The Journal of Neurophysiology Vol. 84 No. 1 July 2000, pp. 65-74
Copyright ©2000 by the American Physiological Society

Modulation of the Startle Response During Human Gait

 ¹Departement of Medical Physics and Biophysics, University of Nijmegen, 6525 EZ Nijmegen; and  ²Nijmegen Institute for Cognition and Information (NICI), Motor Behavior and Rehabilitation Research, University of Nijmegen, 6500 HE Nijmegen, The Netherlands

Nieuwenhuijzen, P.H.J.A., A. M. Schillings, G. P. Van Galen, and J. Duysens. Modulation of the Startle Response During Human Gait. J. Neurophysiol. 84: 65-74, 2000. While many studies have shown that there is a phase-dependent modulation of proprioceptive and exteroceptive reflexes during gait, little is known about such modulation for auditory reflexes. To examine how startle reactions are incorporated in an ongoing gait pattern, unexpected auditory stimuli were presented to eight healthy subjects in six phases of the step cycle during walking on a treadmill at 4 km/h. For both legs, electromyographic activity (EMG) was recorded in the biceps femoris (BF), the rectus femoris (RF), the tibialis anterior (TA), and the soleus (SO). In addition, stance and swing phases of both legs, along with knee angles of both legs and the left ankle angle, were measured. All subjects showed various response peaks. Responses with latencies of ~60 ms (F1), ~85 ms (F2), and ~145 ms (F3) were found. The amplitude of the reflex responses was dependent on the timing of the startle stimulus in the step cycle. Although the startle response habituated rapidly, the phase-dependent modulation pattern generally remained the same. The phase-dependent amplitude modulations were not strictly correlated with the modulation of the background activity. The TA even showed a transition from facilitatory F2 responses during stance to suppressive responses during midswing. Responses were observed in both flexors and extensors, often in coactivation, especially during stance. Furthermore the gait characteristics showed a shortening of the subsequent step cycle and a small decrease in the range of motion of ankle and knees. These results suggest that the responses are adapted to achieve extra stability dependent on the phase of the step cycle. However, even in the first trials, the changes in kinematics were small allowing a smooth progression of gait.