Koji Kazai

Comparison between the lambda response of eye-fixation-related potentials and the P100 component of pattern-reversal visual evoked potentials

The purpose of this study was to compare the lambda response of eye-fixation-related potentials (EFRPs) with the P100 component of pattern-reversal visual-evoked potentials. EFRPs were obtained by averaging EEGs time-locked to the offset of the saccade. The dipole of the lambda response and that of the P100 component were estimated by the dipole-tracing method (Musha & Homma, 1990). The locations of their dipoles at the occipital sites were very close to each other when the difference waveform, which was calculated by subtracting the EFRP to the patternless stimulus from the EFRP to the patterned stimulus, was used for the lambda response. This finding implies that the lambda response and P100 have a common neural generator in the visual cortex. However, the peak latency of the lambda response was shorter than that of P100. The saccades in the EFRP trial were considered to be the cause of the difference.

Larger forward memory displacement in the direction of gravity

An observers memory for the final position of a moving stimulus is shifted forward in the direction of its motion. Observers in an upright posture typically show a larger forward memory displacement for a physically downward motion than for a physically upward motion of a stimulus (representational gravity; Hubbard & Bharucha, 1988). We examined whether representational gravity occurred along the environmentally vertical axis or the egocentrically vertical axis. In Experiment 1 observers in either upright or prone postures viewed egocentrically upward and downward motions of a stimulus. Egocentrically downward effects were observed only in the upright posture. In Experiment 2 observers in either upright or prone postures viewed approaching and receding motions of a stimulus along the line of sight. Only in the prone posture did the receding motion produce a larger forward memory displacement than the approaching motion. These results indicate that representational gravity depends not on the egocentric axis but on the environmental axis.

Integrated effect of stimulation at fixation points on EFRP (eye-fixation related brain potentials)

The purpose of this study was to investigate the integrated effect of stimulation at the fixation points just before and just after saccadic eye-movement (saccade) on eye-fixation related brain potentials (EFRP: P75 and N105). Checkerboard patterns were used as stimuli. In Experiment 1, changes in check sizes between two fixation points enhanced the amplitude of P75, while changes in the phases of patterns between the two points did not affect EFRP. This result showed that EFRP was affected by two fixation points, and that changes in the retinal image between the two points did not necessarily affect EFRP. In Experiment 2, the relationship between EFRP and check size was investigated in detail. A second order relationship between logarithm of check size and the latency of P75, and a linear relationship between logarithm of check size and the amplitude of N105 were found. The effect of check size on the amplitude of P75 which might explain the increased amplitude of P75 observed in Experiment 1 did not appear. These results suggest that EFRP might reflect relative higher processing than peripheral stimulation at one fixation point.

Spatial and temporal variations in eye‐fixation‐related potentials

When electroencephalograms (EEGs) are averaged at eye fixation pauses (at the end of saccades), the specific brain potentials related to the visual information processing can be obtained. This eye-fixation-related potential (EFRP) is a type of event-related potential. In this paper, the authors introduced two systems developed at their laboratory. The first measured the distribution of EFRPs on the scalp with DC amplifiers in order to nullify eye movement artifacts. Three-dimensional EFRP topographies were obtained. The most prominent component (lambda response) of the waveform appeared at occipital sites. The second system was designed to examine the variation in EFRPs over time, using a sliding average method. The dynamic variation in EFRPs could be observed as an animation. The sequence of variations in amplitude was continuously displayed on a screen. This indicated the process of decrease and recovery of the lambda response generated by changes between periods of brightness and darkness.

A continuously lit stimulus is perceived to be shorter than a flickering stimulus during a saccade

When subjects made a saccade across a single-flashed dot, a flickering dot or a continuous dot, they perceived a dot, an array (phantom array), or a line (phantom line), respectively. We asked subjects to localize both endpoints of the phantom array or line and calculated the perceived lengths. Based on the findings of Matsumiya and Uchikawa (2001), we predicted that the apparent length of the phantom line would be larger than that of the phantom array. In Experiment 1 of the current study, contrary to the prediction, the phantom line was found to be shorter than the phantom array. In Experiment 2, we investigated whether the function underlying the filled-unfilled space illusion (Lewis, 1912) instead of the function underlying the saccadic compression could explain the results. Subjects were asked to localize both endpoints of a line or an array while keeping their eyes fixated. Although the results of Experiment 2 showed that the perceived length of a line was shorter than that of an array, the function underlying the filled-unfilled illusion could not fully account for the results of Experiment 1. To explain the present results, we proposed a model for the localization process and discussed its validity.

LAMBDA RESPONSE BY ORIENTATION OF STRIPED PATTERNS

A lambda response is an averaged occipital EEG potential associated with the offsets of saccadic eye movements. In the present experiment, two participants were asked to make horizontal saccades across horizontal or vertical white-black stripes. Analysis of orientations of striped patterns showed horizontal saccades across vertical stripes produced larger amplitudes of the lambda response than did horizontal saccades across horizontal stripes. Therefore, when the lambda response is used as the index of mental workload, it is necessary to take notice of the orientation of stimulus pattern.

Activation of the left superior temporal gyrus of musicians by music-derived sounds.

Previous studies have suggested that professional musicians comprehend features of music-derived sound even if the sound sequence lacks the traditional temporal structure of music. We tested this hypothesis through behavioral and functional brain imaging experiments. Musicians were better than nonmusicians at identifying scrambled pieces of piano music in which the original temporal structure had been destroyed. Bilateral superior temporal gyri (STG) activity was observed while musicians listened to the scrambled stimuli, whereas this activity was present only in the right STG of nonmusicians under the same experimental conditions. We suggest that left STG activation is related to the processing of deviants, which appears to be enhanced in musicians. This may be because of the superior knowledge of musical temporal structure held by this population.