Recently, it has been shown that not only dorsal, but also ventral visual areas may exhibit preserved unconscious processing under CFS (Sterzer et al., 2008). Furthermore, behavioral evidence indicates that unconscious priming extends to images that are assumed to be processed along the ventral stream (Barbot and Kouider, 2012). For those reasons, the apparent dissociation between dorsal and ventral visual brain regions brought about by CFS does not seem to hold, and it can therefore be questioned whether the study by Sakuraba et al. (2012) successfully isolated dorsal stream processes.

The idea of a division between a dorsal and a ventral visual stream is one of the most basic principles of visual processing in the brain (Milner and Goodale, 1995). The ventral stream originates in primary visual cortex and extends along the ventral surface into the temporal cortex; the dorsal stream also arises in primary visual cortex, but continues along the dorsal surface into parietal cortex. The ventral stream (or “vision-for-perception” pathway) is believed to mainly subserve recognition and discrimination of visual shapes and objects, whereas the dorsal stream (or “vision-for-action” pathway) has been primarily associated with visually guided reaching and grasping based on the moment-to-moment analysis of the spatial location, shape, and orientation of objects. It has been proposed, however, that the dorsal stream also processes tools as a category, so that manipulable objects would be processed by those brain regions that are important for the execution of actions. However, because dorsal and ventral visual regions are heavily interconnected, it is difficult to tell in healthy subjects whether information is processed along the dorsal stream only, or whether it is fed to parietal cortex via ventral visual regions.One way to distinguish between these two alternatives has been offered by the use of continuous flash suppression (CFS) in which noise screens or Mondrian masks flashed at 10 Hz to one eye suppress the perception of an image presented to the other eye. It has been suggested that this method selectively disrupts ventral visual processing while leaving dorsal processing intact (Fang and He, 2005). In a series of visual priming experiments using CFS, Almeida and colleagues (2008) found that reaction times to visible tools were shorter when they were preceded by masked tools rather than masked animals. No such priming effect appeared when subjects had to react to pictures of animals. Importantly, this result was limited to CFS: category priming using backward masking (in which the stimulus is followed by the mask, typically on both eyes) demonstrated priming effects for both tools and animals. These results, together with previous neuroimaging findings (Fang and He, 2005), were interpreted as evidence that tools, or manipulable objects, are indeed processed as a category directly along the dorsal stream.A recent paper in The Journal of Neuroscience by Sakuraba and colleagues (2012) addresses an important question left open by this research: if information is already separated into dorsal and ventral streams in early visual cortex, which deals with basic visual features, how is it possible that the visual system can distinguish between manipulable and nonmanipulable objects? Almeida and colleagues (2008) had noted that all tool stimuli used in their experiments had an elongated principle axis, which may explain part of their results. Sakuraba and colleagues (2012) take this idea one step further and explore the processing of elongated shapes and nonelongated tools in more detail. Their results challenge the view that previous findings were specific to the category of tools and add significant new evidence to the question of what attributes of visually presented tools may be processed (presumably in dorsal stream cortical areas) outside of awareness under CFS.