Gap detection in the starling (Sturnus vulgaris)

Summary Gap-detection thresholds of single units were determined from auditory forebrain neurons of the awake starling. Nine different response types were statistically defined from the discharge pattern to a 400 ms broadband noise stimulus. The gap stimuli consisted of two broadband noise bursts which were separated by a gap ranging from 0.4 to 204.8 ms duration. The median minimumdetectable gap for 121 out of 145 units that had a significant threshold 204.8ms was 12.8 ms; 20% of the neurons showed thresholds between 0.4 and 3.2 ms. The neurons of the nine response types differed significantly in their minimum-detectable gaps; neurons with phasic-tonic and phasic excitation exhibited the best (i.e. shortest) minimum-detectable gaps. The neurons of the three different recording areas (field L, NCM and HV) were significantly different in their minimumdetectable gaps; field L neurons showed the best temporal resolution for gaps in broadband noise. Gap-detection thresholds are compared with psychophysical thresholds determined with the same stimuli and the relevance of forebrain units for temporal resolution is discussed.Abbreviations CS control stimulus - HV hyperstriatum ventrale - HVc hyperstriatum ventrale pars caudalis - NB noise burst - NCM neostriatum caudale pars medialis - NS noise stimulus - SGS standard gap series - TW time window     

Neural correlates of behavioral gap detection in the inferior colliculus of the young CBA mouse

The gap detection paradigm is frequently used in psychoacoustics to characterize the temporal acuity of the auditory system. Neural responses to silent gaps embedded in white-noise carriers, were obtained from mouse inferior colliculus (IC) neurons and the results compared to behavioral estimates of gap detection. Neural correlates of gap detection were obtained from 78 single neurons located in the central nucleus of the IC. Minimal gap thresholds (MGTs) were computed from single-unit gap functions and were found to be comparable, 1–2 ms, to the behavioral gap threshold (2 ms). There was no difference in MGTs for units in which both carrier intensities were collected. Single unit responses were classified based on temporal discharge patterns to steady-state noise bursts. Onset and primary-like units had the shortest mean MGTs (2.0 ms), followed by sustained units (4.0 ms) and phasic-off units (4.2 ms). The longest MGTs were obtained for inhibitory neurons (xˉ = 14 ms). Finally, the time-course of behavioral and neurophysiological gap functions were found to be in good agreement. The results of the present study indicate the neural code necessary for behavioral gap detection is present in the temporal discharge patterns of the majority of IC neurons. Accepted: 6 February 1997     

Temporal coding in the auditory receptor of the moth ear

Temporal coding in the moth ear was inferred from the response of the auditory receptor to acoustic stimuli with different temporal characteristics.

Temporal resolution for calling song signals by female crickets, <Emphasis Type="Italic">Gryllus bimaculatus</Emphasis>

A behavioural gap detection paradigm was used to determine the temporal resolution for song patterns by female crickets, Gryllus bimaculatus. For stimuli with a modulation depth of 100% the critical gap duration was 6–8 ms. A reduction of the modulation depth of gaps to 50% led either to an increase or a decrease of the critical gap duration. In the latter case, the critical gap duration dropped to 3–4 ms indicating a higher sensitivity of auditory processing. The response curve for variation of pulse period was not limited by temporal resolution. However, the reduced response to stimuli with a high duty cycle, and thus short pause durations, was in accordance with the limits of temporal resolution. The critical duration of masking pulses inserted into pauses was 4–6 ms. An analysis of the songs of males revealed that gaps (5.8 ms) and masking pulses (6.9 ms) were at detectable time scales for the auditory pathway of female crickets. However, most of the observed temporal variation of song patterns was tolerated by females. Critical cues such as pulse period and pulse duty cycle provided little basis for inter-individual selection by females.     

Temporal modulation transfer functions in auditory receptor fibres of the locust (Locusta migratoria L.)

The temporal resolution of auditory receptors of locusts was investigated by applying noise stimuli with sinusoidal amplitude modulations and by computing temporal modulation transfer functions. These transfer functions showed mostly bandpass characteristics, which are rarely found in other species at the level of receptors. From the upper cut-off frequencies of the modulation transfer functions the minimum integration times were calculated. Minimum integration times showed no significant correlation to the receptor spike rates but depended strongly on the body temperature. At 20 degrees C the average minimum integration time was 1.7 ms, dropping to 0.95 ms at 30 degrees C. The values found in this study correspond well to the range of minimum integration times found in birds and mammals. Gap detection is another standard paradigm to investigate temporal resolution. In locusts and other grasshoppers application of this paradigm yielded values of the minimum detectable gap widths that are approximately twice as large than the minimum integration times reported here.     

The modulation rate transfer function of a harbour porpoise (Phocoena phocoena)

During echolocation, toothed whales produce ultrasonic clicks at extremely rapid rates and listen for the returning echoes. The auditory brainstem response (ABR) duration was evaluated in terms of latency between single peaks: 5.5 ms (from peak I to VII), 3.4 ms (I–VI), and 1.4 ms (II–IV). In comparison to the killer whale and the bottlenose dolphin, the ABR of the harbour porpoise has shorter intervals between the peaks and consequently a shorter ABR duration. This indicates that the ABR duration and peak latencies are possibly related to the relative size of the auditory structures of the central nervous system and thus to the animal’s size. The ABR to a sinusoidal amplitude modulated stimulus at 125 kHz (sensitivity threshold 63 dB re 1 μPa rms) was evaluated to determine the modulation rate transfer function of a harbour porpoise. The ABR showed distinct envelope following responses up to a modulation rate of 1,900 Hz. The corresponding calculated equivalent rectangular duration of 263 μs indicates a good temporal resolution in the harbour porpoise auditory system similar to the one for the bottlenose dolphin. The results explain how the harbour porpoise can follow clicks and echoes during echolocation with very short inter click intervals.     

Two measures of temporal resolution in brown-headed cowbirds (<Emphasis Type="Italic">Molothrus ater</Emphasis>)

Studies of auditory temporal resolution in birds have traditionally examined processing capabilities by assessing behavioral discrimination of sounds varying in temporal structure. Here, temporal resolution of the brown-headed cowbird (Molothrus ater) was measured using two auditory evoked potential (AEP)-based methods: auditory brainstem responses (ABRs) to paired clicks and envelope following responses (EFRs) to amplitude-modulated tones. The basic patterns observed in cowbirds were similar to those found in other songbird species, suggesting similar temporal processing capabilities. The amplitude of the ABR to the second click was less than that of the first click at inter-click intervals less than 10 ms, and decreased to 30% at an interval of 1 ms. EFR amplitude was generally greatest at modulation frequencies from 335 to 635 Hz and decreased at higher and lower modulation frequencies. Compared to data from terrestrial mammals these results support recent behavioral findings of enhanced temporal resolution in birds. General agreement between these AEP results and behaviorally based studies suggests that AEPs can provide a useful assessment of temporal resolution in wild bird species.     

Avidity enhancement of L-selectin bonds by flow: shear-promoted rotation of leukocytes turn labile bonds into functional tethers

L-selectin is a key lectin essential for leukocyte capture and rolling on vessel walls. Functional adhesion of L-selectin requires a minimal threshold of hydrodynamic shear. Using high temporal resolution videomicroscopy, we now report that L-selectin engages its ligands through exceptionally labile adhesive bonds (tethers) even below this shear threshold. These tethers share a lifetime of 4 ms on distinct physiological ligands, two orders of magnitude shorter than the lifetime of the P-selectin-PSGL-1 bond. Below threshold shear, tether duration is not shortened by elevated shear stresses. However, above the shear threshold, selectin tethers undergo 14-fold stabilization by shear-driven leukocyte transport. Notably, the cytoplasmic tail of L-selectin contributes to this stabilization only above the shear threshold. These properties are not shared by P-selectin- or VLA-4-mediated tethers. L-selectin tethers appear adapted to undergo rapid avidity enhancement by cellular transport, a specialized mechanism not used by any other known adhesion receptor.     

Biological Markers of Auditory Gap Detection in Young,Middle-Aged,and Older Adults

The capability of processing rapid fluctuations in the temporal envelope of sound declines with age and this contributes to older adults'' difficulties in understanding speech. Although, changes in central auditory processing during aging have been proposed as cause for communication deficits, an open question remains which stage of processing is mostly affected by age related changes. We investigated auditory temporal resolution in young, middle-aged, and older listeners with neuromagnetic evoked responses to gap stimuli with different leading marker and gap durations. Signal components specific for processing the physical details of sound stimuli as well as the auditory objects as a whole were derived from the evoked activity and served as biological markers for temporal processing at different cortical levels. Early oscillatory 40-Hz responses were elicited by the onsets of leading and lagging markers and indicated central registration of the gap with similar amplitude in all three age groups. High-gamma responses were predominantly related to the duration of no-gap stimuli or to the duration of gaps when present, and decreased in amplitude and phase locking with increasing age. Correspondingly, low-frequency activity around 200 ms and later was reduced in middle aged and older participants. High-gamma band, and long-latency low-frequency responses were interpreted as reflecting higher order processes related to the grouping of sound items into auditory objects and updating of memory for these objects. The observed effects indicate that age-related changes in auditory acuity have more to do with higher-order brain functions than previously thought.     

Temporal modulation transfer functions in the barn owl (<Emphasis Type="Italic">Tyto alba</Emphasis>)

Barn owls (Tyto alba) have evolved several specializations in their auditory system to achieve the high sensory acuity required for prey capture, including superior processing of interaural time differences and phase coding in the auditory periphery. Here, we tested whether barn owls are capable of high temporal resolution that may be a prerequisite for the accuracy in binaural processing. Temporal resolution was measured psychoacoustically and demonstrated in temporal modulation transfer functions. Four barn owls were trained in an operant task with food reward to detect sinusoidal amplitude modulations within an 800-ms gated white-noise burst or 800-ms periods of modulation in continuous white noise (spectrum levels of -5 dB and 15 dB SPL). Within the range of tested amplitude modulation frequencies from 5 Hz to 1280 Hz, barn owls' detection thresholds were lowest at 10-20 Hz. This sensitivity corresponds to an intensity-difference limen of between 0.9 dB and 1.4 dB. For all conditions, temporal modulation transfer functions showed band-pass characteristics with a high-frequency cutoff in the range of 37 Hz to 92 Hz, corresponding to minimum integration times of 4.3 ms and 1.7 ms, respectively. In summary, these data indicate a temporal resolution in the owl's auditory system that is good, but not unusual, compared to other vertebrates.     

Intensity Coding in the Frog Retina : Quantitative relations between impulse and graded activity

The impulse discharge of single on-off neurons and a graded field potential, the proximal negative response (PNR), were simultaneously recorded with an extracellular microelectrode in the inner frog retina. Normalized amplitude-intensity functions for the on-response of the PNR and the neuron's post-stimulus time histogram (PSTH) were nearly coincident and typically showed a dynamic range spanning approximately 2 log units of intensity. Thus a nearly linear relation is found between the amplitude of the PNR and the neuron's PSTH. A neuron's PSTH amplitude and maximum instantaneous frequency of discharge were usually highly correlated, but occasional marked disparities indicate that temporal jitter of the first spike latency is an additional, relatively independent variable influencing PSTH amplitude. It typically changes by a factor of 20–30 over the intensity range. These and other findings have implications for the functional significance of the PNR and the PSTH, for a possible linear link between amacrine and on-off ganglion cells, and for a mechanism of intensity coding in which temporal jitter of latency exerts a major role.     

Functional topography of cat primary auditory cortex: response latencies

Minimum onset latency (L_min) of single- and multiple-unit responses were mapped in the primary auditory cortex (AI) of barbiturate-anesthetized cats. Contralateral L_min for multiple units was non-homogeneously distributed along the dorso-ventral/isofrequency axis of the AI. Responses with shorter latencies were more often located in the central, more sharply tuned region while longer latencies were more frequently encountered in the dorsal and ventral portions of the AI. For single units, a large scatter of L_min values was found throughout the extent of the AI including cortical depth. The relationship between L_min and previously reported spectral, intensity and temporal parameters was analyzed and revealed statistically significant correlations between minimum onset latency and the following response properties in some but not all studied animals: sharpness of tuning of a frequency response area 10 dB above threshold, broadband transient response, strongest response level, monotonicity of rate/level functions, dynamic range, and preferred frequency modulation sweep direction. This analysis suggests that L_min is determined by several independent factors and that the prediction of L_min based on relationships with other spectral and temporal response properties is inherently weak. The spatial distribution and the functional relationship between these response parameters may provide an important aspect of the time-based cortical representation of specific features in the animal's natural environment. Accepted: 13 August 1997     

Responses to features of the calling song by ascending auditory interneurones in the cricket Gryllus campestris

ABSTRACT. In female Gryllus campestris L., three functional types of ascending auditory intemeurones have been studied by recording from them extracellularly in the split cervical connectives using suction electrodes. Type 1 neurones are characterized by an optimal sensitivity to the carrier frequency of the species calling song (4–5 kHz). They copy the syllable and pause structure of the call at all intensities. The patterned spike discharge is observable at least 8 dB above absolute threshold. With suprathreshold stimulation, the neurones exhibit maximal responses (number of spikes/chirp) around the carrier frequency. The intensity response curves are approximately linear in the range of 40–90 dB SPL. The envelope of each syllable is reflected by a corresponding change in the firing rate, and syllable periods of 24ms and longer are resolved. This type can be considered as a neural correlate for phonotactic behaviour of the female where the syllable period has been found to be the most important temporal parameter. Type 2 neurones are most sensitive in the range of 4–6 and 11–13 kHz. They copy the syllable and pause structure of the species calling song at low and moderate intensities. However, the spikes invade the intersyllable pauses, when stimulated with the calling song at higher intensities (above 85 dB). This is particularly apparent at the onset of a chirp series. The slope of the intensity—response curve mimics that of type 1 units. The neurones cannot follow syllable periods shorter than 32 ms. Type 3 neurones differ from types 1 and 2 by a rather broad-band sensitivity in the range of 3–16 kHz, and in copying the chirp as a whole. Even at low stimulus intensities, the intersyllable pauses are filled with spikes, and information about the syllable—pause structure is lost. Stimulation with suprathreshold intensities gives rise to a rather uniform, broad-band response without distinctive peaks. The intensity—response curve is characterized by a higher absolute threshold, and by the reduction in the response magnitude starting above 70–80 dB. These units are not suitable for copying the calling song temporal structure in detail, but would indicate the chirping rhythm. Their strong response in the range of the species courtship song carrier frequency make them suitable to copy the courtship song.     

Behaviorally measured audiograms and gap detection thresholds in CBA/CaJ mice

Tone detection and temporal gap detection thresholds were determined in CBA/CaJ mice using a Go/No-go procedure and the psychophysical method of constant stimuli. In the first experiment, audiograms were constructed for five CBA/CaJ mice. Thresholds were obtained for eight pure tones ranging in frequency from 1 to 42 kHz. Audiograms showed peak sensitivity between 8 and 24 kHz, with higher thresholds at lower and higher frequencies. In the second experiment, thresholds for gap detection in broadband and narrowband noise bursts were measured at several sensation levels. For broadband noise, gap thresholds were between 1 and 2 ms, except at very low sensation levels, where thresholds increased significantly. Gap thresholds also increased significantly for low pass-filtered noise bursts with a cutoff frequency below 18 kHz. Our experiments revised absolute auditory thresholds in the CBA/CaJ mouse strain and demonstrated excellent gap detection ability in the mouse. These results add to the baseline behavioral data from normal-hearing mice which have become increasingly important for assessing auditory abilities in genetically altered mice.     

Echoic Memory: Investigation of Its Temporal Resolution by Auditory Offset Cortical Responses

Previous studies showed that the amplitude and latency of the auditory offset cortical response depended on the history of the sound, which implicated the involvement of echoic memory in shaping a response. When a brief sound was repeated, the latency of the offset response depended precisely on the frequency of the repeat, indicating that the brain recognized the timing of the offset by using information on the repeat frequency stored in memory. In the present study, we investigated the temporal resolution of sensory storage by measuring auditory offset responses with magnetoencephalography (MEG). The offset of a train of clicks for 1 s elicited a clear magnetic response at approximately 60 ms (Off-P50m). The latency of Off-P50m depended on the inter-stimulus interval (ISI) of the click train, which was the longest at 40 ms (25 Hz) and became shorter with shorter ISIs (2.5∼20 ms). The correlation coefficient r² for the peak latency and ISI was as high as 0.99, which suggested that sensory storage for the stimulation frequency accurately determined the Off-P50m latency. Statistical analysis revealed that the latency of all pairs, except for that between 200 and 400 Hz, was significantly different, indicating the very high temporal resolution of sensory storage at approximately 5 ms.     

Directionality of phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens

Summary A dorsal approach to the eighth nerve and free-field stimulation were used to investigate the effect of sound direction and intensity on phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens.Tuning curves of 75 auditory neurons were analyzed (Fig. 2). Amphibian papillar neurons, but not basilar papillar neurons, exhibit significant phase locking to short tone bursts at the characteristic frequency (CF), the degree of phase locking (vector strength) decreasing with the neuron's CF (Figs. 3, 4 and 10E). Vector strength increases with sound pressure level to saturate about 20 dB above threshold, while the preferred firing phase is only slightly affected (Figs. 5 and 6).In contrast, sound direction hardly affects vector strength (Figs. 7, 8, 9A and 10A and C), but has a strong influence on the preferred firing phase (Figs. 7, 8, 9B and C, 10B and D): With respect to anterior tone presentation there are phase lags for ipsilateral and phase leads for posterior and contralateral presentation. Phase differences between both ears show a sinusoidal or cardioid/ovoidal directional characteristic; maximum differences are found with antero-lateral tone presentation (Fig. 11). The directionality of phase locking decreases with the neuron's CF (Fig. 10F) and only slightly changes with sound pressure level (Fig. 12). Thus, phase locking of amphibian papilla neurons can potentially provide intensity-independent information for sound localization.Abbreviations SPL sound pressure level - FTC frequency threshold curve - CF characteristic frequency - TF test frequency - VS vector strength - AP amphibian papilla - BP basilar papilla     

White Noise Masks Some Temporal Parameters of the Auditory Localization of Radially Moving Targets

The temporal parameters of the perception of radially moving sound sources partly masked with broadband internalized noise at an intensity of 40, 46, or 52 dB above the hearing threshold have been studied. The threshold of sound duration necessary for identifying the direction of movement of the sound source (75% correct answers) increases from 135 ms in silence to 285 ms at all intensities of continuous noise studied. The minimum duration of the stimulus beginning with which a subsequent increase in duration does not increase the number of correct responses is the same (385 ms) under all conditions of stimulus presentation. Broadband noise of any intensity increases the time of response to stimuli in the range of durations studied. At a noise of 52 dB, which is close to the threshold of full masking, the reaction time is not increased significantly compared to its estimation at a noise of 46 dB. The minimum duration of the stimulus has proved to be the stablest temporal parameter of the perception of movement of a sound source. Changes in the temporal parameters of sound perception at noise levels close to the threshold of full masking are discussed.     

Songbirds tradeoff auditory frequency resolution and temporal resolution

Physical tradeoffs may in some cases constrain the evolution of sensory systems. The peripheral auditory system, for example, performs a spectral decomposition of sound that should result in a tradeoff between frequency resolution and temporal resolution. We assessed temporal resolution in three songbird species using auditory brainstem responses to paired click stimuli. Temporal resolution was greater in house sparrows (Passer domesticus) than Carolina chickadees (Poecile carolinensis) and white-breasted nuthatches (Sitta carolinensis), as predicted based on previous observations of broader auditory filters (lower frequency resolution) in house sparrows. Furthermore, within chickadees, individuals with broader auditory filters had greater temporal resolution. In contrast to predictions however, temporal resolution was similar between chickadees and nuthatches despite broader auditory filters in chickadees. These results and the results of a model simulation exploring the effect of broadened auditory filter bandwidth on temporal resolution in the auditory periphery strongly suggest that frequency resolution constrains temporal resolution in songbirds. Furthermore, our results suggest that songbirds have greater temporal resolution than some mammals, in agreement with recent behavioral studies. Species differences in temporal resolution may reflect adaptations for efficient processing of species-specific vocalizations, while individual differences within species may reflect experience-based developmental plasticity or hormonal effects.     

Neuronal Adaptation Translates Stimulus Gaps into a Population Code

Neurons in sensory pathways exhibit a vast multitude of adaptation behaviors, which are assumed to aid the encoding of temporal stimulus features and provide the basis for a population code in higher brain areas. Here we study the transition to a population code for auditory gap stimuli both in neurophysiological recordings and in a computational network model. Independent component analysis (ICA) of experimental data from the inferior colliculus of Mongolian gerbils reveals that the network encodes different gap sizes primarily with its population firing rate within 30 ms after the presentation of the gap, where longer gap size evokes higher network activity. We then developed a computational model to investigate possible mechanisms of how to generate the population code for gaps. Phenomenological (ICA) and functional (discrimination performance) analyses of our simulated networks show that the experimentally observed patterns may result from heterogeneous adaptation, where adaptation provides gap detection at the single neuron level and neuronal heterogeneity ensures discriminable population codes for the whole range of gap sizes in the input. Furthermore, our work suggests that network recurrence additionally enhances the network''s ability to provide discriminable population patterns.     

Contrast gain control in auditory cortex

The auditory system must represent sounds with a wide range of statistical properties. One important property is the spectrotemporal contrast in the acoustic environment: the variation in sound pressure in each frequency band, relative to the mean pressure. We show that neurons in ferret auditory cortex rescale their gain to partially compensate for the spectrotemporal contrast of recent stimulation. When contrast is low, neurons increase their gain, becoming more sensitive to small changes in the stimulus, although the effectiveness of contrast gain control is reduced at low mean levels. Gain is primarily determined by contrast near each neuron's preferred frequency, but there is also a contribution from contrast in more distant frequency bands. Neural responses are modulated by contrast over timescales of ～100?ms. By using contrast gain control to expand or compress the representation of its inputs, the auditory system may be seeking an efficient coding of natural sounds.