Publications (13)83.13 Total impact
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Article: Time-invariant reference frames for parietal reach activity.
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ABSTRACT: Neurophysiological studies suggest that the transformation of visual signals into arm movement commands does not involve a sequential recruitment of the various reach-related regions of the cerebral cortex but a largely simultaneous activation of these areas, which form a distributed and recurrent visuomotor network. However, little is known about how the reference frames used to encode reach-related variables in a given "node" of this network vary with the time taken to generate a behavioral response. Here we show that in an instructed delay reaching task, the reference frames used to encode target location in the parietal reach region (PRR) and area 5 of the posterior parietal cortex (PPC) do not evolve dynamically in time; rather the same spatial representation exists within each area from the time target-related information is first instantiated in the network until the moment of movement execution. As previously reported, target location was encoded predominantly in eye coordinates in PRR and in both eye and hand coordinates in area 5. Thus, the different computational stages of the visuomotor transformation for reaching appear to coexist simultaneously in the parietal cortex, which may facilitate the rapid adjustment of trajectories that are a hallmark of skilled reaching behavior.Experimental Brain Research 07/2008; 188(1):77-89. · 2.39 Impact Factor -
Article: Cortical neural prosthesis performance improves when eye position is monitored.
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ABSTRACT: Neural prostheses that extract signals directly from cortical neurons have recently become feasible as assistive technologies for tetraplegic individuals. Significant effort toward improving the performance of these systems is now warranted. A simple technique that can improve prosthesis performance is to account for the direction of gaze in the operation of the prosthesis. This proposal stems from recent discoveries that the direction of gaze influences neural activity in several areas that are commonly targeted for electrode implantation in neural prosthetics. Here, we first demonstrate that neural prosthesis performance does improve when eye position is taken into account. We then show that eye position can be estimated directly from neural activity, and thus performance gains can be realized even without a device that tracks eye position.IEEE Transactions on Neural Systems and Rehabilitation Engineering 03/2008; 16(1):24-31. · 3.44 Impact Factor -
Article: Single-neuron stability during repeated reaching in macaque premotor cortex.
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ABSTRACT: Some movements that animals and humans make are highly stereotyped, repeated with little variation. The patterns of neural activity associated with repeats of a movement may be highly similar, or the same movement may arise from different patterns of neural activity, if the brain exploits redundancies in the neural projections to muscles. We examined the stability of the relationship between neural activity and behavior. We asked whether the variability in neural activity that we observed during repeated reaching was consistent with a noisy but stable relationship, or with a changing relationship, between neural activity and behavior. Monkeys performed highly similar reaches under tight behavioral control, while many neurons in the dorsal aspect of premotor cortex and the primary motor cortex were simultaneously monitored for several hours. Neural activity was predominantly stable over time in all measured properties: firing rate, directional tuning, and contribution to a decoding model that predicted kinematics from neural activity. The small changes in neural activity that we did observe could be accounted for primarily by subtle changes in behavior. We conclude that the relationship between neural activity and practiced behavior is reasonably stable, at least on timescales of minutes up to 48 h. This finding has significant implications for the design of neural prosthetic systems because it suggests that device recalibration need not be overly frequent, It also has implications for studies of neural plasticity because a stable baseline permits identification of nonstationary shifts.Journal of Neuroscience 11/2007; 27(40):10742-50. · 7.11 Impact Factor -
Article: Reference frames for reach planning in macaque dorsal premotor cortex.
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ABSTRACT: When a human or animal reaches out to grasp an object, the brain rapidly computes a pattern of muscular contractions that can acquire the target. This computation involves a reference frame transformation because the target's position is initially available only in a visual reference frame, yet the required control signal is a set of commands to the musculature. One of the core brain areas involved in visually guided reaching is the dorsal aspect of the premotor cortex (PMd). Using chronically implanted electrode arrays in two Rhesus monkeys, we studied the contributions of PMd to the reference frame transformation for reaching. PMd neurons are influenced by the locations of reach targets relative to both the arm and the eyes. Some neurons encode reach goals using limb-centered reference frames, whereas others employ eye-centered reference fames. Some cells encode reach goals in a reference frame best described by the combined position of the eyes and hand. In addition to neurons like these where a reference frame could be identified, PMd also contains cells that are influenced by both the eye- and limb-centered locations of reach goals but for which a distinct reference frame could not be determined. We propose two interpretations for these neurons. First, they may encode reach goals using a reference frame we did not investigate, such as intrinsic reference frames. Second, they may not be adequately characterized by any reference frame.Journal of Neurophysiology 09/2007; 98(2):966-83. · 3.32 Impact Factor -
Article: Movement intention is better predicted than attention in the posterior parietal cortex.
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ABSTRACT: We decoded on a trial-by-trial basis the location of visual targets, as a marker of the locus of attention, and intentions to reach and to saccade in different directions using the activity of neurons in the posterior parietal cortex of two monkeys. Predictions of target locations were significantly worse than predictions of movement plans for the same target locations. Moreover, neural signals in the parietal reach region (PRR) gave better predictions of reaches than saccades, whereas signals in the lateral intraparietal area (LIP) gave better predictions of saccades than reaches. Taking together the activity of both areas, the prediction of either movement in all directions became nearly perfect. These results cannot be explained in terms of an attention effect and support the idea of two segregated populations in the posterior parietal cortex, PRR and LIP, that are involved in different movement plans.Journal of Neuroscience 04/2006; 26(13):3615-20. · 7.11 Impact Factor -
Article: Increasing the performance of cortically-controlled prostheses.
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ABSTRACT: Neural prostheses have received considerable attention due to their potential to dramatically improve the quality of life of severely disabled patients. Cortically-controlled prostheses are able to translate neural activity from cerebral cortex into control signals for guiding computer cursors or prosthetic limbs. Non-invasive and invasive electrode techniques can be used to measure neural activity, with the latter promising considerably higher levels of performance and therefore functionality to patients. We review here some of our recent experimental and computational work aimed at establishing a principled design methodology to increase electrode-based cortical prosthesis performance to near theoretical limits. Studies discussed include translating unprecedentedly brief periods of "plan" activity into high information rate (6.5 bits/s)control signals, improving decode algorithms and optimizing visual target locations for further performance increases, and recording from chronically implanted arrays in freely behaving monkeys to characterize neuron stability. Taken together, these results should substantially increase the clinical viability of cortical prostheses.Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 02/2006; Suppl:6652-6. -
Article: Direction-selective visual responses in macaque superior colliculus induced by behavioral training.
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ABSTRACT: In a previous report, we described a heretofore undetected population of neurons in the intermediate and deep layers of the monkey superior colliculus (SC) that yielded directionally selective visual responses to stimuli presented within the central 4 degrees of the visual field. We observed these neurons in three monkeys that had been extensively trained to perform a visual direction discrimination task in this region of the visual field. The task required the monkeys to report the perceived direction of motion by making a saccadic eye movement to one of two targets aligned with the two possible directions of motion. We hypothesized that these neurons reflect a learned association between visual motion direction and saccade direction formed through extensive training on the direction discrimination task. We tested this hypothesis by searching for direction-selective visual responses in two monkeys that had been trained to perform a similar motion discrimination task in which the direction of stimulus motion was dissociated from the direction of the operant saccade. Strongly directional visual responses were absent in these monkeys, consistent with the notion that extensive training can induce highly specific visual responses in a subpopulation of SC neurons.Neuroscience Letters 09/2004; 366(3):315-9. · 2.11 Impact Factor -
Article: Representation of an abstract perceptual decision in macaque superior colliculus.
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ABSTRACT: We recorded from neurons in the intermediate and deep layers of the superior colliculus (SC) while monkeys performed a novel direction discrimination task. In contrast to the task we used previously, the new version required the monkey to dissociate perceptual judgments from preparation to execute specific operant saccades. The monkey discriminated between 2 opposed directions of motion in a random-dot motion stimulus and was required to maintain the decision in memory throughout a delay period before the target of the required operant saccade was revealed. We hypothesized that perceptual decisions made in this paradigm would be represented in an "abstract" or "categorical" form within the brain, probably in the frontal cortex, and that decision-related neural activity would be eliminated from spatially organized preoculomotor structures such as the SC. To our surprise, however, a small population of neurons in the intermediate and deep layers of the SC fired in a choice-specific manner early in the trial well before the monkey could plan the operant saccade. Furthermore, the representation of the decision during the delay period appeared to be spatial: the active region in the SC map corresponded to the region of space toward which the perceptually discriminated stimulus motion flowed. Electrical microstimulation experiments suggested that these decision-related SC signals were not merely related to covert saccade planning. We conclude that monkeys may employ, in part, a spatially referenced mnemonic strategy for representing perceptual decisions, even when an abstract, categorical representation might appear more likely a priori.Journal of Neurophysiology 06/2004; 91(5):2281-96. · 3.32 Impact Factor -
Article: Neural prosthetic control signals from plan activity.
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ABSTRACT: The prospect of assisting disabled patients by translating neural activity from the brain into control signals for prosthetic devices, has flourished in recent years. Current systems rely on neural activity present during natural arm movements. We propose here that neural activity present before or even without natural arm movements can provide an important, and potentially advantageous, source of control signals. To demonstrate how control signals can be derived from such plan activity we performed a computational study with neural activity previously recorded from the posterior parietal cortex of rhesus monkeys planning arm movements. We employed maximum likelihood decoders to estimate movement direction and to drive finite state machines governing when to move. Performance exceeded 90% with as few as 40 neurons.Neuroreport 04/2003; 14(4):591-6. · 1.66 Impact Factor -
Article: A computational basis to object?
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ABSTRACT: To use an object, we must be able to perceive the spatial relationship between the object's parts. The accepted view of how the brain coherently encodes an object is that some neurons in the frontal cortex employ an object-centered coordinate frame. A new computational model challenges this view, using the rich conceptual framework of neural basis functions.Neuron 02/2003; 37(2):189-90. · 14.74 Impact Factor -
Article: Comparison of neural activity preceding reaches to auditory and visual stimuli in the parietal reach region.
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ABSTRACT: We examined the responses of neurons in the parietal reach region (PRR) during reaches to the remembered locations of auditory or visual stimuli. We found that the firing rate of PRR neurons contained information about the location of auditory and visual stimuli. For neurons tested with visual stimuli, the amount of information remained constant throughout the task. In contrast, for neurons tested with auditory stimuli, the amount of target-location information increased as the trial evolved. During the reach period of the task, the amount of information that was carried by neurons tested with auditory stimuli was not statistically different from the amount carried by neurons tested with visual stimuli. We interpret these data to suggest that the type of information that PRR neurons encode evolves throughout a task.Neuroreport 06/2002; 13(6):891-4. · 1.66 Impact Factor -
Article: Direct visuomotor transformations for reaching.
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ABSTRACT: The posterior parietal cortex (PPC) is thought to have a function in the sensorimotor transformations that underlie visually guided reaching, as damage to the PPC can result in difficulty reaching to visual targets in the absence of specific visual or motor deficits. This function is supported by findings that PPC neurons in monkeys are modulated by the direction of hand movement, as well as by visual, eye position and limb position signals. The PPC could transform visual target locations from retinal coordinates to hand-centred coordinates by combining sensory signals in a serial manner to yield a body-centred representation of target location, and then subtracting the body-centred location of the hand. We report here that in dorsal area 5 of the PPC, remembered target locations are coded with respect to both the eye and hand. This suggests that the PPC transforms target locations directly between these two reference frames. Data obtained in the adjacent parietal reach region (PRR) indicate that this transformation may be achieved by vectorially subtracting hand location from target location, with both locations represented in eye-centred coordinates.Nature 05/2002; 416(6881):632-6. · 36.28 Impact Factor -
Article: Visuo-motor control: Giving the brain a hand
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ABSTRACT: Sensory information is acquired in spatial coordinate systems linked to sense organs, yet movement must be executed in coordinate systems linked to motor effector organs. Neurophysiological experiments are yielding new insights into how the brain transforms coordinate systems to facilitate movement.Current Biology.
Top co-authors
Top Journals
- Journal of Neurophysiology (2)
- Neuroreport (2)
- Journal of Neuroscience (2)
- Nature (1)
- Neuroscience Letters (1)
Institutions
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2007–2008
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Stanford University
- Department of Electrical Engineering
Stanford, CA, USA
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2002–2008
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California Institute of Technology
- Division of Biology
Pasadena, CA, USA
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2003–2004
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Howard Hughes Medical Institute
Chevy Chase, MD, USA
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