Concomitant hypertension, bradycardia, and loss of transcranial electric motor evoked potentials during pedicle hook removal: report of a case.

Neurophysiologic monitors in the form of transcranial electric motor evoked potentials (tceMEPs) and somatosensory evoked potentials (SSEPs) have become widely used modalities to monitor spinal cord function during major orthopedic spine procedures. In combination with invasive and non-invasive clinical monitoring and an anesthesia information management system (AIMS), we promptly recognized an acute change in hemodynamic and neurophysiologic parameters, managed intraoperative spinal cord contusion, and successfully minimized iatrogenic injury to the spinal cord during corrective spine surgery.

Influence of nitrous oxide on posterior tibial nerve cortical somatosensory evoked potentials.

The suppressive effect of the halogenated inhalation anesthesia on cortical somatosensory evoked potentials (cSSEPs) has been well documented. Less studied and appreciated is the effect of nitrous oxide often with a narcotic as an alternative to a potent agent for spinal cord monitoring. This study sought to define more clearly the influence of nitrous oxide on cSSEPs elicited to posterior tibial nerve stimulation. A secondary purpose was to demonstrate the advantage of a total intravenous propofol anesthesia in facilitating uncompromised large-amplitude cSSEPs. Fifty adult patients undergoing anterior cervical discectomy served as the study sample. Brainstem and cortical posterior tibial nerve SSEPs were recorded under two independent anesthesia conditions, namely, nitrous oxide and propofol. Results demonstrated a significant amplitude reduction and latency prolongation with the nitrous oxide versus propofol protocol. cSSEP amplitude with propofol was, on the average, approximately two times larger than that with nitrous oxide. Based on these findings, the use of nitrous-oxide anesthesia is not recommended when limited to monitoring cSSEPs that are already amplitude compromised secondary to existing spinal cord disease.

Relationship between response latency and amplitude for ganglion and geniculate X- and Y-cells in the cat.

This study investigates the relationship between visual response latency and amplitude in the retina and dorsal lateral geniculate nucleus (dLGN) of the anesthetized, paralyzed cat. The discharge rate profiles of retinal ganglion and dLGN X- and Y-cells were measured on a trial by trial basis during repeated stimulation with sinusoidal grating patterns. Latencies of response onsets and peaks were regressed linearly against different measures of response amplitude to determine the extent of covariance. In general, response amplitude was a poor predictor of response latency for both retinal ganglion and geniculate cells. The results suggest that response latency, which changes systematically with stimulus spatial frequency and/or contrast, is not a trivial consequence of discharge rate at either level of the visual system.

Traumatic trochlear nerve palsy diagnosed by magnetic resonance imaging: case report and review of the literature.

Although head trauma is the leading cause of acquired trochlear nerve dysfunction, it receives little attention in the neurosurgical literature. A case is reported of closed head injury that resulted in a right superior oblique palsy in association with incoordination on the left side. Diagnostic imaging revealed a normal cranial computed tomographic scan and a left dorsal midbrain lesion on magnetic resonance imaging scan. The relevant anatomy is reviewed, as well as the action of the superior oblique muscle, its agonists and antagonists, and the clinical manifestations of superior oblique dysfunction. This case is one of the few we are aware of in which a relatively isolated trochlear nerve palsy is the result of a lesion that can be documented by diagnostic imaging, and the first in which the imaging modality is magnetic resonance imaging scan.

Response variability of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat.

1. We measured the variability of neural responses in the dorsal lateral geniculate nucleus (dLGN) of the anesthetized, paralyzed cat during repeated visual stimulation with sinusoidal grating patterns. Results are reported for 11 X-cells and 16 Y-cells recorded in laminae A and A1. 2. The responses of most X- and Y-cells varied markedly from trial to trial. The standard deviations of prestimulus, base-line discharge rate. In contrast, the standard deviations of poststimulus responses increased only slightly or not at all with increases in mean discharge rate. 3. Standard deviations of poststimulus responses to optimal stimuli were about one-third the size of mean discharge rates. Relative variability (standard deviation/mean) increased markedly and in nonlinear fashion with decreases in response amplitude, which resulted in considerable overlap of base-line and poststimulus response distributions when stimuli were less than optimal.

Common and differential effects of attentive fixation on the excitability of parietal and prestriate (V4) cortical visual neurons in the macaque monkey.

The excitability of cortical neurons of prestriate area V4 and area PG of the inferior parietal lobule were examined using the method of single-neuron analysis in awake macaque monkeys. Levels of excitability were measured as the intensity of response to optimal visual stimuli placed in the most responsive region of the cell's receptive field. Physically and retinotopically identical stimuli were delivered during eye movement pauses under 3 conditions: during a no-task state in which the animal was awake and alert, but not receiving or expecting rewards or working in any task; between trials of the task state, the intertrial interval, while the animal awaited the appearance of a fixation target; and during the foreperiod of the task state, as the animal attentively fixated a small target light, waiting to detect its dimming in order to receive liquid reward. Experiments were carried out in 6 hemispheres of 4 monkeys; both V4 and PG were examined through the same chamber placements in 2 hemispheres. A total of 478 neurons in V4 and PG were identified as visual; quantitative studies were done on 146 in V4 and 54 in PG. We found in these experiments a common effect, a 3-4-fold facilitation of the responses of both V4 and PG visual neurons during the task state as compared to in the no-task state, and a differential effect, in that V4 neurons showed a similar 3-4-fold facilitation of responses to stimuli presented during the intertrial interval, whereas PG neuronal responses during this interval were similar to those evoked in the no-task state. We describe the functional properties of V4 neurons studied in the waking state. The findings are discussed in relation to the positions of these 2 areas in the occipitoparietal and occipitotemporal transcortical visual systems and to their respective roles in visuospatial perception and pattern recognition. They are also discussed with regard to the candidate neural mechanisms through which the changes in cortical neuronal excitability might be mediated.

Visual latency of ganglion X- and Y-cells: a comparison with geniculate X- and Y-cells.

Visual response latencies and rise times of X and Y ganglion cells recorded in the optic tract of anaesthetized, paralyzed cats were measured during repeated stimulation with sinusoidal gratings. These measures were compared with visual latencies and rise times of X- and Y-cells in the dorsal lateral geniculate nucleus. Measurements were restricted to individual trials on which the instantaneous discharge rate exceeded a criterion amplitude defined in terms of the statistics of the baseline activity of each cell in order to screen out false alarm responses. The onset and peak latencies of ganglion Y-cells are about 10-15 msec shorter than those of ganglion X-cells at low spatial frequencies (less than 0.25 c/deg) but about 10-20 msec longer at higher spatial frequencies (greater than 0.75 c/deg/). The onset latencies of geniculate X- and Y-cells lag their ganglion counterparts by 10-20 msec. Despite a delay in onsets of geniculate responses, the peak latencies of geniculate and ganglion X-cells are similar, and peak latencies of geniculate Y-cells are even shorter than those of their ganglion inputs. The short latencies of the peak responses of geniculate Y-cells are related to their short response rise times. A functional consequence of the bursty, but fast responses of geniculate Y-cells may be to accelerate the processing of lower spatial frequencies by the retino-geniculate Y-cell pathway.

The effects of monocular deprivation on the visual latency of geniculate X- and Y-cells in the cat.

Monocular lid suture deprivation during early visual development of the cat alters the temporal flow of retinal information as it passes through the dorsal lateral geniculate nucleus. The latencies of Y-cells located in the deprived layers and of both X- and Y-cells in the non-deprived layers are shorter than the latencies of their counterparts in normally reared cats. The visual response onset latencies of X-cells located in the deprived geniculate layers lag those of X-cells in the non-deprived layers.

Visual response latency of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat.

Visual response latencies and rise times of X- and Y-cells in the dorsal lateral geniculate nucleus (dLGN) of anaesthetized, paralyzed cats were measured during repeated stimulation with sinusoidal grating patterns. Measurements were restricted to individual stimulus trials on which the instantaneous discharge rate exceeded a criterion amplitude defined in terms of the baseline activity of each cell. The latencies of response onsets and response peaks were systematically related to the spatial frequency and contrast of the grating stimuli. Response latencies of Y-cells were shortest for gratings of low spatial frequency (0.17 c/deg) and increased monotonically with increases in spatial frequency. Response latencies of X-cells were shortest for gratings of intermediate spatial frequency (0.75 c/deg) and longer for lower and higher spatial frequencies. Latencies decreased monotonically with increases in stimulus contrast from 5 to 40% for both X- and Y-cells. In general, short-latency responses were less variable than long-latency responses. This was true for absolute as well as relative measures of variability. The mean onset and peak latencies of Y-cell responses were 10-15 msec shorter than the corresponding latencies of X-cell responses to stimuli of optimal spatial frequency and contrast. The rise times (latency of response peak minus latency of response onset) of Y-cell responses were consistently shorter than those of X-cells in spite of the higher peak responses of Y-cells. The results of this study are consistent with the idea that low spatial frequency information is passed through the lateral geniculate nucleus more quickly than is high spatial frequency information. These data provide support for models of visual processing wherein a coarse, global analysis of the visual scene by Y-cells precedes a finer, local analysis by X-cells.

Cerebellar targets of visual pontine cells in the cat.

The cerebellum receives visual mossy fiber input from the cerebral cortex via visual cells in the pons. We identified the regions of cat cerebellum that receive cerebral visual input by injecting orthograde tracers among physiologically identified visual pontine cells. Cerebellar labeling following these injections indicates that the contralateral paraflocculus and the rostral folium of the uvula (vermal lobule IX) receive the heaviest projection from cortically activated pontine visual cells. Lighter visual input reaches much of the rest of the contralateral posterior lobe. A second experiment combined, in the same animal, orthograde tracing of the visual corticopontine pathway with retrograde tracing of the pontocerebellar projection. The results of this experiment confirm that the paraflocculus and uvula receive more cortical visual input than do other regions of the cerebellum. This experiment also shows that uvula-projecting and paraflocculus-projecting cells occupy different parts of the ventromedial pons. Uvula-projecting cells cluster immediately adjacent to the ventral and medial borders of the pyramidal tract and near the midline. Paraflocculus-projecting cells lie ventral and medial to the pyramidal tract but displaced from its border. There are few paraflocculus-projecting cells near the midline.