Critical
periods for the development of visual functions
Frank Rösler, Brigitte Röder (D-Marburg)
Experimental and Biological Psychology
Philipps-University Marburg, Germany
Both animal and human studies have revealed that the visual system is
not fully developed at birth but rather shows systematic structural and
functional changes during the first weeks and months of life. Hubel &
Wiesel (1978), for example, showed that the receptive field properties
of neurons in the visual cortex of monkeys depend on the available input
during early developmental stages. In particular, cells which are normally
driven by the input of both eyes become completely monocular, if one eye
was occluded during a critical period after birth. In a similar manner
the density of neurons and synaptic connections in the visual cortex of
rats is strongly dependent on the environmental conditions during early
development, i.e. whether an individual grows up in an enriched or impoverished
environment. On the cellular level it has been shown for both cats and
humans that the density of synaptic connections within visual processing
areas (corpus geniculatum laterale, area 17) first increases and later
decreases in a selective manner. These findings suggest that a fine tuning
of the structure and the functions of the visual system depends on a continuous
interaction with the environment.
In patients suffering from cataracts, which were surgically treated at
some later stage of life, Maurer and colleagues showed that visual perception
is only partially restored. Particularly in patients with bilateral congenital
cataracts basic perceptual functions, as spatial frequency resolution
(contrast sensitivity), temporal flicker frequency resolution, stereoacuity,
and symmetry of optokinetic nystagmus, show permanent deficits. These
basic deficits also seem to affect higher perceptual functions as letter
and face discrimination. Such behavioral observations suggest that a visual
impairment in early life results in permanent abnormalities of the visual
system. Thus, visual perception is acquired by an interaction with the
environment and successful learning of perceptual functions seems to depend
on certain critical epochs during development.