Perceptual Coherence : Hearing and Seeing

Summing Up 423
course of the stimulation. It is only when simple single stimuli are presented
that the spectral-temporal receptive fields are separable. The receptive
fields can change dramatically at different time scales, when the
stimuli are embedded in different backgrounds, and so on. The auditory
cortex has a large adaptive and plastic capacity, and Nelken speculated that
the role of the auditory cortex is to integrate acoustic features into auditory
streams or objects. This integration can extend over different time spans
and is sensitive to the statistics of the environment.
Albright and Stoner (2002) made a similar argument for contextual effects
in vision. They point out that the “meaning” of any local region can be
understood only in terms of the information in other regions. Of special importance
is information revealing surface occlusion or lighting conditions
that underlie depth ordering, figure-ground segregation, and color perception.
As described at many points here, the response of individual cells is
not invariant but depends on the response of neurons whose receptive
fields encode distant retinal points (e.g., Vinje & Gallant, 2002). Although
not discussed explicitly, there must be contextual temporal effects also.
The interpretation of movement within a short time frame would often be
ambiguous without the context provided by prior, contemporaneous, and
subsequent information, as in the moving light displays pioneered by
Johansson (1973).
If we continue to think about the auditory and visual systems in terms of
the question “How does it work?” then we will trapped into conceptualizing
hearing and seeing as very different processes, as expressed in table 1.1.
But if we transform the question into “What are the auditory and visual
systems designed to do?” the essential equivalence becomes apparent. Both
are designed to identify objects, mostly the same object, within a common
space-time framework. From this perspective, the neurological and perceptual
commonalities and interactions are to be expected.