Really elegant study in Nature Neuroscience showing how attention (consciousness for you philosophers…) can modulate perception. Most previous work that I’ve seen has been about how attention changes response time but this study shows how the percept itself can change.
The neat part is that the experimenters found a way of assaying stimulus salience (contrast, in this case) without directly asking. It’s interesting to see how this “Holy Grail of Consciousness” is being scientifically deconstructed bit-by-bit with rather simple experiments. Check out the full article here or click below for the news & views.
Understanding awareness: one step closer
Steven J Luck
Steven Luck is in the Department of Psychology, University of Iowa, Iowa City, Iowa 52242-1407, USA. e-mail: firstname.lastname@example.org
Attention enhances neural and behavioral responses to visual objects, but how does this affect our conscious perception? Attending to an object increases our subjective experience of stimulus contrast, reports a study in this issue.
How does the biophysical machinery of the brain evoke our rich phenomenological experience of the world? This question was once thought to be beyond the range of scientific inquiry, but leading neuroscientists have begun to find interesting answers by studying the neural correlates of awareness1, 2. Most of these experiments examine how brain activity differs when an observer reports being aware versus unaware of a given sensory input. However, these studies ignore the quality of our phenomenological experience (for example, the difference between what it’s like to see blue and what it’s like to see red). This approach to awareness is a bit like an art critic classifying Van Gogh’s Starry Night as “a dark picture” and Monet’s Garden at Giverny as “a light picture,” ignoring the dimensions of color, technique, composition and expression.
In this issue, Carrasco and colleagues3 provide a step toward a richer and yet still rigorous description of awareness. This study addresses phenomenological experience in the context of a very old question about perception: does paying attention to an object change its appearance? Attention is often likened to a spotlight4 or zoom lens5 that brightens or sharpens our perception, but no one has convincingly shown that attention actually changes our phenomenological experience of the world. Many studies have shown that attending to an object amplifies and sharpens neural representations of the object6-8, leading to an improved ability to detect the object and report its properties9, 10. However, these studies do not show that we actually experience attended objects differently from unattended objects.
The ever-present problem in studies of awareness is that observers’ reports of their experience are very easily biased by a variety of cognitive and affective factors. If observers report that an attended object seems brighter than an ignored object, it is usually impossible to know whether they really experienced it as being visually brighter. It is always possible that attention did not influence their perceptual experience, but rather that preconceptions about attention led them?intentionally or unintentionally?to report it as being brighter. Carrasco and colleagues have developed a new procedure for assessing an observer’s experience that markedly reduces the influence of bias on such reports.
In this procedure (Fig. 1), observers were shown two oriented gratings and asked to report the orientation of the higher-contrast grating (the one with brighter brights and darker darks). Thus, the observers explicitly reported the orientation of a grating, and their decision about which grating was higher in contrast was implicit rather than explicit. Attention was manipulated by preceding one of the two gratings with a small dot that automatically attracted attention.
When the two gratings differed greatly in contrast, the attention-capturing dot had no effect: observers simply reported the orientation of the higher-contrast grating. When the two gratings had similar contrasts, however, observers tended to report the orientation of the grating that was preceded by the dot. Thus, the attention-capturing dot changed the appearance of the subsequent grating, increasing its apparent contrast and leading the observers to report its orientation. This finding fits perfectly with previous studies indicating that attention increases neural6-8 and behavioral9, 10 measures of contrast sensitivity. However, these new results allow us to make the stronger conclusion that attention changes the actual phenomenological experience of contrast, making brights appear brighter and darks appear darker.
The innovative aspect of this procedure is that the observers were not directly asked about their perception of contrast, minimizing the possibility of bias. Instead, their perception of contrast was used to determine which of the two orientations should be reported. Consequently, the observers were led to believe that orientation perception, rather than contrast perception, was the focus of the experiment. Although this should have minimized any biases, it is still possible that the attention-capturing dot biased subjects to report the orientation of the grating on the same side of the dot without any change in the appearance of that grating. To rule out this possibility, the authors ran a control experiment, taking advantage of previous findings that the appearance of a dot captures attention for only a brief period of time. In the main experiment, the grating appeared 120 ms after the dot, while attention was still focused. In the control experiment, this interval was increased to 500 ms, allowing attention to fade away before the grating appeared. Although the same top-down bias factors should have been operating at this interval, observers showed no tendency to report the orientation of the grating that was preceded by the dot. This provides strong evidence that changes in sensory processing?and not top-down bias effects?were responsible for the results of the main experiment.
By showing that the focusing of attention in space leads to a change in phenomenological experience, this study confirms the common-sense assertion of William James11 that attention and awareness are intertwined. Much research over the past few decades has been devoted to assessing the neural substrates of attention12, 13, and it may be possible to use the results of this research to help understand the neural substrates of awareness. However, it would be easy to fall prey to the oversimplification of assuming a 1:1 relationship between attention and awareness. For example, given that attention seems to increase contrast sensitivity in area V4 of the primate visual system7, it would be tempting to conclude that neural activity in V4 is directly related to awareness. However, an attention-related change in V4 activity could easily lead to changes in downstream activity that are more directly related to awareness. Moreover, recent evidence suggests that attention and awareness may be dissociable under some conditions14. Thus, although it may be possible to use our growing knowledge of the neural substrates of attention as leverage when studying the neural substrates of awareness, this research will require a sophisticated understanding of both attention and awareness.
Will the present work of Carrasco and colleagues3 change the minds of those philosophers and neuroscientists who believe that we will never be able to measure an individual’s subjective experience? Probably not. No single study will change their minds, and this is appropriate given the difficulty of measuring subjective experience. However, if the new approach stands up under further scrutiny and can be used to address other questions about phenomenological experience, then it may become possible to objectively study subjective experience. Even then, we will not be at the point where we can measure the rich subjective experience evoked by viewing Starry Night or Garden at Giverny, much less understand the neural circuitry that gives rise to this experience. But we will have taken the first step in that direction.