Multiplexing in single cells of the alert monkeys visual cortex during brightness discrimination

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

Multiplexing in the primate motion pathway

This article begins by reviewing recent work on 3D motion processing in the primate visual system. Some of these results suggest that 3D motion signals may be processed in the same circuitry already known to compute 2D motion signals. Such "multiplexing" has implications for the study of visual cortical circuits and neural signals. A more explicit appreciation of multiplexing--and the computations required for demultiplexing--may enrich the study of the visual system by emphasizing the importance of a structured and balanced "encoding/decoding" framework. In addition to providing a fresh perspective on how successive stages of visual processing might be approached, multiplexing also raises caveats about the value of "neural correlates" for understanding neural computation.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

Principles of neural ensemble physiology underlying the operation of brain-machine interfaces

Research on brain-machine interfaces has been ongoing for at least a decade. During this period, simultaneous recordings of the extracellular electrical activity of hundreds of individual neurons have been used for direct, real-time control of various artificial devices. Brain-machine interfaces have also added greatly to our knowledge of the fundamental physiological principles governing the operation of large neural ensembles. Further understanding of these principles is likely to have a key role in the future development of neuroprosthetics for restoring mobility in severely paralysed patients.

Cortical involvement in visual scan in the monkey

Monkeys performed a visual search task for food reward. Green square targets were embedded in 3 x 3 arrays of colored forms. In distinct-feature arrays, all nontarget stimuli were red diamonds, whereas in shared-feature arrays, some nontarget stimuli shared either form (red square) or color (green diamond) with the target. Reaction time was slower for shared-feature arrays and linearly related to the number of shared-feature distractors. Errors were more common in shared-feature arrays, and shared-feature distractors were mistaken for targets more frequently than distinct-feature distractors. Event-related local field potentials were recorded from implanted transcortical electrodes. Significant task-related differences were obtained from association cortex, but not from projection cortex. Results are discussed in terms of the relative contribution of inferotemporal, dorsolateral frontal, and parietal cortex to feature-driven visual scan.

The physiology of attention: participation of cat striate cortex in behavioral choice

In the awake, behaving cat, we have compared response of striate cortex neurons to informative and to noninformative stimuli. A cat pressed a pedal in response to a flashed pattern repeated every 10 s, receiving a reward if the press was within a 0.5-1.5-s post-stimulus window. There were three trial types: 50% of the trials had only the initial informative flash, 25% had an additional physically identical, but unrewarded, flash 500 ms after the first, and 25% had an unrewarded flash 3-5 s after the informative flash. Cats learned to respond only to the rewarded flashes. Neurons were divided into two categories: 27 neurons defined as "primary" showed an early burst of firing 30-70 ms after stimulus onset, and 17 did not. The distinction was arbitrary, since all cells were exposed to the same stimulus. For stimuli preceding a pedal press, stimulus-synchronized histograms of primary neurons had a smaller early burst and more firing before and after it. Response-synchronized histograms showed an abrupt decrease in firing shortly before the pedal press. The effects were stronger for primary cells in the stimulus-synchronized data and stronger for non-primary cells in the response-synchronized data.

"Learned" changes in the responses of the rat barrel field neurons

The effect of pairing two vibrissa stimulations on unit responses of the barrel field of the somatosensory cortex were studied in partially restrained but awake and undrugged rats. Before pairing, one of the stimulations (S2) evoked a stable, short-latency and excitatory response from the recorded unit. Depending on the neuron, the other stimulation (S1), preceding S2 by 500 ms, did or did not have an effect before pairing. In a number of cases, the S1-S2 association produced significant changes in the unit responses: (1) the appearance of an excitatory response to S1 when that stimulus was ineffective before pairing; (2) the modification of pre-existing responses to S1 and/or S2. In all instances these modifications consisted in the decrease or disappearance of the "afferent inhibition" and/or the appearance of long-latency excitatory components. These effects appeared after some 30-100 trials and persisted in some cases up to 20 min after interruption of pairing. Our observations provide the first physiological data on the plasticity of the vibrissa projections in the chronic adult rodent. Though the underlying plastic neural elements and mechanisms remain to be specified, these phenomena suggest that "learned" changes in unit activity may occur in sensory systems and not only in "non-specific" ones.