Single unit responses to linear frequency-modulations in the inferior colliculus of the Greater horseshoe bat,Rhinolophus ferumequinum

Evolution of high duty cycle echolocation in bats

Duty cycle describes the relative ‘on time’ of a periodic signal. In bats, we argue that high duty cycle (HDC) echolocation was selected for and evolved from low duty cycle (LDC) echolocation because increasing call duty cycle enhanced the ability of echolocating bats to detect, lock onto and track fluttering insects. Most echolocators (most bats and all birds and odontocete cetaceans) use LDC echolocation, separating pulse and echo in time to avoid forward masking. They emit short duration, broadband, downward frequency modulated (FM) signals separated by relatively long periods of silence. In contrast, bats using HDC echolocation emit long duration, narrowband calls dominated by a single constant frequency (CF) separated by relatively short periods of silence. HDC bats separate pulse and echo in frequency by exploiting information contained in Doppler-shifted echoes arising from their movements relative to background objects and their prey. HDC echolocators are particularly sensitive to amplitude and frequency glints generated by the wings of fluttering insects. We hypothesize that narrowband/CF calls produced at high duty cycle, and combined with neurobiological specializations for processing Doppler-shifted echoes, were essential to the evolution of HDC echolocation because they allowed bats to detect, lock onto and track fluttering targets. This advantage was especially important in habitats with dense vegetation that produce overlapping, time-smeared echoes (i.e. background acoustic clutter). We make four specific, testable predictions arising from this hypothesis.

Mapping responses to frequency sweeps and tones in the inferior colliculus of house mice

In auditory maps of the primary auditory cortex, neural response properties are arranged in a systematic way over the cortical surface. As in the visual system, such maps may play a critical role in the representation of sounds for perception and cognition. By recording from single neurons in the central nucleus of the inferior colliculus (ICC) of the mouse, we present the first evidence for spatial organizations of parameters of frequency sweeps (sweep speed, upward/downward sweep direction) and of whole-field tone response patterns together with a map of frequency tuning curve shape. The maps of sweep speed, tone response patterns and tuning curve shape are concentrically arranged on frequency band laminae of the ICC with the representation of slow speeds, build up response types and sharp tuning mainly in the centre of a lamina, and all (including high) speeds, phasic response types and broad tuning mainly in the periphery. Representation of sweep direction shows preferences for upward sweeps medially and laterally and downward sweeps mainly centrally in the ICC (either striped or concentric map). These maps are compatible with the idea of a gradient of decreasing inhibition from the centre to the periphery of the ICC and by gradients of intrinsic neuronal properties (onset or sustained responding). The maps in the inferior colliculus compare favourably with corresponding maps in the primary auditory cortex, and we show how the maps of sweep speed and direction selectivity of the primary auditory cortex could be derived from the here-found maps of the inferior colliculus.

Directional selectivity for FM sweeps in the suprageniculate nucleus of the mustached bat medial geniculate body

Mustached bats emit echolocation and communication calls containing both constant frequency (CF) and frequency-modulated (FM) components. Previously we found that 86% of neurons in the ventral division of the external nucleus of the inferior colliculus (ICXv) were directionally selective for linear FM sweeps and that selectivity was dependent on sweep rate. The ICXv projects to the suprageniculate nucleus (Sg) of the medial geniculate body. In this study, we isolated 37 single units in the Sg and measured their responses to best excitatory frequency (BEF) tones and linear 12-kHz upward and downward FM sweeps centered on the BEF. Sweeps were presented at durations of 30, 12, and 4 ms, yielding modulation rates of 400, 1,000, and 3,000 kHz/s. Spike count versus level functions were obtained at each modulation rate and compared with BEF controls. Sg units responded well to both tones and FM sweeps. BEFs clustered at 58 kHz, corresponding to the dominant CF component of the sonar signal. Spike count functions for both tones and sweeps were predominantly non-monotonic. FM directional selectivity was significant in 53-78% of the units, depending on modulation rate and level. Units were classified as up-selective (52%), down-selective (24%), or bi-directional (non-selective, 16%); a few units (8%) showed preferences that were either rate- or level-dependent. Most units showed consistent directional preferences at all SPLs and modulation rates tested, but typically showed stronger selectivity at lower sweep rates. Directional preferences were attributable to suppression of activity by sweeps in the non-preferred direction (~80% of units) and/or facilitation by sweeps in the preferred direction (~20-30%). Latencies for BEF tones ranged from 4.9 to 25.7 ms. Latencies for FM sweeps typically varied linearly with sweep duration. Most FM latency-duration functions had slopes ranging from 0.4 to 0.6, suggesting that the responses were triggered by the BEF. Latencies for BEF tones and FM sweeps were significantly correlated in most Sg units, i.e., the response to FM was temporally related to the occurrence of the BEF in the FM sweep. FM latency declined relative to BEF latency as modulation rate increased, suggesting that at higher rates response is triggered by frequencies in the sweep preceding the BEF. We conclude that Sg and ICXv units have similar, though not identical, response properties. Sg units are predominantly upsweep selective and could respond to either or both the CF and FM components in biosonar signals in a number of echolocation scenarios, as well as to a variety of communication sounds.

Responses to exponential frequency modulations in the rat inferior colliculus

We examined responses to pure tones and exponentially frequency-modulated (FM) stimuli in the inferior colliculus of ketamine anesthetized rats. All units responded to both pure-tone and FM stimulation: units responding selectively to FM stimuli were not found. The comparison between responses to many different FM sweeps revealed that activity was elicited when the instantaneous frequency of a FM sweep entered the unit's pure-tone tuning curve. Units were tuned to the rate of frequency modulation. Most modulation rate transfer functions had bandpass characteristics. Best modulation rates covered a range from 4.8 to 1904 octaves/s with more than 90% between 10 and 400 octaves/s. In contrast to previous studies, modulation direction was not coded in unit responses and only few units demonstrated a weak change in response strength when sweep direction was altered. This is at least partly attributable to the FM stimulus design which, in the present study, was adapted to the logarithmic frequency representation in the rat auditory system and carefully matched to the units' pure-tone responses area. In spite of the close relationship between pure tone and FM response behavior, modulation rate tuning cannot be completely explained on the basis of the units' pure-tone responses.

Responses to pure tones and linear FM components of the CF-FM biosonar signal by single units in the inferior colliculus of the mustached bat

The responses of 682 single-units in the inferior colliculus (IC) of 13 mustached bats (Pteronotus parnellii parnellii) were measured using pure tones (CF), frequency modulations (FM) and pairs of CF-FM signals mimicking the species' biosonar signal, which are stimuli known to be essential to the responses of CF/CF and FM-FM facilitation neurons in auditory cortex. Units were arbitrarily classified into 'reference frequency' (RF), 'FM2' and 'Non-echolocation' (NE) categories according to the relationship of their best frequencies (BF) to the biosonar signal frequencies. RF units have high Q10dB values and are tuned to the reference frequency of each bat, which ranged between 60.73 and 62.73 kHz. FM2 units had BF's between 50 and 60 kHz, while NE units had BF's outside the ranges of the RF and FM2 classes. PST histograms of the responses revealed discharge patterns such as 'onset', 'onset-bursting' (most common), 'on-off', 'tonic-on','pauser', and 'chopper'. Changes in discharge patterns usually resulted from changes in the frequency and/or intensity of the stimuli, most often involving a change from onset-bursting to on-off. Different patterns were also elicited by CF and FM stimuli. Frequency characteristics and thresholds to CF and FM stimuli were measured. RF neurons were very sharply tuned with Q10dB's ranging from 50-360. Most (92%) also responded to FM2 stimuli, but 78% were significantly more sensitive (greater than 5 dB) to CF stimuli, and only 3% had significantly lower thresholds to FM2. The best initial frequency for FM2 sweeps in RF units was 65.35 +/- 2.138 kHz (n = 118), well above the natural frequency of the 2nd harmonic. FM2 and NE units were indistinguishable from each other, but were quite different from RF units: 41% of these two classes had lower thresholds to CF, 49% were about equally sensitive, and 10% had lower thresholds to FM. For FM2 units, mean best initial frequency for FM was 60.94 kHz +/- 3.162 kHz (n = 114), which is closely matched to the 2nd harmonic in the biosonar signal. Very few units (5) responded only to FM signals, i.e., were FM-specialized. The characteristics of spike-count functions were determined in 587 units. The vast majority (79%) of RF units (n = 228) were nonmonotonic, and about 22% had upper-thresholds.(ABSTRACT TRUNCATED AT 400 WORDS)

Patterns of inhibition in auditory cortical cells in awake squirrel monkeys

Two-tone interactions are recorded in the responses of single units in the superior temporal gyrus to contralateral acoustic stimulation of the awake squirrel monkey. Four response types are distinguished based primarily on the nature of the inhibitory responses elicited by two-tone stimuli, and secondarily on such criteria as the patterns of response to single tones and noise stimuli, thresholds, and spontaneous activity levels. Type A units display strong lateral inhibitory influences which may extend up to 2 octaves on either side, or both sides, of the BF. They are sharply tuned at all intensities and exhibit sustained response to single tone stimuli at the BF. The units have nonmonotonic rate-level functions, and show little or no response to broad band noise. Type A units have low spontaneous rates (less than 3 spikes/s) and relatively high thresholds (greater than or equal to 30 dB SPL). Type B units are characterized by relatively high spontaneous rates of activity (greater than 20 spikes/s) and inhibitory responses to single tone stimuli. Broad band noise may evoke strong excitatory response. Type C units summate the responses to the two-tone stimulus, and show little or no inhibitory influences. They have V-shaped tuning curves, monotonic rate-level functions, low thresholds (greater than or equal to 30 dB SPL), moderate spontaneous rates (ca. 10 spikes/s), and a strong and sustained response to noise and single tone stimuli. Type D units show 'temporal inhibition' to two-tone stimuli, in that an excitatory response to the first tone suppresses (adapts or inhibits) the response to the second tone. These units generally have moderate to broad frequency tuning and phasic responses to single tone stimuli. Histological examination of electrode tracks suggests that Type A units are restricted to A1 (and possibly the rostral field) while other types are distributed over all auditory fields.

Time and Frequency domain processing in the inferior colliculus of echolocating bats

Tone bursts and frequency-modulated (FM) signals were presented to Mexican free-tailed bats and tuning curves, discharge patterns, and discharge latencies of single units in the inferior colliculus were recorded. Cells were broadly tuned to tone bursts, with most Q 10 values ranging from 3 to 20. However, in response to FM stimulation the discharges of neurons were closely synchronized to the time of occurrence of restricted frequency components within the FM sweep. These excitatory frequencies (EFs) were generally unaffected by changes in the starting frequency or intensity of the stimulus. Thus, in response to FM signals, the cells exhibited a much greater frequency selectivity than that observed following tone burst stimulation. Across the population of neurons sampled, EFs covering a wide frequency range were found, and the different EFs were represented in a systematic fashion within the colliculus. The frequencies in an FM biosonar signal or echo will thus be neurally represented both by the time of occurrence of neuronal discharges and by the location of the discharging cells within the nucleus. The potential role of this dual frequency coding in spectral and temporal processing of biosonar signals and echoes is discussed, with emphasis on the neural coding of target range.