Jos J. Eggermont

Metrics of saccade responses to visual double stimuli: two different modes.

Earlier studies using visual double stimuli along the horizontal axis have revealed a strong averaging tendency in the saccadic system. This study shows averaging also for equally eccentric double stimuli with a modest difference in direction (delta phi = 30 deg). When the difference is enlarged (delta phi = 90 deg) the response pattern becomes bistable; i.e. the eye jumps near either one or the other stimulus. This bistable response mode is reflected also in saccade amplitude when double stimuli along the horizontal axis have a large difference in eccentricity. It is concluded that the saccadic system response mode to double stimuli depends on interstimulus spacing. Furthermore, both types of response can be shown to exist with double stimuli confined to one visual half-field.

Visuomotor fields of the superior colliculus: a quantitative model.

Electrophysiological and electrical stimulation studies in the monkey have disclosed that both the retinal surface and the metrics of saccades are topographically represented in the superior colliculus. This mapping of sensory and motor space onto the colliculus is nonhomogeneous in that the central region is over-represented in both the visual and the motor map. Single unit studies have revealed that visual receptive fields of collicular neurons are typically quite large and are characterized by a skewed (asymmetrical) sensitivity profile. Analyses by McIlwain [J. Neurophysiol. 38, 219-230 (1975)] in the cat have suggested that this skewness property reflects mainly the spatial distortion inherent in the afferent mapping. In this paper we describe a quantitative model, based on a logarithmic mapping function combined with a Gaussian connectivity function in the colliculus, which can account for the extent and the shape of collicular receptive fields. Collicular neurons in the deeper layers have movement-related bursts of activity for saccades in a limited amplitude and direction range related to their location in the collicular map. These movement fields, like visual receptive fields, may be quite extensive and typically have a skewed profile. In our model, an efferent-mapping function is defined, which relates the locus of a population of recruited cells to the metrics of the ensuing saccade. The parameters of this function, which was taken to be identical with the afferent mapping function, were estimated from Robinsons [Vision Res. 12, 1795-1808 (1972)] electrical stimulation data. Based on the assumption that the population-activity profile resembles a two-dimensional Gaussian function, the shape and the size of movement fields can then be described with just 2 or 3 free parameters. Electrophysiological data recorded from a small sample of collicular visuomotor neurons were used to illustrate the procedure, which we designed to enable application of our model to the experimental data. The best fit was obtained when the mapping function was slightly anisotropic. Suggestions on how the model could be improved and extended are offered in the Discussion.

Latency dependence of colour-based target vs nontarget discrimination by the saccadic system.

First saccade responses to sudden presentations of a target/nontarget stimulus consisting of green and red spots of light have been investigated. This paradigm, which avoids certain ambiguities present in earlier experiments with identical double stimuli, leads to remarkably similar conclusions. We found, again, that the saccadic system had short-latency compromising responses (averaging response mode) when the stimulus pair had a modest direction difference (delta phi). When delta phi was enlarged, first saccades were either directed near the green or the red spot (bistable response mode). In a second series of experiments different instructions, emphasizing either speed or accuracy of response, have been given to investigate the relation between saccade accuracy and latency. It appears that, independent of instruction, there is a fixed relation between saccade metrics and latency. The only way to avoid errors, such as averaging responses, is to delay the saccade. Hypothetical mechanisms underlying this relation are discussed against recent neurophysiological studies.

Reverse-correlation methods in auditory research

Single unit recordings have provided us with a basis for understanding the auditory system, especially about how it behaves under stimulation with simple sounds such as clicks and tones. The experimental as well as the theoretical approach to single unit studies has been dichotomous. One approach, the more familiar, gives a representation of nervous system activity in the form of peri-stimulus-time (PST) histograms, period histograms, iso-intensity rate curves and frequency tuning curves. This approach observes the neural output of units in the various nuclei in the auditory nervous system, and, faced with the random way in which the neurons respond to sound, proceeds by repeatedly presenting the same stimulus in order to obtain averaged results. These are the various histogram procedures (Gerstein & Kiang, 1960; Kiang et al. 1965).

Collicular involvement in a saccadic colour discrimination task

SummaryWe have recorded the neural activity of single superior colliculus (SC) neurons in monkeys engaged in a saccadic target/nontarget discrimination task based on a colour cue. Since correct execution of this task probably depends on cortical signal processing, our experiments are of interest for getting a better insight in the problem of how cortical and subcortical signals, relevant for the visual guidance of saccades, are combined. The experiments were designed to distinguish between two extreme possibilities: 1) The crucial cortical signal affects the saccadic system at or above the level of the SC movement-related cells (serial hypothesis); 2) The colour-based target information bypasses the motor colliculus and affects the saccadic system at a level more downstream (bypass hypothesis). Under conditions where the saccadic system had to select a green target stimulus and to ignore the red nontarget spot, the saccade-related activity in SC visuomotor neurons remained as tightly coupled to the metrics of the saccade as it was in a simple spot-detection task. Since the saccade-related activity of these cells appeared to be based on colour information, we conclude that our data corroborate the serial hypothesis. The initial activity after stimulus onset appeared to be colour nonopponent in all neurons. In some cells the neural activity was quantitatively slightly different for the green target and the red nontarget. Since these minor differences were colour rather than motor response dependent, they were probably not part of the target-selection process. These data suggest the possibility that the decision as to which saccade should be made was largely imposed upon the SC visuomotor cells by an external source. We discuss various possibilities for the origin of the putative intervening signal which orders a saccade by causing a burst in the appropriate SC visuomotor neurons.

Prediction of the responses of auditory neurons in the midbrain of the grass frog based on the spectro-temporal receptive field

The spectro-temporal receptive field (STRF) of an auditory neuron represents those characteristics of the sound stimulus in both the time and frequency domain that affect the firing probability of the neuron. The STRF is determined under stationary stimulus conditions for Gaussian wide-band noise. It has been demonstrated that for some neurons the response to that noise could to a considerable extent be derived from the STRF. In the present study the usefulness of the STRF is tested to predict responses to other stimuli such as noise with different frequency content and to species-specific vocalisations. It appears that the predicted response to vocalisations is at best in qualitative agreement with the actual response.

Spectro-temporal characteristics of single units in the auditory midbrain of the lightly anaesthetised grass frog (Rana temporaria L.) Investigated with noise stimuli

About 30% of the auditory units in the midbrain of the lightly anaesthetised grass frog respond in a sustained way to stationary pseudorandom noise. This response is described by the spectro-temporal receptive field (STRF), the regions in the spectro-temporal domain where the average second-order functional of those parts of the stimulus ensemble that precede the action potentials differ from the average second-order functional of the stimulus ensemble. By means of the STRF frequency selectivity, postactivation suppression and lateral suppression can quantitatively be studied under one and the same experimental condition. Auditory units that respond to stationary noise are localised in those parts of the torus where fibres enter from the olivary nucleus. They are characterised by relatively short latencies to tones and probably represent the first information-processing stage in the torus semicircularis.

Peripheral auditory adaptation and fatigue: A model oriented review☆

A model is introduced for auditory adaptation based on the stochastic models that are widely documented for birth and death processes and used as a vehicle to review single unit and compound action potential studies as well as various models for adaptation and forward masking. It appears that such a model inherently incorporates a relation between the perstimulatory adaptation time constant, the poststimulatory recovery time constant and the ratio between adapted firing rate and onset firing rate. The knowledge of any two of these parameters allows the prediction of the third one. The model takes into account postsynaptic membrane properties. The model based on Markov assumptions is a linear one. Although onset firing rates depend on a nonlinear way upon stimulus level, the above mentioned time constants and the adapted rate to onset rate ratio are intensity independent. The idea is put forward and tested that a comparable situation exists for the depression effects produced by mild auditory fatigue. The apparent findings that time constants tend to be intensity dependent for auditory fatigue is explained on basis of an interaction effect of dependent exponential processes.

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 neurons 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 neurons 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. Thirthy-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.