When I am awake and say I see a face, am able to report its identity; I can identify the colour of its eyes and hair, and judge its emotional expression. There is little reason to doubt that I have a conscious sensation of that face.[6] If we study the properties of visual processing in this condition, we can be pretty sure we are studying the properties of conscious visual perception. This is our “mammal” of consciousness. We can study the properties of this species fairly easily. We can resort to introspection, verbal reports, or more strictly formalized approaches like detection or discrimination tasks. In favour of using introspection is that our introspective idea of consciousness is the very thing we are trying to explain. I would like to understand why the world looks the way it looks in my mind’s eye. This is the explanandum. Even so, we should be cautious in fully “trusting” introspection,[7] and that is where more formal approaches may come in handy.
What would be the proper “fish” of consciousness? Are there conditions where everyone agrees that consciousness is absent? Dreamless sleep (Tononi & Massimini 2008) and anaesthesia (Alkire et al. 2008) seem to be good candidates, although not very useful ones, given that visual stimuli are difficult to deliver, and that one can only resort to objective measures (brain signals) to assess what is still processed or not. Awake subjects are easier to assess in that respect, but there it is hard to find truly unequivocal manipulations of consciousness. “Unequivocal” in this context means that the manipulation can truly be regarded as a manipulation of consciousness, i.e., in the case of vision is a manipulation of visibility (Kim & Blake 2005; Lamme 2006). An example of the latter would be visual masking (Breitmeyer & Ogmen 2000; Enns & Di Lollo 2000). Here, a target stimulus is presented very briefly, and immediately followed by another stimulus, known as the mask. When properly done, this will render the target completely invisible. People will be at chance detecting presence or absence, or in judging another property of the target stimulus. It is safe to assume invisibility in masking, because there is no conceivable reason that could prevent the subject from reporting his visual percept, had he had one: the subject is sitting there, focussing his full attention to the target location, ready to push the button as soon as he sees the target. The not-seeing can therefore not be attributed to the absence of attention, to a lapse of memory, or to any other cognitive function sitting between a potentially conscious sensation and its report (Lamme 2003, 2010a, 2010b). As we are ready to believe the presence of consciousness in the case of someone verbally describing the face he sees, we should be equally ready to believe its absence in the case of masking (or dreamless sleep and anaesthesia).[8]
Another popular paradigm to render stimuli invisible is continuous flash suppression (CFS; Tsuchiya & Koch 2005. Here, the target stimulus is shown to one eye, while the other eye receives a rapid stream of brightly coloured patches, serving as a mask. This typically results in the target stimulus being rendered invisible, although stimuli may “break through” after a while.[9] A third paradigm is dichoptic masking, where two oppositely coloured stimuli are shown to the two eyes, that when properly fused combine into an invisible stimulus (Moutoussis & Zeki 2002; Fahrenfort et al. 2012). From all the available neuropsychological patients, patients suffering from hemianopia due to a V1 lesion (often accompanied by blindsight) are probably the clearest cases of impaired visual consciousness (Weiskrantz 1996).
I select these consciousness manipulations because they seem to be the safest bets for highlighting situations where conscious vision is really absent. The absence of conscious vision in these cases has purely visual origins. There is no other function precluding the report of a potentially present visual sensation, as may be the case in split-brain patients or neglect, or in manipulations like innattentional blindness, change blindness, or the attentional blink (Lamme 2003). The two extreme ends—the mammals and fish of consciousness—may serve as a guideline towards establishing the properties of conscious versus unconscious processing. What are the differences between awake conscious vision and vision in sleep, anaesthesia, blindsight, and the various forms of masking?