Abstract

At suprathreshold levels, detection and awareness of visual stimuli
are typically synonymous in nonclinical populations. But following
postgeniculate lesions, some patients may perform above chance
in forced-choice detection paradigms, while reporting not to see
the visual events presented within their blind field. This phenomenon,
termed “blindsight,” is intriguing because it demonstrates
a dissociation between detection and perception. It is possible,
however, for a blindsight patient to have some “feeling” of the
occurrence of an event without seeing per se. This is termed blindsight
type II to distinguish it from the type I, defined as discrimination
capability in the total absence of any acknowledged
awareness. Here we report on a well-studied patient, D.B., whose
blindsight capabilities have been previously documented. We have
found that D.B. is capable of detecting visual patterns defined by
changes in luminance (first-order gratings) and those defined
by contrast modulation of textured patterns (textured gratings;
second-order stimuli) while being aware of the former but reporting
no awareness of the latter. We have systematically investigated
the parameters that could lead to visual awareness of the
patterns and show that mechanisms underlying the subjective reports
of visual awareness rely primarily on low spatial frequency,
first-order spatial components of the image.