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Biomedical subjects

B J Frost

Publications and source records attributed to B J Frost.

At least 19 recordsLinked to original sources
PubMed2001

Rhenium oxo complexes of a chelating diyne ligand. Synthesis and study of the kinetics of protonation.

A series of oxo complexes, Re(O)X(diyne) (X = I, Me, Et), have been prepared from 2,7-nonadiyne and Re(O)I(3)(PPh(3))(2). Addition of B(C(6)F(5))(3) to Re(O)I(2,7-nonadiyne) (5) results in coordination of the oxo ligand to the boron. The protonation of Re(O)(X)(2-butyne)(2) and Re(O)(X)(2,7-nonadiyne)(2) with a variety of acids has been examined. With 5 and HBF(4)/Et(2)O, the ultimate product was [Re(CH(3)CN)(3)(I)(2,7-nonadiyne)](2+) (7). The conversion of 5 to 7 changes the conformation of the diyne ligand from a "chair" to a "boat" and shifts its propargylic protons considerably downfield in the (1)H NMR. The kinetics of the protonation of Re(O)I(2,7-nonadiyne) (5) by CF(3)SO(3)H in CH(3)CN have been monitored by visible spectroscopy, in a stopped-flow apparatus, and by low temperature (1)H NMR. Two second-order rate constants, presumably successive protonations, were observed in the stopped-flow, k(1) = 11.9 M(-)(1) s(-)(1) and k(2) = 3.8 M(-)(1) s(-)(1). Low temperature (1)H NMR spectroscopy indicated that the resulting solution contained a mixture of two doubly protonated intermediates X and Y, each of which slowly formed the product 7 via an acid-independent process.

Journal Article↗
PubMed2001

An experimental and theoretical investigation of the carbon dioxide insertion process into the tungsten-nitrogen bond of an anionic W(0) complex.

The pyridine bound 2-aminopyridine (2APH) derivative of tungsten pentacarbonyl has been prepared from photogenerated W(CO)5THF and 2APH. Deprotonation of the distal amine group by sodium hydride has provided two complexes, [Na][W(CO)5(2AP)] and [Na]2[W(CO)4(2AP)]2. Both complexes have been characterized by X-ray crystallography with the monomeric derivative being crystallized as its [Na2(18-crown-6)][W(CO)5(2AP)]2 salt which exhibits strong Na+...-NH interactions. Photolysis of W(CO)6 in the presence of excess 2-aminopyridine in THF has led to an efficient synthesis of the chelated neutral derivative, W(CO)4(2APH).2APH, where the extra equivalent of 2APH is hydrogen bonded to its bound counterpart. The 2-aminopyridine molecule of solvation was almost quantitatively removed via aqueous washings. Deprotonation of W(CO)4(2APH) with NaH afforded the amidopyridine derivative which was shown to rapidly undergo reaction with CO2 to yield the chelated carbamate complex, W(CO)4(OC(O)2AP)-. Nevertheless, because of the presence of small quantities of free 2-aminopyridine during the reactions with CO2, we have not been able to conclusively rule out participation by a ligand substitution process involving NC5H4NHCOOH. Ab initio computations were found to substantiate many of these experimental observations. That is, in the monodentate bound W(CO)5(2APH) derivative, binding through the pyridine nitrogen atom is favored by about 29 kJ/mol over the amine nitrogen atom, whereas the opposite site for binding is preferred for the deprotonated amido analogue, W(CO)5(2AP)-. Furthermore, both forms of W(CO)5(2AP)- were found to be more stable than the chelated tungsten tetracarbonyl anion plus CO. On the other hand, CO2 insertion into the W(CO)4(2AP)- anion to provide the chelated carbamate, W(CO)4(OC(O)2AP)-, was thermodynamically favored by >110 kJ/mol. Finally, both experimental and theoretical studies were inconclusive with regard to identifying reaction intermediates during the CO2 insertion pathway which involve prior interactions of CO2 at the amido nitrogen center.

Journal Article↗
PubMed2000

Azimuthal directional sensitivity of prepulse inhibition of the pinna startle reflex in decerebrate rats.

Previous studies have indicated that the auditory midbrain, the inferior colliculus, is important for both sound localization and mediation of prepulse inhibition of the startle reflex. The present study investigated the azimuthal directional sensitivity of prepulse inhibition of the pinna startle reflex in decerebrate rats. The pinna startle reflex was measured by recording multi-unit action potentials from the cervicoauricular muscles. The startling noise burst (94 dB SPL) was produced by a stationary speaker at 0 degrees azimuth, and the non-startling prepulse noise burst (46 dB SPL) was produced by a movable speaker whose direction was changed in the frontal azimuthal plane. The interval between the onset of the prepulse sound and the onset of the startling sound was 100 ms. The pinna reflex to the startling sound was strongly inhibited by the prepulse sound, and the inhibited startle response exhibited a flat azimuthal directional curve. In addition to further confirming that the neural pathways mediating prepulse inhibition are located in the brainstem, the present results indicate that interaural disparities of binaural inputs used for sound localization are not capable of modulating prepulse inhibition of the startle reflex.

Acoustic Stimulation↗
PubMed2000

Head-bobbing in pigeons: how stable is the hold phase?

The head movement of a walking pigeon Columba livia is characterized by two alternating phases, a thrust phase and a hold phase. While the head is rapidly thrust forward during the thrust phase, it has been shown repeatedly that it remains virtually motionless with respect to translation along a horizontal axis (roll axis) during the hold phase. It has been shown that the stabilization during the hold phase is under visual control. This has led to the view that the pigeon's head-bobbing is an optokinetic response to stabilize the retinal image during the hold phase. However, it has never been shown explicitly that the head is really held stable in space with respect to other translatory or rotatory dimensions. Using videography, we show here that this is in fact the case: except for a small but systematic slip that presumably serves as an error signal for retinal image stabilization, the head of the pigeon remains locked in space not only with respect to the horizontal (roll) axis but also with respect to vertical translation (along the yaw axis) and with respect to rotation around the pitch and yaw axes.

Animals↗
PubMed1999

Complex spike activity of Purkinje cells in the ventral uvula and nodulus of pigeons in response to translational optic flow.

The complex spike (CS) activity of Purkinje cells in the ventral uvula and nodulus of the vestibulocerebellum was recorded from anesthetized pigeons in response to translational optic flow. Translational optic flow was produced using a "translator" projector: a mechanical device that projected a translational optic flowfield onto the walls, ceiling, and floor of the room and encompassed the entire binocular visual field. CS activity was broadly tuned but maximally modulated in response to translational optic flow along a "best" axis. Each neuron was assigned a vector representing the direction in which the animal would need to translate to produce the optic flowfield that resulted in maximal excitation. The vector is described with reference to a standard right-handed coordinate system, where the vectors, +x, +y, and +z represent, rightward, upward, and forward translation of the animal, respectively. Neurons could be grouped into four response types based on the vector of maximal excitation. +y neurons were modulated maximally in response to a translational optic flowfield that results from self-motion upward along the vertical (y) axis. -y neurons also responded best to translational optic flow along the vertical axis but showed the opposite direction preference. The two remaining groups responded best to translational optic flow along horizontal axes: -x + z neurons and -x-z neurons. In summary, our results suggest that the olivocerebellar system dedicated to the analysis of translational optic flow is organized according to a reference frame consisting of three approximately orthogonal axes: the vertical axis, and two horizontal axes oriented 45 degrees to either side the midline. Previous research has shown that the rotational optic flow system, the eye muscles, the vestibular semicircular canals and the postural control system all share a similar spatial frame of reference.

Animals↗
PubMed1999

Responses of neurons in the nucleus of the basal optic root to translational and rotational flowfields.

The nucleus of the basal optic root (nBOR) receives direct input from the contralateral retina and is the first step in a pathway dedicated to the analysis of optic flowfields resulting from self-motion. Previous studies have shown that most nBOR neurons exhibit direction selectivity in response to large-field stimuli moving in the contralateral hemifield, but a subpopulation of nBOR neurons has binocular receptive fields. In this study, the activity of binocular nBOR neurons was recorded in anesthetized pigeons in response to panoramic translational and rotational optic flow. Translational optic flow was produced by the "translator" projector described in the companion paper, and rotational optic flow was produced by a "planetarium projector" described by Wylie and Frost. The axis of rotation or translation could be positioned to any orientation in three-dimensional space. We recorded from 37 cells, most of which exhibited a strong contralateral dominance. Most of these cells were located in the caudal and dorsal aspects of the nBOR complex and many were localized to the subnucleus nBOR dorsalis. Other units were located outside the boundaries of the nBOR complex in the adjacent area ventralis of Tsai or mesencephalic reticular formation. Six cells responded best to rotational flowfields, whereas 31 responded best to translational flowfields. Of the rotation cells, three preferred rotation about the vertical axis and three preferred horizontal axes. Of the translation cells, 3 responded best to a flowfield simulating downward translation of the bird along a vertical axis, whereas the remaining 28 responded best to flowfields resulting from translation along axes in the horizontal plane. Seventeen of these cells preferred a flowfield resulting from the animal translating backward along an axis oriented approximately 45 degrees to the midline, but the best axes of the remaining eleven cells were distributed throughout the horizontal plane with no definitive clustering. These data are compared with the responses of vestibulocerebellar Purkinje cells.

Animals↗
PubMed1998

Stereoscopic depth perception in the owl.

It is unclear whether the neural algorithm that underlies stereoscopic vision in birds incorporates both low level (camouflage breaking) and high level (depth ordering) comparisons of information available to each of the eyes. Both visual functions were successfully tested by examining transitive inference performance in two barn owls trained to discriminate static Julesz random dot stereograms, thus demonstrating a capacity to detect relative depth using fine retinal disparity as the sole cue for discrimination. Our behavioral tests provide strong evidence that the barn owl possesses global stereopsis comparable to that found in the macaque monkey--the owl's best stereoacuity was 2 min of arc.

Algorithms↗
PubMed1998

Common reference frame for neural coding of translational and rotational optic flow.

Self-movement of an organism through the environment is guided jointly by information provided by the vestibular system and by visual pathways that are specialized for detecting 'optic flow'. Motion of any object through space, including the self-motion of organisms, can be described with reference to six degrees of freedom: rotation about three orthogonal axes, and translation along these axes. Here we describe neurons in the pigeon brain that respond best to optic flow resulting from translation along one of the three orthogonal axes. We show that these translational optic flow neurons, like rotational optic flow neurons, share a common spatial frame of reference with the semicircular canals of the vestibular system. The three axes to which these neurons respond best are the vertical axis and two horizontal axes orientated at 45 degrees to either side of the body midline.

Animals↗
PubMed1998

Prepulse inhibition following lesions of the inferior colliculus: prepulse intensity functions.

The magnitude of the acoustic startle response can be reduced by a relatively weak sound presented immediately before the startle-eliciting sound; this phenomenon has been termed prepulse inhibition (PPI). Previous studies reported that PPI was present in the decerebrate rat, indicating that the primary neural pathways mediating PPI are located in the brainstem. The present study investigated the effects of focal excitotoxic lesions of the inferior colliculus (IC) on acoustic PPI in rats. In the first part, startle magnitudes were measured in six normal rats as the interstimulus interval (ISI) between the prepulse and startle-eliciting sounds varied between 10 and 100 ms. Prepulse-inhibited startle changed in an ISI-dependent manner with the most effective ISI at 50 ms. In the second part, 21 rats were assigned to three groups: normal unoperated, cortical lesion, and IC lesion. With the ISI fixed at 50 ms, as the prepulse sound level increased from 29 to 49 dB SPL, startle responses decreased quickly in both normal and cortical lesion rats. However, rats with unilateral IC lesions made with ibotenic acid had significantly lower PPI but did not display any increase in startle magnitude. These data suggest that the IC is an important structure in the neural circuit mediating acoustic PPI.

Acoustic Stimulation↗
PubMed1998

Computation of different optical variables of looming objects in pigeon nucleus rotundus neurons.

Three types of looming-selective neurons have been found in the nucleus rotundus of pigeons, each computing a different optical variable related to image expansion of objects approaching on a direct collision course with the bird. None of these neurons respond to simulated approach toward stationary objects. A detailed analysis of these neurons' firing pattern to approaching objects of different sizes and velocities shows that one group of neurons signals relative rate of expansion tau (tau), a second group signals absolute rate of expansion rho (rho), and a third group signals yet another optical variable eta (eta). The rho parameter is required for the computation of both tau and eta, whose respective ecological functions probably provide precise 'time-to-collision' information and 'early warning' detection for large approaching objects.

Animals↗
PubMed1997

Motion processing in pigeon tectum: equiluminant chromatic mechanisms.

Recent psychophysical and neurophysiological studies have suggested that, in mammals, there are interactions between the P (colour processing) and M (motion processing) visual pathways, which were previously believed to be parallel and separate. In this study, the role colour information plays in the coding of object motion was determined in the tectofugal pathway of pigeons. The responses of motion-sensitive neurons in the tectum to moving stimuli formed by chromatic contrast were recorded extracellularly using standard single-unit recording techniques. A moving coloured object was presented on a uniform (opponent coloured) background (e.g. blue-on-yellow, red-on-green and black-on-white). Through systematically manipulation of the luminance contrast between object and background, an equiluminant condition was generated. It was found that, at chromatic equiluminance, the majority of cells maintain some level of response. The mean magnitude of the response at equiluminance was about one-third of the response at maximal contrast to the same chromatic border. These results suggest that tectal units can detect motion of a pattern defined by a pure colour contour, although the strength of output is considerably weaker than that for the movement of patterns formed by luminance contrast.

Animals↗
PubMed1996

Azimuthal sensitivity of rat pinna reflex: EMG recordings from cervicoauricular muscles.

Electromyographic (EMG) responses of the cervicoauricular muscles (CAM) to free-field sounds were recorded in two groups of rats whose brainstems were dissected transversely either at a pretectal or transtectal level. After the rat recovered from anesthesia, wide-band noise pulses were presented and speaker positions were varied systematically in azimuth. Sound levels were set at 10-15 dB above empirically determined threshold for an EMG response to a sound from 0 degree azimuth. In both animal groups, transient CAM EMGs with short latency were produced and three main types of azimuthal sensitivity of CAM EMG response were observed. (1) For the majority of the cases, an inverted "U' type of azimuthal sensitivity was identified: the maximum activity occurred around 0 degree azimuth, but as the speaker was moved toward either the ipsilateral or contralateral fields, the sound-evoked activity declined systematically. This directional tuning is quite different from the passive pinna directionality which is very lateral in the resting positions used in this study. (2) In a small number of cases, the spatial sensitivity curves were not symmetrical about the midline (0 degree azimuth): the EMG response was vigorous in one hemifield and dropped off systematically as the speaker was moved toward extreme positions of the other hemifield. Regardless of shapes of EMG spatial tuning curves, obstruction of either the ipsilateral or contralateral meatus reduced the sound-elicited response dramatically and eliminated the spatial sensitivity. (3) Some cases exhibited an omnidirectional function: the EMG spike rate had no or minor systematical variation as the speaker position was changed in azimuth. The results of this study indicate that with either pretectal or transtectal decerebrate preparations, the acoustically evoked CAM EMG can exhibit an azimuthal sensitivity which is based on binaural processing.

Acoustic Stimulation↗
PubMed1995

Magnocellular and parvocellular divisions of pigeon nucleus isthmi differentially modulate visual responses in the tectum.

The electrophysiological responses of 162 tectal cells to computer-generated visual stimuli were extracellularly recorded from 24 homing pigeons before and after injecting either lidocaine or N-methyl-D-aspartate (NMDA) into the nucleus isthmi pars magnocellularis (Imc) or the nucleus isthmi pars parvocellularis (Ipc). Micro-injections of lidocaine into Imc resulted in a significant reduction of firing rate in 80% of tectal cells, whose excitatory receptive fields (ERFs) were localized within the ERF of the Imc cell where the lidocaine was injected. In contrast, when lidocaine was injected into Ipc under identical circumstances it had no effect on the visually driven activity of 68% of tectal cells. However, when the excitatory amino acid NMDA was injected into Ipc it produced a significant reduction in the visually driven firing of 75% of tectal neurons when their ERFs were within the isthmic ERF, while similar application of NMDA into Imc had no effect on the visually driven response of 94% of tectal neurons. When the ERFs of tectal cells were localized outside the ERF of the isthmic cell where the chemical was injected, Imc-injected lidocaine had no effect in 9 out of 10 tectal cells, whereas Ipc-injected NMDA increased firing in 7 out of 17 tectal cells. Therefore, it is suggested that the Imc-tectal fibers participate in a positive feedback pathway and the Ipc-tectal fibers are involved in a negative feedback pathway.

Anesthetics, Local↗
PubMed1994

The visual response properties of neurons in the nucleus of the basal optic root of the northern saw-whet owl (Aegolius acadicus).

The nucleus of the basal optic root (nBOR) in birds is a component of the accessory optic system (AOS) which is involved in the analysis of visual flowfields normally resulting from self-motion. Using standard extracellular techniques, we recorded from 81 single-unit and multi-unit clusters in the nBOR of the northern saw-whet owl, Aegolius acadicus, an avian species that has a visual system with frontal emphasis. These cells responded best to large patterns of random dots moving either upward (52%), downward (31%) or nasal to temporal (N-T; contralateral visual field; 15%). Only 2 units (2%) preferred temporal to nasal motion. 'Up' units were found in the dorsal portion of the nucleus whereas 'Down' units were located more ventrally. The N-T units were found in both the lateral margin of the nucleus and ventral to the Down units in the lateral half of the nucleus. About half of the units tested (10/19) responded to stimulation of the ipsilateral as well as the contralateral eye. For all but one cell, the direction preference of both eyes was the same in visual space. When compared with previous studies of pigeons (Columba livia) and chickens (Gallus domesticus), these findings reveal that the nBOR in all three avian species have important similarities with respect to direction preference and functional compartmentalization. Furthermore, the high proportion of binocular neurons found in the nBOR of the saw-whet owl is similar to the condition generally reported in frontal eyed mammals and hence may reflect adaptation.

Animals↗
PubMed1993

Connections of the auditory forebrain in the pigeon (Columba livia).

Ascending auditory efferents in birds terminate mainly within Field L2, a cytoarchitectonically distinct region of the caudomedial telencephalon. The organization of Field L2, and that of its flanking regions, L1 and L3, was investigated with 14C-2-deoxyglucose (14C-2-DG), cytochrome oxidase, and both retrograde and anterograde tracing techniques. Field L2 was found to contain a high concentration of cytochrome oxidase. Following auditory stimulation, 14C-2-DG autoradiography revealed that Field L2 consists of two adjacent but seemingly discontinuous zones, designated Field L2a, which lies ventromedially, and Field L2b, which lies dorsolaterally. Termination of thalamic efferents: The thalamic auditory nuclei ovoidalis (Ov) and semilunaris parovoidalis (SPO) project predominantly upon Field L2, and possibly sparsely upon L1, L3 and the overlying hyperstriatum ventrale (HV). Ov subnuclei project upon L2a and SPO projects predominantly upon L2b. The topography of the projections is inverted along the ventromedial-to-dorsolateral axis of L2, and is in accord with an inverted tonotopic representation of frequencies; high frequencies (< 3.5 kHz) being found in the more ventromedial parts of L2a, and low frequencies and broad band responses in L2b. Intra- and extratelencephalic connections: Field L2a also receives a substantial projection from HV, but the efferent projections of L2a appear confined to adjacent "neostriatal" regions. The subsequent projections of L2b were not identified in this study. L1 and L3 project predominantly to the dorsal neostriatum (Nd) caudolateral to Field L, and have fewer projections to the caudomedial paleostriatum and anterior hyperstriatum accessorium. Nd projects massively upon the ventromedial nucleus of the intermediate archistriatum (Aivm), which has bilateral projections upon the caudomedial telencephalon and is the origin of a major descending pathway having dense terminations surrounding the ovoidalis complex (Ov and SPO), MLd, the lateral lemniscal nuclei, and sparse terminations within SPO itself. It is suggested that within the telencephalon the major components of the auditory pathway consist of cell groups which collectively correspond to the populations of neurons found within the auditory cortex of mammals.

Afferent Pathways↗
PubMed1993

Responses of pigeon vestibulocerebellar neurons to optokinetic stimulation. I. Functional organization of neurons discriminating between translational and rotational visual flow.

1. Extracellular recordings were made from 235 neurons in the vestibulocerebellum (VbC), including the flocculus (lateral VbC), nodulus (folium X), and ventral uvula (ventral folium IXc,d), of the anesthetized pigeon, in response to an optokinetic stimulus. 2. The optokinetic stimuli consisted of two black and white random-dot patterns that were back-projected onto two large tangent screens. The screens were oriented parallel to each other and placed on either side of the bird's head. The resultant stimulus covered the central 100 degrees x 100 degrees of each hemifield. The directional tuning characteristics of each unit were assessed by moving the largefield stimulus in 12 different directions, 30 degrees apart. The directional tuning curves were performed monocularly or binocularly. The binocular directional tuning curves were performed with the direction of motion the same in both eyes (in-phase; e.g., ipsi = upward, contra = upward) or with the direction of motion opposite in either eye (antiphase; e.g., ipsi = upward, contra = downward). 3. Mossy fiber units (n = 17) found throughout folia IXa,b and IXc,d had monocular receptive fields and exhibited direction selectivity in response to stimulation of either the ipsilateral (n = 12) or contralateral (n = 5) eye. None had binocular receptive fields. 4. The complex spike (CS) activity of 218 Purkinje cells in folia IXc,d and X exhibited direction selectivity in response to the large-field visual stimulus moving in one or both visual fields. Ninety-one percent of the cells had binocular receptive fields that could be classified into four groups: descent neurons (n = 112) preferred upward motion in both eyes; ascent neurons (n = 14) preferred downward motion in both eyes; roll neurons (n = 33) preferred upward and downward motion in the ipsilateral and contralateral eyes, respectively; and yaw neurons (n = 40) preferred forward and backward motion in the ipsilateral and contralateral eyes, respectively. Within all groups, most neurons (70%) showed an ipsilateral dominance. 5. For most binocular neurons (91%), the maximum depth of modulation occurred with simultaneous stimulation of both eyes, compared with monocular stimulation of the dominant eye alone. For the translation neurons (descent and ascent), binocular inphase stimulation produced the maximum depth of modulation, whereas for the rotation neurons (roll and yaw), binocular antiphase stimulation produced the maximum depth of modulation. 6. There was a clear functional segregation of the translation and rotation neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Responses of pigeon vestibulocerebellar neurons to optokinetic stimulation. II. The 3-dimensional reference frame of rotation neurons in the flocculus.

1. The complex spike activity of Purkinje cells in the flocculus in response to rotational flowfields was recorded extracellularly in anesthetized pigeons. 2. The optokinetic stimulus was produced by a rotating "planetarium projector." A light source was placed in the center of a tin cylinder, which was pierced with numerous small holes. A pen motor oscillated the cylinder about its long axis. This apparatus was placed above the bird's head and the resultant rotational flow-field was projected onto screens that surrounded the bird on all four sides. The axis of rotation of the planetarium could be oriented to any position in three-dimensional space. 3. Two types of responses were found: vertical axis (VA; n = 43) neurons responded best to visual rotation about the vertical axis, and H-135i neurons (n = 34) responded best to rotation about a horizontal axis. The preferred orientation of the horizontal axis was at approximately 135 degrees ipsilateral azimuth. VA neurons were excited by rotation about the vertical axis producing forward (temporal to nasal) and backward motion in the ipsilateral and contralateral eyes, respectively, and were inhibited by rotation in the opposite direction. H-135i neurons in the left flocculus were excited by counterclockwise rotation about the 135 degrees ipsilateral horizontal axis and were inhibited by clockwise motion. Thus, the VA and H-135i neurons, respectively, encode visual flowfields resulting from head rotations stimulating the ipsilateral horizontal and ipsilateral anterior semicircular canals. 4. Sixty-seven percent of VA and 80% of H-135i neurons had binocular receptive fields, although for most binocular cells the ipsilateral eye was dominant. Binocular stimulation resulted in a greater depth of modulation than did monocular stimulation of the dominant eye for 69% of the cells. 5. Monocular stimulation of the VA neurons revealed that the best axis for the contralateral eye was tilted back 11 degrees, on average, to the best axis for ipsilateral stimulation. For the H-135i neurons, the best axes for monocular stimulation of the two eyes were approximately the same. 6. By stimulating circumscribed portions of the monocular receptive fields of the H-135i neurons with alternating upward and downward largefield motion, it was revealed that the contralateral receptive fields were bipartite. Upward motion was preferred in the anterior 45 degrees of the contralateral field, and downward motion, was preferred in the central 90 degrees of the contralateral visual field.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗