In order to determine whether moving visual stimuli
were sufficient to induce the emergence of direction-selective
responses, the animals were exposed to two "training" stimuli
consisting of grating patterns which drifted back and forth across the
visual field perpendicular to the orientation of the grating in
opposite directions. These stimuli were presented to the ferrets for 5
seconds at a time, with intervals of 10 seconds, for a period of 20
minutes. Subsequently, the activity of primary visual cortical neurons
was observed whilst these stimuli were presented again.
For the first 8-10 hours of visual stimulation after
this motion training, no changes were observed in the functional
properties of visual cortical neurons. Most neurons were highly
responsive to the orientation of the stimuli, but their selectivity to
the direction in which the stimuli moved was very weak. Later on, small
groups of cells with a preference for one of the two training stimuli
began to emerge. With time, these responses progressively increased, so
that each group became highly tuned to one or the other training
stimuli (see above figure). The number of neurons selective for each
orientation was also found to increase with time.
To test whether it was the motion of the training
stimuli that induced these changes in activity, the researchers flashed
identical gratings in the ferrets’ visual fields for brief periods of
time. This "flash training" elicited responses in the same cortical
neurons, but the responses did not increase with time. Gratings which
moved in eight directions that differed from those in the training
elicited little response or none at all. This confirmed that the
observed emergence of orientation selectivity was indeed due to
exposure to the training stimuli.
Closer examination of the responses of individual
pyramidal neurons in layer 2/3 of the cortex revealed that the
preferred direction of motion of each changed over time, so that it
became more like the preferences of its neighbours. Prior to training,
most of the cells exhibited uncertain or moderate orientation
preferences. Upon presentation of the training stimuli, however, the
responses of most neurons became more certain, and the neurons
segregated into small domains with a preference for one direction or
the other.
Other interesting functional changes were also
observed. Some neurons maintained their initial moderate preference for
one direction of movement and later increased their response to it,
while others reversed their orientation preference during training. If,
for example, a neuron was surrounded by cells with a preference for the
opposite direction, it was likely to reverse its own preference so that
it matched that of its neighbours. On the other hand, a neuron
surrounded by others with the same preference was unlikely to change
its own preference during training. This suggests that the functinal
grouping of neurons occurs because of some kind of interaction between
neighbouring cells during motion training.
These experiments show that early experience of
moving visual stimuli has a strong and relatively rapid effect on the
functional properties of neurons in the primary visual cortex.
Initially, the ferret primary visual cortex contains an array of
neurons with weak direction preferences, possibly because of light
entering through the closed eye lids. The two training stimuli used,
which consisted of gratings moving in opposite directions, transformed
this array into two highly ordered columns, each containing neurons
with a highly selective preference for one of the directions of
stimulus motion. The study supports the widely-held belief that sensory
experience is essential for proper visual development, but adds some
fascinating details of how it does so. It also raises the question of
exactly how visual cortical neurons interact with each other during
their selection of direction preference.
