In this study we describe visually triggered gamma oscillations in the optic tectum of awake pigeons. This study was motivated by the potential relevance of synchronous oscillatory responses in perceptual binding in a laminated structure other than the mammalian neocortex. Tectal responses were recorded as local field potential and multiunit activity by differential filtering. The local field potential was analysed by computing its autocorrelation function and spectral power with a moving window applied to single response sweeps. The temporal structure of the spike trains was evaluated by computing averaged autocorrelograms. A damped sine wave function was fitted to the autocorrelograms in order to quantify the degree of oscillation of both signals. Epochs of significant oscillatory activity were observed in the local field potential in 60% of the trials (n= 39). In all trials, significant oscillations occupied ∼ 10% of the time the stimulus was present. The oscillatory events in both the local field potential and the multiunit activity had frequencies in the range of 20 – 50 Hz. It is important to emphasize the great variability in the frequency and in the probability of occurrence of the oscillatory responses from trial to trial, which makes the oscillatory behaviour of the tectal activity highly non-stationary. The oscillatory activity we describe in the avian tectum has characteristics similar to those reported in the mammalian neocortex. These findings from a fully awake animal strengthen the universality of oscillations as a possible carrier for synchronization of activity in the constitution of neuronal assemblies.

Key words: avian optic tectum, local field potential, multiunit activity, neuronal assemblies, oscillatory activity, temporal binding

Abstract Multiunit activity was recorded in the optic tectum of awake pigeons with two electrodes at sites varying in depth and separated by 0.3 to 3.0 mm. Autocorrelation and cross-correlation functions were computed from the recorded spike trains to determine temporal relationships in the neuronal firing patterns. Cross-correlation analysis revealed that spatially separate groups of cells in the tectum show synchronous responses to a visual stimulus. Strong synchronization occurred in both superficial and deep layers of the tectum, in general with zero-phase shift. The response synchronization in the avian optic tectum resembles that observed in the mammalian cortex, suggesting that it may subserve common functions in visual processing.

Key words: Temporal coding, Synchronization, Oscillation, Optic tectum, Pigeon