A recent post discussed the possibility put forward by Max Velmans that consciousness consists primarily of first-person subjective experience. Consciousness enters “too late” into brain activity to exert any sort of direction or control, Velmans claims. Consciousness can only observe the results of mental activity rather than participating in or initiating it. But now I’ve just finished reading In Search of Memory (2006) by Nobel prize-winning neurobiologist Eric Kandel. It took me awhile to get into this book (recommended, like Velmans’ book, by Lafayette) but eventually I found it enlightening and fairly entertaining. Kandel begs to differ with Velmans: he argues based on neural evidence that there is conscious mental processing, that it is activated by attention, and that it differs from unconscious processing at the level of biochemical action in the neurons. Here are two examples.
1. Forming a Spatial Map of the Environment
By exploring a new environment mice learn to navigate the space with increasing efficiently, quickly moving toward dark enclosures while avoiding brightly lit open spaces. Mice can also learn about their environments by being required to perform a task; e.g., by finding and sitting in the unmarked space that turns off the bright lights and loud noises that mice tend to avoid. To perform this sort of spatial task requires the mouse to pay more attention while investigating the space than when just having a look around the place. It turns out that mice who learn to perform a task retain their memory of the space much longer than do the casual explorers.
Dopamine enhances the response of neurons to stimuli. Dopamine is produced by cells in the mid-brain. The axons of these dopamine-making cells project into the hippocampus, where inputs from various sensory modalities are integrated and stabilized via long-term potentiation of the neural synapses. When the action of dopamine is blocked in the hippocampus of a mouse that learns to perform a spatial task requiring attention, the mouse’s memory of the spatial layout degrades rapidly. Conversely, when the dopamine receptors are activated in the hippocampus of a casually exploring mouse, the mouse retains its memory of the space far longer than would otherwise be expected. The implication is that dopamine stimulates the responsiveness of attention-directing neural pathways in the hippocampus, which in turn trigger the long-term neural retention of spatial layout in memory.
Conversion of short-term to long-term memory requires the activation of genes in the neurons via neurotransmitters that signal the importance of the stimulus for retention. In response to the biochemical signal, the appropriate genes are turned on in the neuron, causing particular proteins to be produced in the cell that are sent to the cell’s synapses, passing along the signal to cells in the hippocampus. In mice, dopamine is the triggering chemical. But the neurochemical memory cascade is triggered differently depending on the type of learning involved. In spatial memory — e.g., mice exploring a new environment — the dopamine activation is initiated from the top down, from the cerebral cortex to the hippocampus. The implication is that the focused attention required for long-term learning — mental activities typically associated with consciousness — follows a neural pathway that’s different from the diffuse mental activity suitable for short-term retention of environmental information.
A person who looks at a photo of someone whose facial expression indicates fear shows increased activity in the amygdala, the area of the brain that mediates fear. If the face with the fearful expression is presented for a long period of time, giving the person the opportunity to study the face consciously, then the dorsal region of the amygdala is activated. The dorsal region sends information to the autonomic nervous system, initiating the emotional arousal triggering the fight-or-flight response to frightening stimuli. If on the other hand the face is presented unconsciously — so rapidly that the observer cannot even report what type of expression the face is making — then the basolateral nucleus is activated. This area of the amygdala typically receives sensory input and transmits the signal to the cortex, heightening vigilance toward environmental threats.
Participants in the study were administered a questionnaire assessing their baseline level of anxiety. The level of anxiety had no effect on the conscious reaction to the fearful face. However, baseline anxiety was directly associated with the amount of activation in the basolateral nucleus in the unconscious reaction.
By implication, given the time to evaluate a fear-inducing stimulus consciously, everyone is able to activate the top-down triggering of an appropriate fight-or-flight response. Unconsciously, however, already-anxious people respond to fear-inducing stimuli by becoming hypervigilant, thereby increasing their already-high baseline anxiety levels, whereas non-anxious people do not become more anxious by being presented with a transitory fear-inducing stimulus.