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Neurons in the primary auditory cortex (AI) encode complex features of the spectral content of sound, such as direction selectivity. Recent findings of temporal symmetry in AI predict a specific organization of the subcortical input into the cortex that contributes to the emergence of direction selectivity. We demonstrate two subpopulations of neurons in the central nucleus of the inferior colliculus, which differ in their steady-state temporal response profile: lagged and non-lagged. The lagged cells (23%) are shifted in temporal phase with respect to non-lagged cells, and are characterized by an ‘inhibition first’ and delayed excitation in their spectro-temporal receptive fields. Non-lagged cells (77%) have a canonical ‘excitation first’ response. However, we find no difference in the response onset latency to pure tone stimuli between the two subpopulations. Given the homogeneity of tonal response latency, we predict that these lagged cells receive inhibitory input mediated by cortical feedback projections. [ABSTRACT FROM AUTHOR]

Abstract: It is known that anesthesia depresses neural activity and inhibits cortico-cortical interactions and cortical output. Hence, it is important to record from awake animals in order to better understand the full dynamic range of neural responses. We have developed a preparation for chronic, multi-electrode physiological recording in the cortex of the awake ferret. This paper discusses several of the advantages and disadvantages of the technique as well as procedures used to overcome potential complications associated with chronic implants in the ferret. Our solutions are well suited to the special species requirements, yet are also easily generalizable to other species. [Copyright &y& Elsevier]

Five different psychophysical procedures were used to measure level-discrimination (also called intensity discrimination) thresholds for 1-kHz tones at two levels (30 and 90 dB SPL) and two durations (10 and 500 ms). The procedures were the classic transformed up-down staircase method with a two-alternative forced-choice (2AFC) paradigm (UPD), 15- and 50-trial implementations of the method of maximum likelihood (MML) with a cued yes-no paradigm, and 18-trial implementations of ZEST using both cued yes-no and 2AFC paradigms. Results obtained from nine normal listeners show that estimates of level-discrimination thresholds for the four conditions are similar across all five procedures when different points of convergence are accounted for. The variance of threshold estimates within listener and condition was smallest for UPD, largest for the MML with 15 trials, and statistically indistinguishable among the others. The sweat factors ranged from 5.5 for MML with 50 trials to about 1.4 for UPD and ZEST. Simulations show that ideal performance of procedures may be far from real-life experience and that these deviations are likely to depend on complex interactions between listener behavior and parameter choices used for implementing the procedures. Therefore, empirical verification is important for judging the effectiveness of psychophysical procedures. [ABSTRACT FROM AUTHOR]

Pollimyrus adspersus is a fish that uses simple sounds for communication and has auditory specializations for sound-pressure detection. The sounds are species-specific, and the sounds of individuals are sufficiently stereotyped that they could mediate individual recognition. Behavioral measurements are presented indicating that Pollimyrus probably can make species and individual discriminations on the basis of acoustic cues. Interclick interval (ICI; 10-40 ms) and frequency (100-1400 Hz) discrimination was assessed using modulations of the fish's electric organ discharge rate in the presence of a target stimulus presented in alternation with an ongoing base stimulus. Tone frequency discrimination was best in the 200-600-Hz range, with the best threshold of 1.7% ±0.4% standard error at 500 Hz (or 8.5 Hz ±1.9 SE). The just noticeable differences (jnd's) were relatively constant from 100 to 500 Hz (mean 8.7 Hz), then increased at a rate of 13.3 Hz per 100 Hz. For click trains, jnd's increased linearly with ICI. The mean jnd's for 10- and 15-ms ICI were both 300 μs (SE=0.8 ms at 10-ms ICI, SE=0.11 ms at 15-ms ICI). The jnd at 20-ms ICI was only 1.1 ms ±0.25 SE. © 2000 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

In this report we present the first behavioral measurements of auditory sensitivity for Pollimyrus adspersus. Pollimyrus is an electric fish (Mormyridae) that uses both electric and acoustic signals for communication. Tone detection was assessed from the fish's electric organ discharge rate. Suprathreshold tones usually evoked an accelerated rate in naïve animals. This response (rate modulation >=25%) was maintained in a classical conditioning paradigm by presenting a weak electric current near the offset of 3.5-s tone bursts. An adaptive staircase procedure was used to find detection thresholds at frequencies between 100 and 1700 Hz. The mean audiogram from six individuals revealed high sensitivity in the 200-900 Hz range, with the best thresholds near 500 Hz (66.5±4.2 SE dB re: 1 μPa). Sensitivity declined slowly (about 20 dB/octave) above and below this sensitivity maximum. Sensitivity fell off rapidly above 1 kHz (about 60 dB/octave) and no responses were observed at 5 kHz. This behavioral sensitivity matched closely the spectral content of the sounds that this species produced during courtship. Experiments with click trains showed that sensitivity (about 83-dB peak) was independent of inter-click-interval, within the 10-100 ms range. © 2000 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

A study to determine the mechanisms and parameters of the perception of small timing differences in short rhythmic sequences was performed using 13 subjects (which yielded 10 valid subjects) in a completely within-subjects forced choice factorial design. Manipulated variables included three 'base' internote intervals (100, 200, 400 ms-corresponding to different tempos) with respective total durations of the rhythmic figures (1200, 2400, 4800 ms), three categories of two patterns each of varying rhythmic complexity from Povel and Essens [Music Percept. 2, 411-440 (1982)], and six variations in internote note onset timings (0-50 ms). Subjects were played pairs of rhythms, one with the timing variation and one without and then vice versa, and were asked to decide whether the rhythms were the same or different. Scores were based on correct discrimination; reaction time was recorded, but varied too much to be useful. All possible combinations of factors were presented to each subject and order of presentation varied for each subject. Results based on differences between interval variations, plus the strong showing of one of the rhythmic patterns, strongly supported several aspects of a model for an internal clock plus a threshold of 30 ms for detection of timing variation. Except for that one pattern, differences between the patterns did not emerge. This casts doubt on the generality of Povel and Essens' complexity determination of rhythmic figures for pure perception (rather than production). Trends of differences in 'base' interval (or tempo) and thus the total duration of the rhythmic sequences, although not statistically significant, suggested possible confirmation of the limits of precategorical acoustic storage or a model of rhythmic regularity. The unintentional factor of presentation order (within a pair of patterns) turned out to be highly significant, further supporting the model of strongly induced internal clocks. Specifically, 'regular' rhythms preceding variations produced much stronger discrimination, especially at the 30-ms variation threshold. [ABSTRACT FROM AUTHOR]

A maximum-likelihood yes-no procedure was used to measure minimum detectable gaps (MDGs) at 1 and 4 kHz using two track lengths (15 and 30 trials). Results from 11 normal listeners show no difference between MDGs for the two track lengths, and variability of the MDGs did not differ significantly for the track lengths. Results from simulations indicate that the variability of MDGs from real listeners is considerably larger than that imposed by the psychophysical procedure. Additional simulations show that random variability of listeners' response criterion is a compelling explanation for the excess variability. These findings indicate that changes in a listener's threshold over time impose a lower bound on the variability obtainable with a yes-no procedure. They imply that increasing the number of trials in a track, beyond the minimum number required to obtain a stable threshold estimate, offers little or no advantage for the test-retest reliability of a clinical threshold measurement. [ABSTRACT FROM AUTHOR]

The elevation of detection thresholds for a short signal added just after the onset of a masker versus when it is added later during the masker has been called ''overshoot.'' Previous work with broadband maskers suggests masker frequency component near, but not at, the signal frequency produce overshoot. The relative contribution of different frequency regions to the overshoot effect was investigated here with narrow-band noise maskers of different center frequencies. The 350-ms maskers were 1/2 octave wide with center frequencies ranging from 1400 to 8500 Hz. The 10-ms signal was 2500-Hz tone added 4 or 325 ms after the onset of the masker. Masker spectrum levels were 35 dB SPL. The three subjects generally showed little or no overshoot when the masker spectrally overlapped the signal, but showed a maximum overshoot of 8-12 dB for masker bands centered 1/2 to 1 octave above and below the signal frequency; increased signal-masker frequency difference diminished but did not abolish the overshoot effect. Detection thresholds were maximal when the masker band spectrally overlapped the signal and fell as signal-masker frequency separation grew. A similar pattern of results was obtained using low-pass, high-pass, band-pass, and notched noise maskers bounding on a 750-Hz-wide region around the 2500-Hz signal frequency. [Work supported by NIH.] [ABSTRACT FROM AUTHOR]