Martha Nari Havenith
UCL

In your own time – oscillations as an internal clock for visual processing (cancelled)

Wednesday 10th of November 2010 at 04:30pm
508-20 Evans Hall

Please note that this seminar is cancelled due to the speaker's travelling schedule mismatch.

Neurons speak spike timing.  Mechanisms like membrane resonance, coincidence detection and spike-timing dependent plasticity are found in virtually all neuron types, and they produce an acute sensitivity to the relative timing of inputs. However, it is not clear to what extent cortical networks utilize this sensitivity to transmit information.  To do so, communicating neurons require a common timing reference. Oscillations can serve this purpose. 

I present data from multi-electrode recordings in anaesthetized cats, showing that the relative spike times of neurons in the primary visual cortex carry stimulus information at the millisecond level. These spike times, though stimulus-dependent, are not time-locked to the stimulus, but instead referenced to fluctuations in the population activity, particularly to gamma oscillations (20-50 Hz). Although stimulus-dependent spike sequences also occur in the absence of gamma oscillations, their reliability markedly improves with gamma oscillations, underlining the role of oscillations as an internal clock.

In the second part of my talk I present recent work aiming to test the role of oscillations in visual cortical activity more directly. To manipulate oscillations, we introduced the light-activated ion channel protein ChR2 (sensitive to blue light) in parvalbumin-expressing (PV) interneurons in the mouse visual cortex. The network of PV interneurons is crucial for generating oscillations, providing common input that synchronizes principal neurons. Thus, it should be possible to drive population oscillations by activating PV interneurons with rhythmic blue-light stimuli. Somewhat in contrast to that notion, our initial recordings of local-field potentials show that, instead of simply driving cortical oscillations, periodic blue-light stimulation of PV interneurons interacts with the intrinsically generated oscillations of the cortical network. As a result, the degree of entrainment to the blue-light stimulus depends on the frequency and strength of ongoing oscillations. This suggests that the cortex possesses robust mechanisms for calibrating oscillatory activity despite extrinsic disturbances.

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