Symposium 8: Primitives for the Control of Movement in Space

sensory qualities of the stimulus and not by its behavioral significance. We will present data from direction-selective cells in awake behaving macaque monkeys demonstrating influences of attention on the sensory properties of these cells, such as their tuning characteristics and their sensitivity and selectivity. These attentional effects are well matched to the effects of attention in psychophysieal studies we conducted on human subjects and the two can be compared to demonstrate the behavioral significance of the changes in neuronal responses. As one moves up in the processing hierarchy of visual cortex the attentional influences observed physiologically become stronger and there is an increasing shift from a sensory representation of the visual environment to a representation that reflects the current state of behavioral significance of various aspects and elements of the sensory input. These attentional modulations seem to use similar mechanisms (changes of response gain), as the ones employed in sensory aspects of processing, most likely reflecting a tight link between sensory and attentional mechanisms. Furthermore, the magnitude of the attentional modulation seems to be determined by the similarity between the currently attended stimulus features and the features of the stimulus that the cell is responding to, leading us to propose a 'feature-similarity gain model' of attention. Although our investigations have centered on visual motion processing, the effects seem general enough to be applicable to other visual as well as non-visual domains. The issue of translating the planning of arm movements into muscle forces is discussed in relation to the recent discovery of modular structures in the spinal cord. Experiments conducted in decerebrated frogs, revealed that these structures contain circuitry that, when activated, produce precisely balanced contractions in groups of muscles. These synergistic contractions generate forces that direct the limb toward an equilibrium point in space. The force outputs, produced by activating different spinal-cord structures, sum vectorially. The basic elements with which the vertebrate spinal cord constructs one complex behavior were identified by means of a computational analysis. This analysis extracted a small set of muscle synergies from the range of muscle activations generated by cutaneous stimulation of the frog hindlimb. The flexible combination of these synergies was ableto account for the large number of different motor patterns produced by different animals.