Figure 5: Electrical stimulation of the olfactory bulb entrains cortical activity.

(a) Average barrel cortical LFP response to electrical stimulation of the ipsilateral olfactory bulb with a 0.8-Hz rhythmic stimulus. The stimulus consisted of 50 Hz trains of 10-ms current steps (50 μA) repeated every 1.25 s (=0.8 Hz). The arrow points to the onset of the stimulus train. Electrical stimulation artefacts (marked by curly brackets labelled stim.artif.) appear as dense vertical lines truncated at the bottom. LFP signals were averaged across 30 stimulus trains. The LFP increased during bulbar stimulation and decreased again during the inter-train interval. The average LFP response amplitude was measured as the difference between the voltage at the time of the onset and end of the stimulus train in the averaged signal, as marked by the two dashed horizontal lines. (b) Average barrel cortical LFP response to the same stimulus as in a but with a higher stimulus current (80 μA). The LFP increased faster with higher stimulus current. (c,d) Average barrel cortical LFP response to bulbar stimulation (80 μA) at 1.6 Hz (c) and 3.2 Hz (d). In all cases, bulbar stimulation was through a concentric bipolar electrode (SNEX 100x, Kopf Instruments, USA). Stimulus timing was controlled using Spike2 software and the D/A output of the CED 1401 (both Cambridge Electronic Design, UK) and stimulus currents were generated by a battery driven stimulus isolation unit (A380, World Precision Instruments, USA). (e) Average LFP responses measured as described in a to three different stimulus frequencies at two different stimulus currents. Responses to each stimulus frequency-current combination were measured in three mice. Response amplitudes are normalized to each animal’s maximum response to the 0.8 Hz/80 μA stimulus and averaged across the three mice. Response amplitude was a function of stimulus current and stimulus frequency. Error bars show s.e.m.