Spatial Localization for Transientand Intransient Tactile Stimulus
Abstract - When the hand is stimulated during hand movements, humans can localize thestimulated position even without visual guidance. This is because the sensorimotor system
integrates internal information of the hand's locat ion and perceived temporal information of
the stimulus. It is reported that when transient (6 ms) tactile stimulation was presented
before, during or after hand movements, the stimulus was systematically mislocalized [1].
The mislocalization was interpreted as a result of mismatch between internal information andthe physical location of the hand. However, typical environments exhibit a high degree of
spatiotemporal coherence, and a different spatial localization strategy may be more adaptive
to the continuous case. Here, we examine the time courses of localization for both transientand intransient vibration stimuli, to gain further insight into the spatial localization
mechanism of the hand.
Keywords: Hand movement, Spatial localization, Somatosensory system, Internal spatial representation

Masashi Nakatani*,**Junji Watanabe*,**Hideyuki Ando**
Taro Maeda**
Susumu Tachi**Univ. of Tokyo, Japan**NTT Corporation, Japancorresponding tojunji@star.t.u-tokyo.ac.jp

Initial Position (IP)Experiment

Target Position (TP)xApparatus & Procedure:
Three naive male subjects participated in three experiments. A small vibration motor was
mounted on the forefinger of each subject's right hand for delivering vibration. The subject
closed his eyes and made a rapid horizontal hand movement along the aluminum bar, as
shown in Fig. 1.

Stimulus (S)vibration
motorFigure 2 describes the experimental time chart. At the beginning of each trial, eachsubject was cued by an audio signal to make an immediate hand movement. The latency
and duration of the hand movement were about 200 and 400 ms, respectively. In Exp. 1, atransient vibration (duration 20 ms, frequency 130 Hz) was presented at a random timefrom 50 ms to 700 ms after the audio signal. This timing ranged from about 150 ms before
to about 500 ms after the onset of the hand movement. The subject was asked to indicate
the perceived location of the vibration by his forefinger. The position of the subject's
forefinger was measured by a laser sensor (Keyence Corp. LK-500).

400mm hand movements

In Exp. 2 and 3, the same procedure was performed except for the duration of thevibration. An intransient vibration (duration 300 ms) was presented, instead of 20 ms. The
subject was asked to indicate the perceived starting position of the vibration in Exp. 2 and,the ending position in Exp. 3

Fig. 1 Spatial arrangement of experiments
Hand movements

Result:Perceived positions for the transient vibration in Exp. 1 are shown in Fig. 3. The
horizontal axis represents the temporal difference between the vibration time and the hand
movement onset time. The vertical axis represents the localization error of the indicatedposition for the physically presented position of the vibration (the distance between theindicated position minus the presented position from IP). When the subject moves his handfrom Left to Right, a positive value indicates the vibration is localized in the hand
movement direction, while a negative value indicates the mislocalization is in the opposite
direction. When the subject moves his hand from Right to Left, a positive value indicatesthe vibration is localized in the opposite hand movement direction. Blue (Red) dots are theperceived positions when the right hand moves from Left to Right (from Right to Left).
Blue dots have positive values, and Red dots have negative values in the period from-100ms to 400 ms. These data suggest that the vibration is mislocalized during hand
movement in the same direction as the hand movement, despite the direction of themovement.
Perceived positions for intransient (300 ms) vibration in Exp. 2 & 3 are shown in Fig. 4.
Blue (Red) dots indicate perceived positions of the Start Position (End Position) of thevibration. If the Start and End Positions of 300 ms vibration are localized in the same way
as the 20ms transient vibration, time courses of both perceived positions should coincidewith the time course for the transient vibration. Although a similar tendency was observedin the time course for the Start Position, the time course for the End Position is clearly
different [2].
mislocalization
mislocalization
IPTPx
Localization error [mm]2001000-100-200200 ms

BeepExp. 2400 msSAnswer Start position

Exp. 3SAnswer End position
300 msFig. 2 Time chart of Exp.1, 2 and 3400500Spatial localization of an intransient (300 ms) vibrationshows a different trend, depending on which position is
localized (stimulus Starts or Ends). The conventional
localization theory, which assumes integration of internal
hand position information and perceived vibration timing,
cannot explain the difference between the Start and End
Positions. As a reason of this difference, the temporal
information of the End Position may be misjudged, orobservations made with transient stimuli may not necessarily
indicate inaccuracy of the internal information, as shown inseveral lines of visual perception studies [3].

0-100-200-400-200300

Conclusion

100-400-200200

IP

200-300100Time

300-3000S

400

300-100

Exp. 1

Random transient vibration

TPx400Localization error [mm]IPTP-100

0100

200

300

400

500

Stimulus Start (End for Red dots) time
Stimulus time from hand movement onset [ms]of the vibration from hand movement onset [ms]Fig. 3 Results of Experiment 1
Fig. 4 Results of Experiment 2 & 3Blue dots are obtained data when hand moved fromt Left to Right,
Blue dots: subjects answered Start Position
and Red dots are when Right to Left. Circles mean the averaged
Red dots: subjects answered End Position
value of the data in each 50 ms interval.

References[1] P. Dassonville: Haptic localization and the internal representation of thehand in space, Exp. Brain Res., 106, 434-448, 1995.
[2] J. Watanabe, M. Nakatani, H. Ando, T. Maeda and S. Tachi:
Spatial Localization for Transient and Intransient Tactile Stimulus,
in preparation.
[3] J. Watanabe, A. Noritake, T. Maeda, S. Tachi, S. Nishida: Perisaccadic
Perception of Continuous Flickers, Vision Res., 45, 413-430, 2005.