Datensatz

Zusammenfassung

The local field potential (LFP) comprises the low frequency membrane potential fluctuations of an extracellular recording. It is thought to be closely related to frequently used, noninvasively recorded signals such as the EEG or the BOLD signal and has been proven to show some stimulus selectivity in various brain regions. Thus the relationship of the LFP to the spiking activity as well as a comparison of their level of selectivity is of great interest. We simultaneously recorded spiking activity and LFPs from tree shrew primary visual cortex using pairs or triplets of tetrodes separated by between 200 and 1200μm. We map the receptive fields at eccentricities between 10 and 25deg using binary sparse noise and estimate their spatial extent by fitting oriented two dimensional Gaussians to the resulting activation map. We have preliminary data from 49 pairs of simultaneously recorded neurons as well as 18 additional single sites from a total of 14 animals. The visual spread of the receptive field estimated from the trial averaged LFP ( 2.99deg2) was statistically similar to the one for the spiking activity ( 3.06deg2). Estimating the receptive field size using LFP gamma power (30- 90Hz), we found significantly smaller values ( 2.17deg2) compared to the above two conditions (1-way ANOVA: p«0.001). In addition, we found that receptive fields tended to be ellipsoidal: long to short axis ratio: 1.41 (LFP) and 1.36 (spikes) and oriented mostly horizontally: mean angle 5.79deg (LFP) and 6.95deg (spikes). This bias toward horizontal orientations was statistically significant (Rayleigh tests: p « 0.001) for both signal types. Relating the spatial separation between the tetrodes to the respective receptive field centers, we found that 1deg of visual angle corresponded to a cortical distance of around 180μm for both spikes and LFP. Our results suggest that the spatial extent of activation estimated from LFPs can be similar or smaller than the values for spiking activity depending on which features of the LFP are analyzed. Thus, we obtained smaller receptive field sizes using gamma-band oscillations compared to trial-averaged stimulus evoked LFPs. Our findings represent the first detailed investigations of this relationship in tree shrew V1, and are in good general agreement with related work in macaque monkeys.