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PubMed2000

Plastic changes in the auditory cortex induced by intensive frequency discrimination training.

The slow auditory evoked (wave N1m) and mismatch field (MMF) elicited by sequences of pure tones of 1000 Hz and deviant tones of 1050, 1010 and 1005 Hz were measured before, during and 3 weeks after subjects were trained at frequency discrimination for 15 sessions (over 3 weeks) using an odd-ball procedure. The task of the subject was to detect deviants differing by progressively smaller frequency shifts from the standard stimulus. Frequency discrimination improved rapidly in the first week and was followed by small but constant improvements thereafter. N1m and MMF responses to the deviant stimuli increased in amplitude during training. This enhancement persisted until training was finished, but decreased 3 weeks later. The results suggest a plastic reorganization of the cortical representation for the trained frequencies.

Adult↗
PubMed1998

Neuromagnetic studies of human auditory cortex function and reorganization.

Much of our present understanding of sensory processing by the human brain is obtained from studies of patients with sensory impairments. Past auditory studies have focused strongly on the peripheral mechanisms of hearing disorders, and have led to an overuse of stimuli that are good for testing the transfer functions of simple acoustic features but are not suitable for understanding the perception of complex auditory stimuli or stimulus sequences. Magnetoencephalography (MEG) is a non-invasive method for studying how the human brain processes and stores auditory information. The long-term groundwork in building up the basic understanding of cortical dynamics during various "simple" stimulations now allows the use of more complex, real-life-like stimuli, and clinical applications.

Auditory Cortex↗
PubMed1991

Separate mechanisms control spike numbers and inter-spike intervals in transient responses of cat auditory cortex neurons.

In the anesthetized cat, some cortical auditory neurons discharge a train of up to 5 spikes in response to the onset of a characteristic frequency tone pulse. This report provides the first description of the inter-spike intervals (ISIs) in these responses. The ISIs were typically close to 2.0 ms in length, and, as indexed by the standard deviation of the interval length, were very regular. Except at threshold levels of stimulation, mean ISIs were relatively insensitive to both tone amplitude and repetition rate. This was true even over ranges of those variables that exerted dramatic effects on spike numbers and first spike latency. These data suggest that the relative timing of discharges within the spike burst is controlled by a mechanism which is separable from that which determines the number of spikes in them. The brevity of the ISIs suggest that they may be a means of enhancing the salience of the transient response against a background of spontaneous discharges.

Acoustic Stimulation↗
PubMed2001

Anesthesia changes frequency tuning of neurons in the rat primary auditory cortex.

The vast majority of investigations on central auditory processing so far were conducted under the influence of an anesthetic agent. It remains unclear, however, to what extend even basic response properties of central auditory neurons are influenced by this experimental manipulation. We used a combination of chronic recording in unrestrained animals, computer-controlled randomized acoustic stimulation, and statistical evaluation of responses to directly compare the response characteristics of single neurons in the awake and anesthetized state. Thereby we were able to quantify the effects of pentobarbital/chloral hydrate anesthesia (Equithesin) on rat auditory cortical neurons. During Equithesin anesthesia, only a portion of central neurons were active and some of their basic response properties were changed. Only 29% of the neurons still had a frequency response area. Their tuning sharpness was increased under anesthesia. Most changes are consistent with an enhancement of inhibitory influences during Equithesin anesthesia. Thus when describing response properties of central auditory neurons, the animal's anesthetic state has to be taken into account.

Anesthetics, Combined↗
PubMed2004

Topography of corticothalamic projections from the auditory cortex of the rat.

Corticothalamic projections from cortical auditory field to the medial geniculate body (MG) in the rat were systematically examined by making small injections of biocytin in cortical area Te1. All injections, confined to 400 microm in diameter, resulted in two projections terminating in the ventral (MGV) and dorsal divisions (MGD) of the MG. The projections to the MGV were evidently topographic. The rostral and caudal portions of area Te1 projected to the ventromedial and dorsolateral parts of the MGV, respectively, forming narrow bands of terminal axons that extended in the mediolateral direction in the coronal plane of the MGV. The minimum dorsoventral width of the bands ranged approximately from 100 to 300 microm. Besides, the more rostral portion of area Te1 tended to project to the more rostral side of the MGV. The projections to the MGD consistently arborized in its ventral margin made up of the deep dorsal nucleus of the MGD. A similar weak topography along the rostrocaudal direction was observed in the projections to the MGD. Large terminals were occasionally found in the MGD after the injections involving cortical layer V. The distribution of large terminals also appeared topographic along with small terminals that were the major component of labeling. Collaterals of labeled axons produced slabs of terminal field in the thalamic reticular nucleus, which also exhibited a weak topography of distribution. These results provide insights into the structural basis of corticofugal modulations related to the tonotopic organizations in the cortex and MG.

Animals↗
PubMed1977

Binaural columns in the primary field (A1) of cat auditory cortex.

Responses of single neurons and neuron clusters were studied at short intervals along penetrations into the high-frequency (4-25 kHz) representation of A1. The monaural and binaural sensitivities of neurons at best frequency were studied. With the exception of some neurons responsive only to binaural stimulation, the monaural responses of most were classified as contralateral dominant, ipsilateral dominant or equidominant. Binaural interactions of most neurons were classified as summation (binaural response size greater than the monaural response size) or suppression (dominant ear response size greater than the binaural response size). Most neurons arrayed in a column perpendicular to the cortical surface display the same aural dominance and binaural interaction. Summation columns occupy about two-thirds of the area sampled; suppression columns, about one-third. Within most suppression columns, the contralateral ear was dominant. Within summation columns, aural dominance varied. Summation columns appear to be composed of smaller columns differing in aural dominance. The sizes of binaural interaction columns vary considerably; some occupy several square millimeters of cortical surface. At least some binaural interaction columns occupy strips of cortex oriented orthogonal to isofrequency contours.

Animals↗
PubMed1991

Functional topographies in the primary auditory cortex of the cat.

One of the fundamental organizational principles of the mammalian sensory cortex is the topographic representation of the sensory epithelium. A cochleotopic topographic organization has been demonstrated for a number of auditory cortical fields, including the primary auditory field (AI). In the spatial cortical dimension orthogonal to the cochleotopic frequency gradient, systematic arrangements of other functional parameters have been observed. The first evidence of a spatial segregation of functional parameters along the isofrequency domain of cat AI was the discovery of binaural interaction bands. Recent studies show topographic representations of additional functional parameters in the central auditory system of the cat. Among the functional parameters that are evidence of systematic and topographic representations in cat auditory cortical fields are spectral resolution (sharpness of tuning), temporal resolution (preferred repetition rate), and inhibitory properties related to the coding of broadband signals.

Acoustic Stimulation↗
PubMed1983

[Responses of neurons of the auditory cortex in the cat to exposure to tones of different frequencies and electrical stimulation of the corresponding portions of the cochlea].

The characteristic frequencies for responses of the primary auditory cortical neurons (zone AI) whose receptive fields were located in different parts of cochlear basilar membrane were determined in nembutal-anesthetized cats. It is shown that the higher the characteristic frequency of the neuron, the nearer its receptive field to the base of the cochlea. The receptive fields of neurons with characteristic frequency above 4 kHz were located over the first 10 mm of the basilar membrane. Receptive fields of neurons with characteristic frequencies lower than 4 kHz occupied the rest of the basilar membrane. Electrical stimulation of the centre of the receptive field evoked neuronal responses which corresponded to reactions of the same unit produced by the tone of the characteristics frequency. The more the frequency of the tone differed from the characteristic one or the greater the distance between the centre of the receptive field and the point of stimulation, the lower is the probability of the response. The size of the receptive field of neurons with low characteristic frequencies was wider than that of high-frequency neurons. Using paired stimuli it is shown that excitation of cortical neurons was followed by inhibition. This inhibition was more prolonged and effective in responses to tones of characteristic frequency.

Animals↗