Repetition Enhancement for Frequency-Modulated but Not Unmodulated Sounds: A Human MEG Study

BACKGROUND: Decoding of frequency-modulated (FM) sounds is essential for phoneme identification. This study investigates selectivity to FM direction in the human auditory system. METHODOLOGY/PRINCIPAL FINDINGS: Magnetoencephalography was recorded in 10 adults during a two-tone adaptation paradigm with a 200-ms interstimulus-interval. Stimuli were pairs of either same or different frequency modulation direction. To control that FM repetition effects cannot be accounted for by their on- and offset properties, we additionally assessed responses to pairs of unmodulated tones with either same or different frequency composition. For the FM sweeps, N1m event-related magnetic field components were found at 103 and 130 ms after onset of the first (S1) and second stimulus (S2), respectively. This was followed by a sustained component starting at about 200 ms after S2. The sustained response was significantly stronger for stimulation with the same compared to different FM direction. This effect was not observed for the non-modulated control stimuli. CONCLUSIONS/SIGNIFICANCE: Low-level processing of FM sounds was characterized by repetition enhancement to stimulus pairs with same versus different FM directions. This effect was FM-specific; it did not occur for unmodulated tones. The present findings may reflect specific interactions between frequency separation and temporal distance in the processing of consecutive FM sweeps

TRY plant trait database – enhanced coverage and open access

Plant traits - the morphological, anatomical, physiological, biochemical and phenological characteristics of plants - determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait‐based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits - almost complete coverage for ‘plant growth form’. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait–environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives

Stimulus-specific adaptation in field potentials and neuronal responses to frequency-modulated tones in the primary auditory cortex

In order to structure the sensory environment our brain needs to detect changes in the surrounding that might indicate events of presumed behavioral relevance. A characteristic brain response presumably related to the detection of such novel stimuli is termed mismatch negativity (MMN) observable in human scalp recordings. A candidate mechanism underlying MMN at the neuronal level is stimulus-specific adaptation (SSA) which has several characteristics in common. SSA is the specific decrease in the response to a frequent stimulus, which does not generalize to an interleaved rare stimulus in a sequence of events. SSA was so far mainly described for changes in the response to simple pure tone stimuli differing in tone frequency. In this study we provide data from the awake rat auditory cortex on adaptation in the responses to frequency- modulated tones (FM) with the deviating feature being the direction of FM modulation. Adaptation of cortical neurons to the direction of FM modulation was stronger for slow modulation than for faster modulation. In contrast to pure tone SSA which showed no stimulus preference, FM adaptation in neuronal data differed sometimes between upward and downward FM. This, however, was not the case in the local field potential data recorded simultaneously. Our findings support the role of the auditory cortex as the source for change related activity induced by FM stimuli by showing that dynamic stimulus features such as FM modulation can evoke SSA in the rat in a way very similar to FM-induced MMN in the human auditory cortex

Schematic illustration of the stimuli used in both parts of the experiment and experimental conditions.

<p>(A) All stimuli consisted of four harmonic components indicated by the black bars, and were filtered with a Gaussian band-pass as symbolized by the Gaussian curve at the ordinate. The lighter gray shades of the lower and higher components indicate reduced sound intensity due to the filtering, see section on Stimuli for further details. The central sketch depicts an ascending FM sweep as used in experiment 1, the left and right sketches show the non-modulated stimuli used in experiment 2. (B) experimental conditions and procedure for the frequency-modulated sweeps, and (C) unmodulated tones.</p

Grand-averaged event-related fields (ERF).

<p>ERFs for all subjects and sensors for the ‘same’ (on the left) and ‘different’ (on the right) conditions are shown. In parts (A) and (B) of the figure the raw data are shown for the frequency-modulated tones in experiment 1, and (C) and (D) show the ERF for the non modulated-tones in experiment 2 of the study. Dotted grey lines in each of the four graphs indicate the beginning and ending of the stimuli.</p

Source difference waveforms between modulated and unmodulated tones.

<p>Difference waveforms from the source models for the differences between same-different modulated and same-different unmodulated tones are shown in blue and red for the left and right-hemisphere, respectively. The confidence range, obtained with the bootstrapping procedure, is plotted in grey. Significant differences are found across right- and left-hemisphere sensors in response to the second stimulus at 150–300 ms after S2 onset (p<0.001, uncorrected for multiple comparisons). In addition, the same-different waveforms for modulated and unmodulated tones are depicted in purple and green, respectively. Dotted grey lines in each of the four graphs indicate the beginning and ending of the stimuli.</p

Evoked magnetic responses plotted as global field power (GFP).

<p>(A) GFP evoked by the frequency-modulated sweeps averaged across the left- and right-hemisphere sensors. (B) GFP evoked by unmodulated tones over the left- and the right-hemisphere sensors.</p

Discrimination of Direction in Fast Frequency-Modulated Tones by Rats

Fast frequency modulations (FM) are an essential part of species-specific auditory signals in animals as well as in human speech. Major parameters characterizing non-periodic frequency modulations are the direction of frequency change in the FM sweep (upward/downward) and the sweep speed, i.e., the speed of frequency change. While it is well established that both parameters are represented in the mammalian central auditory pathway, their importance at the perceptual level in animals is unclear. We determined the ability of rats to discriminate between upward and downward modulated FM-tones as a function of sweep speed in a two-alternative-forced-choice-paradigm. Directional discrimination in logarithmic FM-sweeps was reduced with increasing sweep speed between 20 and 1,000 octaves/s following a psychometric function. Average threshold sweep speed for FM directional discrimination was 96 octaves/s. This upper limit of perceptual FM discrimination fits well the upper limit of preferred sweep speeds in auditory neurons and the upper limit of neuronal direction selectivity in the rat auditory cortex and midbrain, as it is found in the literature. Influences of additional stimulus parameters on FM discrimination were determined using an adaptive testing-procedure for efficient threshold estimation based on a maximum likelihood approach. Directional discrimination improved with extended FM sweep range between two and five octaves. Discrimination performance declined with increasing lower frequency boundary of FM sweeps, showing an especially strong deterioration when the boundary was raised from 2 to 4 kHz. This deterioration corresponds to a frequency-dependent decline in direction selectivity of FM-encoding neurons in the rat auditory cortex, as described in the literature. Taken together, by investigating directional discrimination of FM sweeps in the rat we found characteristics at the perceptual level that can be related to several aspects of FM encoding in the central auditory pathway