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Biomedical subjects

L N Cooper

Publications and source records attributed to L N Cooper.

27 records · Page 2Linked to original sources
PubMed1987

A physiological basis for a theory of synapse modification.

The functional organization of the cerebral cortex is modified dramatically by sensory experience during early postnatal life. The basis for these modifications is a type of synaptic plasticity that may also contribute to some forms of adult learning. The question of how synapses modify according to experience has been approached by determining theoretically what is required of a modification mechanism to account for the available experimental data in the developing visual cortex. The resulting theory states precisely how certain variables might influence synaptic modifications. This insight has led to the development of a biologically plausible molecular model for synapse modification in the cerebral cortex.

Action Potentials↗
PubMed1987

A relaxation model for memory with high storage density.

We present a relaxation model for memory based on a generalized coulomb potential. The model has arbitrarily large storage capacity and, in addition, well-defined basins of attraction about stored memory states. The model is compared with the Hopfield relaxation model.

Algorithms↗
PubMed1986

Theory of an immune system retrovirus.

Human immunodeficiency virus (HIV; formerly known as human T-cell lymphotropic virus type III/lymphadenopathy-associated virus, HTLV-III/LAV), the retrovirus that infects T4-positive (helper) T cells of the immune system, has been implicated as the agent responsible for the acquired immune deficiency syndrome. In this paper, I contrast the growth of a "normal" virus with what I call an immune system retrovirus: a retrovirus that attacks the T4-positive T cells of the immune system. I show that remarkable interactions with other infections as well as strong virus concentration dependence are general properties of immune system retroviruses. Some of the consequences of these ideas are compared with observations.

Cell Division↗
PubMed1982

A neural model for category learning.

We present a general neural model for supervised learning of pattern categories which can resolve pattern classes separated by nonlinear, essentially arbitrary boundaries. The concept of a pattern class develops from storing in memory a limited number of class elements (prototypes). Associated with each prototype is a modifiable scalar weighting factor (lambda) which effectively defines the threshold for categorization of an input with the class of the given prototype. Learning involves (1) commitment of prototypes to memory and (2) adjustment of the various lambda factors to eliminate classification errors. In tests, the model ably defined classification boundaries that largely separated complicated pattern regions. We discuss the role which divisive inhibition might play in a possible implementation of the model by a network of neurons.

Animals↗
PubMed1982

Theory for the development of neuron selectivity: orientation specificity and binocular interaction in visual cortex.

The development of stimulus selectivity in the primary sensory cortex of higher vertebrates is considered in a general mathematical framework. A synaptic evolution scheme of a new kind is proposed in which incoming patterns rather than converging afferents compete. The change in the efficacy of a given synapse depends not only on instantaneous pre- and postsynaptic activities but also on a slowly varying time-averaged value of the postsynaptic activity. Assuming an appropriate nonlinear form for this dependence, development of selectivity is obtained under quite general conditions on the sensory environment. One does not require nonlinearity of the neuron's integrative power nor does one need to assume any particular form for intracortical circuitry. This is first illustrated in simple cases, e.g., when the environment consists of only two different stimuli presented alternately in a random manner. The following formal statement then holds: the state of the system converges with probability 1 to points of maximum selectivity in the state space. We next consider the problem of early development of orientation selectivity and binocular interaction in primary visual cortex. Giving the environment an appropriate form, we obtain orientation tuning curves and ocular dominance comparable to what is observed in normally reared adult cats or monkeys. Simulations with binocular input and various types of normal or altered environments show good agreement with the relevant experimental data. Experiments are suggested that could test our theory further.

Animals↗
PubMed1979

A theory for the acquisition and loss of neuron specificity in visual cortex.

We assume that between lateral geniculate and visual cortical cells there exist labile synapses that modify themselves in a new fashion called threshold passive modification and in addition, non-labile synapses that contain permanent information. In the theory which results there is an increase in the specificity of response of a cortical cell when it is exposed to stimuli due to normal patterned visual experience. Non-patterned input, such as might be expected when an animal is dark-reared or raised with eyelids sutured, results in a loss of specificity, with details depending on whether noise to labile and non-labile junctions is correlated. Specificity can sometimes be regained, however, with a return of input due to patterned vision. We propose that this provides a possible explanation of experimental results obtained by Imbert and Buisseret (1975); Blakemore and Van Sluyters (1975); Buisseret and Imbert (1976); and Frégnac and Imbert (1977, 1978).

Animals↗
PubMedDate not supplied

Temporally graded retrograde amnesia following separate and combined lesions of the perirhinal cortex and fornix in the rat.

The involvement of the perirhinal cortex and the fornix in retrograde and anterograde amnesia in the rat was investigated in this experiment. Male Sprague-Dawley rats were trained on a series of five visual discrimination problems at distinct time intervals prior to receiving bilateral, electrolytic lesions of the perirhinal cortex or the fornix, combined lesions of both these structures, or sham operations. Following recovery from surgery, rats were retested on the preoperatively learned discrimination problems, as well as learning a new discrimination and discrimination reversal. Results indicated that all animals with lesions exhibited temporally graded retrograde amnesia, whereby memories acquired in the recent past (1-3 weeks) were impaired, and memories acquired in the remote past (6-8 weeks) were spared. There was no difference in the magnitude of retrograde amnesia between the three lesion groups. Animals in the perirhinal, fornix, and combined lesion groups were able to learn a new discrimination problem at a rate comparable to control rats; however, the animals with lesions were impaired at learning the discrimination reversal. The perirhinal, fornix, and combined lesion animals also exhibited a significantly faster forgetting rate over a 2-week retention interval than control rats. These results suggest that medial temporal structures including the perirhinal cortex and the fornix are involved in the consolidation of mnemonic information and that their involvement in this process occurs over a discrete period of time.

Amnesia↗