Correlations between unit firing and EEG in the rat olfactory system

Area of Science:

• Neuroscience
• Computational Neuroscience
• Sensory Systems

Background:

• Awake animals exhibit gamma-range (30-100 Hz) oscillatory bursts in olfactory electroencephalography (EEG).
• These bursts correlate with airflow but lack gamma-range input signals, suggesting local generation within olfactory cortices.
• Potential mechanisms include neuronal feedback interactions or coupled intrinsically oscillatory neurons.

Purpose of the Study:

• To investigate the origin of gamma-range oscillatory bursts in the olfactory system.
• To differentiate between negative feedback and coupled oscillator models for burst generation.

Main Methods:

• Simultaneous recordings of EEG and multi-unit spikes in awake, motivated rats.
• Recordings were performed in four olfactory system areas: olfactory bulb, anterior olfactory nucleus, prepyriform cortex, and lateral entorhinal area.
• Multi-unit electrodes captured spikes from principal output neurons in local neighborhoods corresponding to EEG recordings.

Main Results:

• Neuronal firing probability oscillations matched the dominant EEG frequency across all tested olfactory structures.
• Two distinct neuronal populations were identified: one firing in phase with the EEG, and another with a phase lag/lead of approximately 1/4 cycle.
• These findings align with the predictions of a negative feedback interaction model.

Conclusions:

• The data strongly support a negative feedback mechanism, involving excitatory and inhibitory neuronal interactions, as the generator of olfactory gamma-range oscillations.
• The coupled oscillator model was not supported by the observed phase relationships.
• The findings have implications for understanding oscillatory activity in other cortical systems, such as the visual cortex.