Retinal image motion alone does not control disconjugate postsaccadic eye drift

1. In these experiments, postsaccadic ocular drift was induced by postsaccadic motion of the visual scene. In the most important case, the scene was moved in one eye but not the other. Six human subjects viewed the interior of a full-field hemisphere filled with a random-dot pattern. During training, eye movements were recorded by the electrooculogram. A computer detected the end of every saccade and immediately moved the pattern horizontally in the same or, in different experiments, in the opposite direction as the saccade. The pattern motion was exponential with an amplitude of 25% of the size of the antecedent saccade and a time constant of 50 ms. Before and after 3-4 h of such training, movements of both eyes were measured simultaneously by the eye coil-magnetic field method while subjects looked between stationary targets for calibration, explored the visual pattern with saccades, or made saccades in the dark to measure the effects of adaptation on postsaccadic ocular drift. The amplitude of this drift was expressed as a percentage of the size of the antecedent saccade. 2. In monocular experiments, subjects viewed the random-dot pattern with one eye. The other eye was patched. With two subjects, the pattern drifted backward in the direction opposite to the saccade; with the third, it drifted onward. The induced ocular drift was exponential, always in the direction to reduce retinal image motion, had zero latency, and persisted in the dark. After training, drift in the dark changed by 6.7% in agreement with our prior study with binocular vision, which produced a change of 6.0%. 3. In a dichoptic arrangement, one eye regarded the moveable random-dot pattern; the other, through mirrors, saw a different random-dot pattern (with similar spacing, contrast, and distance) that was stationary. These visual patterns were not fuseable and did not evoke subjective diplopia. In this case, the induced change in postsaccadic drift in the same three subjects was only 4.8%. In all cases the changes in postsaccadic drift were conjugate--they obeyed Hering's law. 4. Normal human saccades are characterized by essentially no postsaccadic drift in the abducting eye and a pronounced onward drift (approximately 4%) in the adducting eye. After training, this abduction-adduction asymmetry was preserved in the light and dark with monocular or dichoptic viewing, indicating again that all adaptive changes were conjugate. 5. When the subjects viewed the adapting stimulus after training, the zero-latency, postsaccadic drift always increased from levels in the dark.(ABSTRACT TRUNCATED AT 400 WORDS)

Site-directed mutagenesis of m1 muscarinic acetylcholine receptors: conserved aspartic acids play important roles in receptor function

Muscarinic acetylcholine receptors contain a region encompassing the second and third transmembrane domains that is rich in conserved aspartic acid residues. To investigate the role of four conserved aspartic acids at positions 71, 99, 105, and 122 in muscarinic receptor function, point mutations in the rat m1 muscarinic receptor gene were made that converted each Asp to Asn, and wild type or mutant genes were stably expressed in Chinese hamster ovary cells that normally lack muscarinic receptors. Substitution of Asp71 or Asp122 with Asn produced mutant receptors that displayed high affinity for carbachol but decreased efficacy and potency, respectively, in agonist-induced activation of phosphoinositide hydrolysis, suggesting that these residues may mediate receptor-GTP binding protein interactions. Substitution of Asp99 or Asp105 with Asn produced marked decreases in ligand binding affinities and/or covalent incorporation of [3H] propylbenzilylcholine mustard, suggesting that these residues may be involved in receptor-ligand interactions.

Tracheal tube biofilm as a source of bacterial colonization of the lung

Biofilm formation in tracheal tubes, its bacterial content, and its interaction with ventilator gas flow were investigated. At least 50 mg (dry weight) of biofilm was found in 30 of 40 tracheal tubes used in intensive care patients for 2 h to 10 days. Electron microscopy showed bacteria in this layer, and quantitative studies showed that bacterial counts could reach up to 10(6)/cm of tube length. Bacteria were cultured from the patient side of 18 of 78 heat and moisture exchanger-microbiological filter units removed from ventilator circuits. Particles were shown to detach from tracheal tube luminal biofilm and were projected up to 45 cm from the tracheal tube tip. Following contamination of the tracheal tube biofilm with a patient's own gastrointestinal flora, entrainment of bacteria in the inspiratory gas flow provides a mechanism for initial and repeated lung colonization.

Visually induced plasticity of postsaccadic ocular drift in normal humans

1. Five human subjects viewed binocularly the interior of a full-field hemisphere filled with a random-dot pattern. During training, eye movements were recorded by the electrooculogram. A computer detected the end of every saccade and immediately moved the pattern horizontally either in the same or, in different experiments, the opposite direction as the saccade. The motion was exponential, its amplitude was 25% of the horizontal component of the antecedent saccade, and its time constant was either 25, 50, or 100 ms in different experiments. Before and after 2-3 h of this experience, movements of both eyes were measured simultaneously by the eye-coil/magnetic-field method while subjects made saccades across the moveable pattern, looked between stationary targets, or made saccades in the dark, to see the effect of such adaptation on postsaccadic eye movements. 2. After 2-3 h (10,000-20,000 saccades) subjects developed a zero-latency, postsaccadic, ocular drift in the dark in the direction of the pattern motion. Three subjects were trained to backward drift, two to onward drift. Drift amplitude in the dark changed by 6% of the saccade size (range: 2-11%). The drift was exponential with an overall time constant of 108 ms. 3. After training, while viewing the adapting pattern motion, the change in the amplitude of the zero-latency drift was approximately 10% (range: 6.5-14%). 4. Increasing the time constant of the pattern motion produced significant increases in the time constant of the ocular drift. 5. The incidence of dynamic overshoot (a tiny, backward saccade immediately following a main saccade) was idiosyncratic and went up in some subjects and down in others with adaptation. These changes did not seem related to modifications of postsaccadic drift. 6. Normal human saccades are characterized by essentially no postsaccadic drift in the abducting eye and a pronounced onward drift (approximately 4%) in the adducting eye. This adduction-adduction asymmetry is largely preserved through adaptation. Thus the changes in drift were conjugate and conformed to Hering's law of equal (change of) innervation. 7. These results agree with those previously demonstrated in the monkey and can similarly be explained by parametric changes in the pulse, slide, and step of normal saccadic innervation.

Visual cancellation of the torsional vestibulo-ocular reflex in humans

Using the eye-coil/magnetic field method, we measured the torsional vestibulo-ocular reflex (VOR) in ten subjects during active head rotations in roll at about 0.5 Hz. In the dark, regardless of instructions or mental effort, the gains (eye velocity/head velocity) had a mean value of around 0.61. When they viewed a visual display that was stationary, gains rose to 0.72. When viewing a visual display that moved in roll with their heads, subjects could decrease their gains to a mean of 0.46. Separate experiments showed that, as expected at this frequency, the optokinetic system made only a weak contribution. It has been proposed that the horizontal VOR is cancelled by the smooth pursuit system. Since there is no torsional pursuit system, some other mechanism must be used to augment or partially cancel the torsional VOR. Attempts to show that imagination could change this gain showed only weak effects. When asked to imagine an earth-fixed scene, gains were around 0.63; when asked to imagine a subject-fixed scene, gains decreased to only 0.60. When allowed to use a tactile contribution to aid the imagination in cancelling the VOR, the gain dropped further but only to 0.57. We conclude that mental effort in the dark has little influence on the torsional VOR but vision does by a mechanism that is not optokinetic or pursuit.

Complications of a pressure sore of the chest wall: case report

A female tetraplegic patient developed a pressure sore of the chest wall leading to an empyema of the lung and respiratory failure. The pressure sore resulted from the commonly practised habit of grasping the upright of the wheel chair with the upper arm in order to gain stability.

Predicting successful brachial plexus block using changes in skin electrical resistance

Skin electrical resistance is determined by the degree of sweating of the skin which is, in turn, related to sympathetic nervous system activity in the area concerned. It is increased when the nerves supplying the area are damaged or blocked by local anaesthetic agents. We have assessed the temporal and spatial relationship between the onset of sympathetic and sensory loss in the hand following brachial plexus block in 44 patients. Skin electrical resistance, measured using a simple ohm meter, has been shown to increase within 2 min of brachial plexus blockade with 1% lignocaine and adrenaline 1:200,000. This increase is an early and reliable indicator of subsequent, and occasionally delayed, sensory loss.

The MR contribution after CT demonstration of supratentorial mass effect without additional localising features

The contribution of magnetic resonance (MR) imaging was retrospectively evaluated in 24 patients in whom CT demonstrated a supratentorial mass effect without significant additional localising features. Using a combination of T1-weighted inversion recovery (IR) and T2-weighted spin echo sequences, the MR images localised the lesion with greater precision than CT in 80% of cases. Areas of contrast enhancement were visible on the IR images in five of nine (56%) patients assessed after intravenous gadolinium-diethylenetriamine pentaacetic acid, where no comparable enhancement was seen on CT. Twenty of the 24 patients subsequently underwent surgery; eight had biopsies and 12 had resections. Magnetic resonance accurately predicted the site of the tumour in all these patients. Three of the remaining four cases were treated with radiotherapy on the basis of the MR findings without a tissue diagnosis being sought. Magnetic resonance produces better localization than CT and provides a sound basis for further diagnostic and therapeutic procedures.

Evolution of neurotransmitter receptor systems

The presence of hormones, neurotransmitters, their receptors and biosynthetic and degradative enzymes is clearly not only associated with the present and the recent past but with the past several hundred million years. Evidence is mounting which indicates substantial conservation of protein structure and function of these receptors and enzymes over these tremendous periods of time. These findings indicate that the evolution and development of the nervous system was not dependent upon the formation of new or better transmitter substances, receptor proteins, transducers and effector proteins but involved better utilization of these highly developed elements in creating advanced and refined circuitry. This is not a new concept; it is one that is now substantiated by increasingly sophisticated studies. In a 1953 article discussing chemical aspects of evolution (Danielli, 1953) Danielli quotes Medawar, "... endocrine evolution is not an evolution of hormones but an evolution of the uses to which they are put; an evolution not, to put it crudely, of chemical formulae but of reactivities, reaction patterns and tissue competences." To also quote Danielli, "In terms of comparative biochemistry, one must ask to what extent the evolution of these reactivities, reaction patterns and competences is conditional upon the evolution of methods of synthesis of new proteins, etc., and to what extent the proteins, etc., are always within the synthetic competence of an organism. In the latter case evolution is the history of changing uses of molecules, and not of changing synthetic abilities." (Danielli, 1953). Figure 4 outlines a phylogenetic tree together with an indication of where evidence exists for both the enzymes that determine the biosynthesis and metabolism of the cholinergic and adrenergic transmitters and their specific cholinergic and adrenergic receptors. This figure illustrates a number of important points. For example, the evidence appears to show that the transmitters and their associated enzymes existed for a substantial period before their respective receptor proteins. While the transmitters and enzymes appear to exist in single cellular organisms, there is no solid evidence for the presence of adrenergic or cholinergic receptors until multicellular organisms where the receptors appear to be clearly associated with specific cellular and neuronal communication (Fig. 4). One can only speculate as to the possible role for acetylcholine and the catecholamine in single cell organisms.(ABSTRACT TRUNCATED AT 400 WORDS)

Smooth pursuit eye movements in schizophrenics: quantitative measurements with the search-coil technique

Eye movements of five schizophrenic and five normal subjects were measured with the magnetic-field search-coil technique. Subjects followed targets moving smoothly at various speeds, either unpredictably in a step-ramp fashion or predictably in a triangular wave. The tracking stimulus was either a small dot or a large, richly-textured image that occupied a large portion of the visual field. Tracking by schizophrenics was abnormal; it was punctuated by catch-up saccades that corrected for smooth following movements of inadequate velocity. We did not, however, find saccadic intrusions, such as square wave jerks. Under all tracking conditions steady-state gains (eye velocity/target velocity) and, in the case of step-ramps, average acceleration in the first 120 ms were lower in patients than in normal subjects. The differences were most pronounced for tracking of the small target, moving at the highest speed tested (30 degree/s), in the nonpredictable, step-ramp waveform. With this stimulus mean steady-state gain was 0.36 (SD +/- 0.12) for the schizophrenic patients and 0.73 (SD +/- 0.11) for the normal subjects. When the target was changed to the large-field stimulus or moved in a predictable (triangular-wave) fashion, tracking improved in both patients and normal subjects, and even more so when these features were combined.

Transition dynamics between pursuit and fixation suggest different systems

The offset of smooth pursuit eye movements is very different from the onset. The onset eye velocity is characterized by overshoot and ringing before settling to steady-state velocity. Yet, at pursuit offset, the eye velocity returns smoothly to zero. One reason for this difference may be that the pursuit system is nonlinear and behaves differently during acceleration and deceleration. After testing four subjects, we found no difference between acceleration and deceleration ringing dynamics, except for decelerations to 0 +/- 2 deg/sec, suggesting that around zero velocity, the central nervous system switches off pursuit and adopts fixation. This supports the idea that fixation and pursuit represent different neurological systems.

Primary structure of rat cardiac beta-adrenergic and muscarinic cholinergic receptors obtained by automated DNA sequence analysis: further evidence for a multigene family

Two cDNA clones, lambda RHM-MF and lambda RHB-DAR, encoding the muscarinic cholinergic receptor and the beta-adrenergic receptor, respectively, have been isolated from a rat heart cDNA library. The cDNA clones were characterized by restriction mapping and automated DNA sequence analysis utilizing fluorescent dye primers. The rat heart muscarinic receptor consists of 466 amino acids and has a calculated molecular weight of 51,543. The rat heart beta-adrenergic receptor consists of 418 amino acids and has a calculated molecular weight of 46,890. The two cardiac receptors have substantial amino acid homology (27.2% identity, 50.6% with favored substitutions). The rat cardiac beta receptor has 88.0% homology (92.5% with favored substitutions) with the human brain beta receptor and the rat cardiac muscarinic receptor has 94.6% homology (97.6% with favored substitutions) with the porcine cardiac muscarinic receptor. The muscarinic cholinergic and beta-adrenergic receptors appear to be as conserved as hemoglobin and cytochrome c but less conserved than histones and are clearly members of a multigene family. These data support our hypothesis, based upon biochemical and immunological evidence, that suggests considerable structural homology and evolutionary conservation between adrenergic and muscarinic cholinergic receptors. To our knowledge, this is the first report utilizing automated DNA sequence analysis to determine the structure of a gene.

Magnetic resonance imaging of the solitary hepatic mass: direct correlation with pathology and computed tomography

Magnetic resonance images (MRI) of the liver were obtained using a combination of short time inversion recovery (STIR) and spin echo (SE) sequences. These were correlated with comparable tissue slices generated from resected specimens obtained at partial hepatectomy. All 10 cases appeared to have solitary masses on contrast enhanced computed tomography (CT). Histological examination revealed five primary tumours (two hepatocellular carcinomas, two haemangiomas and one cholangiocarcinoma) and five metastatic tumours. The STIR images demonstrated a high signal intensity in all areas of viable tumour involvement and reduced signal intensity in regions of confluent necrosis with superimposed haemorrhage or calcification. This sequence also demonstrated additional areas of high signal intensity adjacent to several lesions which were not visible on CT. Microscopy of these regions in the specimens demonstrated no tumour involvement or steatosis and their precise cause remains obscure. All the lesions demonstrated on the CT images were visible on MRI and no additional lesions were discovered on detailed microscopical examination of the specimens. Delineation of the extent of the cholangiocarcinoma was a problem with both techniques. MRI showed no major advantage over CT except for a higher contrast of the lesion compared with normal liver and also a better delineation of the tumour mass.

Loss of the neural integrator of the oculomotor system from brain stem lesions in monkey

Eye movement were recorded from four juvenile rhesus monkeys (Macaca mulatta) before and after the injection of neurotoxins (kainate or ibotenate) in the region of the medial vestibular and prepositus hypoglossi nuclei, an area hypothesized to be the locus of the neural integrator for horizontal eye movement commands. Eye movements were measured in the head-restrained animal by the magnetic field/eye-coil method. The monkeys were trained to follow visual targets. A chamber implanted over a trephine hole in the skull permitted recordings to be made in the brain stem with metal microelectrodes. The abducens nuclei were located and used as a reference point for subsequent neurotoxin injections through cannulas. The effects of these lesions on fixation, vestibuloocular and optokinetic responses, and smooth pursuit were compared with predicted oculomotor anomalies caused by a loss of the neural integrator. Kainate and ibotenate did not create permanent lesions in this region of the brain stem. All the eye movements returned toward normal over the course of a few days to 2 wk. Histological examination revealed that the cannula tips were mainly located between the vestibular and prepositus hypoglossi nuclei, in their rostral 2 mm, bordered rostrally by the abducens nuclei. Dense gliosis clearly demarcated the cannula tracks, but for most injections there were no surrounding regions of neuronal loss. Thus the eye movement disorders were due to a reversible, not a permanent, lesion. The time constant for the neural integrator was determined from the velocity of the centripetal drift of the eyes just after an eccentric saccade in total darkness. For intact animals this time constant was greater than 20 s. Shortly after bilateral injections of neurotoxin, the time constant began to decrease and reached a minimum of 200 ms; every horizontal saccade was followed by a rapid centripetal drift with a time constant of approximately 200 ms. For vertical eye movements, in this acute phase, the time constant was approximately 2.5 s. The vestibuloocular reflex (VOR) was drastically changed by the lesions. A step of constant head velocity in total darkness evoked a step change in eye position rather than in velocity. In the absence of the neural integrator, the step velocity command from the canal afferents was not integrated to produce a ramp of eye position (normal slow phases); rather this signal was relayed directly to the motoneurons and caused a step in eye position. The per- and postrotatory decay of the head velocity signal was decreased to 5-6 s indicating that vestibular velocity storage was also impaired.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptive changes in post-saccadic drift induced by patching one eye

A prior study showed that after horizontal saccades the abducting eye has little post-saccadic drift (about 0.5 deg/sec) while the abducting eye has considerable onward drift (about 1.7 deg/sec). To investigate this further, five subjects patched one eye for three days. This reduced the drift after adducting saccades in the viewing eye to the level of that after abducting saccades. The changes were a combination of conjugate and disconjugate alterations. Decreases in drift in the viewing eye did not cause increases in drift in the covered eye. These changes appear functional in that retinal image slip is decreased in the viewing eye but why this goal is not attained when both eyes habitually view is not understood. Also, post-saccadic drift could depend on which eye was used to view the target.

Hysteresis and slow drift in abducens unit activity

Two trained monkeys made saccadic eye movements to a small visual target. The activity of 39 isolated abducens units, presumed to be motoneurons or abducens internuclear neurons, was recorded in relation to these eye movements. After a calibration trial, a test trial repeatedly elicited 20 degrees horizontal saccades to primary position from either the left or right. On average, the steady-state firing rate at primary position depended on the direction of the saccade. For saccades where the neuron showed a burst in activity during the saccade (on-saccades) the steady-state firing rates were usually higher than for those saccades that showed a pause in activity during the saccade (off-saccades). For the population of units this hysteresis measured 5.4 spikes/s, which may be compared with an average primary-position rate of 97 spikes/s. The average hysteresis for individual units ranged from -2.1 to 18.5 spikes/s. The steady-state firing rate after equal saccades in the same direction and ending at the same position (primary) varied slowly over time. Across all units the variability (standard deviation) ranged from 0.5 to 11.8 spikes/s with a mean of 4.7 spikes/s. Furthermore, for any one unit the variations following on-saccades generally correlated with the variations following the off-saccades. Hysteresis, doubted by many, does exist. Fortunately, it is small enough, 5.5% of typical primary-position rate, that it can be neglected for many purposes. Nevertheless, it poses the interesting theoretical question of how the oculomotor system compensates for hysteresis. The simplest explanation of slow variations in background rate is cocontractive noise: a slow fluctuation in all abducens neurons so that these variations do not result in fluctuations of eye position.