Towards electrophysiological correlates of auditory perception of temporal order.

Recent studies have postulated that the temporal order (TO) of two successive events can be correctly identified if they are separated by an inter-stimulus interval (ISI) of at least 30 ms duration. Using Auditory Evoked Potentials, we tested 21 students for the cortical activation associated with TO detection of two successively presented tones in either 'easy' (ISI=60 ms) or 'difficult' (ISI=10 ms) conditions. The amplitude of P2 component was related to difficulty of TO perception and was significantly higher in 'difficult' than 'easy' condition. Moreover, in 'difficult' condition the correlation analyses revealed a negative association at both Fz and Cz electrodes between P2 amplitudes and the correctness level. Correct responses in this condition were accompanied by lower P2 amplitudes than the incorrect ones.

Coupling of beta and gamma activity in corticothalamic system of cats attending to visual stimuli.

We have previously shown that local field potentials (LFPs) recorded in the lateral geniculate nucleus (LGN) and primary visual cortex (VCx) of the cat contain significantly more beta frequency activity when the animal attends to visual than to auditory stimuli. In the present study we utilised data from the same experiments to calculate the cross-correlation between envelopes of filtered beta (16-24Hz) and gamma (30-45Hz) oscillatory signals recorded at the same sites. Correlation values obtained from visual trials were significantly higher than those calculated for the auditory task. This observation was typical in those LGN and VCx recording sites which corresponded to the central representation of the visual field. The cross-correlation function peaked within a 20 ms time window from the centre of the cross-correlogram. These findings support the hypothesis that beta activity provides an excitatory background for the appearance of oscillations in the gamma band.

Visual classification of X and Y perigeniculate neurons of the cat.

The spike activity of perigeniculate cells evoked by small light spots flashing along the axes of their receptive fields was recorded and presented in response planes. This method allowed the investigated neurons to be grouped into two classes characterized by (1) large receptive fields and phasic-like responses and (2) small fields and tonic responses. The latency measurements for stimulation of the optic chiasma and visual cortex revealed that the cells from the first group are excited by fast, Y fibers and the second by slow, X axons. The spatial tuning curves of the second harmonic component, as measured from the responses of the cells from the two groups for slowly moving square gratings, are also different. We conclude that the X and Y systems of the visual pathway are segregated at the level of the perigeniculate nucleus.

20 Hz bursting beta activity in the cortico-thalamic system of visually attending cats.

It has recently been found (Bekisz and Wróbel 1993) that electroencephalographic recordings from the primary visual cortex and lateral geniculate nucleus of cats attending to visual stimuli contained enhanced activity in the 20 Hz frequency band. Here we present the detailed analysis of this activity. It consisted of short (0.1-1 s) bursts of oscillations which tended to appear simultaneously in the visual cortex and lateral geniculate nucleus. There was an increase of amplitude and frequency of appearance of such burst events in both of the investigated visual centres, which resulted in a power increase in 16-24 Hz band during situation requiring visual attention. The present findings provide additional evidence for cortical influence upon the thalamic information processing.

20 Hz rhythm of activity in visual system of perceiving cat.

Cats attended to either visual or acoustic moving stimuli in order to perceive the location of their disappearance, which signaled reward in left or right foodwell. The analysis of concomitant electroencephalograms revealed an elevation of the 20 Hz band in the Fourier spectra of activity of respective visual and auditory projection cortices. These oscillations appeared only on the trials that ended with a successful response. With use of directed transfer functions we were able to show that during the visual task, the 20 Hz frequency was propagated from the spot of appearance along the visual cortex, as well as towards the lateral geniculate nucleus.