Milner (see citation below) reviews the evidence that the visual-motor control is not conscious.
Visual perception starts at the back of the optical lobe and moves forward in the cortex as processing proceeds. There are two tracks along which visual perception proceeds, called the dorsal stream and the ventral stream. The two streams have few interconnections. The dorsal stream runs from the primary visual cortex to the superior occipito-parietal cortex near the top the the head. The ventral stream runs from the primary visual cortex to the inferior occipito-temporal cortex at the side of the head. Their functions, as far as is known, differ. “The dorsal stream’s principal role is to provide real-time ‘bottom-up’ visual guidance of our movements online. In contrast, the ventral stream, in conjunction with top-down information from visual and semantic memory, provides perceptual representations that can serve recognition, visual thought, planning and memory offline… we have proposed that the visual products of dorsal stream1 processing are not available to conscious awareness—that they exist only as evanescent raw materials to provide the unconscious moment-to-moment sensory calibration of our movements.”
The researchers used three methods in their studies: patients with lesions in their visual system, patients suffering from visual extinction, and fMRI experiments.
One patient had part of their ventral streams destroyed – they could reach and grasp objects that they were not conscious of. The opposite was true of other patients with damage to their dorsal streams – they had difficulties grasping objects that they were consciously aware of.
Visual extinction is a form of spatial neglect. The patient fails to detect a stimulus presented on the side of space opposite the brain damage when and only when there is simultaneously a stimulus on the good side. By carefully arranging an experimental setup, a patient with visual extinction took account of an obstacle that they were not conscious of when reaching for an object. Avoiding an obstacle depends of the dorsal stream because patients with damage to the dorsal stream did not adjust their reaching movements in the presence of obstacles.
There is visual feedback during reaching. “Under normal viewing conditions, the brain continuously registers the visual locations of both the reaching hand and the target, incorporating these two visual elements within a single ‘loop’ that operates like a servomechanism to progressively reduce their mutual separation in space (the ‘error signal’) as the movement unfolds. When the need to use such visual feedback is increased by the occasional introduction of unnoticed perturbations in the location of the target during the course of a reach, a healthy subject will make the necessary adjustments to the parameters of his or her movement quite seamlessly. ..In contrast, a patient with damage to the dorsal
stream was quite unable to take such target changes on board: she first had to complete the reach towards the original location, before then making a post hoc switch to the new target location…It thus
seems very likely that the ability to exploit the error signal between hand and target during reaching is dependent on the integrity of the dorsal stream. ”
The phenomenon of binocular rivalry where the subject has different images projected to the two retinas and is alternatively conscious of one or the other image has been studied with fMRI. It is possible to see which image is conscious by the activity in the ventral stream. But the dorsal stream is able to act on information even if it is not being processed by the ventral stream and therefore not consciously available.
The authors do point out that they are not saying that the dorsal stream plays no role in conscious perception. It may for example have some control over attention.
In the conclusion, they say “according to the model, such ventral-stream processing plays no causal role in the real-time visual guidance of the action, despite our strong intuitive inclination to believe otherwise (what Clark calls ‘the assumption of experienced-based control’). According to the Milner & Goodale model, that real-time guidance is provided through continuous visual monitoring by the dorsal stream of those very same visual inputs that we experience by courtesy of our ventral stream. ”
A.D. Milner (2012). Is visual processing in the dorsal stream accessible to consciousness? Proc R Soc B, 2289-2298 DOI: 10.1098/rspb.2011.2663