Recurring Functional Interactions Predict Network Architecture of Interictal and Ictal States in Neocortical Epilepsy

Human epilepsy patients suffer from spontaneous seizures, which originate in brain regions that also subserve normal function. Prior studies demonstrate focal, neocortical epilepsy is associated with dysfunction, several hours before seizures. How does the epileptic network perpetuate dysfunction during baseline periods? To address this question, we developed an unsupervised machine learning technique to disentangle patterns of functional interactions between brain regions, or subgraphs, from dynamic functional networks constructed from approximately 100 h of intracranial recordings in each of 22 neocortical epilepsy patients. Using this approach, we found: (1) subgraphs from ictal (seizure) and interictal (baseline) epochs are topologically similar, (2) interictal subgraph topology and dynamics can predict brain regions that generate seizures, and (3) subgraphs undergo slower and more coordinated fluctuations during ictal epochs compared to interictal epochs. Our observations suggest that seizures mark a critical shift away from interictal states that is driven by changes in the dynamical expression of strongly interacting components of the epileptic network.

Dynamic Network Drivers of Seizure Generation, Propagation and Termination in Human Neocortical Epilepsy

The epileptic network is characterized by pathologic, seizure-generating 'foci' embedded in a web of structural and functional connections. Clinically, seizure foci are considered optimal targets for surgery. However, poor surgical outcome suggests a complex relationship between foci and the surrounding network that drives seizure dynamics. We developed a novel technique to objectively track seizure states from dynamic functional networks constructed from intracranial recordings. Each dynamical state captures unique patterns of network connections that indicate synchronized and desynchronized hubs of neural populations. Our approach suggests that seizures are generated when synchronous relationships near foci work in tandem with rapidly changing desynchronous relationships from the surrounding epileptic network. As seizures progress, topographical and geometrical changes in network connectivity strengthen and tighten synchronous connectivity near foci-a mechanism that may aid seizure termination. Collectively, our observations implicate distributed cortical structures in seizure generation, propagation and termination, and may have practical significance in determining which circuits to modulate with implantable devices.

The ataxic mouse as a model for studying downbeat nystagmus

Downbeat nystagmus (DBN) is a common eye movement complication of cerebellar disease. Use of mice to study pathophysiology of vestibulocerebellar disease is increasing, but it is unclear if mice can be used to study DBN; it has not been reported in this species. We determined whether DBN occurs in the ataxic mutant tottering, which carries a mutation in the Cacna1a gene for P/Q calcium channels. Spontaneous DBN occurred only rarely, and its magnitude did not exhibit the relationship to head tilt seen in human patients. DBN during yaw rotation was more common and shares some properties with the tilt-independent, gaze-independent component of human DBN, but differs in its dependence on vision. Hyperactivity of otolith circuits responding to pitch tilts is hypothesized to contribute to the gaze-independent component of human DBN. Mutants exhibited hyperactivity of the tilt maculo-ocular reflex (tiltMOR) in pitch. The hyperactivity may serve as a surrogate for DBN in mouse studies. TiltMOR hyperactivity correlates with hyperdeviation of the eyes and upward deviation of the head during ambulation; these may be alternative surrogates. Muscimol inactivation of the cerebellar flocculus suggests a floccular role in the tiltMOR hyperactivity and provides insight into the rarity of frank DBN in ataxic mice.

Eye hyperdeviation in mouse cerebellar mutants is comparable to the gravity-dependent component of human downbeat nystagmus

Humans with cerebellar degeneration commonly exhibit downbeat nystagmus (DBN). DBN has gravity-independent and -dependent components, and the latter has been proposed to reflect hyperactive tilt maculo-ocular reflexes (tilt-MOR). Mice with genetically determined cerebellar ataxia do not exhibit DBN, but they do exhibit tonic hyperdeviation of the eyes, which we have proposed to be the DBN equivalent. As such, the tilt-MOR might be predicted to be hyperactive in these mutant mice. We measured the tilt-MOR in 10 normal C57BL/6 mice and in 6 tottering, a mutant exhibiting ataxia and ocular motor abnormalities due to mutation of the P/Q calcium channel. Awake mice were placed in body orientations spanning 360 degrees about the pitch axis. The absolute, equilibrium vertical angular deviations of one eye were measured using infrared videooculography. In both strains, eye elevation varied quasi-sinusoidally with tilt angle in the range of 90 degrees nose-up to 90 degrees nose-down. Beyond this range the eye returned to a neutral position. Deviation over +/-30 degrees of tilt was an approximately linear function of the projection of the gravity vector into the animal's horizontal plane, and can thus be summarized by its slope (sensitivity). Sensitivity measured 14.9 degrees/g for C57BL/6 and 20.3 degrees/g for tottering, a statistically significant difference. Thus the pitch otolithic reflex of the ataxic mutants is hyperactive relative to controls and could explain tonic hyperdeviation of the eyes, consistent with the idea that the tonic hyperdeviation is analogous to DBN.

Idiosyncratic variations in eye-head coupling observed in the laboratory also manifest during spontaneous behavior in a natural setting

The tendency to generate head movements during saccades varies from person to person. Head movement tendencies can be measured as subjects fixate sequences of illuminated targets, but the extent to which such measures reflect eye-head coupling during more natural behaviors is unknown. We quantified head movement tendencies in 20 normal subjects in a conventional laboratory experiment and in an outdoor setting in which the subjects directed their gaze spontaneously. In the laboratory, head movement tendencies during centrifugal saccades could be described by the eye-only range (EOR), customary ocular motor range (COMR), and the customary head orientation range (CHOR). An analogous EOR, COMR, and CHOR could be extracted from the centrifugal saccades executed in the outdoor setting. An additional six measures were introduced to describe the preferred ranges of eyes-in-head and head-on-torso manifest throughout the outdoor recording, i.e., not limited to the orientations following centrifugal saccades. These 12 measured variables could be distilled by factor analysis to one indoor and six outdoor factors. The factors reflect separable tendencies related to preferred ranges of visual search, head eccentricity, and eye eccentricity. Multiple correlations were found between the indoor and outdoor factors. The results demonstrate that there are multiple types of head movement tendencies, but some of these influence behavior across rather different experimental settings and tasks. Thus behavior in the two settings likely relies on common neural mechanisms, and the laboratory assays of head movement tendencies succeed in probing the mechanisms underlying eye-head coupling during more natural behaviors.

Eye orientation during static tilts and its relationship to spontaneous head pitch in the laboratory mouse

Both eye position and head orientation are influenced by the macular (otolith) organs, via the tilt maculo-ocular reflex (tiltMOR) and the vestibulo-collic reflexes, respectively. The mechanisms that control head position also influence the rest position of the eye because head orientation influences eye position through the tiltMOR. Despite the increasing popularity of mice for studies of vestibular and ocular motor functions, relatively little is known in this species about tiltMOR, spontaneous orientation of the head, and their interrelationship. We used 2D video oculography to determine in C57BL/6 mice the absolute horizontal and vertical positions of the eyes over body orientations spanning 360 degrees about the pitch and roll axes. We also determined head pitch during ambulation in the same animals. Eye elevation varied approximately sinusoidally as functions of pitch or roll angle. Over the central +/-30 degrees of pitch, sensitivity and gain in the light were 31.7 degrees/g and 0.53, respectively. The corresponding values for roll were 31.5 degrees/g and 0.52. Absolute positions adopted in light and darkness differed only slightly. During ambulation, mice carried the lambda-bregma plane at a downward pitch of 29 degrees , corresponding to a horizontal eye position of 64 degrees and a vertical eye position of 22 degrees . The vertical position is near the center of the range of eye movements produced by the pitch tiltMOR. The results indicate that the tiltMOR is robust in this species and favor standardizing pitch orientation across laboratories. The robust tiltMOR also has significant methodological implications for the practice of pupil-tracking video oculography in this species.

Eye movements of the murine P/Q calcium channel mutant tottering, and the impact of aging

Mice carrying mutations of the gene encoding the ion pore of the P/Q calcium channel (Cacna1a) are an instance in which cerebellar dysfunction may be attributable to altered electrophysiology and thus provide an opportunity to study how neuronal intrinsic properties dictate signal processing in the ocular motor system. P/Q channel mutations can engender multiple effects at the single neuron, circuit, and behavioral levels; correlating physiological and behavioral abnormalities in multiple allelic strains will ultimately facilitate determining which alterations of physiology are responsible for specific behavioral aberrations. We used videooculography to quantify ocular motor behavior in tottering mutants aged 3 mo to 2 yr and compared their performance to data previously obtained in the allelic mutant rocker and C57BL/6 controls. Tottering mutants shared numerous abnormalities with rocker, including upward deviation of the eyes at rest, increased vestibuloocular reflex (VOR) phase lead at low stimulus frequencies, reduced VOR gain at high stimulus frequencies, reduced gain of the horizontal and vertical optokinetic reflex, reduced time constants of the neural integrator, and reduced plasticity of the VOR as assessed in a cross-axis training paradigm. Unlike rocker, young tottering mutants exhibited normal peak velocities of nystagmus fast phases, arguing against a role for neuromuscular transmission defects in the attenuation of compensatory eye movements. Tottering also differed by exhibiting directional asymmetries of the gains of optokinetic reflexes. The data suggest at least four pathophysiological mechanisms (two congenital and two acquired) are required to explain the ocular motor deficits in the two Cacna1a mutant strains.

Amplitudes of head movements during putative eye-only saccades

The mechanisms allowing humans and other primates to dissociate head and eye movements during saccades are poorly understood. A more precise knowledge of head movement behavior during apparent eye-only saccades may provide insight into those mechanisms. We studied the distributions of head amplitude in normal humans. In half of the subjects, these distributions indicated the presence of a population of minor ("residual") head movements during eye-only saccades, distinct from the continuum of head movements generated during frank eye-head saccades. Like full-sized head movements, the residual movements grew in proportion to target eccentricity, indicating their drive is derived from the premotor command for the saccade. Furthermore, their amplitudes related most strongly to the head amplitudes obtained when subjects produced full-sized head movements and were reduced when subjects were instructed to perform exclusively eye-only saccades. Both observations suggest that the drive for residual head movements originates downstream of the point in which the head movement command diverges from the generalized gaze shift command. The results are consistent with a model of head control in which a neural gate prevents the common gaze shift command from reaching the head premotor circuitry whenever an eye-only saccade is desired. However, the gate is either imperfect or the multiple pathways that relay gaze shift signals to the head motor circuitry allow for the gate to be circumvented. The results underscore the need for physiological studies to probe neuronal activity related to neck activation during eye-only saccades.

Overlapping gaze shifts reveal timing of an eye–head gate

The ability to dissociate eye movements from head movements is essential to animals with foveas and fovea-like retinal specializations, as these species shift the eyes constantly, and moving the head with each gaze shift would be impractical and energetically wasteful. The processes by which the dissociation is effected remain unclear. We hypothesized that the dissociation is accomplished by means of a neural gate, which prevents a common gaze-shift command from reaching the neck circuitry when eye-only saccades are desired. We further hypothesized that such a gate would require a finite period to reset following opening to allow a combined eye-head saccade, and thus the probability of generating a head movement during a saccade would be augmented when a new visual target (the 'test' target) appeared during, or soon after, a combined eye-head saccade made to an earlier, 'conditioning' target. We tested human subjects using three different combinations of targets-a horizontal conditioning target followed by a horizontal test target (H/H condition), horizontal conditioning followed by vertical test (H/V), and vertical conditioning followed by horizontal test (V/H). We varied the delay between the onset of the conditioning head movement and the presentation of the test target, and determined the probability of generating a head movement to the test target as a function of target delay. As predicted, head movement probability was elevated significantly at the shortest target delays and declined thereafter. The half-life of the increase in probability averaged 740, 490, and 320 ms for the H/H, H/V, and V/H conditions, respectively. For the H/H condition, the augmentation appeared to outlast the duration of the conditioning head movement. Because the augmentation could outlast the conditioning head movement and did not depend on the head movements to the conditioning and test targets lying in the same directions, we could largely exclude the possibility that the augmentation arises from mechanical effects. These results support the existence of the hypothetical eye-head gate, and suggest ways that its constituent neurons might be identified using neurophysiological methods.

Inhibited head movements: A risk of combining phoning with other activities?

Studies of cellular phone use while driving have attributed impaired performance to the distractions of conversation. We determined that holding an inactive phone to the ear reduces the probability of eccentric head positions, potentially indicating reduced ability to monitor the visual surround. This effect may constitute a risk of cellular phone use independent of conversation and peculiar to handheld models.

Image-shifting optics for a nystagmus treatment device

Acquired pendular nystagmus (APN) complicates multiple sclerosis and other neurological disorders, causes visual impairment, and is often refractory to available treatments. Vision could be improved by an optical aid that shifts the seen world in lockstep with the APN. An essential component of such a device is the image-shifting mechanism, which must be light, accurate, suitable for battery operation, and capable of image shifting at the frequencies and amplitudes seen in APN. We determined that a three-lens image-shifting mechanism used in commercial image-stabilizing lenses has the potential to satisfy all these requirements. In combination with software designed to track nystagmus, the optical mechanism proved capable of improving visual acuity in 12 normal subjects experiencing simulated two-dimensional nystagmus. Acuity was restored to within an average of 0.12 logMAR (range 0.0-0.22) of the subjects' values without the simulated nystagmus. These results support the feasibility of an assistive device for patients with APN.

Application of adaptive filters to visual testing and treatment in acquired pendular nystagmus

Acquired pendular nystagmus (APN) complicates multiple sclerosis and other neurological disorders, causes visual impairment, and frequently resists treatment. Vision could be improved by a visual aid that gates or shifts the seen world in lockstep with the APN. Since the pathological oscillations are embedded in normal eye movements, such a device must track the nystagmus selectively. We evaluated the ability of an adaptive filter to perform this tracking and improve acuity when coupled to either of two devices--a shutter that permitted brief glimpses of the world synchronized with the nystagmus, or simulated image-shifting optics. In 10 normal subjects whose decimal acuity averaged 1.46 +/- 0.20, acuity fell to 0.36 +/- 0.08 under viewing conditions simulating APN. The synchronized shutter restored acuity to 0.60 +/- 0.12, while image-stabilization raised it to 1.17 +/- 0.13. Adaptive filters provide a practical means by which to track nystagmus. The most effective visual aid would couple such filters to image-stabilizing optics.

The influence of future gaze orientation upon eye-head coupling during saccades

Mammals with foveas (or analogous retinal specializations) frequently shift gaze without moving the head, and their behavior contrasts sharply with "afoveate" mammals, in which eye and head movements are strongly coupled. The ability to move the eyes without moving the head could reflect a gating mechanism that blocks a default eye-head synergy when an attempted head movement would be energetically wasteful. Based upon such considerations of efficiency, we predicted that for saccades to targets lying within the ocular motor range, the tendency to generate a head movement would depend upon a subject's expectations regarding future directions of gaze. We tested this hypothesis in two experiments with normal human subjects instructed to fixate sequences of lighted targets on a semicircular array. In the target direction experiment, we determined whether subjects were more likely to move the head during a small gaze shift if they expected that they would be momentarily required to make a second, larger shift in the same direction. Adding the onward-directed target increased significantly the distribution of final head positions (customary head orientation range, CHOR) observed during fixation of the primary target from 16.6+/-4.9 degrees to 25.2+/-7.8 degrees. The difference reflected an increase in the probability, and possibly the amplitude, of head movements. In the target duration experiment, we determined whether head movements were potentiated when subjects expected that gaze would be held in the vicinity of the target for a longer period of time. Prolonging fixation increased CHOR significantly from 53.7+/-18.8 degrees to 63.2+/-15.9 degrees. Larger head movements were evoked for any given target eccentricity, due to a narrowing in the gap between the x-intercepts of the head amplitude:target eccentricity relationship. The results are consistent with the idea that foveate mammals use knowledge of future gaze direction to influence the coupling of saccadic commands to premotor circuitry of the head. While the circuits ultimately mediating the coupling may lie within the brainstem, our results suggest that the cerebrum plays a supervisory role, since it is a likely seat of expectation regarding target behavior. Eye-head coupling may reflect separate gating and scaling mechanisms, and changes in head movement tendencies may reflect parametric modulation of either mechanism.

Proteolytic specificity of Lactobacillus delbrueckli subsp. bulgaricus influences functional properties of mozzarella cheese

Low-moisture part-skim Mozzarella cheeses were manufactured from 2% fat milk and aged for 21 d. Treatments included cheeses made with one of three different strains of Lactobacillus delbrueckii ssp. bulgaricus in combination with a single strain of Streptococcus thermophilus. A fourth, control treatment consisted of cheeses made with only S. thermophilus. Although total proteolytic ability of these strains, as indicated by the o-phthaldialdehyde analysis, was similar in each of the three strains of L. bulgaricus, these strains exhibited different proteolytic specificities toward the peptide, alpha(s1)-CN (f 1-23). On the basis of their alpha(s1)-CN (f 1-23) cleavage patterns and a previously described classification, these strains were assigned to the groups I, III, and V. The objective of this study was to investigate the influence of lactobacilli proteolytic systems, based on specificity toward alpha(s1)-CN (f 1-23), on functionality of part-skim Mozzarella cheese. Moisture, fat, protein, salt-in-moisture, and moisture in nonfat substances content of cheeses made with groups I, III, and V strain were similar. Control cheese had a lower moisture content than did other treatments. Significant differences were observed in functional properties between cheeses manufactured using groups III and V strains. Cheeses made with groups I and III strains were similar in their meltability, hardness, cohesiveness, melt strength, and stretch quality. Meltability and cohesiveness increased with age, while melt strength and stretch quality decreased with age for all cheeses. Additionally, HPLC showed that total peak areas of water-soluble peptides derived from cleavage of alpha(s1)-CN (f 1-23) by different strains of lactobacilli could be highly correlated to meltability and stretch characteristics of cheeses made with those strains.