The evaluation of interhemispheric transfer time (IHTT) in adults

The goal of the present study was to develop an objective technique to measure interhemispheric transfer time (IHTT) of linguistic stimuli using late auditory evoked potentials to develop normative data in adults. Nine participants, five females and four males (M = 25.22) were included in this study. Each participant had their hearing tested and electrodes were placed on the forehead, tip of the nose, below the right eye and several places on the scalp. The results revealed that when comparing electrode sites (CZ, C3, and C4), waves (P1-N1-P2) and ears (right ear and left ear) there was no statistically significant effect for electrode sites and ears; however, there were for waves. There also were no significant interactions when comparing electrodes to waves, waves to ears, or electrodes to waves to ears. There was also comparison to determine which waves were significantly different from the others. Analysis did not indicate any statistically significant differences between waves P1-N1-P2 when compared for the right versus the left sides. Overall results revealed consistently shorter latencies when the left ear was stimulated compared to when the right ear was stimulated. These results were unexpected and further research is needed with a larger sample size to fully understand how the human auditory system works

Time-frequency analysis of visual evoked potentials for interhemispheric transfer time and proportion in callosal fibers of different diameters

This study is an extension of the experimental research of Nalcaci et al., who presented 16 subjects with a reversal of checkerboard pattern as stimuli in the right visual field or left visual field and recorded EEG at O1, O2, P3, and P4. They applied the chosen bandpass filters (4-8, 8-15, 15-20, 20-32 Hz) to the VEPs of subjects and obtained four different components for each VEP. The first aim of this study is to improve the previous report using some methods in time-frequency domain to estimate interhemispheric delays and amplitudes in a time window. Using the improved estimates of interhemispheric delays, the second aim is to estimate the proportion of callosal fibers of different diameters that are activated by visual stimuli by comparing amplitudes of VEPs in different frequency bands. If the relation between frequency components of VEP and delays for callosal fibers of different dimension were reliable, it would give us an opportunity to deal with amplitude of bandpass-filtered VEPs in order to see approximately the proportion of these fibers activated by a certain stimulus. By using frequency-dependent shifts in time and maximizing the cross correlation of direct VEP (DVEP-VEP obtained from contralateral hemisphere)-indirect VEP (IVEP-VEP obtained from ipsilateral hemisphere) pairs in the time-frequency domain, we examined the delay not only at P100 and N160 peaks but along a meaningful time interval as well. Furthermore, by shifting back the IVEP according to the delay estimated at each time window, both the amplitudes and energies of the synchronized DVEP-IVEP pairs were compared at the chosen frequency bands. The percentages of IVEPs at each band was then examined further in conjunction with the distribution of axon diameters in the posterior pole of the CC, questioning the relation between the distributions of the axon diameters and activations at each band. We established an energy definition to express the activation in the fibers. When the energy percentages of IVEPs in theta and alpha were totaled, they were found to be between 76.2% and 81.6%, which is close to the value 74-77% for fibers of 0.4-1 mum in diameter obtained from anatomical study of human CC. The sum of energy percentages in the beta1 and beta2 bands was between 20.1% and 24.2%, which probably reflects the proportion of activation of callosal fibers 1-3 mum in diameter