Study examines nonlinear dynamic properties of neural population activity in primary visual cortex of rats

The purpose of our study was to investigate the nonlinear dynamic properties of neural ensemble activity in the primary visual cortex of rats following differential visual stimuli. The relative wavelet energy (RWE), wavelet entropy (WS), and the mean WS were extracted from LFPs recorded in rat V1 during three distinct visual stimuli: low ambient light, a uniform grey computer screen, and simple pictures of common scenes. The RWE of the γ band (31-62.5 Hz) is much higher in response to pictures, at least during certain periods of image presentation, and shows semi-periodic fluctuations. The wavelet entropy also showed time related changed during the stationary visual stimuli, indicating alternating periods of disorder and order, possibly corresponding to different phases of visual processing. In general, images conducive to perception (pictures) evoked dynamic LFPs with alternating periods of low and high entropy, possibly reflecting periods of neuronal asynchrony associated with individual feature extraction and more ordered activity associated with integration (perception and recognition).

There were significant differences in the time-evolving WS between the three forms of visual input studied, reflecting the distinct neuronal response patterns evoked. The stimulation with simple pictures was associated with alternating periods of high and low WS, indicating that visual processing involves periods of ordered (synchronized) and disordered (or complexly ordered) activity in V1. We suggest that the periods of disorder correspond to visual processing, while ordered activity may correspond to recognition.

In conclusion, the parameters RWE and WS quantify certain high-level characteristics of neural population activity in rats and so may help decipher the complex encoding rules that underlie visual processing and object recognition.