Anton Plotkin

3-D magnetic tracking of a single subminiature coil with a large 2-D array of uniaxial transmitters

In this paper, we describe a novel system and method for magnetic tracking of a single subminiature coil, which can be used, for example for intrabody navigation of flexible medical instruments.

Neural activity at the human olfactory epithelium reflects olfactory perception

Organization of receptive surfaces reflects primary axes of perception. In vision, retinal coordinates reflect spatial coordinates. In audition, cochlear coordinates reflect tonal coordinates. However, the rules underlying the organization of the olfactory receptive surface are unknown. To test the hypothesis that organization of the olfactory epithelium reflects olfactory perception, we inserted an electrode into the human olfactory epithelium to directly measure odorant-induced evoked responses. We found that pairwise differences in odorant pleasantness predicted pairwise differences in response magnitude; that is, a location that responded maximally to a pleasant odorant was likely to respond strongly to other pleasant odorants, and a location that responded maximally to an unpleasant odorant was likely to respond strongly to other unpleasant odorants. Moreover, the extent of an individuals perceptual span predicted their span in evoked response. This suggests that, similarly to receptor surfaces for vision and audition, organization of the olfactory receptor surface reflects key axes of perception.

Sniffing enables communication and environmental control for the severely disabled

Paradoxically, improvements in emergency medicine have increased survival albeit with severe disability ranging from quadriplegia to “locked-in syndrome.” Locked-in syndrome is characterized by intact cognition yet complete paralysis, and hence these individuals are “locked-in” their own body, at best able to communicate using eye blinks alone. Sniffing is a precise sensory-motor acquisition entailing changes in nasal pressure. The fine control of sniffing depends on positioning the soft palate, which is innervated by multiple cranial nerves. This innervation pattern led us to hypothesize that sniffing may remain conserved following severe injury. To test this, we developed a device that measures nasal pressure and converts it into electrical signals. The device enabled sniffs to control an actuator with speed similar to that of a hand using a mouse or joystick. Functional magnetic resonance imaging of device usage revealed a widely distributed neural network, allowing for increased conservation following injury. Also, device usage shared neural substrates with language production, rendering sniffs a promising bypass mode of communication. Indeed, sniffing allowed completely paralyzed locked-in participants to write text and quadriplegic participants to write text and drive an electric wheelchair. We conclude that redirection of sniff motor programs toward alternative functions allows sniffing to provide a control interface that is fast, accurate, robust, and highly conserved following severe injury.

Magnetic Eye Tracking: A New Approach Employing a Planar Transmitter

A new scleral search coil (SSC) tracking approach employing a planar transmitter has been developed theoretically and tested experimentally. A thin and flat transmitter is much more convenient in installation, operation, and maintenance than the conventional large cubic one. A planar transmitter also increases the mobility of SSC systems, simplifies their accommodation in a limited clinical space, enables bedside testing, and causes no visual distractions and no discomfort to the users. Moreover, it allows tracking not only the SSC orientation, but also its location, which is very important for many medical and scientific applications. The suggested approach provides the speed and precision that are required in SSC applications. The experimental results show that it can be used for the diagnosis of vestibular disorders. The tracking precision is in good agreement with its theoretical estimation.

A Mechanistic Link between Olfaction and Autism Spectrum Disorder

Summary Internal action models (IAMs) are brain templates for sensory-motor coordination underlying diverse behaviors [1]. An emerging theory suggests that impaired IAMs are a common theme in autism spectrum disorder (ASD) [2–4]. However, whether impaired IAMs occur across sensory systems and how they relate to the major phenotype of ASD, namely impaired social communication [5], remains unclear. Olfaction relies on an IAM known as the sniff response, where sniff magnitude is automatically modulated to account for odor valence [6–12]. To test the failed IAM theory in olfaction, we precisely measured the non-verbal non-task-dependent sniff response concurrent with pleasant and unpleasant odors in 36 children—18 with ASD and 18 matched typically developing (TD) controls. We found that whereas TD children generated a typical adult-like sniff response within 305 ms of odor onset, ASD children had a profoundly altered sniff response, sniffing equally regardless of odor valance. This difference persisted despite equal reported odor perception and allowed for 81% correct ASD classification based on the sniff response alone (binomial, p < 0.001). Moreover, increasingly aberrant sniffing was associated with increasingly severe ASD (r = −0.75, p < 0.001), specifically with social (r = −0.72, p < 0.001), but not motor (r < −0.38, p > 0.18), impairment. These results uncover a novel ASD marker implying a mechanistic link between the underpinnings of olfaction and ASD and directly linking an impaired IAM with impaired social abilities.

Magnetic Tracking of Eye Motion in Small, Fast-Moving Animals

Here, we present a new approach for the magnetic tracking of eye motion in small, fast-moving animals. We employ a thin, flat magnetic tracking transmitter instead of the conventional bulky, cubic-shape transmitting frame. The new transmitter enables convenient access to the tracked animal, causes no visual distractions, and occupies much less space. We also employ a tiny solenoidal search coil instead of the conventional scleral search coils. Such a small solenoidal search coil attached laterally to the eye does not limit the peripheral field of view and allows the animal to perform its standard behavioral tasks. The flat transmitter comprises eight transmitting coils that allow us to monitor not only the orientation of a search coil but also its location. To test the efficiency of the new approach, we have measured the location and orientation of the solenoidal search coils attached to the eyes and head of an archer fish during swimming, targeting, and shooting. The size of the coils attached to the fish eyes was 2 mm in diameter and 2 mm in length, and the size of the coil attached to the fish head was 4 mm in diameter and 4 mm in length. The transmitter size was 60 cm times 60 cm times 2 cm. At a 25 cm from the transmitter, we have obtained the tracking resolution of 3 millidegree and 8.3 mu m rms for a 200-Hz bandwidth. Such a performance is good enough to precisely monitor the fastest component in the fish eyes movements. The fish with the search coils on the eyes and head correctly hits the target up to 20 times during an experimental session, which is similar to the shooting rate of the fish without the search coils. This implies that our new design does not introduce much discomfort for the fish.

A Tube-Core Orthogonal Fluxgate Operated in Fundamental Mode

In this paper, we suggest applying fundamental-mode operation to orthogonal fluxgates with tube cores. Excitation current in these fluxgates flows through a toroidal coil wound around the tube core, whereas in the orthogonal fluxgates with amorphous wires, it flows directly through the core. Having no excitation current inside the core reduces its heating and, hence, decreases the fluxgate thermal drift. Employing the toroidal coil also allows decreasing the excitation current by simply increasing the number of coil turns, while keeping the same intensity of the excitation field. Our experiments have shown a much higher efficiency of the new operating mode as compared with the second-harmonic mode. Adding a great enough dc bias to the ac excitation has caused a dramatic noise reduction. This effect is especially pronounced at relatively low frequencies, below 10 kHz. The fluxgate resolution in the fundamental mode, 10 pT/radicHz at 1 Hz , is by a factor of 30 better than in the second-harmonic mode. The sensitivity in the fundamental mode exceeds by a factor of 12.5 the sensitivity in the second-harmonic mode. We have also observed an about two times lower thermal drift of the fluxgate output. We have also found in this work that the phase noise of the excitation current is the main contributor to the fluxgate noise at low frequencies. It contributes about 67% to the fluxgate noise power density near the fundamental.

A New Calibration Procedure for Magnetic Tracking Systems

In this study, we suggest a new approach for the calibration of magnetic tracking systems that allows us to calibrate the entire system in a single setting. The suggested approach is based on solving a system of equations involving all the system parameters. These parameters include: 1) the magnetic positions of the transmitting coils; 2) their magnetic moments; 3) the magnetic position of the sensor; 4) its sensitivity; and 5) the gain of the sensor output amplifier. We choose a set of parameters that define the origin, orientation, and scale of the reference coordinate system and consider them as constants in the above system of equations. Another set of constants is the sensor output measured at a number of arbitrary positions. The unknowns in the above equations are all the other system parameters. To define the origin and orientation of the reference coordinate system, we first relate it to a physical object, e.g., to the transmitter housing. We then use special supports to align the sensor with the edges of the transmitter housing and measure the sensor output at a number of aligned positions. To define the scale of the reference coordinate system, we measure the distance between two arbitrary sensor locations with a precise instrument (a caliper). This is the only parameter that should be calibrated with the help of an external measurement tool. To illustrate the efficiency of the new approach, we applied the calibration procedure to a magnetic tracking system employing 64 transmitting coils. We have measured the systematic tracking errors before and after applying the calibration. The systematic tracking errors were reduced by an order of magnitude due to applying the new calibration procedure.

Compensation of the thermal drift in the sensitivity of fundamental-mode orthogonal fluxgates

A method is suggested that reduces the temperature coefficient of the sensitivity of fundamental-mode orthogonal fluxgates by an order of magnitude. For the background magnetic fields greater than 20μT, the method provides even better reduction, down to 100ppm∕°C, which is comparable with the temperature coefficient of conventional parallel-type fluxgates operated in closed-loop configuration. The fluxgate prototype has demonstrated a 20-fold reduction of the thermal drift in its sensitivity. The suggested method is solely based on the processing of signals generated by fundamental-mode orthogonal fluxgates in open-loop configuration and does not require any additional hardware. This makes the open-loop fundamental-mode orthogonal fluxgates competitive with closed-loop parallel-type ones in terms of both resolution and accuracy, while still keeping them simpler than the parallel fluxgates.

Detection of response to command using voluntary control of breathing in disorders of consciousness

Background: Detecting signs of consciousness in patients in a vegetative state/unresponsive wakefulness syndrome (UWS/VS) or minimally conscious state (MCS) is known to be very challenging. Plotkin et al. (2010) recently showed the possibility of using a breathing-controlled communication device in patients with locked in syndrome. We here aim to test a breathing-based “sniff controller” that could be used as an alternative diagnostic tool to evaluate response to command in severely brain damaged patients with chronic disorders of consciousness (DOC). Methods: Twenty-five DOC patients were included. Patients’ resting breathing-amplitude was measured during a 5 min resting condition. Next, they were instructed to end the presentation of a music sequence by sniffing vigorously. An automated detection of changes in breathing amplitude (i.e., >1.5 SD of resting) ended the music and hence provided positive feedback to the patient. Results: None of the 11 UWS/VS patients showed a sniff-based response to command. One out of 14 patients with MCS was able to willfully modulate his breathing pattern to answer the command on 16/19 trials (accuracy 84%). Interestingly, this patient failed to show any other motor response to command. Discussion: We here illustrate the possible interest of using breathing-dependent response to command in the detection of residual cognition in patients with DOC after severe brain injury.