What is conscious and unconscious difference

Conscious perception as a dynamic and plastic process

Research Report 2010 - Max Planck Institute for Brain Research

Melloni, Lucia; Schwiedrzik, Caspar M.
Neurophysiology (Singer)
Max Planck Institute for Brain Research, Frankfurt am Main
Which factors determine whether a stimulus is consciously perceived or unconsciously processed? At the MPI for Brain Research, it is investigated how existing knowledge affects perception and the neuronal processes on which perception is based. It is also examined whether conscious perception can be learned. The results show that conscious perception does not only depend on how much information a stimulus provides. Rather, it is the result of a plastic and integrative process, in the course of which currently incoming information interacts with previously acquired knowledge.
Which factors determine whether a stimulus is consciously perceived or unconsciously processed? Here it is investigated how previous knowledge affects perception and its underlying neuronal processes. Furthermore, it is investigated whether conscious perception can be learned. The results show that conscious perception is not solely due to the amount of information that a stimulus carries. Rather, conscious perception is the result of a plastic, integrative process during which current information interacts with previously acquired knowledge.

The question of how conscious perception emerges from the activity of neurons is one of the great puzzles in neuroscience. We have the impression that we clearly perceive everything that is in the space around us, but the impression is deceptive. It is now known that a large part of the information that is received by our sensory organs is processed without us becoming aware of it. What makes the difference between conscious and unconscious perception has not yet been conclusively clarified. What is certain is that strong stimuli normally always reach consciousness. Weaker stimuli, on the other hand, are often only perceived if they are actively paid attention. The studies presented here investigate the question of whether there are other factors besides intensity and attention that determine whether a stimulus is consciously perceived or unconsciously processed. In particular, it examines how a previous experience affects subjective perception and which neural processes and circuits are based on it.

Our perception is seldom determined solely by the information that a stimulus carries. Rather, the information that has just been received is integrated with existing information. For example, it is easier to recognize an animal in its natural environment despite being camouflaged if you know which animal you are looking for. Here, the information already available about the animal is used to make a prediction and thus speed up the search. The first study presented here examines how predictability interacts with the strength of a stimulus and how this leads to conscious perception of the stimulus [1]. For this purpose, visual stimuli were initially made unrecognizable by means of noise. By gradually reducing the noise component, the stimulus slowly becomes more visible until it is finally clearly recognizable. After each step, the test person states whether they saw the stimulus. The noise component is then increased again step by step. Once the test person has recognized the stimulus, this allows him to see the stimulus afterwards despite the higher proportion of noise.

Electroencephalography can now be used to compare the neural processes that distinguish the stimuli that are seen from those that are not seen. In this way, the effect that the meanwhile clear recognizability has on the neural processes on which the conscious perception is based can be recorded: In the phase in which the test subject has not yet clearly perceived the stimulus, the conscious perception only through the If information present in the stimulus itself is determined, the first differences in the neural processes between seen and unseen passages are found around 300 milliseconds (ms) after the presentation of the stimulus. On the other hand, this differentiation is already found at 200 ms, if the test person has already clearly recognized the stimulus beforehand and can therefore predict what the stimulus will look like. This shows that previous experience has a strong influence on whether we are consciously aware of a stimulus. In addition, the time shift of the neural correlates of conscious perception from 300 ms without prediction to 200 ms with prediction indicates that the processes on which conscious perception is based do not have a fixed temporal relationship to the stimulus. Rather, they depend on how quickly current and existing information converge. This means that the processes that lead to conscious perception are much more flexible than previously assumed (Fig. 1).


The previous results show that an experience made recently can strongly influence the current perception. The following research investigates whether perception can also be influenced by long-term, systematic practice [2]. It is now established that the processing of stimuli in the cerebral cortex is highly plastic even in adulthood. In this way, test subjects can be trained to distinguish extremely small differences between stimuli. However, it has not yet been investigated whether such training also has an effect on the subjective perception of the stimuli or whether the improvements in the ability to distinguish between the stimuli can be attributed solely to unconscious processing. This question is also clinically relevant: some stroke patients who have become blind in part of their visual field as a result of damage to the visual cortex can distinguish between stimuli that fall in the blinded part of the visual field. However, these patients state that they are not consciously aware of the stimuli. Rehabilitation attempts have so far shown that targeted exercise can improve the ability to process the stimuli falling into the blinded visual field, and in rare cases this is accompanied by an increasing subjective perception of the stimuli [3]. This indicates that conscious perceptions, like other cognitive skills, can be changed through targeted training. This question can be pursued in healthy test subjects if easily recognizable stimuli are restricted in their visibility by quickly successive presentations (“masking”). In this way, conditions can be created in which the test subject cannot distinguish the presented target stimuli. At the same time, the test subjects stated that the stimulus was invisible to them. In the course of several days of training, the ability to differentiate between target stimuli increases. The question now is whether the training also causes an increase in subjective perception. In fact, after training, the test subjects state that they recognize the target stimuli more clearly (Fig. 2).


This shows that conscious perception can be improved through targeted training, even if the stimulus is initially subjectively invisible. This shows once again how flexible the neural processes are on which conscious perception is based. These results not only open up a new area of ​​research on the plasticity of conscious perception, but also provide important information for rehabilitation programs, the aim of which is to exploit the plasticity of the brain, for example to alleviate perceptual deficits after brain lesions.

In summary, the studies presented here show that the processes on which conscious perception is based are flexible and even plastic. On the one hand, experiences made shortly before the current stimulus can change perception and its neural correlates [1]. On the other hand, long-term training can lead to stimuli that are not consciously perceived under normal circumstances finding access to our subjective feelings [2]. This suggests that conscious perception is a process that should not be examined in isolation, but rather in its complex interplay with other cognitive processes that collectively determine what we perceive. Current studies will shed light on how these interactions are implemented at the neural level and what changes the training effects on conscious perception are based on.

L. Melloni, C. M. Schwiedrzik, N. Müller, E. Rodriguez, W. Singer:
Expectations change the signatures and timing of electrophysiological correlates of perceptual awareness.
Journal of Neuroscience 31 (4), 1386-1396 (2010).
C. M. Schwiedrzik, W. Singer, L. Melloni:
Sensitivity and perceptual awareness increase with practice in metacontrast masking.
Journal of Vision 9 (10), 1-18 (2009).
A. Sahraie, C. T. Trevethan, M. J. MacLeod, A. D. Murray, J. A. Olson, L. Weiskrantz:
Increased sensitivity after repeated stimulation of residual spatial channels in blindsight.
Proceedings of the National Academy of Science USA 103 (40), 14971-14976 (2006).