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J W Gnadt

Publications and source records attributed to J W Gnadt.

11 recordsLinked to original sources
PubMed1991

Sensorimotor transformation during eye movements to remembered visual targets.

For eye movements made to visual targets, the brain must transform the retinotopic coordinate frame of the visual system to that of the oculomotor plant. Ideally, responses should exactly match target demands. However, during eye movements to remembered targets, responses are spatially distorted. The transformation does not retain accurate retinotopic registration, having both constant and variable components of error. Generally, the constant pattern of distortion appears as a hypermetria for upward saccades and a hypometria for downward movements. Most of the error accumulates during the first 800 msec of memory-contingent delay. The results are interpreted with respect to theories of how spatial information may be coded and transformed.

Animals↗
PubMed1991

Saccade-related activity in the lateral intraparietal area. I. Temporal properties; comparison with area 7a.

1. The cortex of the inferior parietal lobule (IPL) contains neurons whose activity is related to saccadic eye movements. The exact role of the IPL in relation to saccades remains, however, unclear. In this and the companion paper, we approach this problem by quantifying many of the spatial and temporal parameters of the saccade-related (S) activity. These parameters have hitherto been largely unstudied. 2. The activity of single neurons was recorded from Macaca mulatta monkeys while they were performing a delayed-saccade task. The analysis presented here is based on 161 neurons recorded from the lateral intraparietal area (LIP), a recently defined subdivision of the IPL; and 54 neurons recorded from the neighboring part of the IPL, area 7a. Overall, 409 IPL neurons were isolated in this study. 3. The typical activity of IPL neurons during the delayed-saccade task has three basic phases: light sensitive (LS), memory (M), and S. These basic phases are common to neurons of both areas LIP and 7a. In each phase (LS, M, and S), individual neurons may or may not be active. Most LIP neurons, however, are active in more than one phase. 4. To compare the activity levels of different neurons, the actual firing rate was weighted by each neuron's background level, yielding an "activity index" for each neuron, in each phase of the task. We calculated the activity index for the LS and M phases and for three phases related to the saccade: a presaccadic (Pre-S), a saccade-coincident (S-Co), and a postsaccadic (Post-S) phase. For area LIP neurons the median values of the activity index were high for the LS, M, Pre-S, and S-Co activities, and slightly lower in the Post-S period. In area 7a the median values were low for the LS phase and, in particular, for the M and Pre-S phases, somewhat higher coincident with the saccade, and high post-saccadically. 5. In area LIP, in each phase, 49-63% of the neurons had excitatory activity, and 10-17% had inhibitory responses. 6. In contrast, in area 7a excitatory responses were most frequent in the Post-S phase (56%). Excitation was particularly infrequent during M (28%) and Pre-S (22%). The incidence of inhibitory responses varied too (4-18%). The time course of inhibition was roughly opposite that of excitation; the highest frequency of inhibitory responses occurred during the saccade.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Saccade-related activity in the lateral intraparietal area. II. Spatial properties.

1. Single-neuron activity was recorded from the inferior parietal lobule (IPL) of Macaca mulatta monkeys while they were performing delayed saccades and related tasks. Temporal characteristics of this activity were presented in the companion paper. Here we focus on the spatial characteristics of the activity. The analysis was based on recordings from 145 neurons. All these neurons were from the lateral intraparietal area (LIP), a recently defined subdivision of the IPL. 2. Delayed saccades were made in eight directions. Direction-tuning curves were calculated for each neuron, during each of the following activity phases that were described in the companion paper: light sensitive (LS), delay-period memory (M), and saccade related (S); the latter further partitioned into presaccadic (Pre-S), saccade coincident (S-Co), and postsaccadic (Post-S). 3. Width and preferred direction were calculated for each direction-tuning curve. We studied the distributions of widths and preferred directions in LIP's neuronal population. In each case we included only neurons that showed clear excitatory activity in the phases in question. 4. Width was defined as the angle over which the response was higher than 50% of its maximal net value. Width distributions were similar for all phases studied. Widths varied widely from neuron to neuron, from very narrow (less than 45 degrees) to very wide (close to 360 degrees). Median widths were approximately 90 degrees in all phases. 5. Preferred-direction distributions were also similar for various phases. All directions were represented in each distribution, but contralateral directions were more frequent (e.g., 69% for S-Co). 6. For each neuron the alignment of the preferred directions of its various phases was determined. Distributions of alignments were calculated (again, phases that were not clearly excitatory were disregarded). On the level of the neuronal population LS, M, and Pre-S were well aligned with each other. S-Co was also aligned with these phases, but less precisely. 7. A set of "narrowly tuned" neurons was selected by imposing a constraint of narrow (width, less than 90 degrees) LS and S-Co direction tuning. In this set of neurons, the LS and S-Co preferred directions were very well aligned (median, 12 degrees). The fraction of narrowly tuned neurons in the population was 40% (25/63). Thus, in a large subpopulation of area LIP, a fairly precise alignment exists between sensory and motor fields. 8. An additional set of 82 area LIP neurons were recorded while the monkey performed delayed saccades to 32 targets located on small, medium, and large imaginary circles.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Eye position effects on visual, memory, and saccade-related activity in areas LIP and 7a of macaque.

We studied the effect of eye position on the light-sensitive, memory, and saccade-related activities of neurons of the lateral intraparietal area and area 7a in the posterior parietal cortex of rhesus monkeys. A majority of the cells showed significant effects of eye position, for each of the 3 types of response. The direction tuning of the light-sensitive, memory and saccade responses did not change with eye position but the magnitude of the response did. Since previous work showed a similar effect for the light-sensitive response of area 7a neurons (Andersen and Mountcastle, 1983; Andersen et al., 1985b), the present results indicate that this modulating effect of eye position may be a general one, as it is found in 3 types of responses in 2 cortical areas. Gain fields were mapped by measuring the effect of eye position on the magnitude of the response at 9 different eye positions for each neuron. The gain fields were usually planar or largely planar for all 3 types of response in both areas, indicating that the magnitude of the response usually varies linearly with both horizontal and vertical eye position. A similar observation was made previously for the gain fields of the light-sensitive response of area 7a neurons (Andersen et al., 1985b). Although gain fields sloped in all directions for the population of cells, the gain field slopes of the light-sensitive, memory and saccade responses for individual cells were usually similar. It is proposed that these eye position effects play an important role in making coordinate transformations for visually guided movement.

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PubMed1989

Abducens internuclear neurons carry an inappropriate signal for ocular convergence.

1. Single-unit recording studies in alert Rhesus monkeys characterized the vergence signal carried by abducens internuclear neurons. These cells were identified by antidromic activation and the collision of spontaneous with antidromic action potentials. The behavior of abducens internuclear neurons during vergence was compared with that of horizontal burst-tonic fibers in the medial longitudinal fasciculus (MLF) and to that of a large sample of unidentified abducens cells (presumably both motoneurons and internuclear neurons). 2. The results indicate that abducens internuclear neurons and lateral rectus motoneurons behave similarly during vergence eye movements: the majority of both groups of cells decrease their firing rate for convergence eye movements: a minority show no change for vergence. This finding is strongly supported by recordings of horizontal burst-tonic fibers in the MLF, the majority of which decrease their activity significantly for convergence eye movements. 3. These findings indicate that a net inappropriate vergence signal is sent to medial rectus motoneurons via the abducens internuclear pathway. Because medial rectus motoneurons increase their activity appropriately during symmetrical convergence, this inappropriate MLF signal must be overcome by a more potent direct vergence input. 4. Overall, both abducens internuclear neurons and lateral rectus motoneurons decrease their activity for convergence less than would be expected based on their conjugate gain. This implies that some degree of co-contraction of the lateral and medial rectus muscles occurs during convergence eye movements. 5. Some horizontal burst-tonic MLF fibers decrease their activity more for convergence than any recorded abducens neuron. These fibers may arise from cells in the nucleus prepositus hypoglossi or vestibular nuclei.

Abducens Nerve↗
PubMed1989

Lidocaine-induced unilateral internuclear ophthalmoplegia: effects on convergence and conjugate eye movements.

1. To characterize the vergence signal carried by the medial longitudinal fasciculus (MLF), it was subjected to reversible blockade by small injections of 10% lidocaine hydrochloride. The effects of these blockades on both conjugate and vergence eye movements were studied. 2. With this procedure, experimentally induced internuclear ophthalmoplegia (INO) and its effects on conjugate eye movements could be studied acutely, without possible contamination from long-term oculomotor adaptation. In the eye contralateral to the MLF blockade, saccadic and horizontal smooth-pursuit eye movements were normal. Horizontal abducting nystagmus, often seen in patients with INO, was not observed in this eye. 3. As previously reported for INO, profound oculomotor deficits were seen in the eye ipsilateral to the MLF blockade. During maximal blockade, adducting saccades and horizontal smooth-pursuit movements in this eye did not cross the midline. Adducting saccades were reduced in amplitude and peak velocity and showed significantly increased durations. Abducting saccades, which were slightly hypometric, displayed a marked postsaccadic centripetal drift. 4. The eye ipsilateral to the blockade displayed a pronounced, upward, slow drift, whereas the eye contralateral to the blockade showed virtually no drift. Furthermore, although vertical saccades to visual targets remained essentially conjugate, the size of the resetting quick phases in each eye was related to the amplitude of the slow phase movement in that eye. Thus the eye on the affected side displayed large quick phases, whereas the eye on the unaffected side showed only slight movements. On occasion, unilateral downbeating nystagmus was seen. This strongly suggests that the vertical saccade generators for the two eyes can act independently. 5. The effect of MLF blockade on the vergence gain of the eye on the affected side was investigated. As a measure of open-loop vergence gain, the relationship of accommodative convergence to accommodation (AC/A) was measured before, during, and after reversible lidocaine block of the MLF. After taking conjugate deficits into account, the net vergence signal to the eye ipsilateral to the injection was found to increase significantly during the reversible blockade. 6. The most parsimonious explanation for this increased vergence signal is suggested by the accompanying single-unit study. This study showed that abducens internuclear neurons, whose axons course in the MLF, provide medial rectus motoneurons with an appropriate horizontal conjugate eye position signal but an inappropriate vergence signal. Ordinarily, this incorrect vergence signal is overcome by another, more potent, v

Animals↗
PubMed1988

Memory related motor planning activity in posterior parietal cortex of macaque.

Unit recording studies in the lateral bank of the intraparietal cortex (area LIP) have demonstrated a response property not previously reported in posterior cortex. Studies were performed in the Rhesus monkey during tasks which required saccadic eye movements to remembered target locations in the dark. Neurons were found which remained active during the time period for which the monkey had to withhold eye movements while remembering desired target locations. The activity of the cells was tuned for eye movements of specific direction and amplitude, and it was not necessary for a visual stimulus to fall within the response field. The responses appeared to represent a memory-related motor-planning signal encoding motor error. The relation of the activity to the behavior of the animal suggests that the response represents the intent to make eye movements of specific direction and amplitude.

Animals↗
PubMed1986

Cholinergic brainstem mechanisms of REM sleep in the rat.

Injection of carbachol into the brainstem of rats produced an increase in rapid eye movement (REM) sleep which was site- and dose-dependent. Effective locations for carbachol to stimulate REM sleep included the pontine reticular formation at the level of the trigeminal motor nucleus and the dorsal parabrachial area in the caudal midbrain. The carbachol effect in the caudal pons was dose-dependent. Additionally, this effect was blocked by concomitant administration of the muscarinic antagonist atropine. Control experiments suggested that the drug-induced phenomenon appeared to be an increase in normal physiological REM sleep.

Animals↗
PubMed1986

Di-isopropyl-fluorophosphate (DFP): acute toxicity and sleep.

Acute administration of the organophosphate diisopropyl-fluorophosphate (DFP) produces behavioral and physiological symptoms indicative of excessive cholinergic stimulation. This behavioral toxicity was found to be incompatible with the occurrence of sleep, despite the fact that chronic administration of DFP has been shown to increase the rapid eye movement stage of sleep. DFP was found to decrease all stages of sleep and to increase wakefulness in a dose-dependent manner. Atropine sulfate, at doses of 3.0 mg/kg, was ineffective in blocking the DFP effects upon sleep.

Animals↗
PubMed1985

The acetylcholinesterase inhibitor di-isopropyl-fluorophosphate increases REM sleep in rats.

In this experiment, rats were treated chronically with moderate doses of the acetylcholinesterase inhibitor di-isopropyl-fluorophosphate (DFP). After an initial injection of 1.0 mg/kg DFP, the rats received a 0.5 mg/kg injection every third day thereafter for a total of 5 injections (13 days). Following the treatment regimen, the rats were found to have increased amounts of REM sleep compared to vehicle control rats. The time spent awake and in slow wave sleep was relatively unaffected. The increase in REM sleep appears to be due to increased numbers of REM sleep episodes and not an increase in the average length of the REM sleep episodes. Furthermore, the increased REM sleep does not appear to be due to REM rebound or to disruptions of circadian rhythm.

Animals↗