Purvis Bedenbaugh

Auditory cortical neuron response differences under isoflurane versus pentobarbital anesthesia.

Response properties of the middle layers of feline primary auditory cortex neurons to simple sounds were compared for isoflurane versus pentobarbital anesthesia in a within subject study control design. Initial microelectrode recordings were made under isoflurane anesthesia. After a several hour washout period, recordings were repeated at spatially matched locations in the same animal under pentobarbital. The median spatial separation between matched recording locations was 50 microns. Excitatory frequency tuning curves (n=71 pairs) to tone bursts and entrainment to click train sequences (n=64 pairs) ranging from 2 to 38 Hz were measured. Characteristic frequency and BW10 and BW30 were not different under either anesthetic. The spontaneous rate was slightly decreased (P<0.05) for isoflurane (median 4.2 spikes/s) compared to pentobarbital (median 5.8 spikes/s). Minimum median threshold and latency were elevated by 12 dB and 2 ms, respectively, under isoflurane. Entrainment to click sequences assumed a lowpass filter profile under both anesthetics, but was markedly impoverished under isoflurane. Responses to click sequences under isoflurane were phasic to the first click but had very poor following to subsequent elements. Compared to pentobarbital, isoflurane appears to have a profound impact on response sensitivity and temporal response properties of auditory cortical neurons.

Sensory representation abnormalities that parallel focal hand dystonia in a primate model.

In our hypothesis of focal dystonia, attended repetitive behaviors generate aberrant sensory representations. Those aberrant representations interfere with motor control. Abnormal motor control strengthens sensory abnormalities. The positive feedback loop reinforces the dystonic condition. Previous studies of primates with focal hand dystonia have demonstrated multi-digit or hairy-glabrous responses at single sites in area 3b, receptive fields that average ten times larger than normal, and high receptive field overlap as a function of horizontal distance. In this study, we strengthen and elaborate these findings. One animal was implanted with an array of microelectrodes that spanned the border between the face and digits. After the animal developed hand dystonia, responses in the initial hand representation increasingly responded to low threshold stimulation of the face in a columnar substitution. The hand-face border that is normally sharp became patchy and smeared over 1 mm of cortex within 6 weeks. Two more trained animals developed a focal hand dystonia variable in severity across the hand. Receptive field size, presence of multi-digit or hairy-glabrous receptive fields, and columnar overlap covaried with the animals ability to use specific digits. A fourth animal performed the same behaviors without developing dystonia. Many of its physiological measures were similar to the dystonic animals, but receptive field overlap functions were minimally abnormal, and no sites shared response properties that are normally segregated such as hairy-glabrous combined fields, or multi-digit fields. Thalamic mapping demonstrated proportionate levels of abnormality in thalamic representations as were found in cortical representations.

Multiunit normalized cross correlation differs from the average single-unit normalized correlation

As the technology for simultaneously recording from many brain locations becomes more available, more and more laboratories are measuring the cross-correlation between single-neuron spike trains, and between composite spike trains derived from several undiscriminated cells recorded on a single electrode (multiunit clusters). The relationship between single-unit correlations and multiunit cluster correlations has not yet been fully explored. We calculated the normalized cross-correlation (NCC) between single unit spike trains and between small clusters of units recorded in the rat somatosensory cortex. The NCC between small clusters of units was larger than the NCC between single units. To understand this result, we investigated the scaling of the NCC with the number of units in a cluster. Multiunit cross-correlation can be a more sensitive detector of neuronal relationship than single-unit cross-correlation. However, changes in multiunit cross-correlation are difficult to interpret uniquely because they depend on the number of cells recorded on each electrode and because they can arise from changes in the correlation between cells recorded on a single electrode or from changes in the correlation between cells recorded on two electrodes.

Plasticity in Primary Auditory Cortex of Monkeys with Altered Vocal Production

Response properties of primary auditory cortical neurons in the adult common marmoset monkey (Callithrix jacchus) were modified by extensive exposure to altered vocalizations that were self-generated and rehearsed frequently. A laryngeal apparatus modification procedure permanently lowered the frequency content of the native twitter call, a complex communication vocalization consisting of a series of frequency modulation (FM) sweeps. Monkeys vocalized shortly after this procedure and maintained voicing efforts until physiological evaluation 5-15 months later. The altered twitter calls improved over time, with FM sweeps approaching but never reaching the normal spectral range. Neurons with characteristic frequencies <4.3 kHz that had been weakly activated by native twitter calls were recruited to encode self-uttered altered twitter vocalizations. These neurons showed a decrease in response magnitude and an increase in temporal dispersion of response timing to twitter call and parametric FM stimuli but a normal response profile to pure tone stimuli. Tonotopic maps in voice-modified monkeys were not distorted. These findings suggest a previously unrecognized form of cortical plasticity that is specific to higher-order processes involved in the discrimination of more complex sounds, such as species-specific vocalizations.

Representation of consonant–vowel syllables in mammalian auditory cortex: Effects of background noise and speaking rate

The representation of synthetic and naturally spoken consonant–vowel (CV) syllables was investigated in the auditory cortex of the cat and the squirrel monkey. Spatially discrete and temporally synchronized patterns of neuronal activation are systematically distributed across the tonotopic axis. This activation pattern is largely determined by the dynamics of the signal’s spectro‐temporal envelope (such as the energy modulation associated with plosive consonants and the onset of voicing). The spatio‐temporal activity is strongly influenced by sound pressure level, resulting in significant changes in neuronal response strength and distribution (analogous to those associated with sinusoidal signals). In contrast, background noise has a much smaller impact on the excitation pattern, except at levels sufficiently high as to cause extensive suppression of overall activity. Varying the speaking rate (i.e., shortening vocalic durations and the interval separating CV syllables) was systematically investigated. Fo...

Coding of amplitude modulation in the central auditory system

The coding of temporal information, as reflected in the response to amplitude‐modulated signals, changes dramatically between the auditory midbrain and the auditory cortex. Most neurons in the inferior colliculus are tuned to specific AM rates between ten and several hundred Hertz. This tuning is expressed either in their capacity to phaselock their response to the AM rate or by their overall response magnitude. In the primary auditory cortex, this capacity is largely reduced to AM rates below 20 Hz. Other cortical fields show even lower AM limiting rates and usually only one field (the anterior field in the cat, and the posterior field in New World monkeys) shows a slightly higher following capacity. The consequences of this overall reduction in repetition coding capacity for the representation of temporal properties of complex signals, such as fundamental frequency or details of the temporal envelope, are profound. A second property of the temporal coding in the midbrain, its systematic distribution alo...