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Kevin A Davis

Publications and source records attributed to Kevin A Davis.

5 recordsLinked to original sources
PubMed2005

Contralateral effects and binaural interactions in dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) receives afferent input from the auditory nerve and is thus usually thought of as a monaural nucleus, but it also receives inputs from the contralateral cochlear nucleus as well as descending projections from binaural nuclei. Evidence suggests that some of these commissural and efferent projections are excitatory, whereas others are inhibitory. The goals of this study were to investigate the nature and effects of these inputs in the DCN by measuring DCN principal cell (type IV unit) responses to a variety of contralateral monaural and binaural stimuli. As expected, the results of contralateral stimulation demonstrate a mixture of excitatory and inhibitory influences, although inhibitory effects predominate. Most type IV units are weakly, if at all, inhibited by tones but are strongly inhibited by broadband noise (BBN). The inhibition evoked by BBN is also low threshold and short latency. This inhibition is abolished and excitation is revealed when strychnine, a glycine-receptor antagonist, is applied to the DCN; application of bicuculline, a GABAA-receptor antagonist, has similar effects but does not block the onset of inhibition. Manipulations of discrete fiber bundles suggest that the inhibitory, but not excitatory, inputs to DCN principal cells enter the DCN via its output pathway, and that the short latency inhibition is carried by commissural axons. Consistent with their respective monaural effects, responses to binaural tones as a function of interaural level difference are essentially the same as responses to ipsilateral tones, whereas binaural BBN responses decrease with increasing contralateral level. In comparison to monaural responses, binaural responses to virtual space stimuli show enhanced sensitivity to the elevation of a sound source in ipsilateral space but reduced sensitivity in contralateral space. These results show that the contralateral inputs to the DCN are functionally relevant in natural listening conditions, and that one role of these inputs is to enhance DCN processing of spectral sound localization cues produced by the pinna.

Acoustic Stimulation↗
PubMed2004

Decreased cortical gray and cerebral white matter in male patients with familial bipolar I disorder.

BACKGROUND: Previous MRI studies of bipolar disorder have failed to consistently demonstrate cortical gray or cerebral white matter tissue loss, as well as sulcal or ventricular enlargement. The inconsistencies are most likely due to the clinical and gender heterogeneity of the study populations as well as the different MRI acquisition and processing techniques. The objective of this study was to determine if there was a cortical gray matter and cerebral white matter deficit as well as sulcal and ventricular enlargement in a homogeneous sample of euthymic male patients with familial bipolar I disorder. METHODS: MRI tissue segmentation was utilized to obtain cortical gray matter, cerebral white matter, ventricular cerebrospinal fluid (CSF), and sulcal CSF volumes in 22 euthymic males with familial bipolar I disorder and 32 healthy male control subjects. RESULTS: Relative to the controls, the familial bipolar I patients demonstrated: (1) significant reductions of both cortical gray matter and cerebral white matter volumes; and (2) significant increases in both sulcal and ventricular CSF volumes. In the bipolar group, there was a significant negative correlation between cortical gray matter volume and sulcal CSF volume. LIMITATIONS: Small sample size, retrospective interviews, possible medication effects. CONCLUSIONS: These results provide evidence for significant cortical gray matter and cerebral white matter deficits and associated sulcal and ventricular enlargement in euthymic males with familial bipolar I disorder.

Adolescent↗
PubMed2003

Auditory processing of spectral cues for sound localization in the inferior colliculus.

The head-related transfer function (HRTF) of the cat adds directionally dependent energy minima to the amplitude spectrum of complex sounds. These spectral notches are a principal cue for the localization of sound source elevation. Physiological evidence suggests that the dorsal cochlear nucleus (DCN) plays a critical role in the brainstem processing of this directional feature. Type O units in the central nucleus of the inferior colliculus (ICC) are a primary target of ascending DCN projections and, therefore, may represent midbrain specializations for the auditory processing of spectral cues for sound localization. Behavioral studies confirm a loss of sound orientation accuracy when DCN projections to the inferior colliculus are surgically lesioned. This study used simple analogs of HRTF notches to characterize single-unit response patterns in the ICC of decerebrate cats that may contribute to the directional sensitivity of the brain's spectral processing pathways. Manipulations of notch frequency and bandwidth demonstrated frequency-specific excitatory responses that have the capacity to encode HRTF-based cues for sound source location. These response patterns were limited to type O units in the ICC and have not been observed for the projection neurons of the DCN. The unique spectral integration properties of type O units suggest that DCN influences are transformed into a more selective representation of sound source location by a local convergence of wideband excitatory and frequency-tuned inhibitory inputs.

Acoustic Stimulation↗
PubMed2002

Evidence of a functionally segregated pathway from dorsal cochlear nucleus to inferior colliculus.

Type O units in the central nucleus of the inferior colliculus (ICC) of decerebrate cats are excited by best frequency (BF) tones near threshold, but are inhibited by high-level tones at all frequencies. Dorsal cochlear nucleus (DCN) principal cells display similar response map features and project directly to the ICC, and are thus supposed to be the dominant source of excitatory input for type O units. To test this hypothesis, the responses of type O units were compared before and after two pharmacological manipulations. When DCN to ICC axons were blocked by pressure injections of lidocaine, most type O units (approximately 80%) were silenced or showed substantially reduced activity, but some units showed increased activity. All of the former units had low maximal rates to BF tones, whereas the latter units had high rates. When local circuit inhibitory mechanisms in the ICC were blocked by iontophoretic application of bicuculline or strychnine, type O unit responses also fell into two classes: low-rate units that showed increased spontaneous and driven activities and high-rate units that showed, in addition, altered response map features. Taken together, these results demonstrate that low-rate type O units are part of a functionally segregated pathway initiated by the DCN, whereas high-rate type O units are created at the level of the ICC.

Acoustic Stimulation↗