The fate of visible features of invisible elements

Transcranial magnetic stimulation (TMS)-induced Blindsight of Orientation is Degraded Conscious Vision

Patients with blindsight are blind due to an early visual cortical lesion, but they can discriminate stimuli presented to the blind visual field better than chance. Studies using transcranial magnetic stimulation (TMS) of early visual cortex have tried to induce blindsight-like behaviour in neurologically healthy individuals, but the studies have yielded varied results. We hypothesized that previous demonstrations of TMS-induced blindsight may result from degraded awareness of the stimuli due to the use of dichotomous visibility scales in measuring awareness. In the present study, TMS was applied to early visual cortex during an orientation discrimination task and the subjective scale measuring awareness was manipulated: The participants reported their conscious perception either using a dichotomous scale or a 4-point Perceptual Awareness Scale. Although the results with the dichotomous scale replicated previous reports of blindsight-like behaviour, there was no evidence of TMS-induced blindsight for orientation when the participants used the lowest rating of the 4-point graded scale to indicate that they were not aware of the presence of the stimulus. Moreover, signal detection analyses indicated that across participants, the individual's sensitivity to consciously discriminate orientation predicted behaviour on reportedly unconscious trials. These results suggest that blindsight-like discrimination of orientation in neurologically healthy individuals does not occur for completely invisible stimuli, that is, when the observers do not report any kind of consciousness of the stimulus. TMS-induced blindsight for orientation is likely degraded conscious vision.

Masked blindsight in normal observers: Measuring subjective and objective responses to two features of each stimulus

Recent visual masking studies that have measured visual awareness with graded subjective scales have often failed the show any evidence for unconscious visual processing in normal observers in a paradigm similar to that used in studies on blindsight patients. Without any reported awareness of the target, normal observers typically cannot discriminate target's features better than chance. The present study examined processing of color and orientation by measuring graded awareness and forced-choice discriminations for both features in each trial. When no awareness for either feature was reported, discrimination of each feature succeed better than expected by chance, even when the other feature was incorrectly discriminated in the same trial. However, the characteristics of the mask determined whether or not masked blindsight was observed. We conclude that when the processing channels are free from intra-channel interference, unbound or weakly bound features can guide behaviour without any reported awareness in normal observers.

Localizing non-retinotopically moving objects

How does the brain determine the position of moving objects? It turns out to be rather complex to answer this question when we realize that the brain has to solve the motion correspondence problem in two kinds of reference frames: Retinotopic and non-retinotopic ones. We show that visual objects are mislocalized along a non-retinotopic motion direction. Observers viewed two successive movie frames each consisting of an outlined square and two target elements inside the square. In the non-retinotopic condition the elements as well as the square moved vertically while two bars also centripetally or centrifugally moved. In the retinotopic condition the vertical movement of them was removed from the stimuli. The task of the observers was to judge a relative position of the elements. Consequently, the elements were mislocalized in the direction of both retinotopic and non-retinotopic motion, although the mislocalization was significantly larger in the retinotopic than in the non-retinotopic conditions. The results suggest that non-retinotopic as well as retinotopic motion processing contributes to the determination of perceived positions of moving objects.