PICTURES MISTAKEN FOR NOISES
It is known that visual cortical cells respond to repeated presentations of the same stimulus with a high variability. However it is believed that neural activity in primary visual cortex is almost exclusively driven by external sensory inputs. In such a view, the observed variability has been considered to be “noise” owing to random spontaneous activity within the cortex.
Recent researches demonstrate that this kind of spontaneous activity show a high spatio-temporal coherence. It has been postulated that patterns from this activity might shape neural responses during natural viewing condition. Jozsef Fiser, Chiayu Chiu & Michael Weliky from the University of Rochester examined the relationship between spontaneous activity and the response of primary visual cortical neurons to dynamic natural-scene and random-noise film images in awake, freely viewing ferrets from the time of eye opening to maturity (Small modulation of ongoing cortical dynamics by sensory input during natural vision. Nature [No 7008] 431, 573-578, 2004).
The authors conclude that in both the developing and mature visual cortex, sensory evoked neural activity represents the modulation and triggering of ongoing circuit dynamics by input signals rather than directly reflecting the structure of the input signal itself.