Identification of the neural correlates that comprise
the necessary substrata for visual experience remains
elusive and controversial. Neurological findings suggests
that the human striate cortex (Vl) is an indispensable
component of a neural substratum subserving static achromatic
form perception in its own right and not simply as a central
distributor of retinally derived information to extrastriate
visual areas. This view is further supported by physiological
evidence in primates that the finest-grained conjoined
representation of spatial detail and retinotopic localization
that underlies phenomenal visual experience for local
brightness discriminations is selectively represented
at cortical levels by the activity of certain neurons
in VI.
At first glance, however, support for these ideas would
appear to be undermined by incontrovertible neurological
evidence (visual hemineglect and the simultanagnosias)
and recent psychophysical results on "crowding" that confirm
that activation of neurons in VI may, at times, be insufficient
to generate a percept. Moreover, a recent proposal suggests
that neural correlates of visual awareness must project
directly to those in executive space, thus
automatically excluding VI from a related perceptual space
because Vl lacks such direct projections. Both sets of
concerns are, however, resolved within the context of
adaptive resonance theories.
Recursive loops, linking the dorsal lateral geniculate
nucleus (LGN) through successive cortical visual areas
to the temporal lobe by means of a series of ascending
and descending pathways, provide a neuronal substratum
at each level within a modular framework for mutually
consistent descriptions of sensory data. At steady state,
such networks obviate the necessity that neural correlates
of visual experience project directly to those
in executive space because a neural phenomenal
perceptual space subserving form vision is continuously
updated by information from an object recognition space
equivalent to that destined to reach executive space.
Within this framework, activity in VI may engender percepts
that accompany figure-ground segregations only when dynamic
incongruities are resolved both within and between ascending
and descending streams. Synchronous neuronal activity
on a short timescale within and across cortical areas,
proposed and sometimes observed as perceptual correlates,
may also serve as a marker that a steady state has been
achieved which, in turn, may be a requirement for the
longer time constants that accompany the emergence and
stability of perceptual states compared to the faster
dynamics of adapting networks and the still faster dynamics
of individual action potentials. Finally, the same consensus
of neuronal activity across ascending and descending pathways
linking multiple cortical areas that in anatomic sequence
subserve phenomenal visual experiences and object recognition
may underlie the normal unity of conscious experience.
However, as Damasio (1999) has emphasized, there is a
necessity for a neural representation of an image to interact
with representations of a sense of self not only to secure
a sense of image ownership but perhaps also to engender
an image interpretation. Pertinent questions arise as
to whether the representations of sense of self are coextant
with and possibly equivalent to parietal lobe representations
that command selective spatial attention to visual details
with respect to maps of eye, head, or body position. Finally,
although such sense of self- image interactions may provide
yet another necessary condition for the emergence
of phenomenal vision, the sufficient conditions
to specify such experience remain unknown.
