No abstract

Peripheral auditory neurons are tuned to single frequencies of sound. In the central auditory system, excitatory (or facilitatory) and inhibitory neural interactions take place at multiple levels and produce neurons with sharp level-tolerant frequency-tuning curves, neurons tuned to parameters other than frequency, cochleotopic (frequency) maps, which are different from the peripheral cochleotopic map, and computational maps. The mechanisms to create the response properties of these neurons have been considered to be solely caused by divergent and convergent projections of neurons in the ascending auditory system. The recent research on the corticofugal (descending) auditory system, however, indicates that the corticofugal system adjusts and improves auditory signal processing by modulating neural responses and maps. The corticofugal function consists of at least the following subfunctions. auditory system ͉ descending system ͉ learning and memory ͉ plasticity ͉ tonotopic map T he central auditory system creates many physiologically distinct types of neurons for auditory signal processing. Their response properties have been interpreted to be produced by divergent and convergent interactions between neurons in the ascending auditory system. Until recently, the contribution of the descending (corticofugal) system to the shaping (or even creation) of their response properties has hardly been considered. Recent findings indicate that the corticofugal system plays important roles in shaping or even creating the response properties of central auditory neurons and in reorganizing cochleotopic (frequency) and computational (e.g., echo-delay) maps. Therefore, the understanding of the neural mechanisms for auditory signal processing is incomplete without the exploration of the functional roles of the corticofugal system. In this article, we first enumerate several types of neurons and computational maps created in the bat's central auditory system and then describe the anatomy and physiology of the corticofugal system. Neurons Tuned to Acoustic Parameters Characterizing Biosonar Signals and Cochleotopic and Computational MapsAll peripheral neurons are tuned to single frequencies (best frequencies, BFs). In the central auditory system, excitatory, inhibitory, and facilitatory neural interactions take place at multiple levels and produce neurons with sharp level-tolerant (the width of a frequency tuning curve is narrow regardless of sound levels) frequency tuning curves (1) and also neurons tuned to specific values of parameters other than frequency. Some of these neurons apparently are related to the processing of biosonar signals. They are latency-constant, phasic on-responding neurons (2, 3); paradoxical latency-shift neurons (4); durationtuned neurons (5) amplitude coordinates for the fine spatio-temporal representation of periodic frequency and amplitude modulations of echoes from flying insects (9). In the superior colliculus of the big brown bat, there is a space map, and some neurons are tuned to a sound source at ...

It is generally accepted that in mammals visual information is sent to the brain along functionally specialized parallel pathways, but whether the mouse visual system uses similar processing strategies is not known. It is important to resolve this issue because the mouse brain provides a tractable system for developing a cellular and molecular understanding of disorders affecting spatiotemporal visual processing. We have used single unit recordings in mouse primary visual cortex to study whether individual neurons are more sensitive to one set of sensory cues than another. Our quantitative analyses show that neurons with short response latencies have low spatial acuity and high sensitivity to contrast, temporal frequency and speed, whereas neurons with long latencies have high spatial acuity, low sensitivities to contrast, temporal frequency and speed. These correlations suggest that neurons in mouse V1 receive inputs from a weighted combination of parallel afferent pathways with distinct spatiotemporal sensitivities.

In the big brown bat (Eptesicus fuscus), conditioning with acoustic stimuli followed by electric leg-stimulation causes shifts in frequency-tuning curves and best frequencies (hereafter BF shifts) of collicular and cortical neurons, i.e., reorganization of the cochleotopic (frequency) maps in the inferior colliculus (IC) and auditory cortex (AC). The collicular BF shift recovers 180 min after the conditioning, but the cortical BF shift lasts longer than 26 h. The collicular BF shift is not caused by conditioning, as the AC is inactivated during conditioning. Therefore it has been concluded that the collicular BF shift is caused by the corticofugal auditory system. The collicular and cortical BF shifts both are not caused by conditioning as the somatosensory cortex is inactivated during conditioning. Therefore it has been hypothesized that the cortical BF shift is mostly caused by both the subcortical (e.g., collicular) BF shift and the activity of nonauditory systems such as the somatosensory cortex excited by an unconditioned leg-stimulation and the cholinergic basal forebrain. The main aims of our present studies are to examine whether acetylcholine (ACh) applied to the AC augments the collicular and cortical BF shifts caused by the conditioning and whether atropine applied to the AC abolishes the cortical BF shift but not the collicular BF shift, as expected from the preceding hypothesis. In the awake bat, we made the following findings. ACh applied to the AC augments not only the cortical BF shift but also the collicular BF shift through the corticofugal system. Atropine applied to the AC reduces the collicular BF shift and abolishes the cortical BF shift which otherwise would be caused. ACh applied to the IC significantly augments the collicular BF shift but affects the cortical BF shift only slightly. ACh makes the cortical BF shift long-lasting beyond 4 h, but it cannot make the collicular BF shift long-lasting beyond 3 h. Atropine applied to the IC abolishes the collicular BF shift. It reduces the cortical BF shift but does not abolish it. Our findings favor the hypothesis that the BF shifts evoked by the corticofugal system, and an increased ACh level in the AC evoked by the basal forebrain are both necessary to evoke a long-lasting cortical BF shift.

In the big brown bat, Eptesicus fuscus, the response properties of neurons and the cochleotopic (frequency) maps in the auditory cortex (AC) and inferior colliculus can be changed by auditory conditioning, weak focal electric stimulation of the AC, or repetitive delivery of weak, short tone bursts. The corticofugal system plays an important role in information processing and plasticity in the auditory system. Our present findings are as follows. In the AC, best frequency (BF) shifts, i.e., reorganization of a frequency map, slowly develop and reach a plateau Ϸ180 min after conditioning with tone bursts and electric-leg stimulation. The plateau lasts more than 26 h. In the inferior colliculus, on the other hand, BF shifts rapidly develop and become the largest at the end of a 30-min-long conditioning session. The shifted BFs return (i.e., recover) to normal in Ϸ180 min. The collicular BF shifts are not a consequence of the cortical BF shifts. Instead, they lead the cortical BF shifts. The collicular BF shifts evoked by conditioning are very similar to the collicular and cortical BF shifts evoked by cortical electrical stimulation. Therefore, our working hypothesis is that, during conditioning, the corticofugal system evokes subcortical BF shifts, which in turn boost cortical BF shifts. The cortical BF shifts otherwise would be very small. However, whether the cortical BF shifts are consequently boosted depends on nonauditory systems, including nonauditory sensory cortices, amygdala, basal forebrain, etc., which determine the behavioral relevance of acoustic stimuli.associative learning ͉ conditioning ͉ hearing ͉ reorganization of tonotopic map ͉ somatosensory cortex T he response properties of neurons and the sensory map in a sensory cortex and subcortical sensory nucleus can be changed by conditioning, learning of a discrimination task, or focal cortical electrical stimulation (see refs. 1-3 for reviews). In the central nucleus of the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus, the best frequency (BF) of a neuron shifts toward the frequency of a conditioned tone that is followed by an unconditioned electric leg stimulus (ES l ). The BF shift of Ϸ1.5 kHz lasts up to 180 min after a 30-min-long conditioning session. Inactivation of the primary auditory cortex (AC) during the conditioning abolishes the collicular BF shift that otherwise would be evoked (4). Focal electrical stimulation of the AC evokes collicular (5, 6) and cortical BF shifts (6, 7) very similar in the amount and duration (retention) to the collicular BF shift evoked by the conditioning. These data indicate that the collicular BF shift evoked by the conditioning is produced by the corticofugal system but do not indicate whether the collicular BF shift is due to the cortical BF shift or vice versa. It has not yet been determined which is the case, because it is very difficult, if not impossible, to inactivate corticofugal fibers selectively without inactivating thalamocortical fibers. One of the aims of the present paper is to p...