Unconscious processing of orientation and color without primary visual cortex

Abstract

In humans, the primary visual cortex (V1) is essential for conscious vision. However, even without V1 and in the absence of awareness, some preserved ability to accurately respond to visual inputs has been demonstrated, a phenomenon referred to as blindsight. We used transcranial magnetic stimulation (TMS) to deactivate V1, producing transient blindness for visual targets presented in a foveal, TMS-induced scotoma. Despite unawareness of these targets, performance on forced choice discrimination tasks for orientation (experiment 1) and color (experiment 2) were both significantly above chance. In addition to demonstrating that TMS can be successfully used to induce blindsight within a normal population, these results suggest a functioning geniculoextrastriate visual pathway that bypasses V1 and can process orientation and color in the absence of conscious awareness.

Fig. 1.

Unconscious processing of orientation. (a) A schematic illustration of the sequence of trial events in experiment 1. To examine orientation processing without primary visual cortex and in the absence of awareness, a horizontally or vertically oriented line was presented while suppressing primary visual cortex processing using TMS. When subjects were unaware of the stimulus, they were asked to guess the orientation and to provide a confidence rating for their guess. (b) The mean accuracy in guessing the stimulus orientation on unaware trials. Despite the absence of awareness, guessing performance was significantly above chance. (c) The correlation between accuracy in guessing the stimulus orientation on unaware trials and the confidence ratings for those guesses.

Fig. 2.

Unconscious processing of color. (a) A schematic illustration of the sequence of trial events in experiment 2. To examine color processing without primary visual cortex and in the absence of awareness, a red or green disk was presented while suppressing primary visual cortex processing using TMS. When subjects were unaware of the stimulus, they were asked to guess the color and to provide a confidence rating for their guess. (b) The mean accuracy in guessing the stimulus color on unaware trials. Despite the absence of awareness, guessing performance was significantly above chance. (c) The correlation between accuracy in guessing the stimulus color on unaware trials and the confidence ratings for those guesses.