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K D MacDonald

Publications and source records attributed to K D MacDonald.

8 recordsLinked to original sources
PubMed2000

Intracellular correlates of fast (>200 Hz) electrical oscillations in rat somatosensory cortex.

Oscillatory activity in excess of several hundred hertz has been observed in somatosensory evoked potentials (SEP) recorded in both humans and animals and is attracting increasing interest regarding its role in brain function. Currently, however, little is known about the cellular events underlying these oscillations. The present study employed simultaneous in-vivo intracellular and epipial field-potential recording to investigate the cellular correlates of fast oscillations in rat somatosensory cortex evoked by vibrissa stimulation. Two distinct types of fast oscillations were observed, here termed "fast oscillations" (FO) (200-400 Hz) and "very fast oscillations" (VFO) (400-600 Hz). FO coincided with the earliest slow-wave components of the SEP whereas VFO typically were later and of smaller amplitude. Regular spiking (RS) cells exhibited vibrissa-evoked responses associated with one or both types of fast oscillations and consisted of combinations of spike and/or subthreshold events that, when superimposed across trials, clustered at latencies separated by successive cycles of FO or VFO activity, or a combination of both. Fast spiking (FS) cells responded to vibrissae stimulation with bursts of action potentials that closely approximated the periodicity of the surface VFO. No cells were encountered that produced action potential bursts related to FO activity in an analogous fashion. We propose that fast oscillations define preferred latencies for action potential generation in cortical RS cells, with VFO generated by inhibitory interneurons and FO reflecting both sequential and recurrent activity of stations in the cortical lamina.

Action Potentials↗
PubMed1996

Thalamic modulation of high-frequency oscillating potentials in auditory cortex.

Perhaps the most widely recognized but least understood electrophysiological activity of the cerebral cortex is its characteristic electrical oscillations. Recently, there have been efforts to understand the mechanisms underlying high-frequency gamma oscillations(approximately 40 Hz) because they may coordinate sensory processing between populations of cortical cells. High-resolution cortical recordings show the gamma oscillations are constrained to sensory cortex, that they occur independently in auditory and somatosensory cortex, and that they are phase-locked between primary and secondary sensory cortex. As yet, the mechanism of their neurogenesis is unknown. Whereas cortical neurons can produce gamma oscillations without subcortical input, they may also be modulated by the thalamus and basal forebrain. Here we report that the neural generator of gamma oscillations in auditory cortex seems to be intracortical, serving to synchronize interactions between the primary and secondary areas. The acoustic thalamus directly modulates these oscillations, which are inhibited by stimulation of the dorsal and ventral divisions of the medial geniculate nucleus (MGd and MGv) and evoked by stimulation of the adjacent posterior intralaminar nucleus (PIL).

Animals↗
PubMed1996

Inter- and intra-hemispheric spatiotemporal organization of spontaneous electrocortical oscillations.

1. Two 64-channel epipial electrode arrays were positioned on homologous locations of the right and left hemisphere, covering most of primary and secondary auditory and somatosensory cortex in eight lightly anesthetized rats. Array placement was verified with the use of cytochrome oxidase histochemistry. 2. Middle-latency auditory and somatosensory evoked potentials (MAEPs and MSEPs, respectively) and spontaneous oscillations in the frequency range of 20-40 Hz (gamma oscillations) were recorded and found to be spatially constrained to regions of granular cortex, suggesting that both phenomena are closely associated with sensory information processing. 3. The MAEP and MSEP consisted of an initial biphasic sharp wave in primary auditory and somatosensory cortex, respectively, and a similar biphasic sharp wave occurred approximately 4-8 ms later in secondary sensory cortex of the given modality. Averaged gamma oscillations also revealed asynchronous activation of sensory cortex, but with a shorter 2-ms delay between oscillations in primary and secondary regions. Although the long latency shift of the MAEP and MSEP may be due in part to asynchronous activation of parallel thalamocortical projections to primary and secondary sensory cortex, the much shorter shift of gamma oscillations in a given modality is consistent with intracortical coupling of these regions. 4. Gamma oscillations occurred independently in auditory and somatosensory cortex within a given hemisphere. Furthermore, time series averaging revealed that there was no phase-locking of oscillations between the sensory modalities. 5. Gamma oscillations were loosely coupled between hemispheres; oscillations occurring in auditory or somatosensory cortex of one hemisphere were often associated with lower-amplitude oscillations in homologous contralateral sensory cortex. Yet, the fact that time series averaging revealed no interhemispheric phase-locking suggests that the corpus callosum may not coordinate the bilateral gamma oscillations, and that a thalamic modulatory influence may be involved.

Animals↗
PubMed1996

Volume and pressure modes of mechanical ventilation in pediatric patients.

There are few controlled pediatric studies comparing the various modes of ventilation in terms of patient outcomes. Thus at this time the choice of ventilator mode depends largely on the apparatus available, the patient's disease state, and personal preference based on one's experience. The next generation of ventilators may well allow the use of the best of both modes, setting both pressure and volume minimums and maximums, safely meeting ventilation targets. Today's challenges are to become familiar with the various modes of ventilators available, understand the developing physiology of the lung and lung disease pathophysiology, and incorporate all this into proper ventilator strategies to prevent ventilator-induced lung injury.

Adolescent↗
PubMed1995

High frequency (gamma-band) oscillating potentials in rat somatosensory and auditory cortex.

An 8 x 8 multichannel electrode array was used to record epipial field potentials, spontaneous gamma oscillations, and the interaction between single trial evoked potentials and ongoing gamma activity in rat somatosensory and auditory Cortex. Array placement over both these cortical regions was verified using cytochrome oxidase histochemistry. Replicating earlier findings, the epipial middle latency auditory and somatosensory evoked potentials (MAEP and MSEP, respectively) consisted of a stereotyped pattern of activation characterized by a spatially confined biphasic sharp wave followed by more diffuse slow wave components whose areal distribution adhered closely to established boundaries of primary and secondary sensory cortex. Spontaneous gamma activity, while exhibiting far more spatiotemporal variation, was also centered on primary and secondary sensory cortex and was significantly attenuated at intercalated dysgranular regions. A modality specificity of gamma activity was also demonstrated in the present study, where spindles occurred independently in auditory and somatosensory cortex. Furthermore, following presentation of a single click or vibrissal displacement, spontaneous gamma activity was suppressed and subsequently enhanced only in the modality stimulated. We conclude that in the lightly anesthetized rodent, spontaneous gamma oscillations are not a global neocortical phenomena, but are instead restricted to the same areas of sensory cortex participating in evoked potentials. However, unlike the MAEP and MSEP which are dominated by systematic activation of parallel thalamocortical projections, the marked spatiotemporal variability of gamma spindles suggests a more complex neurogenesis, probably including dominant contributions from intracortical neural circuitry.

Animals↗