We report here on specific ways in which synaptic long-term potentiation (LTP) affects the response selectivity of primary sensory cortical cells. LTP increases synaptic efficacy by incremental “steps,” up to a “ceiling” at which additional bursts of afferent stimulation cause no further potentiation. Endogenous and exogenous agents have been shown to modulate these two paramenters of LTP, raising the question of the functional implications associated with the sizes of steps and ceiling. We provide an analytical treatment of the effects of these two physiological LTP parameters on the behavior of simulated olfactory (piriform) cortex target cells in response to a range of inputs. A target cell's receptive field, i.e., the set of input patterns to which the cell responds, is broadened with potentiation of the cell's synapses, and is broadened more when the LTP step size is smaller, and when the LTP ceiling is higher. Moreover, the effects of step size and ceiling interact, and their joint relationship to receptive field breadth is nonlinear. Values of step size and ceiling are identified that balance the tradeoff between learning rate and receptive field breadth for particular sensory recognition tasks, and these model values are compared to corresponding known and inferred physiological values.

If Synaptic long-term potentiation (LTP) represents a memory storage mechanism, its induction and expression characteristics may constitute rules governing encoding and read-out of memory in cortical circuitry, The presence of variants of the LTP effect in different anatomical networks provides grounds for predictions about the types of memory operations to which potentiation contributes. Computer modeling studies incorporating the complex rules for LTP induction and the characteristics of expressed potentiation can be used to make such predictions specific. We review ttie types of synaptic plasticity found in the successive stages of the corticohippocampal pathway, and present results indicating that LTP does participate in definably different forms of memory, suggesting a classification of memory types differing somewhat from categories deduced from behavioral studies. Specifically, the results suggest that subtypes of memory operate serially, in an “assembly line” of specialized functions, each of which adds a unique aspect to the processing of memories. The effects of lesions on the encoding versus expression of memory can be interpreted from the perspective of this hypothesis.

Single-unit recording of layer II—III cells in olfactory (piriform) cortex was performed on awake, unrestrained rats actively engaged in learning novel odors in an olfactory discrimination task. Five of the 67 cells tested had very brief monophasic action potentials and high spontaneous firing rates (30–80 Hz); it is suggested that these units were interneurons. The remainder of the neurons had broader spikes and did not discharge for prolonged periods. Thirty-nine percent of the broad spike cells responded to at least one and usually more of the odors presented to the rats during either of the first two trials on which that odor was present, but, in most cases, these responses occurred only very infrequently over the course of subsequent trials. Six percent of the broad-spike group, how ever, continued firing robustly to a single odor but not to others. From these results it appears that most cells in piriform cortex do not respond to most odors, i.e., coding is exceedingly sparse. A subgroup of the predominant broad-spike cell type does react to several odors but this response drops out with repeated exposure, perhaps because of training. However, a few members of this class (a small fraction of the total cell population) do go on responding to a particular odor, thus exhibiting a form of odor specificity. The results are discussed with regard to predictions from recently developed models of the olfactory cortex.

Computer simulations of layers I and II of pirifonn (olfactory) cortex indicate that this biological network can generate a series of distinct output responses to individual stimuli, such that different responses encode different levels of information about a stimulus. In particular, after learning a set of stimuli modeled after distinct groups of odors, the simulated network's initial response to a cue indicates only its group or category, whereas subsequent responses to the same stimulus successively subdivide the group into increasingly specific encoding of the individual cue. These sequences of responses amount to an automated organization of perceptual memories according to both their similarities and differences, facilitating transfer of learned information to novel stimuli without loss of specific information about exceptions. Human recognition performance robustly exhibits such multiple levels: a given object can be identified as a vehicle, as an automobile, or as a Mustang. The findings reported here suggest that a function as apparently complex as hierarchical recognition memory, which seems suggestive of higher ‘cognitive’ processes, may be a fundamental intrinsic property of the operation of this single cortical cell layer in response to naturally-occurring inputs to the structure. We offer the hypothesis that the network function of superficial cerebral conical layers may simultaneously acquire and hierarchically organize information about the similarities and differences among perceived stimuli. Experimental manipulation of the simulation has generated hypotheses of direct links between the values of specific biological features and particular attributes of behavior, generating testable physiological and behavioral predictions.