Consciousness absent and present: a neurophysiological exploration

Backward masking was used to investigate the amount of neuronal activity that occurs in the macaque inferior temporal visual cortex when faces can just be identified. It is shown that the effect of the pattern mask is to interrupt neuronal activity in the inferior temporal visual cortex. This reduces the number of action potentials that occur to a given stimulus, and decreases even more the information that is available about which stimulus was shown because the variance of the spike counts is increased. When the onset of the mask follows the onset of the test stimulus by 20 ms, each neuron fires for approximately 30 ms, provides on average 0.06 bits of information, and human observers perform at approximately 50% better than chance in forced choice psychophysics, yet say that they are guessing, and frequently report that they are unable to consciously see the face and identify which face it is. At a longer Stimulus Onset Asynchrony of 40 ms, the neurons fire for approximately 50 ms, the amount of information carried by a single neuron is 0.14 bits, and human observers are much more likely to report conscious identification of which face was shown. The results quantify the amount of neuronal firing and information that is present when stimuli can be discriminated but not reported on consciously, and the additional amount of neuronal firing and information that is required for humans observers to consciously identify the faces. It is suggested that the threshold for conscious visual perception may be set to be higher than the level at which small but significant information is present in neuronal firing, so that the systems in the brain that implement the type of information processing involved in conscious thoughts are not interrupted by small signals that could be noise in sensory pathways.