Tactile Coactivation-Induced Changes in Spatial Discrimination Performance

Abstract

We studied coactivation-based cortical plasticity at a psychophysical level in humans. For induction of plasticity, we used a protocol of simultaneous pairing of tactile stimulation to follow as closely as possible the idea of Hebbian learning. We reported previously that a few hours of tactile coactivation resulted in selective and reversible reorganization of receptive fields and cortical maps of the hindpaw representation of the somatosensory cortex of adult rats (Godde et al., 1996). In the present study, simultaneous spatial two-point discrimination was tested on the tip of the right index finger in human subjects as a marker of plastic changes. After 2 hr of coactivation we found a significant improvement in discrimination performance that was reversible within 8 hr. Reduction of the duration of the coactivation protocol revealed that 30 min was not sufficient to drive plastic changes. Repeated application of coactivation over 3 consecutive days resulted in a delayed recovery indicating stabilization of the improvement over time. Perceptual changes were highly selective because no transfer of improved performance to fingers that were not stimulated was found. The results demonstrate the potential role of sensory input statistics (i.e., their probability of occurrence and spatiotemporal relationships) in the induction of cortical plasticity without involving cognitive factors such as attention or reinforcement.

Improvement and Decline in Tactile Discrimination Behavior after Cortical Plasticity Induced by Passive Tactile Coactivation

“…The position of the solenoid on the fingertip was chosen to enclose the skin locations that were used for testing discrimination performance and for tactile stimulation in fMRI recordings. The timing of the coactivation protocol was the same as in our previous studies (Godde et al, 2000(Godde et al, , 2003. To prevent habituation during stimulation over several hours, stimuli were applied with interstimulus intervals between 8 and 1761 msec in random order, resulting in a mean stimulation frequency of 1.7 Hz.…”

“…Contrary to the view that learning requires top-down modulation by attention or reinforcement, these purely input-dependent changes also lead to perceptual improvement, the amount of which is predictable from the amount of reorganization (Godde et al, 2000(Godde et al, , 2003Pleger et al, 2001). However, the cortical mechanisms of this type of passive learning, as well as principle differences and similarities with active learning processes, are not well understood, and it might be argued that improvement in discrimination performance is based on unspecific and transient sensitization caused by stronger cortical activation.…”

Cortical topography of intracortical inhibition influences the speed of decision making

“…S1). Moreover, in recent experiments on changes in tactile acuity after coactivating fingers of the right, dominant hand, measurable changes of acuity of the left, nonstimulated hand had never been observed (22,23,(25)(26)(27)(28)(29), which argues for a substantial locality of coactivation-induced changes and supports the view that after repeated testing, RT effects are due to practice.…”

“…Coactivation is a form of repetitive stimulation, which follows the idea of Hebbian learning: Synchronous neural activity, necessary to drive plastic changes, is evoked by tactile "co"-activation of the skin. Several studies have shown that after a few hours of coactivation, tactile discrimination abilities were improved (22,23,25,(27)(28)(29). To provide further evidence for the Hebbian nature of coactivation, we used a modified version of the coactivation protocol consisting of a single, small stimulation site instead of one large area.…”

“…1) (21). RTs strongly differ among individual fingers, displaying an inverted U-shaped function in which the RTs for the middle fingers of each hand are significantly slower than those of other fingers.To address the role of altered topographic maps in the speed of perceptual decision making, we induced plastic changes to the cortical hand representation by applying a so-called tactile coactivation protocol (22)(23)(24). Coactivation is a form of repetitive sensory stimulation based on the induction of presumably Hebbian learning mechanisms (24-26).…”