In a simple-reaction task in which subjects respond as quickly as possible to a visual cue, pre-motor time (PMT) is measured as the delay between stimulus onset and onset of myoelectric activity using electromyography (EMG). A previous study reported that PMT can be subdivided into two factors: elapsed time from visual stimulation to the primary motor cortex (M1) through the visual cortex (visuo-motor related time [VMRT]), and from M1 to the muscle (motor evoked potentials [MEP] latency) [1]. Thus, VMRT is calculated as PMT minus MEP latency. The VMRT is likely to reflect the time of visuo-motor integration and control [2]. VMRT differs in individuals [1]; however, the factors that define the length of time for VMRT are unclear.

Peak latency of P300 (P3) in event-related potentials is well recognized to be associated with reaction time; this has been supported by a number of studies using electroencephalograms (EEG) [3-7]. The P3 component links to cognitive processing of context updating in the particular higher order brain area [8-10], and peak latency is considered a measure of the processing speed via stimulus evaluation [11]. Given that P3 latency reflects the speed of central nervous processing, it seems useful to assess the length of VMRT. Moreover, VMRT, including processing of the visual system, requires consideration of the elapsed speed from visual stimulation to visual cortex activation.

The objective of the present study was to assess the relationship of response time and P3 latency with a simple-reaction task, that is, to examine the length of VMRT. In addition, we also investigated the latency in visual evoked potentials (VEP), that is, potentials indicating neural activities related to visual processing [12-14].

Seven male subjects who were experienced players in decision-making sports, such as baseball, kendo, judo, basketball, or tennis, were included. All subjects had regularly attended local competitions for more than 8 years. Subjects had a mean age ± standard deviation (SD) of 24.7 ± 3.6 years, height of 167.5 ± 5.3 cm, and weight of 64.1 ± 9.1 kg. Informed consent was obtained before beginning the experiment, which was conducted according to the Declaration of Helsinki. The experimental procedures were approved by the Ethics Committee of the National Institute of Fitness and Sports in Kanoya.

Subjects were comfortably seated in an experimental chair in an electrically shielded, sound-attenuated room. Their forearms rested on an armrest while their hands were kept in a neutral position. After a warning signal was presented, a visual signal was presented by a red light-emitting diode (LED) that was 1 m in front of subjects, at eye level. When the LED lit up, subjects were asked to contract the right flexor pollicis brevis muscle as quickly as possible in reaction to the visual signal and then to relax the muscle. The inter-stimulus interval was varied randomly from 2 to 6 sec from the warning signal to avoid an anticipation effect for timing of the visual signal. Subjects were requested to avoid eye blinking during the task and to keep the right flexor pollicis brevis muscle relaxed between each trial. Complete muscle relaxation and eye movements were confirmed online via audiovisual feedback by EMG activity and an electrooculogram (EOG). The trials including eye blinks or other signal artifacts were excluded, and each subject performed the task until 50 artifact-free trials were obtained.

During the task, EEG was recorded with Ag/AgCl electrodes from Fz, Cz, Pz and Oz according to the international 10 - 20 system with reference to linked earlobes. An EOG was recorded using electrodes above and below the left eye. The EEG and EOG signal were sampled at 1000 Hz, filtered with a low-pass frequency of 300 Hz, and recordings were performed with a 32-channel digital DC EEG amplifier (NuAmps model; Compumedics NeuroScan, Charlotte, NC). The EMG activity of the right flexor pollicis brevis muscle was recorded using surface EMG electrodes. A DL-141 single-differential, parallelbar configuration (4 Assist, Inc., Japan) was put on the skin surface over the muscle. The EMG signal was filtered during acquisition with a bandwidth of 5 to 500 Hz and a gain of ×1000. The signal was digitized at 1000 Hz (16 bit, PowerLab, AD Instruments, Japan), recorded, and stored for off-line analysis (Chart 6, AD Instruments) on a personal computer. The MEP elicited by transcranial magnetic stimulation (TMS) was recorded from the flexor pollicis brevis muscle at rest. TMS was performed using a figure-eight coil (outer diameter 70 mm) connected to a monophasic Magstim 200 stimulator (Magstim, 200, UK). To define the optimal stimulation position (hot spot) for the muscle activation, the coil was placed over the left hemisphere, approximately 5 cm lateral to the Cz with the handle pointing backward and 45˚ laterally from the middle line. The site at which TMS of a slightly suprathreshold intensity consistently elicited the largest MEP in the muscle was marked as the motor hotspot. TMS was defined as 1.1 to 1.2 times the intensity at which TMS evoked MEP at 50% probability in 10 to 15 trials [15]. We provided the stimulus at 5-sec intervals.

Response time was measured from the recorded light signal and EMG. PMT was defined as the period between the onset of visual stimulus and the first deflection of response activity noted on EMG (Figure 1A). The VMRT was calculated as PMT minus MEP latency (from TMS stimulus to onset of MEP; Figure 1B), which was identified as the period between the onset of visual signal and M1 [1]. The analysis epoch for stimulus-locked EEG was 1000 msec including a pre-stimulus (light) baseline period of 100 msec. Peak latency of P3 was measured at 250 - 500 msec from Fz, Cz, and Pz. VEP components (N1, P1, N2, and P2) were recorded from Oz, and the peak latency was measured at 50 - 100 msec, 70 - 140 msec, 100 - 190 msec and 130 - 260 msec, respectively. All data were expressed as mean ± SD (Table 1). Correlation analysis was conducted to evaluate relationship between P3, VEP latencies, and response time, especially in VMRT.

Table 2 shows the correlation coefficient analyses results between the response time and each of P3 and VEP latencies. Significant positive correlations were observed between Fz-, Pz-P3 latency and PMT, VMRT (r = 0.77 - 0.85, P < 0.05; Table 2 and Figure 2(a)), and no significant correlations were observed between Cz-P3 latency. In the VEP, significant negative correlations were observed only between Oz-P2 latency and PMT, VMRT (r = −0.76, P < 0.05; Table 2 and Figure 2(b)). There was no significant correlation between MEP latency and each of P3 and VEP latencies.