Altered stimulus representation in rat auditory cortex is not causal for loss of consciousness under general anaesthesia

BACKGROUND

Current concepts suggest that impaired representation of information in cortical networks contributes to loss of consciousness under anaesthesia. We tested this idea in rat auditory cortex using information theory analysis of multiunit responses recorded under three anaesthetic agents with different molecular targets: isoflurane, propofol, and dexmedetomidine. We reasoned that if changes in the representation of sensory stimuli are causal for loss of consciousness, they should occur regardless of the specific anaesthetic agent.

METHODS

Spiking responses were recorded with chronically implanted microwire arrays in response to acoustic stimuli incorporating varied temporal and spectral dynamics. Experiments consisted of four drug conditions: awake (pre-drug), sedation (i.e. intact righting reflex), loss of consciousness (a dose just sufficient to cause loss of righting reflex), and recovery. Measures of firing rate, spike timing, and mutual information were analysed as a function of drug condition.

RESULTS

All three drugs decreased spontaneous and evoked spiking activity and modulated spike timing. However, changes in mutual information were inconsistent with altered stimulus representation being causal for loss of consciousness. First, direction of change in mutual information was agent-specific, increasing under dexmedetomidine and decreasing under isoflurane and propofol. Second, mutual information did not decrease at the transition between sedation and LOC for any agent. Changes in mutual information under anaesthesia correlated strongly with changes in precision and reliability of spike timing, consistent with the importance of temporal stimulus features in driving auditory cortical activity.

CONCLUSIONS

The primary sensory cortex is not the locus for changes in representation of information causal for loss of consciousness under anaesthesia.

Acceleration of pentylenetetrazol seizure kindling associated with induction of sensitized visual responses evoked by strobe stimulation

Exposure of normal adult rats of a variety of species to trains of light flashes leads to acquisition of an enduring high amplitude visual cortical response [Uhlrich DJ, Manning KA, O'Laughlin ML, Lytton WW (2005) Photic-induced sensitization: acquisition of an augmenting spike-wave response in the adult rat through repeated strobe exposure. J Neurophysiol 94:3925-3937]. The photically-induced sensitized response exhibits epileptiform characteristics, including spike-wave morphology, tendency to generalize across the brain, and sensitivity to the anti-epileptic drug ethosuximide. These findings and anecdotal clinical reports raise the possibility that certain sensory stimulation could induce neural plastic changes that affect seizures in some individuals. We hypothesize that photic-induced sensitization can prime seizure-related neural circuitry, resulting in exacerbation of seizures. To test this we compared seizure kindling rates using the pentylenetetrazol (PTZ) model of epileptogenesis in sensitized and unsensitized adult Sprague-Dawley rats. Experimental group rats were sensitized by exposure to repetitive stroboscopic stimulation over 4-6 days until the sensitized photic response fully developed and response magnitude stabilized at its highest plateau. Rats then received a sub-convulsive injection of PTZ (24 mg/kg i.p.) every other day until they attained class 5 seizures. Control rats were not strobed or sensitized, but were otherwise treated identically. Chronic electrodes overlying the dura in occipital cortex recorded the primary visual response. Similar electrodes near the border of somatosensory and motor cortex (SM) were used to record spread of the sensitized response to a patently non-visual region. Rat behavior was monitored by direct observation and digital audio/video recording. All control rats and seven of 14 photically sensitized rats kindled seizures at rates consistent with those reported previously. However, the seven other photically sensitized rats displayed markedly accelerated seizure kindling. Rats with accelerated kindling showed greater spread of the sensitized visual response to somato-motor cortex and, when tested in a post hoc experiment, exhibited a higher likelihood of photo-triggered seizures. These results indicate that photic-induced sensitization in susceptible individuals can prime neural circuitry involved in the generation of PTZ-kindled seizures.

Photic-induced sensitization: eye-specific neural plasticity and effect of behavioral state

We reported previously that exposure to repetitive visual stimulation in ordinary adult rats results in acquisition of an enduring increase in magnitude and change in character of visual cortical responses. This sensitization is consistent with experience-dependent neuroplastic changes, but could also reflect alterations in response with behavioral state during testing. The aim of this study was to distinguish the contributions of behavioral state and neural plasticity in this photic-induced sensitization. We used repetitive light-emitting diode flashes delivered monocularly and recorded electrocorticographically in the albino rat in which retino-geniculo-cortical projections are predominantly crossed. This enabled comparison of visual responses of sensitized visual circuitry associated with one eye to responses from effectively unsensitized circuitry associated with the second eye at similar time points in an animal, thus providing an internal control for behavioral state. Following sensitization, monocular stimulation of one eye produced the characteristic high amplitude driven spike-wave response in corresponding contralateral visual cortex, but not ipsilateral cortex. Expression of the sensitized driven response was optimal in the quiet awake state and suppressed during active exploration, drowsiness, or anesthesia. When the animal was in the quiet awake state, producing sensitized responses to stimulation of the first eye, no such response was observed on alternate trials upon stimulation of the second eye. Only after extended exposure of the second eye did the high amplitude driven spike-wave response in contralateral visual cortex develop. The data further suggest some degree of sensitization of ipsilateral pathways may accompany monocular stimulation and that effects of monocular sensitization could include suppression in pathways related to the unstimulated eye. Thus, while behavioral state influences expression of the sensitized driven visual response, the eye-specific nature of the effect provides strong evidence that response enhancement reflects neuroplasticity in visual pathways and not a more general change in behavioral state during testing.

Photic-induced sensitization: acquisition of an augmenting spike-wave response in the adult rat through repeated strobe exposure

It is well established that patterns of sensory input can affect neuroplastic changes during early development. The scope and consequences of experience-dependent plasticity in the adult are less well understood. We studied the possibility that repeated exposure to trains of stroboscopic stimuli could induce a sensitized and potentially aberrant response in ordinary individuals. Chronic electrocorticographic recording electrodes enabled measurement of responses in awake, freely moving animals. Normal adult rats, primarily Sprague-Dawley, were exposed to 20-40 strobe trains per day after a strobe-free adaptation period. The common response to strobe trains changed in 34/36 rats with development of a high-amplitude spike-wave response that emerged fully by the third day of photic exposure. Onset of this sensitized response was marked by short-term augmentation of response to successive strobe flashes. The waveform generalized across the brain, reflected characteristics of the visual stimulus, as well as an inherent 6- to 8-Hz pacing, and was suppressed with ethosuximide administration. Spike-wave episodes were self-limiting but could persist beyond the strobe period. Sensitization lasted 2-4 wk after last strobe exposure. The results indicate visual stimulation, by itself, can induce in adult rats an enduring sensitization of visual response with epileptiform characteristics. The results raise the question of the effects of such neuroplastic change on sensation and epileptiform events.

Comparison of cholinergic and histaminergic axons in the lateral geniculate complex of the macaque monkey

The cholinergic and histaminergic projections have important neuromodulatory functions in the ascending visual pathways, so we compared the pattern and mode of innervation of the two projections in the lateral geniculate complex (dorsal lateral geniculate nucleus and pregeniculate nucleus) of the macaque monkey. Brain tissue from macaques was immunoreacted by means of antibodies to choline acetyltransferase (ChAT) or to histamine and processed for light and electron microscopy. A dense plexus of thin, highly branched ChAT-immunoreactive axons laden with varicosities was found in all layers of the dLGN including the koniocellular laminae and in the pregeniculate nucleus. ChAT label was more dense in magnocellular layers 1 and 2 than in parvocellular layers 3-6 and relatively sparse in the interlaminar zones. Varicosities associated with the cholinergic axons had an average of three conventional asymmetric synapses per varicosity, and these appeared to contact dendrites of both thalamocortical cells and interneurons. Histamine-immunoreactive axons were distributed homogeneously throughout all laminar and interlaminar zones of the dLGN, but were denser in the pregeniculate nucleus than in the dLGN. Histaminergic axons branched infrequently and were typically larger in caliber than cholinergic axons. The overwhelming majority of varicosities were found en passant and rarely displayed conventional synapses, despite the abundance of synaptic vesicles, and were not associated preferentially with specific cellular structures. The innervation of the macaque dLGN complex by cholinergic and histaminergic systems is consistent with their proposed role in state dependent modulation of thalamic activity. The dense and highly synaptic innervation by cholinergic axons supports the proposal of additional involvement of these axons in functions related to eye movements.

Muscarinic receptor subtypes in the lateral geniculate nucleus: a light and electron microscopic analysis

Neural activity in the dorsal lateral geniculate nucleus of the thalamus (DLG) is modulated by an ascending cholinergic projection from the brainstem. The purpose of this study was to identify and localize specific muscarinic receptors for acetylcholine in the DLG. Receptors were identified in rat and cat tissue by means of antibodies to muscarinic receptor subtypes, ml-m4. Brain sections were processed immunohistochemically and examined with light and electron microscopy. Rat DLG stained positively with antibodies to the m1, m2,and m3 receptor subtypes but not with antibodies to the m4 receptor subtype. The m1 and m3 antibodies appeared to label somata and dendrites of thalamocortical cells. The m1 immunostaining was pale, whereas m3-positive neurons exhibited denser labeling with focal concentrations of staining. Strong immunoreactivity to the m2 antibody was widespread in dendrites and somata of cells resembling geniculate interneurons. Most m2-positive synaptic contacts were classified as F2-type terminals, which are the presynaptic dendrites of interneurons. The thalamic reticular nucleus also exhibited robust m2 immunostaining. Cat DLG exhibited immunoreactivity to the m2 and m3 antibodies. The entire DLG stained darkly for the m2 receptor subtype, except for patchy label in the medial interlaminar nucleus and the ventralmost C laminae. The staining for m3 was lighter and was distributed more homogeneously across the DLG. The perigeniculate nucleus also was immunoreactive to the m2 and m3 subtype-specific antibodies. Immunoreactivity in cat to the m1 or m4 receptor antibodies was undetectable. These data provide anatomical evidence for specific muscarinic-mediated actions of acetylcholine on DLG thalamocortical cells and thalamic interneurons.

Histamine-immunoreactive neurons and their innervation of visual regions in the cortex, tectum, and thalamus in the primate Macaca mulatta

The histaminergic system is involved in the control of arousal in the brain and may impact significantly on visual processing. However, little is known about the histaminergic innervation of visual areas, or the histamine system in the primate brain, in general. We examined in Macaca mulatta the location of histamine-immunoreactive neurons and the innervation of important cortical and subcortical visual areas by histamine-immunoreactive axons. Brain sections were treated with an antibody to histamine and processed with standard immunohistological procedures. Histamine-immunoreactive neurons (20-45 microns in diameter) were localized bilaterally in the hypothalamus, particularly in ventral, lateral, posterior, and perimammillary hypothalamic areas. These hypothalamic cells appear to provide the sole neural source of histamine in the macaque brain. A plexus of varicose histamine-immunoreactive axons was present throughout the superior colliculus, the dorsal and ventral lateral geniculate nuclei of the thalamus, the reticular nucleus of the thalamus, the lateral posterior/pulvinar complex, and the visual cortex, including areas 17, 18, and the nearby extrastriate cortex. The axons nearly homogeneously innervated every region and layer in these structures, except for an increase in density in layer 1 of the visual cortex and in the superficial-most layers of the superior colliculus. Histaminergic axons broadly innervated every visual region examined. In comparison with the other aminergic and the cholinergic projection systems, which show considerable projection specificity, the histaminergic projection exhibited great homogeneity. The breadth of the distribution of histaminergic axons ensures that virtually all levels of visual processing in the primate can be influenced, either directly or indirectly, by the neuromodulatory effects of histamine.

Projection of individual axons from the pretectum to the dorsal lateral geniculate complex in the cat

The dorsal lateral geniculate nucleus of the thalamus transmits visual signals from the retina to the cortex. Within the lateral geniculate nucleus, the ascending visual signals are modified by the actions of a number of afferent pathways. One such projection originates in the pretectum and appears to be active in association with oculomotor activity. Much remains unknown about the pretectal-geniculate projection. Our purpose was to examine for the first time individual axon arbors from the pretectum that project to the lateral geniculate nucleus, describing their topography and nuclear and laminar targets. We made injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the cat pretectum, targeting the nucleus of the optic tract. Serial 40 microns coronal sections were processed by using immunohistochemistry to reveal labeled axons that were then serially reconstructed using light microscopy. Pretectal-geniculate axons appeared morphologically heterogeneous in terms of swelling size, branching patterns, and laminar target. Most axons innervated the geniculate A laminae. A separate, smaller population innervated the C laminae. All axons exhibited substantially greater spread medial-laterally than rostral-caudally in the lateral geniculate nucleus, displaying a topographical organization for visual field elevation, but not azimuth. Many pretectal axons that projected to the LGN also innervated adjacent structures, including the medial interlaminar nucleus, the perigeniculate nucleus, and/or the pulvinar. These results indicate that the projection from the pretectum to the dorsal lateral geniculate nucleus is heterogeneous, is semitopographical, and may coordinate neural activity in the lateral geniculate nucleus and in neighboring visual thalamic structures in association with oculomotor events.

Histaminergic and non-histamine-immunoreactive mast cells within the cat lateral geniculate complex examined with light and electron microscopy

Mast cells and their location in the cat lateral geniculate complex of the thalamus were examined by means of histamine immunohistochemistry and the mast cell stain pinacyanol erythrosinate. Brain sections from seven normal adult pigmented cats were processed for light or electron microscopy. Histamine-containing and pinacyanol erythrosinate-stained mast cells were widespread throughout the dorsal and ventral lateral geniculate nuclei and the surrounding regions. Mast cells were especially numerous rostrally in the complex and in the geniculate C laminae. The cells were found consistently in association with blood vessels, ranging from capillary size to vessels c. 150 microns diameter, and twice as often with arterioles as with venules. Large clusters of many mast cells associated with single blood vessels were seen. Individual mast cells were typically 8 microns in diameter and somewhat oval, although multipolar and crescent-shaped cells were also seen, up to twice as long. The amount of histamine labeling varied across cells. When histamine-labeled material was secondarily stained with pinacyanol erythrosinate, many mast cells were double labeled. In addition, there was a small population of mast cells that stained only with pinacyanol erythrosinate, but was otherwise identical to the histamine-immunoreactive mast cells. Electron microscopic examination showed that the mast cells lie on the brain side of the blood-brain barrier. Mast cells were found in close proximity to the thalamic neuropil, primarily apposed to the processes of astrocytes, but also apposed to neural elements. The distinctive electron-dense cytoplasmic granules in the fully granulated, mature state were largely amorphous in appearance and as large as 700 nm in diameter. Histamine was dispersed throughout some granules and contained within restricted areas of other granules. In degranulated mast cells, large, irregularly shaped, electron-lucent granules were seen fused with the cell membrane on the neuropil side, as well as the lumen side of the mast cell. More mast cells were observed at the electron microscopic level than were expected from the light level observations, which suggests that, despite the numbers of mast cells labeled, these results may still underestimate the total mast cell population present in this region of the thalamus. Mast cells, by their numbers, their distribution and the potent chemical substances they contain, may significantly influence vascular and neural function, directly and indirectly, in the cat lateral geniculate complex.

Not looking while leaping: the linkage of blinking and saccadic gaze shifts

Many vertebrates generate blinks as a component of saccadic gaze shifts. We investigated the nature of this linkage between saccades and blinking in normal humans. Activation of the orbicularis oculi, the lid closing muscle, EMG occurred with 97% of saccadic gaze shifts larger than 33 degrees. The blinks typically began simultaneously with the initiation of head and/or eye movement. To minimize the possibility that the blinks accompanying saccadic gaze shifts were reflex blinks evoked by the wind rushing across the cornea and eye-lashes as the head and eyes turned, the subjects made saccadic head turns with their eyes closed. In this condition, orbicularis oculi EMG activity occurred with all head turns greater than 17 degrees in amplitude and the EMG activity began an average of 39.3 ms before the start of the head movement. Thus, one component of the command for large saccadic gaze shifts appears to be a blink. We call these blinks gaze-evoked blinks. The linkage between saccadic gaze shifts and blinking is reciprocal. Evoking a reflex blink prior to initiating a voluntary saccadic gaze shift dramatically reduces the latency of the initiation of the head movement.

The distribution of histaminergic axons in the superior colliculus of the cat

The histaminergic projection from the hypothalamus to the superior colliculus was examined immunohistochemically in the cat brain using an antibody to histamine. The source of histaminergic fibers in the brain is a group of neurons in the posterior hypothalamus, located primarily in ventrolateral and periventricular regions and collectively referred to as the tuberomammillary nucleus. All laminae of the superior colliculus--including the superficial, intermediate, and deep layers, as well as the central gray--were blanketed with histamine-immunoreactive axonal fibers. Overall, labeling in the superior colliculus was moderately dense compared to other locations in the cat brain, with some variation in fiber density. Individual labeled fibers resembled histaminergic fibers described previously in the brain. Labeled axonal fibers showed infrequent branching and were beaded with numerous en passant varicosities that were typically 1 micron or smaller, but as large as 2.5 micron in diameter. Varicosity size differed significantly at different depths in the colliculus. The histaminergic projection appears to be separate from a previously reported, apparently non-histaminergic projection from neurons in the dorsal hypothalamic area to discrete regions of intermediate and deep colliculus. These results indicate that the histaminergic projection from the tuberomammillary nucleus of the hypothalamus projects extensively throughout the superior colliculus. Histamine, which is believed to act as a neuromodulator in the brain, is in a position to influence sensory and motor-related processes in every layer of the cat superior colliculus.

Sex differences in dependency: a comparison of objective and projective measures

The relationship between scores on two widely used measures of dependency--one objective and one projective--was examined in a sample of 102 undergraduate subjects (60 women and 42 men). Consistent with previous studies in this area, significant sex differences were found on the objective measure of dependency, with women obtaining higher dependency scores than men. However, as predicted, men and women obtained comparable scores on the projective measure of dependency. Additional analyses confirmed that scores on the objective and projective measures of dependency were significantly intercorrelated in both men and women, although the magnitudes of these correlations were somewhat larger in women than in men. Implications of these results for the assessment of dependent personality traits in research and clinical settings are discussed.

The histaminergic innervation of the lateral geniculate complex in the cat

The histaminergic innervation of the thalamic dorsal and ventral lateral geniculate nuclei and the perigeniculate nucleus of the cat was examined immunohistochemically by means of an antibody to histamine. We find histamine-immunoreactive neurons in the cat brain are concentrated in the ventrolateral portion of the posterior hypothalamus, confirming a previous report. However, this cell group also spreads into medial, dorsal, and extreme lateral regions of the posterior hypothalamus and extends as far rostral as the optic chiasm. Histamine-labeled fibers cover all regions of the lateral geniculate complex, but the density of labeling varies. The ventral lateral geniculate nucleus (vLGN) is most densely labeled, the A laminae of the dorsal lateral geniculate are sparsely labeled, and the geniculate C laminae and the perigeniculate nucleus show intermediate amounts of label. Thus, histaminergic fibers demonstrate a predilection for zones innervated by the W-cell system. Labeled fibers exhibit few branchings and numerous en passant swellings, lending a beaded appearance. The vLGN showed more instances of fibers with larger-sized swellings (up to 2 microns). Following injections of biotinylated tracers into the hypothalamus, we find labeled fibers throughout the lateral geniculate complex. The anterogradely labeled fibers resemble the histaminergic fibers in morphology, distribution, and relative bouton size. Thus, the hypothalamus appears to be the source of the histaminergic fibers in the lateral geniculate complex. Histamine-labeled fibers in the dorsal lateral geniculate nucleus (dLGN) exhibit uncommon ultrastructural morphology. Many extremely large, round, or elliptical vesicles fill the fiber swellings. Swellings are directly apposed to a variety of other dendritic and axonal profiles, but thus far no convincing synaptic contacts have been seen. The distribution and appearance of these histaminergic fibers resembles those reported for serotonergic fibers. Our results support the idea that histamine works nonsynaptically as a neuromodulator in the lateral geniculate complex, affecting the level of visual arousal.

Pattern of extraocular muscle activation during reflex blinking

Studies in humans and rabbits suggest that cocontraction of extraocular muscles occurs with reflex and voluntary blinks. We determined the pattern of extraocular muscle activity elicited by blink-evoking visual and trigeminal stimuli by electromyographically recording antagonistic pairs of extraocular muscles in alert rabbits. In addition, we recorded the activity of antidromically identified oculomotor motoneurons in response to the same stimuli in alert rabbits. The data demonstrate that all extraocular muscles except the superior oblique transiently increase their activity in response to blink-evoking stimuli. The pattern of extraocular muscle activity with reflex blinks mirrors that occurring in the lid-closing muscle, orbicularis oculi, but the latency of extraocular muscle activation is longer.

A role for the basal ganglia in nicotinic modulation of the blink reflex

In humans and rats we found that nicotine transiently modifies the blink reflex. For blinks elicited by stimulation of the supraorbital branch of the trigeminal nerve, nicotine decreased the magnitude of the orbicularis oculi electromyogram (OOemg) and increased the latency of only the long-latency (R2) component. For blinks elicited by electrical stimulation of the cornea, nicotine decreased the magnitude and increased the latency of the single component of OOemg response. Since nicotine modified only one component of the supraorbitally elicited blink reflex, nicotine must act primarily on the central nervous system rather than at the muscle. The effects of nicotine could be caused by direct action on lower brainstem interneurons or indirectly by modulating descending systems impinging on blink interneurons. Since precollicular decerebration eliminated nicotine's effects on the blink reflex, nicotine must act through descending systems. Three lines of evidence suggest that nicotine affects the blink reflex through the basal ganglia by causing dopamine release in the striatum. First, stimulation of the substantia nigra mimicked the effects of nicotine on the blink reflex. Second, haloperidol, a dopamine (D2) receptor antagonist, blocked the effect of nicotine on the blink reflex. Third, apomorphine, a D2 receptor agonist, mimicked the effects of nicotine on the blink reflex.

Eyelid movements in facial paralysis

We studied the eyelid movements of six patients with unilateral, isolated, facial paralysis using the magnetic search coil. The most significant abnormality consisted of a reduction in the magnitude of the orbicularis oculi contraction with slowing of the peak velocity of the closing phase of the blink. The closing phase blink velocity, which normally increases linearly as a function of amplitude (main sequence), in our patients displayed a relationship characterized by a slow saturating power function that fell off the main sequence. The contralateral normal lid in some cases can show adaptive signs of hyperactivity during the closing phase of the blink. Lid saccades showed a small but consistent decrease in amplitude and velocity compared with the contralateral unaffected eyelid. Unlike the closing phase of the blink, peak velocities of lid saccades stayed on the main sequence. In this study, we discuss how the eyelid executes downward lid saccades based purely on a passive mechanism.

Spermidine acetylation in response to a variety of stresses in Escherichia coli

Heat shock, cold shock, ethanol, and alkaline shift, but not hydrogen peroxide, stimulate the accumulation of monoacetylspermidine in Escherichia coli. Acetylation occurs with nearly equal frequencies at both the N1 and N8 positions of this ubiquitous polycation. Spermidine acetylation does not appear to be associated with known stress regulons, such as htpR, oxyR, and SOS. E. coli, capable of acetylating spermidine, constitutively express a spermidine acetyltransferase activity during all phases of growth, and this activity is unaffected by cold shock. A mutant strain, incapable of acetylating spermidine, does not express this enzyme activity but grows at an identical rate as the parent strain at 37 degrees C. These results demonstrate that the monoacetylation of spermidine in E. coli is regulated by some mechanism other than a stress-inducible acetyltransferase and is not essential for growth of these cells. They suggest that polyamine acetylation is involved in the responses of these organisms to a variety of chemical and physical stresses.

Eyelid movements. Mechanisms and normal data

This study provides a comprehensive description of upper eyelid movement in normal human subjects. Using the magnetic search coil technique to monitor lid position and modified skin electrodes to record orbicularis oculi electromyographic (EMG) activity, the authors found that the basic eyelid movements, blinks, and saccadic lid movements, can be uniquely and reliably characterized by their amplitude-maximum velocity relationships. The data show that similar increases in levator palpebrae activity produce the upward lid movements that accompany upward saccadic eye movements as well as the upward phase of a blink. The lid movements that accompany downward saccadic eye movements arise almost exclusively from the passive downward forces and relaxation of the levator palpebrae muscle. In contrast, active orbicularis oculi contraction and the passive downward forces act together to generate lid closure with a blink. These normative data and techniques provide the basis for the clinical analysis of lid motility by which abnormal lid movements can be compared with normal lid kinematics.

Eyelid movements before and after botulinum therapy in patients with lid spasm

Quantitative analysis of lid motility is presented for 4 individuals with hemifacial spasm and 1 with Meige's syndrome. The data were obtained, by means of a magnetic search coil technique, prior to and 1 week after injection of botulinum toxin into the orbicularis oculi muscle. Before treatment, the peak velocity of blink-related lid lowering and lid raising was slower than normal, yet lid saccades were normal. After botulinum treatment, significant decreases occurred in (1) the amplitude of blinks and lid saccades, and (2) the peak velocity of the blink down-phase. Botulinum treatment significantly alters blink lid-lowering kinematics, while saccadic lid-lowering kinematics are normal, providing further evidence that the orbicularis oculi muscle does not play a primary role in downward lid saccades.

Retrograde transneuronal transport properties of fragment C of tetanus toxin

The atoxic fragment C of tetanus toxin reliably undergoes retrograde direct and transneuronal transport, but the full extent of its transport capabilities has not been examined. The primary visual pathways provide an excellent system for investigating, for the first time, the possibility of anterograde direct and transneuronal transport of fragment C. Following injection into the eye of the rat and rabbit, fragment C, localized with a monoclonal antibody, underwent anterograde direct transport to all retinorecipient areas. From these areas, fragment C exhibited retrograde transneuronal transport, strongly and reliably labeling regions that project to retinorecipient areas, including layers V and VI of visual cortex, the parabigeminal nucleus, the suprageniculate nucleus, and the reticular thalamus. In contrast, the absence of fragment C in regions receiving only input from, but not projecting to, retinorecipient areas, most notably layer IV of visual cortex, provides strong evidence for the lack of anterograde transneuronal transport. Thus, while fragment C can be induced to undergo anterograde direct transport by injection into the eye, it exhibits only retrograde transneuronal transport. These characteristics suggest that fragment C of tetanus toxin is a consistent and reliable retrograde transneuronal marker for the elucidation of central nervous pathways.

A pharmacological distinction between the long and short latency pathways of the human blink reflex revealed with tobacco

In three, normal, human subjects, tobacco smoking was used as a pharmacological probe to modify differentially the direct and indirect pathways underlying the blink reflex. The latency of the indirect R2 component of the orbicularis oculis electromyogram evoked by electrical stimulation of the trigeminal supraorbital nerve transiently increased 20-80% after smoking, while the latency of the shorter latency, direct R1 component remained constant. The magnitude of both components of the blink reflex transiently decreased. The data demonstrate that tobacco smoking can differentially alter the long and short latency components of the blink reflex, and suggest that these effects result from modifications of central pathways sensitive to nicotine.

A model system for motor learning: adaptive gain control of the blink reflex

An important aspect of the control of movement is how the nervous system produces adaptive gain modification. To investigate this problem in a simple motor system, we studied lid movement and orbicularis oculis muscle activity in human and rabbit subjects during adaptation of reflex eye blinks. The gain of the reflex could be increased or decreased, depending upon the nature of the adaptive stimulus. Since these gain changes could persist upon removal of the adapting stimulus, adaptation appears to result from a modification of the neural program subserving the blink reflex. The orbicularis oculis electromyogram revealed that the neural modifications producing adaptive gain changes predominantly altered the longer latency components of the reflex, while the short latency components remained unchanged. Moreover, in two other paradigms that modulate the gain of reflexes, habituation and reflex modification, similar changes also occurred primarily in the longer latency components of the blink reflex. This result suggests that modification of neurons in longer latency, indirect pathways, may underlie different forms of motor learning.

The effects of tobacco smoking on smooth pursuit eye movements

It has recently been shown that tobacco smoking in normal human subjects induces a transient primary-position upbeat nystagmus. We studied the effects of tobacco smoking on smooth pursuit eye movements and found defects in both vertical and horizontal tracking during the first 5 minutes after smoking one cigarette. The smooth pursuit defect consisted of a reduction in upward tracking velocity and the superposition of saccadic square-wave jerks on both vertical and horizontal tracking eye movements. The degree of impairment in upward smooth pursuit correlated with the intensity of tobacco-induced nystagmus present when recording in darkness. We suggest that these alterations are due to summation of nystagmus on normal tracking eye movements rather than primary defects in the smooth pursuit system.

Blink-related eye movements

Eye movements that accompany a blink have been measured in human subjects by the use of a visual-persistence method. With straight-ahead binocular viewing, each eye typically rotates nasalward and downward 1-2 deg during the closing phase of a blink. These eye movements are more rapid than the lid movements as recorded by high-speed photography. In fact, the eyes have already completed their initial rotation and started back again before the lids are fully closed. With off-center viewing, a blink causes each eye to rotate toward its primary position of regard. Indeed, if the eye is already in that position when the blink starts, the eye moves very little. With eyelids taped open, an eye tracker can be used, and records confirming the visual persistence tracings are obtained. Sequential photography of the cornea in profile reveals that the eye moves inward and back out again during a blink. The amplitude of this retraction is typically less than 1 mm; and its time course, slower than that of the rotational eye movements, parallels the closure and opening of the lids. In normal conditions of viewing there is no evidence of conjugate saccades, or of any large, upward rotation of the eyes (Bell's phenomenon) that was once believed to take place during a blink.

Tobacco-induced primary-position upbeat nystagmus

The effects of tobacco smoking on the eye movements of normal human subjects were studied using direct current electrooculography and the magnetic search coil technique. Tobacco induced a transient primary-position upbeat nystagmus in the dark, which was suppressed by visual fixation. It obeyed Alexander's law and was associated with oblique upward fast phases that alternated from side to side. Tobacco-induced nystagmus exhibited a latency of onset at 40 to 90 seconds, a duration of 10 to 20 minutes, and maximum slow-phase velocities at 2 to 3 minutes. We suggest that tobacco induces primary-position upbeat nystagmus through the excitatory effects of nicotine on the central vestibular pathways.

Different forms of blinks and their two-stage control

The purpose of this paper is to examine blink kinematics and the neural basis of blinks evoked reflexively by different kinds of stimuli. The kinematics of the upper lid movement and the electromyographic response of lid muscles levator palpebrae and orbicularis oculi were recorded in the rabbit during trigeminally and visually-evoked blinks. We find that there is a basic, kinematic difference between blinks. A blink in response to an airpuff is more rapidly accomplished and achieves a higher velocity than does an equal amplitude blink in response to a flash of light. The two forms of the reflex blink result from differences in the nature and timing of activity in antagonistic lid muscle motoneurons. Nevertheless, most characteristics of blink neural control are common to both reflex blinks. Most importantly, it appears that blinks are produced by two-stage neural control, an early component that is preprogrammed and a late component that is under stimulus control.

Eye-movement-dependent loss in vision and its time course during vergence

The attenuation of vision that has long been known to accompany saccadic eye movement has a significant component that is not attributable to visual masking or image smear, and this suppression of vision is now associated with nonsaccadic movement. The purpose of the present experiment was to determine the time course of visual suppression during a vergence eye movement. Suppression was evaluated psychophysically in human observers by measuring their loss of sensitivity to a brief, full-field decrement of light during 6 degrees-9 degrees convergence. Vergence-related suppression was similar in total duration and amplitude to saccadic and blink-related suppression. Since these other forms of oculomotor activity are vastly different in speed and total duration, it is unlikely that suppression results directly from the activity itself. Instead, these results support the hypothesis that a common, more centrally originating, suppression of vision occurs during eye movements, including saccades, eyeblinks, and vergence. Thus, while vision during eye movements can often be reduced through masking and smearing effects, the movement-dependent visual suppression measured in these experiments is a more generally occurring event.

Blinking and associated eye movements in humans, guinea pigs, and rabbits

Recordings of upper eyelid movements in humans, guinea pigs, and rabbits demonstrated that all three species displayed qualitatively similar patterns of eyelid movement. The relation between amplitude, duration, and maximum velocity in rabbits and humans was nearly identical. Guinea pig blinks were faster than those of rabbit and man. Electromyographic (EMG) recordings in humans demonstrated that the orbicularis oculis muscle participated in downward movement of the upper eyelid during blinks and eyelid closure but did not participate actively in the downward lid movement occurring with gaze changes. When looking straight ahead, the estimated stiffness and viscosity of the upper eyelid were 10 g/mm and 0.38 g X s X mm-1 for humans and 1.17 g/mm and 0.062 g X s X mm-1 for rabbits. Upward and abducting rotations of the eye accompanied blinks in rabbits and guinea pigs. Simultaneously, the eyeball retracted (translational movement) into the orbit. These translational and rotational eye movements resulted from contraction of the retractor bulbi muscle and cocontraction of antagonistic extraocular muscles. The data suggested that humans also retracted the eye during voluntary blinks. The retraction produced a rotation of the eye toward a "primary position" rather than a rotation in one specific direction. The relationship between the maximum velocity, duration, and amplitude of the down phase of a blink may be expressed as a single equation, maximum velocity = c X average velocity, where c is a constant. The same relationship, with a similar value for c, also describes saccadic eye movements and rapid skeletal movements. This implies that all three movements employ comparable neural mechanisms.

Vergence eye movements and visual suppression

Visual sensitivity was measured during vergence eye movements in order to determine whether a suppression of vision similar to that associated with saccades is also present during vergence. Suppression was evaluated psychophysically by determining sensitivity to briefly presented, full-field decrements of light in a Ganzfeld. Subjects were rougly 0.5 log unit less sensitive when stimuli were presented at the beginning of a 2-3 deg convergent or divergent eye movement, than during steady fixation. Thus, the concept of saccadic suppression must be broadened to include visual suppression that also accompanies nonsaccadic eye movements. These results support the hypothesis that vision is affected by signals that accompany initiation of oculomotor activity.

Preferential looking vision testing: application to evaluation of high-risk, prematurely born infants and children

We studied the application of preferential looking (PL) procedures to evaluation of visual performance of 59 prematurely born infants and young children. Neurological abnormalities were present in 29 of the 59 patients. Ophthalmic examination revealed normal eyes in 20 of the patients; retinopathy of prematurity (20 patients) and strabismus (13 patients) were the most prevalent ophthalmological disorders; the remaining 6 patients had a variety of ophthalmic anomalies. Results indicate that PL testing of such patients is best accomplished after age 2 months. Patients with developmental delays can be tested, but they may have poor PL performance that is not accounted for by ophthalmic abnormalities. Differences in PL acuities between right and left eyes were indicative of amblyopia in some of the strabismic patients; sequential PL acuities monitored occlusion therapy in these pre-verbal youngsters.

Saccadic suppression under conditions of whiteout

We have measured the impairment of vision that accompanies a saccadic eye movement under whiteout conditions. Translucent plastic diffusers were fitted around the eyes to provide a luminous field without perceivable contours. Visual sensitivity of three subjects was tested by means of 10 msec luminance decrements of variable amplitude. We found that sensitivity was lower, by 0.7 to 1.1 log units, when the eyes were making 16 degree saccades than when they were at rest. Comparable amounts of saccadic suppression occurred under more usual Ganzfeld conditions. We therefore conclude that such minimal contours as are present in the Ganzfeld--fixation guides, minor surface blemishes, and the blurred outlines of the subject's nose and brow--have little effect on suppression. These results are consistent with the hypothesis that a centrally originating inhibitory process accompanies the initiation of the saccade.

Dark-adapted sensitivity, rhodopsin content, and background adaptation in pcd/pcd mice

Adaptation to steady background lights has been investigated in pcd/pcd mice, a mutant strain with retinal degeneration. The hyperbolic stimulus/response functions of the scotopic b-wave of the electroretinogram show progressive changes. For the dark-adapted eyes the decrease in log sensitivity between ages 1 and 12 months is related linearly to the decreasing rhodopsin content. The observed decline in amplitude of maximum responses from dark-adapted eyes begins only after age 5 months and is accompanied by gradual prolongation of the time to the peak of half-maximum b-wave responses. At all ages, the sensitivity of response is changed little by increments of steady red background lights; the greatest slope of log sensitivity vs. log background plots is about +0.2. (In normal mice the slope is about +0.9.) The pcd/pcd b-wave results do not fit the normal empirical relation that is thought to reflect neural processing in the distal retina. Thus the present results suggest that neural processing is abnormal in pcd/pcd retinas.

Measuring visual acuity in infants

This paper reviews the course of development of visual acuity in human infants and young children. Researchers have devised methods based on optokinetic nystagmus, visually evoked cortical potentials and preferential looking to assess visual acuity in infants and preverbal youngsters. During the first postnatal year and early childhood, acuity, measured by any of these methods, improves with increasing age. Each of these methods, has now been applied to the evaluation of clinical cases. Also presented are the results of a quick and simple test based on preferential looking, which has been incorporated into our clinical evaluation of infants. The data so far indicate that infants with normal eyes pass the test while infants with ocular problems which would interfere with vision fail. For evaluation of large numbers of infants this test appears to have the potential to assist nonspecialized personnel in the early identification of ophthalmic abnormalities.

Infant vision testing by a behavioral method

A behavioral method of screening binocular infant vision called forced choice preferential looking (FPL) has been developed. Clinical trials of the FPL test for young infants are reported here. The test aids nonspecialized personnel in early identification of bilateral ocular abnormalities and anomalies of binocular cooperation (strabismus). It is not possible to screen for monocular eye disease or amblyopia with this test. Modifications of the FPL test may offer new ways of assessing other aspects of visual function early in life.

A behavioral method for efficient screening of visual acuity in young infants. II. Clinical application

Visual performance in 130 infants was assessed in a clinical setting with the forced-choice preferential looking (FPL) method described by Dobson et al.5 Over 90% of infants completed testing. Testing usually required less than 10 min. One group of infants tested also underwent complete ophthalmic examination. The second, larger group was screened with handlight examinations and FPL testing; any abnormalities detected were evaluated by full ophthalmic examination. Results so far indicate that the FPL test accurately identifies babies with binocular visual problems and that, when coupled with a handlight examination, it provides efficient screening for ocular problems in infants 0 through 16 weeks of age (postterm).