Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Comparative Study
Short-latency eye movements evoked by near-threshold galvanic vestibular stimulation.
To investigate whether the primary planes of eye and body responses to galvanic vestibular stimulation (GVS) are congruent, we have measured the binocular, three-dimensional eye movements (scleral coil technique) to bilateral bipolar GVS in six normal human subjects. Stimulation intensities were kept deliberately low in order to characterize the response to near-threshold intensities of stimulation (0.1-0.9 mA) that had been used previously to characterise body postural responses. Stimuli were applied for 4 s, but only the early responses that occurred within the initial 300 ms of turning the current on or off were measured. ⋯ However, weak horizontal eye movements, which became more prominent as the stimulus intensity was increased to 0.9 mA, were also observed. This suggests that an additional weak rotational component about the yaw axis, or a component of lateral translation in the frontal plane, is contained in the GVS-evoked signal. The overall pattern of eye movement suggests that semicircular canal afferents contribute to these low-intensity GVS responses.
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Comparative Study
Innocuous cooling can produce nociceptive sensations that are inhibited during dynamic mechanical contact.
In a previous study of the heat grill illusion, sensations of burning and stinging were sometimes reported when the skin was cooled by as little as 2 degrees C. Informal tests subsequently indicated that these nociceptive sensations were experienced if cooling occurred when the stimulating thermode rested on the skin, but not when the thermode was cooled and then touched to the skin. In experiment 1 subjects judged the intensity of thermal (cold/warm) and nociceptive (burning/stinging) sensations when the volar surface of the forearm was cooled to 25 degrees C (1) via a static thermode (Static condition), or (2) via a cold thermode touched to the skin (Dynamic condition). ⋯ Overall, the data show that mild cooling can produce nociceptive sensations that are suppressed under conditions of dynamic mechanical contact. The latter observation suggests that cold is perceived differently during active contact with objects than during passive heat loss to the environment. Hypotheses about the physiological basis of the nociceptive sensations at mild temperatures and their possible role in the phenomena of paradoxical heat and synthetic heat are discussed.