractors. As the rotation of picture stimuli seems to have little effect to reduce distractor interference, in the present study we used the character (katakana) identification task to ensure the effect of stimulus rotation. When the distractors were rotated (180 degrees), no NP was observed whether the targets were upright (Experiment 1) or rotated (Experiment 2). On the other hand, significant NP was observed when the distractors were upright and the targets were rotated (Experiment 3). These results suggest that the inhibitory mechanism of attention may not operate on the rotated distractor characters.

[Migration-of Kanji characters and their radicals in rapid serial visual presentation].

We investigated the spatio-temporal characteristics of visual attention. In rapid serial visual presentation (RSVP) tasks using Kanji characters, we found temporal migration of characters (Experiment 1) and radicals (Experiment 2). In Experiment 1, the migration was more frequent for the word condition than for the pseudoword condition. Migration with the distractor preceding the target was more frequent than that with the distractor following the target in both experiments. Temporal character and radical migration suggests that characters in words and radicals of characters are separately integrated in RSVP situations where attention is overloaded. However, the hypothesis of independent feature integration is rejected. This dependency means that the attentional units of words and characters might exist.

[Temporal illusory conjunction in RSVP tasks: effects of target complexity].

We examined the complexity effects of stimuli and tasks on temporal illusory conjunction in the Rapid Serial Visual Presentation (RSVP) tasks. Subjects were asked to report the partially defined target. We found that error patterns greatly depended on the visual complexity of stimuli. In the case of visually complex stimuli such as Kanji characters consisting of many strokes, pre-target intrusion errors were dominant. In contrast, post-target intrusion errors mainly occurred for visually simple stimuli such as alphabetic letters. Moreover, when target letters were not defined by a specific feature, pre-target intrusion errors were dominant for alphabetic letters. These results were discussed in view of the limited capacity of visual attention.

A 120-MHz ultrasound probe for tissue imaging.

Ultrasound transducers with center frequency above 100 MHz are expected to be used for future diagnostic tissue characterization because of their high lateral resolution. We have fabricated a 120-MHz transducer that consists of a ZnO piezoelectric film on a sapphire substrate that has a concave acoustic lens. The lateral resolution was calculated as 13 microns. The insertion loss of the transducer, defined as the difference between the received voltage and the transmitted one, was -45 dB. The 6-dB handwidth of the received signal was approximately 40 MHz. The transducer was mounted in a rod-shaped probe to ensure contact with in vivo tissue, because of the low penetration of ultrasound in the high frequency region. While the probe is rotated and moved along its axis mechanically, the transducer receives backscattered ultrasound from the surrounding tissue on a cylindrical plane that is kept a constant distance from the probe surface. The feasibility of this high-frequency tissue imaging probe has been demonstrated by obtaining preliminary images of an in vitro bovine kidney.

Illusory line motion in visual search: attentional facilitation or apparent motion?

A line, presented instantaneously, is perceived to be drawn from one end when a dot is flashed at that end prior to the presentation of the line. Although this phenomenon, called illusory line motion, has been attributed to accelerated processing at the locus of attention, preattentive (stimulus-driven) motion mechanisms might also contribute to the line-motion sensation. We tested this possibility in an odd-target-search task. The stimulus display consisted of two, four, or eight pairs of dots and lines. All lines were presented on the same side of the dots (eg right), except for the target line, which was presented on the opposite side (left). Subjects were asked to report the presence or absence of the target, which was presented in half of the trials. Low error rates for target detection (about 10%) even when the display consisted of eight dot-line pairs (ie display size was eight) indicated that illusory line motion could be perceived simultaneously at many locations. The interstimulus interval (ISI) between the dots and lines (0-2176 ms) and the contrast polarity (both dots and lines were brighter than the background, or dots were darker and lines were brighter) were also manipulated. When an ISI of a few hundred milliseconds was inserted, target detection was nearly impossible with larger display sizes. When the contrast polarity was changed, the target-detection performance was impaired significantly, even with no ISI. Moreover, it was found that the effects of display size, ISI, and contrast polarity were comparable in searches for a two-dot apparent-motion target. These results support the idea that preattentive, apparent-motion mechanisms, as well as attentional mechanisms, contribute to illusory line motion.

Processing of Kanji and Kana characters within Japanese words.

Two experiments were conducted to examine the temporal relationship between whole-word and character processing in Japanese. Two kinds of Japanese script were used as stimuli: Kanji, an ideographic script, and Kana, a syllabic script. In Experiment 1, Kanji word processing and Kanji character processing were examined using a successive visual comparison task. The whole-word advantage (Marmurek, 1986) was not found. In Experiment 2, Kana words and characters were used. As in Experiment 1, no whole-word advantage was found. The absence of the whole-word advantage is attributed to the fact that Kanji and Kana characters represent higher level units of processing than alphabetic letters. The difference between word and nonword processing was shown by manipulations of the visual similarity and the homophony between stimuli presented successively. Visual similarity affected both words and nonwords, whereas homophony affected words only.