Neural activity within a localized brain region elicits an increase in blood flow in that region supplying oxygen to active cells. Although this neurovascular coupling is widely used to monitor human brain function, its properties and underlying mechanisms are still unknown. We investigated the relationship between neuronal activity and oxygen metabolism, in the lateral geniculate nucleus of the anaesthetized cat with simultaneous extracellular recordings and spectrophotometry.

Presentation of a visual stimulus (sinusoidal drifting grating-100% contrast) increased oxyhemoglobin levels by 30%. This increment was related to neuronal responses and showed some interesting properties:

1. Temporal domain: Augmentation in oxyhemoglobin levels started 4 second after neuronal responses, peaked at 10 seconds, and showed a very slow adaptation-like evolution in the range of minutes, not accompanied by a decrease in neuronal responses. Interestingly, in about 30% of the recordings, oxyhemoglobin levels showed a post-stimulus depression than lasted up to 60 seconds.

2. Amplitude: Oxyhemoglobin levels showed a linear relationship with stimulus contrast without signs of saturation. However, neuronal responses saturated with a contrast above 30%. The fact that neuron responses reached the maximum at 30% contrast and vascular did not, strongly suggests that contrast saturation is an active process that requires activation of specific neural circuits not detectable in the extracellular recordings but picked up by the level of oxygen consumption.

These results show important differences between regulatory mechanisms of vascular and neuronal responses that might be relevant to optical and fMRI studies. Supported: MECBFU2005-00502