Amplitude spectrum representation in the Doppler-shifted-CF processing area of the auditory cortex of the mustache bat.

The mustache bat, Pteronotus parnellii rubiginosus, emits orientation sounds containing a long constant-frequency (CF) component that is ideal for echo detection and Doppler shift measurement. About 30 percent of the primary auditory cortex of this bat is chiefly devoted to processing the second harmonic of the CF component in Doppler-shifted echoes. In this Doppler-shifted-CF processing area, single neurons recorded in any electrode penetration perpendicular to the cortical surface have nearly identical best frequencies and best amplitudes (or best pressure levels) at which the neurons show maximum excitation. The best frequency and best amplitude vary systematically with the location of the neurons in the cerebral cortex, so that there are tonotopic and "amplitopic" representation axes, which are radial and eccentric, respectively. In other words, the best-frequency and best-amplitude contours are eccentric and radial, respectively. The amplitude spectrum of a signal is thus represented in the coordinates of amplitude and frequency parallel to the cortical surface. This amplitude spectrum representation is disproportionate according to perceptual significance, so that a signal of 61.5 to 62.0 kilohertz and 30 to 50 decibels SPL (sound pressure level) is projected to a larger area than other signals. Just outside this Doppler-shifted-CF processing area, neurons are found which are specialized for responding to a particular information-bearing element or a particular combination of information-bearing elements in orientation sounds and echoes consisting of CF and frequency-modulated components.

Echo-ranging neurons in the inferior colliculus of bats.

Bats measure the distance to an object in terms of the time lag between their outgoing orientation sounds and the returning echo. For measurement of the time lag, the latency of response of a neuron to a stimulus must be nearly constant regardless of the stimulus amplitude and envelope. Otherwise, a large error would be introduced into the measurement. Bats have neurons that are specialized for echo ranging.

Disproportionate tonotopic representation for processing CF-FM sonar signals in the mustache bat auditory cortex.

The extent of cortical representation of the peripheral sensory field depends on its importance for species behavior. The orientation sound of the mustache bat (Pteronotus parnellii rubiginosus) invariably consists of long constant-frequency and short frequency-modulated components and is indispensable for its survival. A disproportionately large part of the auditory cortex of this bat is occupied by neurons processing the predominant components in the orientation signal and Doppler-shifted echoes. This disproportionate cortical representation related to features of biologically significant signals is comparable to that in the somatosensory and visual systems in many mammals, but it has not previously been observed in the auditory system.

Coordinated activities of middle-ear and laryngeal muscles in echolocating bats.

The middle-ear muscles and laryngeal muscles of the little brown bat (Myotis lucifugus) are highly developed. When the bat emits orientation sounds, action potentials of middle-ear muscles appear approximately 3 milliseconds after those of the laryngeal muscles; this activity of middle-ear muscles attenuates the vocal self-stimulation and improves the performance of the echolocation system. When an acoustic stimulus is delivered, both types of muscles contract; action potentials of the laryngeal muscles appear approximately 3 milliseconds after those of the middle-ear muscles. These two groups of muscles are apparently activated in a coordinated manner not only by the nerve impulses from the vocalization center, but also by those from the auditory system.

Site of neural attenuation of responses to self-vocalized sounds in echolocating bats.

Bats of the genus Myotis emit intense orientation sounds for echolocation. If such sounds directly stimulated their ears, the detection of echoes from short distances would be impaired. In addition to the muscular mechanism in the middle ear, the bat has a neural mechanism in the brain for attenuation of responses to self-vocalized orientation and nonorientation sounds. This neural attenuating mechanism operates in the nucleus of the lateral lemniscus, reducing its activity by about 15 decibels, and it is synchronized with vocalization.

Neural attenuation of responses to emitted sounds in echolocating rats.

Bats of the family Vespertilionidae enmit strong ultrasonic pulses for echolocation. If such sounds directly stimulate their ears, the detection of echoes from short distances would be impaired. The responses of lateral lemniscal neurons to emitted sounds were found to be much smaller than those to playback sounds, even when the response of the auditory nerve was the same to both types of sounds. Thus, responses to self-vocalized sounds were attenuated between the cochlear nerve and the inferior colliculus. The mean attenuation was 25 decibels. This neural attenuating mechanism is probably a part of the mechanisms for effective echo detection.

Corticofugal modulation of time-domain processing of biosonar information in bats.

The Jamaican mustached bat has delay-tuned neurons in the inferior colliculus, medial geniculate body, and auditory cortex. The responses of these neurons to an echo are facilitated by a biosonar pulse emitted by the bat when the echo returns with a particular delay from a target located at a particular distance. Electrical stimulation of cortical delay-tuned neurons increases the delay-tuned responses of collicular neurons tuned to the same echo delay as the cortical neurons and decreases those of collicular neurons tuned to different echo delays. Cortical neurons improve information processing in the inferior colliculus by way of the corticocollicular projection.

Frequency sensitivity of single auditory neurons in the gecko Coleonyx variegatus.

Although acoustic communication is not pronounced in reptiles, analysis of single auditory neurons in the medulla oblongata shows that the cochlea is a frequency analyser. Auditory neurons of the lizard Coleonyx variegatus respond to acoustic stimuli over a range of less than 0.1 to 17 kilohertz and are maximally responsive between 0.8 and 2.0 kilohertz. The frequencies to which they are most sensitive differ from neuron to neuron, ranging from 0.11 to 4 kilohertz. Some neurons have an inhibitory area which greatly overlaps the response area, so that inhibitory areas do not seem to sharply tune the response area at this level of the auditory tract. The inhibitory area is responsible for producing in some neurons a phasic response and nonmonotonic relation between sound intensity and number of impulses. The response pattern shows a tendency to change from tonic to phasic in more advanced auditory centers. This may serve to code rapid changes in the acoustic stimuli.

Cortical computational maps control auditory perception.

Mustached bats orient and find insects by emitting ultrasonic pulses and analyzing the returning echoes. Neurons in the Doppler-shifted constant-frequency (DSCF) and frequency-modulated (FM-FM) areas of the auditory cortex form maps of echo frequency (target velocity) and echo delay (target range), respectively. Bats were trained to discriminate changes in echo frequency or delay, and then these areas were selectively inactivated with muscimol. Inactivation of the DSCF area disrupted frequency but not delay discriminations; inactivation of the FM-FM area disrupted delay but not frequency discriminations. Thus, focal inactivation of specific cortical maps produces specific disruptions in the perception of biosonar signals.

Target range-sensitive neurons in the auditory cortex of the mustache bat.

Echolocating bats determine distance to targets by the time delay between their emitted biosonar pulses and the returning echoes. By varying the delay between synthetic pulses and echoes in stimulus pairs at various repetition rates and durations, neurons have been found in the auditory cortex of the mustache bat (Pteronotus parnellii rubiginosus) which are sensitive to target range during the search, approach, and terminal phases of prey capture or landing. Two classes of range-sensitive neurons were found: (i) tracking neurons, whose best delay for response to an echo following the emitted pulse becomes shorter and narrower as the bat closes in on the target, and (ii) range-tuned neurons, whose best delay is constant, and which respond to the target only when it is within a certain narrow fixed range. Range-tuned neurons are specialized for processing echoes only during a particular period of the search, approach, or terminal phases of echolocation, and they provide support for a theory of ranging in bats that incorporates groups of neurons with a spectrum of preferred echo delays to detect target distance.

Neural axis representing target range in the auditory cortex of the mustache bat.

In echolocating bats, the primary cue for determining distance to a target is the interval between an emitted orientation sound and its echo. Whereas frequency is represented by place in the bat cochlea, no anatomical location represents of primary range. Target range is coded by the time interval between grouped discharges of primary auditory neurons in response to both the emitted sound and its echo. In the frequency-modulated-signal processing area of the auditory cortex of the mustache bat (Pteronotus parnellii rubiginosus), neurons respond poorly or not at all to synthesized orientation sounds or echoes alone but respond vigorously to echoes following the emitted sound with a specific delay from targets at a specific range. These range-tuned neurons are systemically arranged along the rostrocaudal axis of the frequency-modulated-signal processing area according to the delays to which they best respond, and thus represent target range in terms of cortical organization. The frequency-modulated-signal processing area therefore shows odotopic representation.

Cortical neurons sensitive to combinations of information-bearing elements of biosonar signals in the mustache bat.

The auditory cortex of the mustache bat, Pteronotus parnellii rubiginosus, is composed of functional divisions which are differently organized to be suited for processing the elements of its biosonar signal according to their biological significance. Unlike the Doppler-shifted-CF (constant frequency) processing area, the area processing the frequency-modulated components does not show clear tonotopic and amplitopic representations, but consists of several clusters of neurons, each of which is sensitive to a particular combination (or combinations) of information-bearing elements of the biosonar signal and echoes. The response properties of neurons in the major clusters indicate that processing of information carried by the frequency-modulated components of echoes is facilitated by the first harmonic of the emitted biosonar signal. The properties of some of these neurons suggest that they are tuned to a target which has a particular cross-sectional area and which is located at a particular distance.

Aural representation in the Doppler-shifted-CF processing area of the auditory cortex of the mustache bat.

In the mustache bat (Pteronotus pamellii rubiginosus) the frequency and amplitude of an acoustic signal are represented in the coordinates parallel to the surface of the Doppler-shifted-CF (constant frequency) processing area ofthe primary auditory cortex. In this area all cortical neurons studied were excited by contralateral stimuli, and almost all of them were either excited or inhibited by ipsilateral stimuli. These are called E-E (ipsilateral and contralateral excitatory) and I-E (ipsilateral inhibitory and contralateral excitatory) neurons, respectively. The I-E neurons are directionally sensitive, while the E-E neurons are not. The E-E neurons are equally sensitive to echoes between 30 degrees contralateral and 30 degrees ipsilateral. Of the electrode penetrations orthogonal to the Doppler-shifted-CF processing area, 57 percent were characterized by either E-E or I-E neurons. Thus, there are at least two types of binaural columns: E-E columns, mainly located in a ventral part of the Doppler-shifted-CF processing area, where neurons are tuned to weak echoes; and IE columns, mainly distributed in a dorsal part, where neurons are tuned to moderate to intense echoes. Therefore, neurons tuned to weaker echoes integrate or even multiply faint signals from both ears for effective detection of a distant small target, while neurons tuned to moderate to intense echoes are suited for processing directional information and are stimulated when a bat approaches a target at short range. The Doppler-shifted-CF processing area may be considered to consist of two functional subdivisions.

Harmonic-sensitive neurons in the auditory cortex of the mustache bat.

Human speech and animal sounds contain phonemes with prominent and meaningful harmonics. The biosonar signals of the mustache bat also contain up to four harmonics, and each consists of a long constant-frequency component followed by a short frequency-modulated component. Neurons have been found in a large cluster within auditory cortex of this bat whose responses are facilitated by combinations of two or more harmonically related tones. Moreover, the best frequencies for excitation of these neurons are closely associated with the constant-frequency components of the biosonar signals. The properties of these neurons make them well suited for identifying the signals produced by other echolocating mustache bats. They also show how meaningful components of sound are assembled by neural circuits in the central nervous system and suggest a method by which sounds with important harmonics (or formants) may be detected and recognized by the brain in other species, including humans.

SHAPE ESTIMATION OF BOWTIE FILTERS BASED ON THE LUMINESCENCE FROM POLYETHYLENE TEREPHTHALATE RESIN BY X-RAY IRRADIATION.

In this study, we devised a novel method estimating the bowtie filter shapes by imaging luminescence from a polyethylene terephthalate (PET) resin with X-ray irradiation in a computed tomography (CT) scanner. The luminescence distribution of the PET resin corresponding to the thickness of bowtie filter was imaged using a charge-coupled device camera. On the assumption that the material of bowtie filter is aluminium (Al), the shape of bowtie filters was estimated from the correlation between Al attenuation curves and the angular-dependent luminance attenuation profiles according to the thickness of bowtie filters. Dose simulations based on the estimated bowtie filter shapes were performed using head and body PMMA phantoms with 16 and 32 cm in diameter. The simulated values of head and body weighted CT dose index (CTDIw) based on bowtie filter shape by the luminescence imaging method agreed within ~9% with the measured values by a dosemeter.

Corticofugal modulation of the midbrain frequency map in the bat auditory system.

The auditory system, like the visual and somatosensory systems, contains topographic maps in its central neural pathways. These maps can be modified by sensory deprivation, injury and experience in both young and adult animals. Such plasticity has been explained by changes in the divergent and convergent projections of the ascending sensory system. Another possibility, however, is that plasticity may be mediated by descending corticofugal connections. We have investigated the role of descending connections from the cortex to the inferior colliculus of the big brown bat. Electrical stimulation of the auditory cortex causes a downward shift in the preferred frequencies of collicular neurons toward that of the stimulated cortical neurons. This results in a change in the frequency map within the colliculus. Moreover, similar changes can be induced by repeated bursts of sound at moderate intensities. Thus, one role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience.

MEASUREMENT OF INTERNAL RADIATION DOSE DISTRIBUTION IN CT EXAMINATIONS USING POLYETHYLENE TEREPHTHALATE RESIN.

This study proposes a new dosimetry method for the estimation of the internal radiation dose distribution of a subject undergoing computed tomography (CT) examinations. In this novel method, dose distribution of a subject by CT scans was estimated based on radiophotoluminance distribution with polyethylene terephthalate (PET) resin which was cut to the average head size of a Japanese 1-year-old child. The difference in dose distribution depending on the type of bowtie filter was visualized by imaging luminance distribution with the PET phantom using a charge-coupled device camera. Dose distribution images simulated from a water phantom of the same size as the PET phantom were compared with the luminance distribution images. The linear correlation was demonstrated between luminance of the PET phantom and the simulated water dose. In comparison with the simulated water doses and the converted water doses from luminance of the PET phantom, the relative differences were within 20%.

Cortical maps for hearing and egocentric selection for self-organization.

The auditory cortex of the mustached bat shows complex multiple frequency maps, because cortical neurons in several areas are tuned to particular combinations of signal elements. Different types of combination-sensitive neurons form computational maps in which information-bearing parameters for echolocation (biosonar) are represented systematically. Neuronal response properties and multiple-frequency and computational maps were considered to be created solely by divergent-convergent interactions of neurons in the 'ascending' system. However, we have found that the 'corticofugal' system also plays an important role. Neurons in an iso-best frequency (or echo delay) 'minicolumn' of the auditory cortex augment the auditory responses of subcortical neurons tuned to that frequency (or echo delay) and sharpen their tuning. At the same time, they reduce the response and shift the tuning of subcortical neurons tuned to other frequencies (or echo delays) away from the best frequency (or delay) of the cortical neurons. Cortical neurons mediate a highly focused positive feedback, incorporated with widespread lateral inhibition, via corticofugal projections. This 'egocentric selection' is expected to play an important role in self-organizing the central auditory system.

Distribution of response types across entire hemispheres of the mustached bat's auditory cortex.

AILDD1 AC'1 The responses of neurons in the mustached bat's auditory cortex are specialized to extract particular information from biosonar signals. For this study, we mapped response properties across entire hemispheres in several animals. These experiments enabled us to construct a standard map that aided in determining the connections among the areas, as described subsequently. The mapping also yielded quantitative data regarding the relative sizes of areas and the proportion of cortex devoted to different response types. We identified six response types that were distributed in 11 areas. Eight areas, comprising two-thirds of the auditory cortex, contained neurons sensitive to particular components in biosonar signals. Most were facilitated by combinations of frequency modulated and constant frequency biosonar signal components (FMs and CFs, respectively). There were three major types of combination-sensitive neurons: FM-FM, CF/CF, and FM-CF. Each type of combination sensitivity occurred in multiple areas. The largest proportion were FM-FM neurons (approximately 30% of all neurons in auditory cortex), followed by FM1-CF2 (approximately 23%) and CF/CF (approximately 11%). In the other three areas comprising approximately one-third of the auditory cortex, most neurons responded well to frequencies not contained in biosonar signals.

Connections among functional areas in the mustached bat auditory cortex.

Connections among functional areas in the mustached bat's auditory cortex were examined by placing anatomical tracers in physiologically defined locations. We identified at least two and probably three channels connecting the various areas. One channel is formed by interconnections among areas containing neurons sensitive to frequency-modulated components (FMs) of the pulse and echo. These neurons are tuned to echo delay, a cue for target range, and thus define a ranging channel. An additional one or two channels are formed by interconnections among areas that contain neurons sensitive to the constant frequency components (CFs) of echoes. These neurons are of two main types: either sensitive to CFs of both pulse and echo (CF/CF neurons) or sensitive to a pulse FM and echo CF (FM-CF neurons). There was only a weak connection between the largest area of each type, suggesting they lie in different channels. Connections among areas in the ranging channel and echo CF-sensitive channel(s) were weak. Thus, the interconnections among functional areas in the mustached bat's auditory cortex define parallel channels for processing different types of biosonar information. Most corticocortical connections were patchy, in a manner suggestive of a columnar organization. The average width of the patches was approximately 360 microm. Based on the sizes of the functional areas, we estimate the auditory cortex contains a total of approximately 150 columns. Individual areas contain from as many as approximately 20 to as few as 1-4 columns. Each area had abundant projections outside of the auditory cortex. Connections within the cortex included the frontal, anterior cingulate, retrosplenial and perirhinal cortices, and the claustrum. Subcortical targets included the amygdyla, auditory thalamus, pons, pretectum, superior and inferior colliculi, and central gray. Projections within the cortex were of modest strength compared with several of the subcortical projections. Thus, the auditory areas themselves are the primary source of cortically processed biosonar information to the rest of the brain.