Although they usually occur together, top-down attention and consciousness are separate processes according to a review of experimental evidence.
There is too much information arriving through the senses for all of it to receive priority perception. Top-down attention selects, in light of current behavioral goals, a portion of the input defined by a circumscribed region in space (spatial or focal attention), but a particular feature (feature-based attention), or by an object (object-based attention) for further processing. Consciousness does not appear to select but to integrate information, in order to summarize all relevant information of the current situation into a compact form. This integrated summary can used used by planning, error detection, decisions, language, memory and cognition. From this viewpoint, we can regard selective focal attention as an analyzer and consciousness as a synthesizer. If they have different functions they are likely to be dissociated under some circumstances.
The authors give a set of four types of events:
Top-down attention is not required and can be found without consciousness: formation of afterimages, rapid vision (less than 120 ms), zombie behaviours.
Top-down attention is not required but give rise to consciousness: pop-out, iconic memory, gist, animal/gender detection, partial reportability.
Top-down attention required but can be found without consciousness: priming, adaptation, processing of objects, visual search, thoughts.
Top-down attention required and give rise to consciousness: working memory, detection and discrimination of unexpected/unfamiliar stimuli, full reportability.
The second and third group show that attention is not necessary or sufficient for consciousness.
The bulk of the paper is a review of the relevant evidence. I pick a few items that seemed of particular interest to me to highlight.
Gist: In a dual-task paradigm when attention is focused at one spot, a peripheral stimulus can still be detected, as can the gist of a scene and characteristics like the gender of a face. Interestingly, what is considered a change in gist and what is not, seems to be affected by expertise. This suggests that consciousness without attention develops in response to extensive experience with a particular class of images. Also, observers often do perceive the gist of the scene and can accurately perceive the category of the object (whether it is a face, a natural scene, a letter, etc.). Even with a mere 30 ms exposure to natural scenes, followed by a mask, observers can clearly perceive their gist…within these 30 ms, top-down attentional bias could not have taken effect.
Neural activity: In priming experiments attention to invisible stimuli and visibility of unattended stimuli both enhanced the priming effects, but via distinctive neuronal mechanisms. Different neural activity has been found for for visibility (54-64 Hz from 250 500 ms in contralateral occipital) and attention (76-90 Hz from 350-500 ms in parietal sensors).
There are some striking parallels between the two-stream hypothesis for perception and action on the one hand and the division between attention and consciousness on the other hand. Attention primarily reduces the complexity of incoming input so that the brain can process it online and in real time. This might/could be a function of Milner and Goodale’s (2008) dorsal visual stream for action. In fact, the pre-motor theory of attention argues that visual attention evolved from the need to plan to move the eyes to one location. Overt eye movements and covert attention are closely related in both neural and functional ways. In terms of anatomical structure, front-parietal areas have been implicated in the control of attention, which are, of course, part of the dorsal, vision-for-action pathway. On the other hand, the ventral, vision-for-perception pathway has been linked to consciousness.
The two streams work together except under unusual conditions. This link of the streams to attention and consciousness is without doubt an over-simplification but an interesting starting point for investigations.
Here is the abstract:
Recent research has slowly corroded a belief that selective attention and consciousness are so tightly entangled that they cannot be individually examined. In this review, we summarize psychophysical and neurophysiological evidence for a dissociation between top-down attention and consciousness. The evidence includes recent findings that show subjects can attend to perceptually invisible objects. More contentious is the finding that subjects can become conscious of an isolated object, or the gist of the scene in the near absence of top-down attention; we critically re-examine the possibility of complete absence of top-down attention. We also cover the recent flurry of studies that utilized independent manipulation of attention and consciousness. These studies have shown paradoxical effects of attention, including examples where top-down attention and consciousness have opposing effects, leading us to strengthen and revise our previous views. Neuroimaging studies with EEG, MEG, and fMRI are uncovering the distinct neuronal correlates of selective attention and consciousness in dissociative paradigms. These findings point to a functional dissociation: attention as analyzer and consciousness as synthesizer. Separating the effects of selective visual attention from those of visual consciousness is of paramount importance to untangle the neural substrates of consciousness from those for attention.
van Boxtel, J., Tsuchiya, N., & Koch, C. (2010). Consciousness and Attention: On Sufficiency and Necessity Frontiers in Psychology, 1 DOI: 10.3389/fpsyg.2010.00217