(65/2544) Quantitative analysis of substantia nigra pars reticulata activity during a visually guided saccade task.
Several lines of evidence suggest that the pars reticulata subdivision of the substantia nigra (SNr) plays a role in the generation of saccadic eye movements. However, the responses of SNr neurons during saccades have not been examined with the same level of quantitative detail as the responses of neurons in other key saccadic areas. For this report, we examined the firing rates of 72 SNr neurons while awake-behaving primates correctly performed an average of 136 trials of a visually guided delayed saccade task. On each trial, the location of the visual target was chosen randomly from a grid spanning 40 degrees of horizontal and vertical visual angle. We measured the firing rates of each neuron during five intervals on every trial: a baseline interval, a fixation interval, a visual interval, a movement interval, and a reward interval. We found four distinct classes of SNr neurons. Two classes of neurons had firing rates that decreased during delayed saccade trials. The firing rates of discrete pausers decreased after the onset of a contralateral target and/or before the onset of a saccade that would align gaze with that target. The firing rates of universal pausers decreased after fixation on all trials and remained below baseline until the delivery of reinforcement. We also found two classes of SNr neurons with firing rates that increased during delayed saccade trials. The firing rates of bursters increased after the onset of a contralateral target and/or before the onset of a saccade aligning gaze with that target. The firing rates of pause-bursters increased after the onset of a contralateral target but decreased after the illumination of an ipsilateral target. Our quantification of the response profiles of SNr neurons yielded three novel findings. First, we found that some SNr neurons generate saccade-related increases in activity. Second, we found that, for nearly all SNr neurons, the relationship between firing rate and horizontal and vertical saccade amplitude could be well described by a planar surface within the range of movements we sampled. Finally we found that for most SNr neurons, saccade-related modulations in activity were highly variable on a trial-by-trial basis. (+info)
(66/2544) Neuronal correlates for preparatory set associated with pro-saccades and anti-saccades in the primate frontal eye field.
Diversity in behavioral responses to sensory stimuli has been attributed to variations in preparatory set. Variability in oculomotor responses toward identical visual stimuli has been well documented, but the neuronal processes underlying this variability are poorly understood. Here, we report evidence for set-related activity for saccadic eye movements in single neurons in the frontal eye field (FEF) in monkeys trained on a task in which they either had to look toward a visual stimulus (pro-saccade) or away from the stimulus (anti-saccade) depending on a previous instruction. A portion of FEF neurons were identified as neurons projecting directly to the superior colliculus (SC) with antidromic activation techniques. Saccade-related neurons in the FEF had lower prestimulus and stimulus-related activity on anti-saccade trials compared with pro-saccade trials. The level of prestimulus activity correlated with saccadic reaction times, express saccade occurrence, and errors in the anti-saccade task. In addition, saccade-related activity in the FEF was higher for pro-saccades than for anti-saccades. These results demonstrate that the direct descending pathway from the FEF to the SC carries preparatory set-related activity for pro-saccades and anti-saccades. The results also provide insights into the neuronal basis of variations in saccadic reaction times and in the control of the prepotent response to glance to a flashed stimulus. (+info)
(67/2544) Immediate neural plasticity shapes motor performance.
The consolidation of motor skills necessitates long-lasting changes in the nervous system. For the most part, plasticity has been documented in motor systems after training and long-term adaptation. However, there has been no demonstration of immediate neural changes associated with the rapid adaptation of motor behavior required to interact with a dynamic environment. To address this issue, we explored the changes in performance (reaction time) of rhesus monkeys that executed saccadic eye movements to one of two visual stimuli while monitoring the preparatory activity of neurons in the superior colliculus, a structure close to the motor output. Similar to the well established sequential effects observed in human manual responses, each monkey displayed reaction times to target locations that were organized in a sequential pattern, becoming progressively shorter with each preceding repeated movement and longer with each preceding nonrepeated movement. This sequential pattern of performance modification was associated with concordant changes in the preparatory activity of superior colliculus neurons in advance of the saccadic target presentation. These data indicate that neural properties are continuously shaped by use-related experience in a manner consistent with the progressive adaptation of motor behavior. (+info)
(68/2544) Activity of smooth pursuit-related neurons in the monkey periarcuate cortex during pursuit and passive whole-body rotation.
Smooth pursuit and vestibularly induced eye movements interact to maintain the accuracy of eye movements in space (i.e., gaze). To understand the role played by the frontal eye fields in pursuit-vestibular interactions, we examined activity of 110 neurons in the periarcuate areas of head-stabilized Japanese monkeys during pursuit eye movements and passive whole-body rotation. The majority (92%) responded with the peak of their modulation near peak stimulus velocity during suppression of the vestibuloocular reflex (VOR) when the monkeys tracked a target that moved with the same amplitude and phase and in the same plane as the chair. We classified pursuit-related neurons (n = 100) as gaze- velocity if their peak modulation occurred for eye (pursuit) and head (VOR suppression) movements in the same direction; the amplitude of modulation during one less than twice that of the other; and modulation was lower during target-stationary-in-space condition (VOR x1) than during VOR suppression. In addition, we examined responses during VOR enhancement (x2) in which the target moved with equal amplitude as, but opposite direction to, the chair. Gaze-velocity neurons responded maximally for opposite directions during VOR x2 and suppression. Based on these criteria, the majority of pursuit-related neurons (66%) were classified as gaze-velocity with preferred directions uniformly distributed. Because the majority of the remaining cells (32/34) also responded during VOR suppression, they were classified as eye/head-velocity neurons. Thirteen preferred pursuit and VOR suppression in the same direction; 13 in the opposite direction, and 6 showed biphasic modulation during VOR suppression. Eye- and gaze-velocity sensitivity of the two groups of cells were similar; mean (+/- SD) was 0.53 +/- 0.30 and 0.50 +/- 0.44 spikes/s per degrees /s, respectively. Gaze-velocity (but not eye/head-velocity) neurons showed significant correlation between eye- and gaze-velocity sensitivity, and both groups maintained their responses when the tracking target was extinguished briefly. The majority of pursuit-related neurons (28/43 = about 65%) responded to chair rotation in complete darkness. When the monkeys fixated a stationary target, more than half of cells tested (21/40) discharged in proportion to the velocity of retinal motion of a second laser spot (mean velocity sensitivity = 0.20 +/- 0.16 spikes/s per degrees /s). Preferred directions of individual cells to the second spot were similar to those during pursuit. Visual responses to the second spot movement were maintained even when it was extinguished briefly. These results indicate that both retinal image- and gaze-velocity signals are carried by single periarcuate pursuit-related neurons, suggesting that these signals can provide target-velocity-in-space and gaze-velocity commands during pursuit-vestibular interactions. (+info)
(69/2544) Disparity sensitivity of frontal eye field neurons.
Information about depth is necessary to generate saccades to visual stimuli located in three-dimensional space. To determine whether monkey frontal eye field (FEF) neurons play a role in the visuo-motor processes underlying this behavior, we studied their visual responses to stimuli at different disparities. Disparity sensitivity was tested from 3 degrees of crossed disparity (near) to 3 degrees degrees of uncrossed disparity (far). The responses of about two thirds of FEF visual and visuo-movement neurons were sensitive to disparity and showed a broad tuning in depth for near or far disparities. Early phasic and late tonic visual responses often displayed different disparity sensitivity. These findings provide evidence of depth-related signals in FEF and suggest a role for FEF in the control of disconjugate as well as conjugate eye movements. (+info)
(70/2544) Comparing extraocular motoneuron discharges during head-restrained saccades and head-unrestrained gaze shifts.
Burst neurons (BNs) in the paramedian pontine reticular formation provide the primary input to the extraocular motoneurons (MNs) during head-restrained saccades and combined eye-head gaze shifts. Prior studies have shown that BNs carry eye movement-related signals during saccades and carry head as well as eye movement-related signals during gaze shifts. Therefore MNs receive signals related to head motion during gaze shifts, yet they solely drive eye motion. Here we addressed whether the relationship between MN firing rates and eye movements is influenced by the additional premotor signals present during gaze shifts. Neurons in the abducens nucleus of monkeys were first studied during saccades made with the head stationary. We then recorded from the same neurons during voluntary combined eye-head gaze shifts. We conclude that the activity of MNs, in contrast to that of BNs, is related to eye motion by the same dynamic relationship during head-restrained saccades and head-unrestrained gaze shifts. In addition, we show that a standard metric-based analysis [i.e., counting the number of spikes (NOS) in a burst] yields misleading results when applied to the same data set. We argue that this latter approach fails because it does not properly consider the system's dynamics or the strong interactions between eye and head motion. (+info)
(71/2544) The effects of skill on the eye-hand span during musical sight-reading.
The eye-hand span (EHS) is the separation between eye position and hand position when sight-reading music. It can be measured in two ways: in notes (the number of notes between hand and eye; the 'note index'), or in time (the length of time between fixation and performance; the 'time index'). The EHSs of amateur and professional pianists were compared while they sight-read music. The professionals showed significantly larger note indexes than the amateurs (approximately four notes, compared to two notes), and all subjects showed similar variability in the note index. Surprisingly, the different groups of pianists showed almost identical mean time indexes (ca. 1 s), with no significant differences between any of the skill levels. However, professionals did show significantly less variation than the amateurs. The time index was significantly affected by the performance tempo: when fast tempos were imposed on performance, all subjects showed a reduction in the time index (to ca. 0.7 s), and slow tempos increased the time index (to ca. 1.3 s). This means that the length of time that information is stored in the buffer is related to performance tempo rather than ability, but that professionals can fit more information into their buffers. (+info)
(72/2544) Auditory saccade impairment after central thalamus lesions.
Visual and auditory saccades were studied in three patients with an isolated lesion located in the central thalamus. Visual saccades proved to be normal, whereas for auditory stimuli, the amplitude of the first saccade was asymmetric: saccades ipsilateral to the lesion were significantly smaller than those directed to the contralateral side. The patients were able to make a corrective saccade and hence to improve gain and to decrease gain asymmetry. It is suggested that patients were able to localise auditory targets correctly, but did not correctly take into account eye position during the saccade, probably as a consequence of an inaccurate efference copy (corollary discharge) signal. The findings are in keeping with the hypothesis that the central thalamus deals with saccades that are based on extraretinal signals. (+info)