Effect of tactile stimulus frequency on time perception: the role of working memory.

In most models of interval timing, there is a central clock, which is considered to be highly protected from the effects of external stimuli. However, many studies have reported such effects and different theories are proposed to explain the observations. These include the effect of arousal, attention sharing, memory load and information processing on central clock as well as change in the speed of the pacemaker. In this study, we used regular vibrotactile stimuli with different frequencies in a "duration reproduction task" to investigate the effect of stimulus content on interval timing. Results showed that subjects overestimated the duration as a function of test stimulus frequency. A significant correlation between increasing the test frequency and overestimation of subjective time was observed. We further investigated the effect of blank and filled gaps with various durations on time estimation. Analysis revealed that regardless of gap duration, subjective time increased in the filled gap condition, compared to the blank gap. This effect was independent from contextual stimuli and correlated to the mean number of stimuli during the temporal interval rather than rate of stimulus presentation.

Independent coding of movement duration in a repetitive non-visually guided movement.

Temporal information is an embedded feature of our sensory and motor experiences. How is temporal information encoded in the brain? In the two-stage theory of timing, an explicit representation of timing is responsible for the movement initiation while movement duration is coded implicitly. We investigated the correlation of movement duration and amplitude in a repetitive one-dimensional non-visually guided movement to find out if temporal information could be coded independently from movement. Subjects were asked to learn the distance between two points by moving their hands repeatedly along the distance between two sticks, while they could not see their hands and hand path. After a training phase, a delay of either 2 or 20 s was imposed and the subjects were asked to reproduce the learned distance. There was no correlation between distance difference and time difference in either delay condition. In the 20 s delay experiment, in comparison to the 2 s delay experiment, there was a significant increase in distance reproduction error. However, there was no significant change in time differences in either of the experiments. In addition, the time difference between the training and test trials was independent from the direction of the distance difference (i.e., overshot, undershot, or accurate). In conclusion, time may be coded as an independent measure after the delay period, so it should be a kind of explicitly coded information.

Kinesthetic memory in distance reproduction task: importance of initial hand position information.

The role of the initial hand position in planning and implementation of a goal-directed movement is a matter of debate. We designed a non-visually guided repetitive reaching movement task to investigate the role of proprioceptive information of the initial or end hand position in implementation of kinesthetic memory. The experimental design consisted four experiments. Each experiment consisted two phases: the training phase and the test phase. In the training phase the subjects were asked to learn and memorize the distance between start and end points by moving their hands ten times between them. In the test phase, the end point was removed, and starting from the same initial position, the subjects were asked to reproduce the memorized distance. In all experiments, varied conditions called "mask phase" preceded the test phase: a 5-s delay in experiment 1, shifting both initial and end points while keeping the distance constant in experiment 2, shifting only the end point, while keeping the initial point constant in experiment 3 and shifting only the initial point and keeping the end point constant in experiment 4. In comparison to experiment 1 (which was considered as the control group), no significant error in distance reproduction occurred in experiment 3, while experiments 2 and 4 showed significant error. These findings mean that although the distance information was available during the mask phase of experiment 2, subjects could not efficiently use this information to reproduce the distance. The other two experiments suggest that this error was caused by changing the initial hand position and not the end point. We conclude that in a kinesthetic-guided distance reproduction task, the initial hand position is more important for implementation of distance memory rather than the end point.

Resistance of peripheral and sub-cortical somatosensory pathway to electrical noise.

The somatosensory system is vulnerable to large amounts of noise distortion. But how does the central nervous system distinguish the peripheral inputs which carry information to the brain from that which does not possess information? To address this question we studied the effect of electrical stimulation of the median nerve on tactile spatial frequency perception in healthy subjects and Parkinson's disease (PD) patients. Subjects were categorized in two groups (healthy and PD patients) and were asked to report if a test tactile frequency pattern (TFP) was the same as the reference TFP given to the other hand. In each case stimulation was either present or absent on the median nerve of the hand holding the test pattern. We observed no impairment of tactile performance in the presence of electrical stimulation of the median nerve. This result together with previous work on direct stimulation of the somatosensory relay nucleus of the thalamus in which the same result of no impairment of the tactile discrimination task was observed suggest a high degree of noise tolerance exists in the somatosensory pathway.