The physiological basis of mental images.

An important hypothesis in neurophysiology supposes that sensory information is encoded in spatial patterns of neural activity. A test of this hypothesis in rabbits reveals that the spatial pattern of encephalographic (EEG) activity of the olfactory bulb depends less on odor than on expectation of odor. Analysis of the structure and nonlinear dynamics of the olfactory system suggests that the EEG manifests a neural activity pattern that is an active filter corresponding to what ethologists call a "chemical search image." This first glimpse at the physical aspect of what may be a primitive form of mental image allows us to infer some of its properties. It is self-organized; input destabilizes the bulb, and the event runs a course that is shaped by the initial conditions, the sensory input, the chemical state of the bulb from centrifugal input, and the record of past input embedded in synaptic connections. The event is analog, but not an analog of input. Its intensity pattern exists in two dimensions but it is not a picture. Its bases in feedback and resonance make it connotative and ascriptive, not denotative or descriptive. The neural-mental image is an operator, not an operand. It gathers neurons by the millions into coherent activity and creates information, which it transmits to the next stage in the brain, thereby helping to shape behavior. It cannot be understood without reference to both what the subject does and what is done to it. Its function in the bulb is to regulate olfactory input with minimal and nonspecific centrifugal control.