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D C Fitzpatrick

Publications and source records attributed to D C Fitzpatrick.

13 recordsLinked to original sources
PubMed1998

Distribution of response types across entire hemispheres of the mustached bat's auditory cortex.

AILDD1 AC'1 The responses of neurons in the mustached bat's auditory cortex are specialized to extract particular information from biosonar signals. For this study, we mapped response properties across entire hemispheres in several animals. These experiments enabled us to construct a standard map that aided in determining the connections among the areas, as described subsequently. The mapping also yielded quantitative data regarding the relative sizes of areas and the proportion of cortex devoted to different response types. We identified six response types that were distributed in 11 areas. Eight areas, comprising two-thirds of the auditory cortex, contained neurons sensitive to particular components in biosonar signals. Most were facilitated by combinations of frequency modulated and constant frequency biosonar signal components (FMs and CFs, respectively). There were three major types of combination-sensitive neurons: FM-FM, CF/CF, and FM-CF. Each type of combination sensitivity occurred in multiple areas. The largest proportion were FM-FM neurons (approximately 30% of all neurons in auditory cortex), followed by FM1-CF2 (approximately 23%) and CF/CF (approximately 11%). In the other three areas comprising approximately one-third of the auditory cortex, most neurons responded well to frequencies not contained in biosonar signals.

Animals↗
PubMed1998

Connections among functional areas in the mustached bat auditory cortex.

Connections among functional areas in the mustached bat's auditory cortex were examined by placing anatomical tracers in physiologically defined locations. We identified at least two and probably three channels connecting the various areas. One channel is formed by interconnections among areas containing neurons sensitive to frequency-modulated components (FMs) of the pulse and echo. These neurons are tuned to echo delay, a cue for target range, and thus define a ranging channel. An additional one or two channels are formed by interconnections among areas that contain neurons sensitive to the constant frequency components (CFs) of echoes. These neurons are of two main types: either sensitive to CFs of both pulse and echo (CF/CF neurons) or sensitive to a pulse FM and echo CF (FM-CF neurons). There was only a weak connection between the largest area of each type, suggesting they lie in different channels. Connections among areas in the ranging channel and echo CF-sensitive channel(s) were weak. Thus, the interconnections among functional areas in the mustached bat's auditory cortex define parallel channels for processing different types of biosonar information. Most corticocortical connections were patchy, in a manner suggestive of a columnar organization. The average width of the patches was approximately 360 microm. Based on the sizes of the functional areas, we estimate the auditory cortex contains a total of approximately 150 columns. Individual areas contain from as many as approximately 20 to as few as 1-4 columns. Each area had abundant projections outside of the auditory cortex. Connections within the cortex included the frontal, anterior cingulate, retrosplenial and perirhinal cortices, and the claustrum. Subcortical targets included the amygdyla, auditory thalamus, pons, pretectum, superior and inferior colliculi, and central gray. Projections within the cortex were of modest strength compared with several of the subcortical projections. Thus, the auditory areas themselves are the primary source of cortically processed biosonar information to the rest of the brain.

Animals↗
PubMed1997

A neuronal population code for sound localization.

The accuracy with which listeners can locate sounds is much greater than the spatial sensitivity of single neurons. The broad spatial tuning of auditory neurons indicates that a code based on the responses of ensembles of neurons, a population code, must be used to determine the position of a sound in space. Here we show that the tuning of neurons to the most potent localization cue, the interaural time difference in low-frequency signals (< approximately 2kHz), becomes sharper as the information ascends through the auditory system. We also show that this sharper tuning increases the efficiency of the population code, in the sense that fewer neurons are required to achieve a given acuity.

Animals↗
PubMed1997

Neurons sensitive to interaural temporal disparities in the medial part of the ventral nucleus of the lateral lemniscus.

The ventral nucleus of the lateral lemniscus (VNLL) is implicated in processing monaural sounds, because its neurons receive input chiefly from the contralateral cochlear nucleus. However, we demonstrate here that a region of the VNLL contains a distinct population of neurons that process binaural sounds and are sensitive to interaural temporal disparities (ITDs). Responses of single neurons were recorded from unanesthetized rabbits by using metal electrodes or micropipettes loaded with dextran tagged with either biotin or a fluorescent label. Reconstructions of recording sites based on a few marks indicated that ITD-sensitive neurons were located in a medial region of VNLL that has a low density of neurons or in the adjacent reticular formation. In one animal the locations of five ITD-sensitive neurons were marked directly by injection of dextrans with different tags. All of these neurons lay in the medial region of the VNLL. The ITD-sensitive neurons of the VNLL had characteristic responses. Most neurons responded only at the onset of contralaterally or binaurally presented tones; many did not respond to ipsilateral stimulation alone and did not follow dynamic changes in the ITD. The presence of ITD-sensitive neurons in the VNLL that responded only at the onset of tones suggests that this center plays a role in the localization of transient sounds.

Acoustic Stimulation↗
PubMed1997

Sensitivity to interaural temporal disparities of low- and high-frequency neurons in the superior olivary complex. I. Heterogeneity of responses.

Interaural temporal disparities (ITDs) are a cue for localization of sounds along the azimuth. Listeners can detect ITDs in the fine structure of low-frequency sounds and also in the envelopes of high-frequency sounds. Sensitivity to ITDs originates in the main nuclei of the superior olivary complex (SOC), the medial and lateral superior olives (MSO and LSO, respectively). This sensitivity is believed to arise from bilateral excitation converging on neurons of the MSO and ipsilateral excitation converging with contralateral inhibition on neurons of the LSO. Here we investigate whether the sensitivity of neurons in the SOC to ITDs can be adequately explained by one of these two mechanisms. Single and multiple units (n = 124) were studied extracellularly in the SOC of unanesthetized rabbits. We found units that were sensitive to ITDs in the fine structure of low-frequency (<2 kHz) tones and also units that were sensitive to ITDs in the envelopes of sinusoidally amplitude-modulated high-frequency tones. For both categories there were "peak-type" units that discharged maximally at a particular ITD across frequencies or modulation frequencies. These units were consistent with an MSO-type mechanism. There were also "trough-type" units that discharged minimally at a particular ITD. These units were consistent with an LSO-type mechanism. There was a general trend for peak-type units to be located in the vicinity of the MSO and for trough-type units to be located in the vicinity of the LSO. Units of both types appeared to encode ITDs within the estimated free-field range of the rabbit (+/-300 micros). Many units had varying degrees of irregularities in their responses, which manifested themselves in one of two ways. First, for some units there was no ITD at which the response was consistently maximal or minimal across frequencies. Instead there was an ITD at which the unit consistently responded at some intermediate level. Second, a unit could display considerable jitter from frequency to frequency in the ITD at which it responded maximally or minimally. Units with irregular responses had properties that were continuous with those of other units. They therefore appeared to be variants of peak- and trough-type units. The irregular responses could be modeled by assuming additional phase-locked inputs to a neuron in the MSO or LSO. The function of irregularities may be to shift the ITD sensitivity of a neuron without requiring changes in the anatomic delays of its inputs.

Acoustic Stimulation↗
PubMed1997

Sensitivity to interaural temporal disparities of low- and high-frequency neurons in the superior olivary complex. II. Coincidence detection.

In the companion paper we demonstrated that neurons in the superior olivary complex that were sensitive to interaural temporal disparities (ITDs) could be divided into two broad categories: peak type and trough type. Within these broad categories, many neurons exhibited various types of irregularities in their responses. In the present paper we devise three criteria to determine whether all types of neurons act as coincidence detectors. Each criterion relies on a comparison between the synchrony of the responses to the wave-forms at either ear and the "interaural synchrony," i.e., the response to a cyclically varying ITD. First, a neuron should exhibit synchrony to both the ipsilateral and contralateral waveforms over the entire range, to which it is sensitive to ITDs. Second, the ITD that elicits maximal discharge should be equal to the delay required to bring the ipsilateral and contralateral waveforms into coincidence. Third, the strength of interaural synchrony should be predicted by the strengths of synchrony to the waveforms at either ear. We found that most neurons of all types in the superior olivary complex met these criteria. Thus coincidence detection is a basic operating principle for all forms of ITD sensitivity.

Acoustic Stimulation↗
PubMed1995

Articulated external fixation of pilon fractures: the effects on ankle joint kinematics.

The effect of the Orthofix articulated ankle external fixator on ankle and subtalar joint kinematics and fracture fragment motion was investigated in fresh cadaver specimens using biplanar radiographic analysis. The kinematic testing was performed for the normal ankle (i.e., no fixation) and for three alternate fixator hinge orientations. The fixator applications simulated a horizontal ankle axis (the current clinically preferred orientation), an axis coincident with a previously defined approximate ankle axis, and an axis located using a mechanical axis finder. The horizontal fixator application significantly disturbed normal ankle kinematics. Aligning the fixator hinge with an approximate ankle axis caused significant distortion of motion about only two of six possible rotational axes. Aligning the fixator hinge with the (specimen-specific) ankle axis determined by the axis finder most closely matched the motion of the normal ankle. For pilon fractures simulated by a transverse osteotomy, there appeared to be no physiologically significant fracture fragment motions, regardless of fracture stability or fixator orientation.

Ankle Injuries↗
PubMed1995

Neural responses to simple simulated echoes in the auditory brain stem of the unanesthetized rabbit.

1. In most natural environments, sound waves from a single source will reach a listener through both direct and reflected paths. Sound traveling the direct path arrives first, and determines the perceived location of the source despite the presence of reflections from many different locations. This phenomenon is called the "law of the first wavefront" or "precedence effect." The time at which the reflection is first perceived as a separately localizable sound defines the end of the precedence window and is called "echo threshold." The precedence effect represents an important property of the auditory system, the neural basis for which has only recently begun to be examined. Here we report the responses of single neurons in the inferior colliculus (IC) and superior olivary complex (SOC) of the unanesthetized rabbit to a sound and its simulated reflection. 2. Stimuli were pairs of monaural or binaural clicks delivered through earphones. The leading click, or conditioner, simulated a direct sound, and the lagging click, or probe, simulated a reflection. Interaural time differences (ITDs) were introduced in the binaural conditioners and probes to adjust their simulated locations. The probe was always set at the neuron's best ITD, whereas the conditioner was set at the neuron's best ITD or its worst ITD. To measure the time course of the effects of the conditioner on the probe, we examined the response to the probe as a function of the conditioner-probe interval (CPI). 3. When IC neurons were tested with conditioners and probes set at the neuron's best ITD, the response to the probe as a function of CPI had one of two forms: early-low or early-high. In early-low neurons the response to the probe was initially suppressed but recovered monotonically at longer CPIs. Early-high neurons showed a nonmonotonic recovery pattern. In these neurons the maximal suppression did not occur at the shortest CPIs, but rather after a period of less suppression. Beyond this point, recovery was similar to that of early-low neurons. The presence of early-high neurons meant that the overall population was never entirely suppressed, even at short CPIs. Taken as a whole. CPIs for 50% recovery of the response to the probe among neurons ranged from 1 to 64 ms with a median of approximately 6 ms. 4. The above results are consistent with the time course of the precedence effect for the following reasons. 1) The lack of complete suppression at any CPI is compatible with behavioral results that show the presence of a probe can be detected even at short CPIs when it is not separately localizable. 2) At a CPI corresponding to echo threshold for human listeners (approximately 4 ms CPI) there was a considerable response to the probe, consistent with it being heard as a separately localizable sound at this CPI. 3) Full recovery for all neurons required a period much longer than that associated with the precedence effect. This is consistent with the relatively long time required for conditioners and probes to be heard with equal loudness. 5. Conditioners with either the best ITD or worst ITD were used to determine the effect of ITD on the response to the probe. The relative amounts of suppression caused by the two ITDs varied among neurons. Some neurons were suppressed about equally by both types of conditioners, others were suppressed more by a conditioner with the best ITD, and still others by a conditioner with the worst ITD. Because the best ITD and worst ITD presumably activate different pathways, these results suggest that different neurons receive a different balance of inhibition from different sources. 6. The recovery functions of neurons not sensitive to ITDs were similar to those of ITD-sensitive, neurons. This suggests that the time course of suppression may be common among different IC populations. 7. We also studied neurons in the SOC. Although many showed binaural interactions, none were sensitive to ITDs. Thus the response of this population may not be

Acoustic Stimulation↗
PubMed1995

An articulated ankle external fixation system that can be aligned with the ankle axis.

Aligning an articulated ankle external fixator with the ankle axis located using a mechanical axis finder has been shown to preserve normal ankle joint kinematics while the fixed hinge device is attached. However, several problems exist preventing the clinical application of this finding for fractures of the tibial plafond. We initiated a series of studies to resolve these issues. First, the accuracy of the mechanical axis finder in biological systems was quantified by comparing it to that of a computationally derived helical axis. Second, a prototype fixator design was developed in the biomechanics lab to increase the versatility of intraoperative fixator placement. Finally, a radiographic method of locating the ankle axis was developed which is based on talar morphology independent of the fractured tibia. The prototype fixator has been accurately aligned along the ankle axis in cadaveric specimens using this method. Open reduction and internal fixation (ORIF) is the accepted method of treatment for tibial plafond fractures. It holds the advantage of sufficient fracture fixation to permit early joint motion. Good results have been reported using this method, but some authors have reported complication rates up to 50%. The wide surgical approaches required, in conjunction with preexisting soft tissue injury, are thought to significantly increase the risk of soft tissue complications. In response to these problems, many investigators are beginning to utilize external fixation as an alternate treatment modality. One external fixation system which has shown particularly good results is a monolateral cross-ankle articulated fixator (Orthofix SRL., Verona, Italy) which allows motion at the ankle joint as the plafond fracture is healing (Figure 1).(ABSTRACT TRUNCATED AT 250 WORDS)

Ankle Joint↗
PubMed1994

Cell types in the mustached bat auditory cortex.

Cells in the auditory cortex of the mustached bat were studied with Golgi stains. No cell types appeared to be unique to the mustached bat auditory cortex or to specialized functional areas, but the laminar proportions and distributions of cell types were somewhat different from that reported for primary sensory cortex of other species. Two major cell types were distinguished, those with dendritic spines and those without spines. Non-spiny neurons were concentrated deep in layer III/IV and in layer V, and had three types of dendritic patterns, multipolar, bitufted and bipolar. Many of the non-spiny neurons were large; some nearly equaled the largest pyramidal neurons in size. Five types of spiny neurons were identified, pyramidal cells, extraverted pyramidal cells, 'spiny stellate-like' neurons, and multiform cells. In the narrow, densely packed, 'accentuated' layer II, slightly more than half of the spiny neurons were extraverted pyramidal cells, which are characterized by multiple, widely diverging apical dendrites. The high concentration of layer II extraverted pyramidal neurons is consistent with descriptions of the 'accentuated' layer II previously reported in other bat species and 'basal' insectivores. The remaining spiny neurons in layer II, and the preponderance of spiny neurons in layers III-VI, were typical pyramidal neurons that had single apical dendrites and tufts of basal dendrites. The thalamic recipient zone (deep layer III/IV) contained few candidates for spiny stellate cells, so a major constituent of the thalamic recipient zone in primary sensory cortex of many species is only a minor cellular component in the mustached bat auditory cortex.

Animals↗
PubMed1993

Combination-sensitive neurons in the primary auditory cortex of the mustached bat.

In the mustached bat, Pteronotus parnellii, neurons in the primary auditory cortex (AI) have been thought to respond primarily to single frequencies, as in other mammals. However, neurons in the Doppler-shifted constant-frequency (DSCF) area, a part of the mustached bat's AI that contains an overrepresentation of the prominent CF2 component of the biosonar signal, were found to show facilitative responses to combinations of different frequencies in the pulse and echo. The essential components for facilitation were the pulse FM1 and the echo CF2. The FM1-CF2 facilitation was sensitive to echo delays, indicating that DSCF neurons respond better to targets within particular ranges. On average, the longest discriminable echo delay, based on increased impulse counts due to facilitation, corresponded to a target range of 4.3 m, and the most discriminable delay corresponded to a target 3.6 m distant. Since mustached bats first show a behavioral response to a target at a distance of 3-4 m, DSCF neurons are suited to signal the presence of an insect within this behaviorally important range. DSCF neurons were broadly tuned to echo delay, with the average minimum discriminable echo delay corresponding to a target range of 1.9 m, and the delay tuning of the neurons followed (tracked) changes in pulse duration, indicating that facilitation occurs during much of the approach phase of insect pursuit when target characterization is presumably occurring. These results show that AI neurons in the mustached bat are specialized to respond to complex, behaviorally relevant stimuli during the search and approach phases of insect pursuit.

Acoustic Stimulation↗
PubMed1991

Are the initial frequency-modulated components of the mustached bat's biosonar pulses important for ranging?

1. FM-FM neurons in the auditory cortex of the mustached bat, Pteronotus parnellii, are specialized to process target range. They respond when the terminal frequency-modulated component (TFM) of a biosonar pulse is paired with the TFM of the echo at a particular echo delay. Recently, it has been suggested that the initial FM components (IFMs) of biosonar signals may also be important for target ranging. To examine the possible role of IFMs in target ranging, we characterized the properties of IFMs and TFMs in biosonar pulses emitted by bats swung on a pendulum. We then studied responses of FM-FM neurons to synthesized biosonar signals containing IFMs and TFMs. 2. The mustached bat's biosonar signal consists of four harmonics, of which the second (H2) is the most intense. Each harmonic has an IFM in addition to a constant-frequency component (CF) and a TFM. Therefore each pulse potentially consists of 12 components, IFM1-4, CF1-4, and TFM1-4. The IFM sweeps up while the TFM sweeps down. 3. The IFM2 and TFM2 depths (i.e., bandwidths) were measured in 217 pulses from four animals. The mean IFM2 depth was much smaller than the mean TFM2 depth, 2.87 +/- 1.52 (SD) kHz compared with 16.27 +/- 1.08 kHz, respectively. The amplitude of the IFM2 continuously increased throughout its duration and was always less than the CF2 amplitude, whereas the TFM2 was relatively constant in amplitude over approximately three-quarters of its duration and was often the most intense part of the pulse. The maximum amplitude of the IFM2 was, on average, 11 dB smaller than that of the TFM2. Because range resolution increases with depth and the maximum detectable range increases with signal amplitude, the IFMs are poorly suited for ranging compared with the TFMs. 4. FM-FM neurons (n = 77) did not respond or responded very poorly to IFMs with depths and intensities similar to those emitted on the pendulum. The mean IFM2 depth at which a just-noticeable response appeared was 4.48 +/- 1.98 kHz. Only 14% of the pulses emitted on the pendulum had IFM2 depths that exceeded the mean IFM2 depth threshold of FM-FM neurons. 5. Most FM-FM neurons responded to IFMs that had depths comparable with those of TFMs. However, when all parameters were adjusted to optimize the response to TFMs and then readjusted to maximize the response to IFMs, 52% of 27 neurons tested responded significantly better to the optimal TFMs than to the optimal IFMs (P less than 0.05, t test).(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗