Biomedical subjects

Troy A Hackett

Publications and source records attributed to Troy A Hackett.

9 recordsLinked to original sources

PubMed2006

Cortical connections of the auditory cortex in marmoset monkeys: core and medial belt regions.

The auditory cortex of primates contains a core region of three primary areas surrounded by a belt region of secondary areas. Recent neurophysiological studies suggest that the belt areas medial to the core have unique functional roles, including multisensory properties, but little is known about their connections. In this study and its companion, the cortical and subcortical connections of the core and medial belt regions of marmoset monkeys were compared to account for functional differences between areas and refine our working model of the primate auditory cortex. Anatomical tracer injections targeted two core areas (A1 and R) and two medial belt areas (rostromedial [RM] and caudomedial [CM]). RM and CM had topographically weighted connections with all other areas of the auditory cortex ipsilaterally, but these were less widespread contralaterally. CM was densely connected with caudal auditory fields, the retroinsular (Ri) area of the somatosensory cortex, the superior temporal sulcus (STS), and the posterior parietal and entorhinal cortex. The connections of RM favored rostral auditory areas, with no clear somatosensory inputs. RM also projected to the lateral nucleus of the amygdala and tail of the caudate nucleus. A1 and R had topographically weighted connections with medial and lateral belt regions, infragranular inputs from the parabelt, and weak connections with fields outside the auditory cortex. The results indicated that RM and CM are distinct areas of the medial belt region with direct inputs from the core. CM also has somatosensory input and may correspond to an area on the posteromedial transverse gyrus of humans and the anterior auditory field of other mammals.

Acoustic Stimulation↗

PubMed2006

Thalamic connections of the auditory cortex in marmoset monkeys: core and medial belt regions.

In this study and its companion, the cortical and subcortical connections of the medial belt region of the marmoset monkey auditory cortex were compared with the core region. The main objective was to document anatomical features that account for functional differences observed between areas. Injections of retrograde and bi-directional anatomical tracers targeted two core areas (A1 and R), and two medial belt areas (rostromedial [RM] and caudomedial [CM]). Topographically distinct patterns of connections were revealed among subdivisions of the medial geniculate complex (MGC) and multisensory thalamic nuclei, including the suprageniculate (Sg), limitans (Lim), medial pulvinar (PM), and posterior nucleus (Po). The dominant thalamic projection to the CM was the anterior dorsal division (MGad) of the MGC, whereas the posterior dorsal division (MGpd) targeted RM. CM also had substantial input from multisensory nuclei, especially the magnocellular division (MGm) of the MGC. RM had weak multisensory connections. Corticotectal projections of both RM and CM targeted the dorsomedial quadrant of the inferior colliculus, whereas the CM projection also included a pericentral extension around the ventromedial and lateral portion of the central nucleus. Areas A1 and R were characterized by focal topographic connections within the ventral division (MGv) of the MGC, reflecting the tonotopic organization of both core areas. The results indicate that parallel subcortical pathways target the core and medial belt regions and that RM and CM represent functionally distinct areas within the medial belt auditory cortex.

Animals↗

PubMed2006

Fluorescent retrograde axonal tracing of the facial nerve.

OBJECTIVE: The objective of this study was to investigate retrograde fluorescent axonal labeling of motor nerves as an aid to surgical dissection in otolaryngology-head and neck surgery. METHODS: Cholera toxin B subunit conjugated to a fluorescent dye was injected into the facial musculature of New Zealand white rabbits. At variable time points after injection, the facial nerves were harvested and analyzed for fluorescence in the peripheral axon. RESULTS: All injected animals demonstrated axonal fluorescence. No animal demonstrated facial weakness. Fluorescence was detected a maximum of 20 mm from the distal end of the nerve. Time periods greater than 48 hours did not substantially increase transport distance. CONCLUSIONS: Cholera toxin B subunit fluorescent conjugates, when administered intramuscularly, reliably label the distal portion of the facial nerve. In vivo labeling of a motor nerve may have significant potential for identifying a nerve in surgery. This work represents a preliminary investigation into the adaptation of in vivo fluorescence techniques as an aid to surgical dissection. Further research to refine this technique should be supported.

Animals↗

PubMed2005

Entropy analysis of neuronal spike train synchrony.

Although vector strength (VS) and the Rayleigh tests are widely used to quantify neuronal firing synchrony to cyclic events, their use is valid only for singly peaked, unimodal distributions. In this report, we propose a new method to quantify synchrony, applicable to both unimodal and multimodal distributions. We also propose a statistical test to examine temporal structure under a null hypothesis of no synchrony.

Action Potentials↗

PubMed2005

Neurosurgical access to cortical areas in the lateral fissure of primates.

In this report, a method is presented for gaining direct access to cortical areas within the lateral fissure of primates for neuroanatomical tracer injections and electrode array implantation. Compared to areas on the surface of the brain, the anatomical and physiological properties of areas within the fissure are poorly understood. Typically, access to these areas is indirectly achieved by ablating or passing through intervening areas. To enable direct experimental access, a neurosurgical technique was developed in primates whereby the banks of the lateral fissure were retracted with sparing of the vascular network and intervening areas. In some animals, anatomical tracers were directly injected into target fields without contamination of other areas. In others, multichannel electrode arrays were implanted into target areas for chronic recording of neural activity. Since, these techniques could be adapted for exploration of areas within other sulci, the approach represents an important advance in efforts to elucidate the functional organization of the primate cerebral cortex.

Animals↗

PubMed2004

A comparison of neuron response properties in areas A1 and CM of the marmoset monkey auditory cortex: tones and broadband noise.

The purpose of this study was to compare response properties of two adjacent areas of the marmoset monkey auditory cortex. Multiunit responses to 50 ms tones and broadband noise bursts (BBN) were recorded in the core area, A1, and the caudomedial belt area, CM, of ketamine-anesthetized animals. Neurons in A1 and CM exhibited robust low-threshold short-latency responses to BBN and tones, whereas neurons in adjoining lateral belt areas were poorly responsive or unresponsive to tones and noise. Except for a population of broadly tuned units in CM, the characteristic frequency (CF) could be determined for all recording sites in A1 and CM. Both areas were tonotopically organized and shared a high CF border. Whereas the tonotopic gradient in A1 was smooth and continuous across the field, the gradient in CM was discontinuous, and the intermediate CF range was underrepresented. For BBN stimuli, rate level functions were largely monotonic in A1 and CM. Response profiles were also similar in both areas. As a population, neurons in CM were distinguished from A1 by significantly shorter response latencies, lower thresholds, and broader tuning bandwidth at higher intensities. The results indicated that, while A1 and CM represent anatomically and physiologically distinct areas, their response profiles under anesthesia overlapped considerably compared with the lateral belt areas. Therefore refinements of current models of the primate auditory cortex may be needed to account for differences in organization among the auditory belt areas.

Acoustic Stimulation↗

PubMed2004

Multivariate receptive field mapping in marmoset auditory cortex.

We describe a novel method for estimation of multivariate neuronal receptive fields that is based on least-squares (LS) regression. The method is shown to account for the relationship between the spike train of a given neuron, the activity of other neurons that are recorded simultaneously, and a variety of time-varying features of acoustic stimuli, e.g. spectral content, amplitude, and sound source direction. Vocalization-evoked neuronal responses from the marmoset auditory cortex are used to illustrate the method. Optimal predictions of single-unit activity were obtained by using the recent-time history of the target neuron and the concurrent activity of other simultaneously recorded neurons (R: 0.82 +/- 0.01, approximately 67% of variance). Predictions based on ensemble activity alone (R: 0.63 +/- 0.18) were equivalent to those based on the combination of ensemble activity and spectral features of the vocal calls (R: 0.61 +/- 0.24). This result suggests that all information derived from the spectrogram is embodied in ensemble activity and that there is a high level of redundancy in the marmoset auditory cortex. We also illustrate that the method allows for quantification of relative and shared contributions of each variable (spike train, spectral feature) to predictions of neuronal activity and describe a novel "neurolet" transform that arises from the method and that may serve as a tool for computationally efficient processing of natural sounds.

Acoustic Stimulation↗

PubMed2003

Auditory cortical neurons respond to somatosensory stimulation.

The prevailing hierarchical model of cortical sensory processing holds that early processing is specific to individual modalities and that combination of information from different modalities is deferred until higher-order stages of processing. In this paper, we present physiological evidence of multisensory convergence at an early stage of cortical auditory processing. We used multi-neuron cluster recordings, along with a limited sample of single-unit recordings, to determine whether neurons in the macaque auditory cortex respond to cutaneous stimulation. We found coextensive cutaneous and auditory responses in caudomedial auditory cortex, an area lying adjacent to A1, and at the second stage of the auditory cortical hierarchy. Somatosensory-auditory convergence in auditory cortex may underlie effects observed in human studies. Convergence of inputs from different sensory modalities at very early stages of cortical sensory processing has important implications for both our developing understanding of multisensory processing and established views of unisensory processing.

Acoustic Stimulation↗

PubMed2003

Anatomical mechanisms and functional implications of multisensory convergence in early cortical processing.

Recent findings in both monkeys and humans indicate that multisensory convergence occurs in low-level cortical structures generally believed to be unisensory in function. In an in-depth treatment of this theme, this paper reviews anatomical and physiological findings relating to the convergence of visual, somatosensory and auditory signals at early stages of auditory cortical processing. We discuss the potential anatomical sources of the input, and the types of known projections, and attempt to integrate this information with the current hierarchical model of auditory processing. Finally, we consider the functional implications of multisensory integration in early sensory processing.

Animals↗