Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Recent investigations have revealed the kinematics of horizontal saccades are less variable near the end of the trajectory than during the course of execution. Converging evidence indicates that oculomotor networks use online sensorimotor feedback to correct for initial trajectory errors. It is also known that oculomotor networks express saccadic corrections with decreased efficiency when responses are made toward memorized locations. ⋯ In contrast, untimed memory-guided saccades exhibited diminished kinematic evidence for online corrections. We conclude that motor timekeeping and sequencing contributed to efficient saccadic corrections. These results contribute to an evolving view of the interactions between motor planning and spatial working memory, as they relate to oculomotor control.
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Comparative Study
Discovering affordances that determine the spatial structure of reach-to-grasp movements.
Extensive research has identified the affordances used to guide actions, as originally conceived by Gibson (Perceiving, acting, and knowing: towards an ecological psychology. Erlbaum, Hillsdale, 1977; The ecological approach to visual perception. Erlbaum, Hillsdale, 1979/1986). ⋯ The affordance structure of this task was found to eliminate the collision risk and thus reduced safety margins in MGA and TGA to zero for larger objects. The results emphasize the role of affordances in determining the structure and scaling of reach-to-grasp actions. Finally, we report evidence supporting the opposition vector as an appropriate unit of analysis in the study of grasping and a unit of action that maps directly to affordance properties.
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The mode of synaptic transmission in the vestibular periphery, between type I hair cells and their associated calyx terminal, has been the subject of much debate. The close and extensive apposition of pre- and post-synaptic elements has led some to suggest potassium (K(+)) accumulates in the intercellular space and even plays a role in synaptic transmission. During patch clamp recordings from isolated and embedded hair cells in a semi-intact preparation of the mouse cristae, we noted marked differences in whole-cell currents. ⋯ As predicted, the K(+) accumulation effects were reduced; however, a fit of our data with a classic diffusion model suggested K(+) permeability, rather than the intercellular space, had been altered by the hyperosmotic change. These results support the notion that under depolarizing conditions substantial K(+) accumulation occurs in the space between type I hair cells and calyx. The extent of K(+) accumulation during normal synaptic transmission, however, remains to be determined.
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By sensing three-dimensional (3D) head rotation and electrically stimulating the three ampullary branches of a vestibular nerve to encode head angular velocity, a multichannel vestibular prosthesis (MVP) can restore vestibular sensation to individuals disabled by loss of vestibular hair cell function. However, current spread to afferent fibers innervating non-targeted canals and otolith end organs can distort the vestibular nerve activation pattern, causing misalignment between the perceived and actual axis of head rotation. We hypothesized that over time, central neural mechanisms can adapt to correct this misalignment. ⋯ In addition, the extent of disconjugacy between the two eyes progressively improved during the same time window. These results indicate that the central nervous system rapidly adapts to multichannel prosthetic vestibular stimulation to markedly improve 3D aVOR alignment within the first week after activation. Similar adaptive improvements are likely to occur in other species, including humans.
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Hemispheric asymmetry reduction in older adults (HAROLD) has been reported in previous imaging studies that employed not only cognitive, but also motor tasks. However, whether age-related reductions in asymmetry of hemispheric activations affect the symmetry of motor behavior in older adults remains largely untested. We now examine the effect of aging on lateralization of motor adaptation and transfer by investigating adaptation to novel visuomotor transformations in both old and young age groups. ⋯ Naïve performance with one arm and the same-arm performance following opposite arm adaptation were compared to determine the extent of transfer in each age group. Our results showed that interlimb transfer of initial direction information only occurred from the nondominant to dominant arm in young adults, whereas it occurred in both directions in older adults. Our findings clearly indicate substantially reduced asymmetry in visuomotor adaptation in older adults, and suggest that this reduced motor asymmetry might be related to diminished hemispheric lateralization for motor control.