The spectro-temporal receptive field. A functional characteristic of auditory neurons

Quantitative characterisation procedure for auditory neurons based on the spectro-temporal receptive field

Studies dealing with auditory information processing often present the dynamic spectrum of the sound stimulus (sonogram) in addition to the stimulus waveform. The sonogram, presenting the spectral and temporal properties of the sound in a combined way, reflects properties that are assumed relevant in central information processing. For 12 neurons recorded from the midbrain of the grass frog the sonogram of a Gaussian wide-band noise stimulus was correlated with the output of the neuron to that noise. From this input-output correlogram the spectro-temporal receptive field (STRF) was calculated. The STRF reflects those spectral and temporal properties of the stimulus that influence the firing probability of the neuron. A quantitative procedure was developed to calculate the neuron's response as far as it could be derived from the STRF. This procedure basically consisted of a convolution between STRF and the sonogram of the stimulus followed by a summation over the various frequency bands. In this way it proved possible to estimate to what extent the STRF characterised the neuron's firing behaviour. Heuristic approaches, in which the neuron was modelled to a parallel series of band-pass filters, a summator and a static nonlinearity, representing a spike-generating mechanism, resulted in a considerable improvement of the characterisation.

Stimulus dependent neural correlations in the auditory midbrain of the grassfrog (Rana temporaria L.)

Few-unit recordings were obtained using metal microelectrodes. Separation into single-unit spike trains was based on differences in spike amplitude and spike waveform. For that purpose a hardware microprocessor based spike waveform analyser was designed and built. Spikes are filtered by four matched filters and filter outputs at the moments of spike occurrence are read by a computer and used for off-line separation and spike waveform reconstruction. Thirty-one double unit recordings were obtained and correlation between the separated spike trains was determined. After stimulus correction correlation remained in only 8 of the double unit records. It appeared that in most cases this neural correlation was stimulus dependent. Continuous noise stimulation resulted in the strongest neural correlation remaining after correction for stimulus coupling, stimulation with 48 ms duration tonepips presented once per second generally did not result in a significant neural correlation after the correction procedure for stimulus lock. The usefulness of the additive model for neural correlation and the correction procedure based thereupon is discussed.

Statistical and dimensional analysis of the neural representation of the acoustic biotope of the frog

The field of investigation is the neural representation of acoustic stimuli occurring in the natural environment of the frog. The point of departure is the description of a stimulus ensemble consisting of natural sounds: the acoustic biotope. A relation of statistical and dimensional structure of the acoustic biotope is indicated. The animal used in the neurophysiological experiments is the grass frog, Rana temporaria L.; microelectrode recordings are made in the auditory midbrain. A method is described to determine the existence of a relation between acoustic stimulus and neural events. The form of this relation has been investigated by first- and second-order stimulus-event correlation. While the first one does not give significant results, the second one leads to the spectrotemporal receptive field of the neuron for natural stimuli. Questions are formulated to estimate the value of this receptive field as a functional descriptor of the neuron. Finally, an outline is sketched for a synthetic construction of the bioacoustic space from neuroacoustic subspaces.

A comparison of the spectro-temporal sensitivity of auditory neurons to tonal and natural stimuli

The spectro-temporal sensitivity of auditory neurons has been investigated experimentally by averaging the spectrograms of stimuli preceding the occurrence of action potentials or neural events ( the APES : Aertsen et al., 1980, 1981). The properties of the stimulus ensemble are contained in this measure of neural selectivity. The spectro-temporal receptive field (STRF) has been proposed as a theoretical concept which should give a stimulus-invariant representation of the second order characteristics of the neuron's system function (Aertsen and Johannesma, 1981). The present paper investigates the relation between the experimental and the theoretical description of the neuron's spectro-temporal sensitivity for sound. The aim is to derive a formally based stimulus-normalization procedure for the results of the experimental averaging procedure. Under particular assumptions, regarding both the neuron and the stimulus ensemble, an integral equation connecting the APES and the STRF is derived. This integral expression enables to calculate the APES from the STRF by taking into account the stimulus spectral composition and the characteristics of the spectrogram analysis. The inverse relation, i.e. starting from the experimental results and by application of a formal normalization procedure arriving at the theoretical STRF, is effectively hindered by the nature of the spectrogram analysis. An approximative "normalization" procedure, based on intuitive manipulation of the integral equation, has been applied to a number of single unit recordings from the grassfrog's auditory midbrain area to tonal and natural stimulus ensembles. The results indicate tha spectrogram analysis, while being a useful real-time tool in investigating the spectro-temporal transfer properties of auditory neurons, shows fundamental shortcomings for a theoretical treatment of the questions of interest.