IV. Motion recognition
Although we showed in the foregoing section
that birds can discriminate between different types of movement, it is
another thing entirely to say that they discriminate them on the basis
of movement cues. A horizontal movement of an object takes it to different
parts of visual space than a vertical movement, and it might be this locational
difference that makes discrimination between them possible. How could we
show that it was movement, itself, not the consequences of movement, that
a bird was discriminating?
We have already cited some evidence that argues
in this direction. For example, Emmerton's (1986) failure to find directional
invariance in most Lissajous figure discriminations argues strongly that
it is the motion of the dot, not its trajectory, that is being discriminated.
Similar Cook and Katz's (1999) evidence that moving objects were discriminated
better than the same objects at rest implies that motion contributes information.
However, the strongest demonstration that discrimination
between types of movement, and especially complex types of movement, can
be achieved using movement cues alone is to replace with stimuli with representations
based on a handful of points of light, the arrangement Johansson (1973)
described as a "biological motion" display. Dittrich et al. (1998, Experiment
3) showed that pigeons could be trained in movement category discriminations
using scenes of pecking and walking movements by other pigeons presented
under point-light conditions. Ryan, Lea, Alklind, and Dittrich (2001) extended
this result to chickens, using the identical stimuli as in the pigeon study,
of which two examples of Dittrich et al's full-detail (pecking
/ walking) and point-light
(pecking / walking)
displays can be viewed here.
Eleven of 16 birds reached criterion, with a
final performance of 80% correct within 27 sessions and the remaining birds
reached at least 60% within 30 sessions. Fifteen of the birds were then
transferred to a second discrimination in which the alternative kind of
stimuli were used. In half of the cases compatible transfer (full-light
walking stimuli replaced by point-light walking stimuli) was tested and
in the remaining cases incompatible transfer was used (full-light walking
stimuli replaced by point-light pecking stimuli). Compatible transfer results
were better than incompatible one indicating that indeed discrimination
transfer between full-light and point-light information seemed possible.
Movement cues alone seemed sufficient to enable a discrimination between
Furthermore, we can show that such pure motion
cues must play a part in discrimination even of full detail displays. In
Dittrich et al.'s Experiment 2, birds that had been trained to discriminate
pecking and walking with full-light video scenes were then tested with
point-light scenes of the same movement categories. There was some transfer
of the discrimination. Using a slightly different testing technique, Lea
et al. also obtained transfer between point-light and full-light displays.
They tested both for transfer from full-detail to point-light, and for
the reverse, and found only slight differences in the extent of transfer
between the two. Transfer to full-light displays seem easier.
Next section: Motion