Primary visual coding can be characterized by the receptive field (RF) properties of single neurons. Subject of this paper is our search for a global, second coding step beyond the RF-concept that links related features in a visual scene. In recent models of visual coding, oscillatory activities have been proposed to constitute such linking signals. We tested the neurophysiological relevance of this hypothesis for the visual system. Single and multiple spikes as well as local field potentials were recorded simultaneously from several locations in the primary visual cortex (A17 and A18) using 7 or 19 individually advanceable fiber-microelectrodes (250 or 330 microns apart). Stimulus-evoked (SE)-resonances of 35-85 Hz were found in these three types of signals throughout the visual cortex when the primary coding channels were activated by their specific stimuli. Stimulus position, orientation, movement direction and velocity, ocularity and stationary flicker caused specific SE-resonances. Coherent SE-resonances were found at distant cortical positions when at least one of the primary coding properties was similar. Coherence was found 1) within a vertical cortex column, 2) between neighbouring hypercolumns, and 3) between two different cortical areas. We assume that the coherence of SE-resonances is mediated by recurrent excitatory intra- and inter-areal connections via phase locking between assemblies that represent the linking features of the actual visual scene. Visually related activities are, thus, transiently labelled by a temporal code that signalizes their momentary association.

Brosch, Michael and Christoph E. Schreiner. Time course of different auditory streams (Bregman 1990). Numerous psyforward masking tuning curves in cat primary auditory cortex. chophysical studies have demonstrated that the temporal J. Neurophysiol. 77: 923-943, 1997. Nonsimultaneous two-tone stimulus context affects the perceptual quality of individual interactions were studied in the primary auditory cortex of anesthe-auditory events. Detection thresholds of individual sounds tized cats. Poststimulatory effects of pure tone bursts (masker) on can be elevated (Lüscher and Zwislocki 1947), and the subthe evoked activity of a fixed tone burst (probe) were investigated. jective pitch and loudness (Stevens and Davis 1938) of audiThe temporal interval from masker onset to probe onset (stimulus tory events can be altered by preceding and succeeding onset asynchrony), masker frequency, and intensity were parametsounds. Although the perceptual consequences of the temporically varied. For all of the 53 single units and 58 multiple-unit clusters, the neural activity of the probe signal was either inhibited, ral stimulus context have been studied intensively, little infacilitated, and/or delayed by a limited set of masker stimuli. The formation is available on the central neural mechanisms and stimulus range from which forward inhibition of the probe was neural structures underlying the processing of time-varying induced typically was centered at and had approximately the size stimuli.of the neuron's excitatory receptive field. This ''masking tuning Among the different neural structures, the auditory cortex curve'' was usually V shaped, i.e., the frequency range of inhibiting has been recognized as playing an important role in the masker stimuli increased with the masker intensity. Forward inhibiprocessing of temporal stimulus sequences. The main experition was induced at the shortest stimulus onset asynchrony between mental support for this comes from behavior-lesion studies masker and probe. With longer stimulus onset asynchronies, the on cats and monkeys. After ablation of large parts of the frequency range of inhibiting maskers gradually became smaller.Recovery from forward inhibition occurred first at the lower-and temporal lobe, subjects are significantly impaired in discrimhigher-frequency borders of the masking tuning curve and lasted inating changes in the temporal patterns of tone sequences the longest for frequencies close to the neuron's characteristic fre- ( content. The differential sensitivity to temporal sound sequences studies have shown that neurons respond in a time-locked within the receptive field of cortical cells as well as across different fashion to repetition rates of up to several elements per seccells could contribute to the neural processing of temporally struc-ond (Creutzfeldt et al. 1980;Eggermont 1991;

A central tenet in brain research is that early sensory cortex is modality specific, and, only in exceptional cases, such as deaf and blind subjects or professional musicians, is influenced by other modalities. Here we describe extensive cross-modal activation in the auditory cortex of two monkeys while they performed a demanding auditory categorization task: after a cue light was turned on, monkeys could initiate a tone sequence by touching a bar and then earn a reward by releasing the bar on occurrence of a falling frequency contour in the sequence. In their primary auditory cortex and posterior belt areas, we found many acoustically responsive neurons whose firing was synchronized to the cue light or to the touch or release of the bar. Of 315 multiunits, 45 exhibited cue light-related firing, 194 exhibited firing that was related to bar touch, and 268 exhibited firing that was related to bar release. Among 60 single units, we found one neuron with cue light-related firing, 21 with bar touch-related firing, and 36 with release-related firing. This firing disappeared at individual sites when the monkeys performed a visual detection task. Our findings corroborate and extend recent findings on cross-modal activation in the auditory cortex and suggests that the auditory cortex can be activated by visual and somatosensory stimulation and by movements. We speculate that the multimodal corepresentation in the auditory cortex has arisen from the intensive practice of the subjects with the behavioral procedure and that it facilitates the performance of audiomotor tasks in proficient subjects.

With a multielectrode system, we explored neuronal activity in the gamma range (>40 Hz) in the primary and caudomedial auditory cortex of six anesthetized macaque monkeys. Stimuli were tone bursts of 100- to 500-ms duration that were presented at sound pressure levels of 40-60 dB and were varied over a wide range of frequencies. These stimuli induced gamma oscillations, not phase-locked to the onset of stimulation, in 465 of 616 multiunit clusters and at 321 of 422 sites at which field potentials were recorded. Occurrence of gamma activity was stimulus dependent. It was mostly seen when the stimulus was at the units' preferred frequency. The incidence of gamma activity decreased with increasing difference between stimulus frequency and preferred frequency. gamma activity emerged 100-900 ms after stimulus onset with highest incidence ~120 ms. Amplitudes of stimulus-induced gamma oscillations in field potentials were, on average, almost twice the amplitude of spontaneously occurring gamma oscillations. gamma activity at different sites within the primary and the caudomedial auditory field could be synchronized at near-zero phase. Synchrony depended on the spatial distance and on the receptive fields similarity of pairs of units. It decreased with increasing distance between recording sites and increased with similarity of preferred frequencies of the pairs of units. The results indicate that stimulus-induced gamma oscillations originate from sources in the auditory cortex. They further suggest that gamma oscillations may provide a mechanism utilized in many parts of the sensory cortex, including the auditory cortex, to integrate neurons according to the similarity of their receptive fields.

It is well established that the tone-evoked response of neurons in auditory cortex can be attenuated if another tone is presented several hundred milliseconds before. The present study explores in detail a complementary phenomenon in which the tone-evoked response is enhanced by a preceding tone. Action potentials from multiunit groups and single units were recorded from primary and caudomedial auditory cortical fields in lightly anesthetized macaque monkeys. Stimuli were two suprathreshold tones of 100-ms duration, presented in succession. The frequency of the first tone and the stimulus onset asynchrony (SOA) between the two tones were varied systematically, whereas the second tone was fixed. Compared with presenting the second tone in isolation, the response to the second tone was enhanced significantly when it was preceded by the first tone. This was observed in 87 of 130 multiunit groups and in 29 of 69 single units with no obvious difference between different auditory fields. Response enhancement occurred for a wide range of SOA (110-329 ms) and for a wide range of frequencies of the first tone. Most of the first tones that enhanced the response to the second tone evoked responses themselves. The stimulus, which on average produced maximal enhancement, was a pair with a SOA of 120 ms and with a frequency separation of about one octave. The frequency/SOA combinations that induced response enhancement were mostly different from the ones that induced response attenuation. Results suggest that response enhancement, in addition to response attenuation, provides a basic neural mechanism involved in the cortical processing of the temporal structure of sounds.