According to the traditional view, the main
role of neurons in the primary visual cortex is to provide
a faithful representation of contrast-defined structures
in the visual environment for the rest of the cortical
visual system through their firing pattern. This firing
pattern is contaminated by noise due to spontaneous activity
of the cells, but such noise can be overcome by averaging
across cells. Such a view is in sharp contrast with the
important role attributed to spontaneous activity before
eye opening, when correlated ongoing activity is thought
to be instrumental in developing functional links and
structures in the visual system. How can spontaneous activity
change from necessity to nuisance at the moment of eye
opening?
To gain insight into this puzzle, Dr. Fiser
recorded from the primary visual cortex of awake ferrets
at different ages before and after eye opening. Before
eye opening he recorded spontaneous activity in complete
darkness while the animal was resting. For the age groups
with eyes opened, he added two more interleaved visually
driven conditions: random noise stimuli and natural scene
movies. Dr. Fiser found, first, that spontaneous activity
was not random at any age group but instead showed a well-defined
spatio-temporal structure that developed according to
a clear pattern across age groups from slow irregular
bursting to fast, correlated, oscillatory patterns. Second,
the temporal and spatial correlational structure of the
neural activity was only slightly modulated by the incoming
visual stimulation and was constrained to a much larger
extent by the internal spontaneous activity. These results
suggest a very different view of activity in the primary
visual cortex of adult behaving animals. Namely, spontaneous
activity is not noise, but rather is generated by the
cortical and subcortical systems and represents the internal
status of the visual system, reflecting the perceptual
state of the animal. Moreover, the incoming visual signal
is not coded faithfully in the primary visual cortex,
but rather this signal dynamically modulates the evoked
internal states represented there by the ongoing activity.
