Neural Correlate - Laboratory Studies

Laboratory Studies

Neurophysiological studies in animals provided some insights on the neural correlates of conscious behavior. Vernon Mountcastle, in the early 1960s, set up to study this set of problems, which he termed "the Mind/Brain problem", by studying the neural basis of perception in the somatic sensory system. His labs at Johns Hopkins were among the first, along with Edward V.Evarts at NIH, to record neural activity from behaving monkeys. Struck with the elegance of SS Stevens approach of magnitude estimation, Mountcastle's group discovered three different modalities of somatic sensation shared one cognitive attribute: in all cases the firing rate of peripheral neurons was linearly related to the strength of the percept elicited. More recently, Ken H. Britten, William T. Newsome, and C. Daniel Salzman have shown that in area MT of monkeys, neurons respond with variability that suggests they are the basis of decision making about direction of motion. They first showed that neuronal rates are predictive of decisions using signal detection theory, and then that stimulation of these neurons could predictably bias the decision. Such studies were followed by Ranulfo Romo in the somatic sensory system, to confirm, using a different percept and brain area, that a small number of neurons in one brain area underlie perceptual decisions.

Other lab groups have followed Mountcastle's seminal work relating cognitive variables to neuronal activity with more complex cognitive tasks. Although monkeys cannot talk about their perceptions, behavioral tasks have been created in which animals made nonverbal reports, for example by producing hand movements. Many of these studies employ perceptual illusions as a way to dissociate sensations (i.e., the sensory information that the brain receives) from perceptions (i.e., how the consciousness interprets them). Neuronal patterns that represent perceptions rather than merely sensory input are interpreted as reflecting the neuronal correlate of consciousness.

Using such design, Nikos Logothetis and colleagues discovered perception-reflecting neurons in the temporal lobe. They created an experimental situation in which conflicting images were presented to different eyes (i.e., binocular rivalry). Under such conditions, human subjects report bistable percepts: they perceive alternatively one or the other image. Logothetis and colleagues trained the monkeys to report with their arm movements which image they perceived. Interestingly, temporal lobe neurons in Logothetis experiments often reflected what the monkeys' perceived. Neurons with such properties were less frequently observed in the primary visual cortex that corresponds to relatively early stages of visual processing. Another set of experiments using binocular rivalry in humans showed that certain layers of the cortex can be excluded as candidates of the neural correlate of consciousness. Logothetis and colleagues switched the images between eyes during the percept of one of the images. Surprisingly the percept stayed stable. This means that the conscious percept stayed stable and at the same time the primary input to layer 4, which is the input layer, in the visual cortex changed. Therefore layer 4 can not be a part of the neural correlate of consciousness. Mikhail Lebedev and their colleagues observed a similar phenomenon in monkey prefrontal cortex. In their experiments monkeys reported the perceived direction of visual stimulus movement (which could be an illusion) by making eye movements. Some prefrontal cortex neurons represented actual and some — perceived displacements of the stimulus. Observation of perception related neurons in prefrontal cortex is consistent with the theory of Christof Koch and Francis Crick who postulated that neural correlate of consciousness resides in prefrontal cortex. Proponents of distributed neuronal processing may likely dispute the view that consciousness has a precise localization in the brain.