Prefrontal neurons coding suppression of specific saccades

The prefrontal cortex has been implicated in the suppression of unwanted behavior, based upon observations of humans and monkeys with prefrontal lesions. Despite this, there has been little direct neurophysiological evidence for a mechanism that suppresses specific behavior. In this study, we used an oculomotor delayed match/nonmatch-to-sample task in which monkeys had to remember a stimulus location either as a marker of where to look or as a marker of where not to look. We found a group of neurons in both the frontal eye field and the caudal prefrontal cortex that carried signals selective for the forbidden stimulus. The activity of these "don't look" neurons correlated with the monkeys' success or failure on the task. These results demonstrate a frontal signal that is related to the active suppression of one action while the subject performs another.     

Source of the eye-movement signal reaching real-motion cells

The stability of visual perception despite eye movements suggests the existence in the visual system of neurons able to recognize whether the movement of a retinal image is due to the actual movement of an object or is self-induced by the ocular movement. We found neurons of this type in several areas of the monkey visual cortex and named them "real-motion" cells. Extracellular recordings were carried out from single neurons of the cortical prestriate area V3A of two awake macaque monkeys. Eighty-seven neurons were studied by comparing their responses during stimulus movement across the stationary receptive field, and receptive-field movement across the stationary stimulus. This visual stimulation was presented against a uniform visual background, in darkness or against a textured background. Neurons which were not real-motion in light (45/87) maintained their behaviour in darkness, while about 40% of real-motion cells lost this behaviour in darkness. Both real-motion and non real-motion cells maintained the same behaviour when tested against a uniform or textured visual background but often, texture increased the difference in the response that real-motion cells showed between stimulus and eye movement. These data suggest that the eye-movement signal which reaches real-motion cells and is responsible for their behaviour may be either retinal or extraretinal in nature. This double innervation is in good agreement with perceptual phenomena related to the detection of movement in the visual field.     

蜜蜂视叶和脑中的方向选择水平运动检测

在蜜蜂被刺激眼的同侧视叶内记录方向选择前进和后退水平运动灵敏的细胞反应。水平前进运动灵敏细胞对同侧前进运动的反应为很强的兴奋和去极化,以及去极化伴随有锋电位发放,同侧的后退运动引起抑制和超极化。在仅刺激对侧眼时,发放的频率不依赖于运动。水平后奶退运动灵敏的细胞对同侧水平后退运动反应出很强的兴奋和去极化,其去极化上伴随有锋电位发放,锋电位达不到零电位而且在其终点没有回射,同侧的前进运动几乎没有反应。     

Pyramidal unit activity in unanesthetized cats

Pyramidal unit activity in unanesthetized cats at rest and during voluntary movement was recorded by a microelectrode technique from the motor cortex for the forelimb. Some pyramidal neurons were not spontaneously active. The conduction velocity along the axon of these neurons was sometimes high (up to 71.5 m/sec), sometimes low (up to 11.2 m/sec). The remaining pyramidal neurons had spontaneous activity with a mean frequency of 1.29 to 43 spikes/sec. Analysis of interspike interval histograms of spontaneous activity and of autocorrelation histograms showed grouping of the spikes into volleys in most pyramidal neurons (irrespective of the conduction velocity). During voluntary movements the change in the activity of many pyramidal units correlated with changes in the EMG. The firing rate of the pyramidal neurons under these circumstances began to rise at least 50 msec before the increase in amplitude of the EMG and it remained high throughout the movement. The firing rate of most neurons during movement was 40–60/sec. The results are compared with those obtained by other workers who studied pyramidal unit activity of monkeys during voluntary movement.     

Pan politicus. Review of Chimpanzee Politics: Power and Sex Among rhe Apes,by Frans de Waal. New York,Harper & Row, 1982, 223 pp., $16.50

This report describes periodic oscillations in electroencephalographic (EEG) and behavioral activity with a cycle length of 15–30 seconds in chair-restrained squirrel monkeys (Saimiri sciureus). These oscillations consisted of alternating episodes of vigilance, characterized by visual scanning and motor movement, and inattentiveness, characterized by behavioral quiescence with little eye or limb movement. During vigilance the EEG exhibited low-amplitude, high-frequency (> 16 Hz) activity. During quiescent periods, a high-amplitude synchronized EEG was present with activity in the 8–16-Hz band predominating. The presence or frequency of this EEG and behavioral periodicity was not modified by time of day, as no difference was found between morning and afternoon recording sessions. Although the factors or mechanisms responsible for this rhythm are unclear, it should be noted by those investigators studying the behavior or neurophysiology of Saimiri sciureus in the laboratory setting.     

The control of gaze (3). Neurological defects

Eye movements serve vision, which has two different aims: changing images using saccades, i.e. rapid eye movements, and stabilizing new images on the retina using slow eye movements. Eye movements are performed by ocular motor nuclei in the brainstem, on which supranuclear pathways--originating in the cerebral cortex, cerebellum and vestibular structures--converge. It is useful for the neurologist to know the clinical abnormalities of eye movements visible at the bedside since such signs are helpful for localization. Eye movement paralysis may be nuclear or infranuclear (nerves), involving all types of eye movements, i.e. saccades as well as the vestibulo-ocular reflex (VOR), or supranuclear, in which case the VOR is usually preserved. Lateral eye movements are organized in the pons, with paralysis of adduction (and preservation of convergence) when the lesion affects the medial longitudinal fasciculus (internuclear ophthalmoplegia), paralysis of conjugate lateral eye movements when the lesion affects the abducens nucleus (VI) and the "one-and-a-half" syndrome when both these structures are involved. Vertical eye movements are organized in the midbrain, with ipsilateral oculomotor (III) paralysis and contralateral paralysis of the superior rectus muscle when the third nerve nucleus is unilaterally damaged, supranuclear upward gaze paralysis when the posterior commissure is unilaterally damaged and supranuclear downward gaze paralysis (often coupled with upward gaze paralysis) when the mesencephalic reticular formations are bilaterally damaged. Numerous types of abnormal eye movements exist, of which nystagmus is the most frequent and usually due to damage to peripheral or central vestibular pathways. Cerebral hemispheric or cerebellar damage results in subtle eye movement abnormalities at the bedside, in general only detected using eye movement recordings, because of the multiplicity of eye movement pathways at these levels and their reciprocal compensation in the case of a lesion. Lastly, eye movements can also help the neuroscientist to understand the organization of the brain. They are a good model of motricity allowing us, using eye movement recordings, to study the afferent pathways of the cortical areas that trigger them, and thus to analyze relatively complex neuropsychological processes such as visuo-spatial integration, spatial memory, motivation and the preparation of motor programs.     

Differences in orientation and receptive field position between supra- and infragranular cells of cat striate cortex and their possible functional implications

On the postlateral gyrus of the cat striate cortex the cells' preferred orientation and the location of their receptive fields was measured as a function of cortical depth in penetrations as parallel as possible to the radiating fibres. In most penetrations the majority of infragranular cells showed orientation preferences 45 degrees-90 degrees different from the preferred orientations of supragranular cells. In addition, aggregate receptive fields from the same eye of supra- and infragranular cells were spatially shifted against each other. Using different columnar models these results are discussed in terms of spatial contrast enhancement for two parallel mechanisms in upper and lower layers, determined for pattern discrimination and movement detection.     

Modeling the effect of sleep regulation on a neural mass model

In mammals, sleep is categorized by two main sleep stages, rapid eye movement (REM) and non-REM (NREM) sleep that are known to fulfill different functional roles, the most notable being the consolidation of memory. While REM sleep is characterized by brain activity similar to wakefulness, the EEG activity changes drastically with the emergence of K-complexes, sleep spindles and slow oscillations during NREM sleep. These changes are regulated by circadian and ultradian rhythms, which emerge from an intricate interplay between multiple neuronal populations in the brainstem, forebrain and hypothalamus and the resulting varying levels of neuromodulators. Recently, there has been progress in the understanding of those rhythms both from a physiological as well as theoretical perspective. However, how these neuromodulators affect the generation of the different EEG patterns and their temporal dynamics is poorly understood. Here, we build upon previous work on a neural mass model of the sleeping cortex and investigate the effect of those neuromodulators on the dynamics of the cortex and the corresponding transition between wakefulness and the different sleep stages. We show that our simplified model is sufficient to generate the essential features of human EEG over a full day. This approach builds a bridge between sleep regulatory networks and EEG generating neural mass models and provides a valuable tool for model validation.     

Time-of-day modulation of homeostatic and allostatic sleep responses to chronic sleep restriction in rats

To study sleep responses to chronic sleep restriction (CSR) and time-of-day influences on these responses, we developed a rat model of CSR that takes into account the polyphasic sleep patterns in rats. Adult male rats underwent cycles of 3 h of sleep deprivation (SD) and 1 h of sleep opportunity (SO) continuously for 4 days, beginning at the onset of the 12-h light phase ("3/1" protocol). Electroencephalogram (EEG) and electromyogram (EMG) recordings were made before, during, and after CSR. During CSR, total sleep time was reduced by ～60% from baseline levels. Both rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS) during SO periods increased initially relative to baseline and remained elevated for the rest of the CSR period. In contrast, NREMS EEG delta power (a measure of sleep intensity) increased initially, but then declined gradually, in parallel with increases in high-frequency power in the NREMS EEG. The amplitude of daily rhythms in NREMS and REMS amounts was maintained during SO periods, whereas that of NREMS delta power was reduced. Compensatory responses during the 2-day post-CSR recovery period were either modest or negative and gated by time of day. NREMS, REMS, and EEG delta power lost during CSR were not recovered by the end of the second recovery day. Thus the "3/1" CSR protocol triggered both homeostatic responses (increased sleep amounts and intensity during SOs) and allostatic responses (gradual decline in sleep intensity during SOs and muted or negative post-CSR sleep recovery), and both responses were modulated by time of day.     

The role of the posterior parietal cortex in coordinate transformations for visual-motor integration

Lesion to the posterior parietal cortex in monkeys and humans produces spatial deficits in movement and perception. In recording experiments from area 7a, a cortical subdivision in the posterior parietal cortex in monkeys, we have found neurons whose responses are a function of both the retinal location of visual stimuli and the position of the eyes in the orbits. By combining these signals area 7 a neurons code the location of visual stimuli with respect to the head. However, these cells respond over only limited ranges of eye positions (eye-position-dependent coding). To code location in craniotopic space at all eye positions (eye-position-independent coding) an additional step in neural processing is required that uses information distributed across populations of area 7a neurons. We describe here a neural network model, based on back-propagation learning, that both demonstrates how spatial location could be derived from the population response of area 7a neurons and accurately accounts for the observed response properties of these neurons.     

On the Vertigo Due to Static Magnetic Fields

Vertigo is sometimes experienced in and around MRI scanners. Mechanisms involving stimulation of the vestibular system by movement in magnetic fields or magnetic field spatial gradients have been proposed. However, it was recently shown that vestibular-dependent ocular nystagmus is evoked when stationary in homogenous static magnetic fields. The proposed mechanism involves Lorentz forces acting on endolymph to deflect semicircular canal (SCC) cupulae. To investigate whether vertigo arises from a similar mechanism we recorded qualitative and quantitative aspects of vertigo and 2D eye movements from supine healthy adults (n = 25) deprived of vision while pushed into the 7T static field of an MRI scanner. Exposures were variable and included up to 135s stationary at 7T. Nystagmus was mainly horizontal, persisted during long-exposures with partial decline, and reversed upon withdrawal. The dominant vertiginous perception with the head facing up was rotation in the horizontal plane (85% incidence) with a consistent direction across participants. With the head turned 90 degrees in yaw the perception did not transform into equivalent vertical plane rotation, indicating a context-dependency of the perception. During long exposures, illusory rotation lasted on average 50 s, including 42 s whilst stationary at 7T. Upon withdrawal, perception re-emerged and reversed, lasting on average 30 s. Onset fields for nystagmus and perception were significantly correlated (p<.05). Although perception did not persist as long as nystagmus, this is a known feature of continuous SSC stimulation. These observations, and others in the paper, are compatible with magnetic-field evoked-vertigo and nystagmus sharing a common mechanism. With this interpretation, response decay and reversal upon withdrawal from the field, are due to adaptation to continuous vestibular input. Although the study does not entirely exclude the possibility of mechanisms involving transient vestibular stimulation during movement in and out of the bore, we argue these are less likely.     

Posterior parietal cortex encodes autonomously selected motor plans

The posterior parietal cortex (PPC) of rhesus monkeys has been found to encode the behavioral meaning of categories of sensory stimuli. When animals are instructed with sensory cues to make either eye or hand movements to a target, PPC cells also show specificity depending on which effector (eye or hand) is instructed for the movement. To determine whether this selectivity retrospectively reflects the behavioral meaning of the cue or prospectively encodes the movement plan, we trained monkeys to autonomously choose to acquire a target in the absence of direct instructions specifying which effector to use. Activity in PPC showed strong specificity for effector choice, with cells in the lateral intraparietal area selective for saccades and cells in the parietal reach region selective for reaches. Such differential activity associated with effector choice under identical stimulus conditions provides definitive evidence that the PPC is prospectively involved in action selection and movement preparation.     

Depth sensitivity of binocular cortical neurons of behaving monkeys.

Activity of neurons if foveal striate and prestriate cortex of trained rhesus monkeys was recorded with metal microelectrodes. While animals fixated a small spot at a given fixation distance (38 or 57 cm), bright or dark bars moving across a frontoparallel plane were presented at different depths in a range of +/- 10 cm about the fixation distance. Almost all cells showed binocular interaction. Neurons with balanced ocularity (approximately equal monocular responses) usually facilitated each other and were tuned to depth around the plane of fixation often with inhibitory flanks nearer and further. Neurons with unbalanced ocularity either inhibited each other or had asymmetric depth sensitivity profiles, i.e. activation by stimuli in front and suppression by stimuli behind the fixation plane (near cells) or vice versa (far cells). Thus striate and prestriate cortex of the monkey contains four subsets of binocular cells which may contribute to depth perception.     

Human fast negative EEG potentials before express saccades

Fast negative EEG potentials preceding fast regular saccades and express saccades were studied by the method of backward averaging under conditions of monocular stimulation of the right and left eye. "Step" and "gap" experimental paradigms were used for visual stimulation. Analysis of parameters of potentials and their spatiotemporal dynamics suggests that, under conditions of the increased attention and optimal readiness of the neural structures, express saccades appear when the previously chosen program of the future eye movement coincides with the actual target coordinates. We assumed that the saccade latency decreases at the expense of the involvement of the main oculomotor areas of motor and saccadic planning in its initiation; an express saccade can be initiated also by means of direct transmission of the signal from the cortex to the brainstem saccadic generator passing by the superior colliculus. Moreover, anticipating release from the central fixation and attention distraction are necessary for the successful initiation of an express saccade.     

Predictability of EEG interictal spikes.

To determine whether EEG spikes are predictable, time series of EEG spike intervals were generated from subdural and depth electrode recordings from four patients. The intervals between EEG spikes were hand edited to ensure high accuracy and eliminate false positive and negative spikes. Spike rates (per minute) were generated from longer time series, but for these data hand editing was usually not feasible. Linear and nonlinear models were fit to both types of data. One patient had no linear or nonlinear predictability, two had predictability that could be well accounted for with a linear stochastic model, and one had a degree of nonlinear predictability for both interval and rate data that no linear model could adequately account for.     

Differences between monkey visual cortex cells in triplet and ghost doublet informational symbols relationships

On the basis of the recent discovery that precisely replicating triplets of impulses present in All-Interval histograms of spike trains generated by visual cortex cells of Rhesus monkeys are surrounded by multiple copies of ghost doublets of such triplets, we have examined and compared in detail, the spike trains generated by four complex cells in the striate cortex of curarized monkeys with respect to: (1) The number of precisely replicating triplet patterns embedded in trains of discharges generated in response to specific Hubel-Wiesel stimulation; (2) The effect of time separating the occurrence of such replicating triplets on the number and time distribution of their ghost doublets; (3) The effect of decreasing the precision criterion for the detection of replicating (parent) triplets (from the standard 0.14 ms criterion to 0.5 ms) on the relationships between triplets and their ghosts and (4) The comparison of the distributions in time of ghost doublets around the first and second copies of triplets when the time intervals separating them were greater than or less than 0.5 s. We found that the precision of replication of triplets varies somewhat from one cell to another, and that ghosts doublets are more copiously associated with replicating triplets emitted near in time to each other than with triplets emitted after larger time intervals, except in the case of one cell. In order to assess the statistical significance of our findings, we systematically shuffled the order of occurrence of intervals in every burst of all the records of one of the studied cells and repeated the analysis. Both the number of replicating triplets and of associated ghost doublets is significantly depressed (but not totally obliterated) by the above shuffling procedure. Finally, further implications based on a model of neural information transmission in the form of temporal correlations between spikes are discussed.     

Monitoring and control of action by the frontal lobes

Success requires deciding among alternatives, controlling the initiation of movements, and judging the consequences of actions. When alternatives are difficult to distinguish, habitual responses must be overcome, or consequences are uncertain, deliberation is necessary and a supervisory system exerts control over the processes that produce sensory-guided movements. We have investigated these processes by recording neural activity in the frontal lobe of macaque monkeys performing a countermanding task. Distinct neurons in the frontal eye field respond to visual stimuli or control the production of the movements. In the supplementary eye field and anterior cingulate cortex, neurons appear not to control directly movement initiation but instead signal the production of errors, the anticipation and delivery of reinforcement, and the presence of processing conflict. These signals form the core of current models of supervisory control of sensorimotor processes.     

Retinotopic organization of striate and extrastriate visual cortex in the golden hamster (Mesocricetus auratus).

The visual topography within striate and lateral extrastriate visual cortices was studied in adult hamsters. The cortical areas 17 and 18a in the left hemisphere were electrophysiologically mapped upon stimulation of the right eye, correlating receptive field positions in the visual field with cortical recording sites. Reference lesions were placed at selected cortical sites. Like in rats and other mammals, the lateral extrastriate cortex contained multiple representations of the visual field. Rostral area 18a contained the rostrolateral maps, with medial and lateral divisions. More caudally and sharing a common border with V1, maps in lateromedial, posterolateral and posterior areas were found. More laterally and forming a "third tier" of visual maps, anterolateral, laterolateral-anterior, laterolateral and laterolateral-posterior areas were found. There was also an indication of a possible pararhinal map. The plan so defined is virtually identical to that of rats. The results may be useful to understand a basic mammalian plan in the organization of the visual cortex.     

Sleep-Stage Correlates of Hippocampal Electroencephalogram in Primates

It has been demonstrated in the rodent hippocampus that rhythmic slow activity (theta) predominantly occurs during rapid eye movement (REM) sleep, while sharp waves and associated ripples occur mainly during non-REM sleep. However, evidence is lacking for correlates of sleep stages with electroencephalogram (EEG) in the hippocampus of monkeys. In the present study, we recorded hippocampal EEG from the dentate gyrus in monkeys overnight under conditions of polysomnographical monitoring. As result, the hippocampal EEG changed in a manner similar to that of the surface EEG: during wakefulness, the hippocampal EEG showed fast, desynchronized waves, which were partly replaced with slower waves of intermediate amplitudes during the shallow stages of non-REM sleep. During the deep stages of non-REM sleep, continuous, slower oscillations (0.5–8 Hz) with high amplitudes were predominant. During REM sleep, the hippocampal EEG again showed fast, desynchronized waves similar to those found during wakefulness. These results indicate that in the monkey, hippocampal rhythmic slow activity rarely occurs during REM sleep, which is in clear contrast to that of rodents. In addition, the increase in the slower oscillations of hippocampal EEG during non-REM sleep, which resembled that of the surface EEG, may at least partly reflect cortical inputs to the dentate gyrus during this behavioral state.     

A deficit in covert attention after parietal cortex inactivation in the monkey

Although the parietal cortex has been repeatedly implicated in controlling attention, the nature and importance of this contribution remain unclear. Here we show that inactivating the lateral intraparietal area in monkeys delays the detection of a visual target located in the contralateral visual field. This effect was observed using different visual scene configurations, e.g., with distractors that differ in number or that differ from the target by a conjunction of shape and color or by a single feature. Since eye movements were not allowed during the searching tasks, these results argue for an unambiguous role of the parietal cortex in the top-down control of attentional deployment in space.