The present report is concerned with an experimental test of the ability of a neurophysiological model to predict perceptual phenomena. The experiment involved obtaining measures of the brightness of pairs of photic pulses under different conditions of pulse duration and pulse separation. The procedure for reporting will be to present a brief summary of the model; to develop predictions from the model, and then to report the experiment.
THE MODEL Summary of the ModelIt has been noted that an isolated shock to the optic nerve of the rabbit is followed by an extended sequence of changes in the evoked potential of the optic cortex in the form of a train of waves of gradually decreasing amplitude. A resemblance can be noted between the latter period of this evoked potential and the human cortical "alpha rhythm" (Bishop, 1936). Brazier (1962), recording human EEGs with the aid of a computer, has presented data showing long lasting wave trains following an isolated flash to the retina (Fig. I). Bishop and Clare have constructed a model of cortical processes consistent with such findings (see Bartley, 1959). The model has relevance not only for single pulse phenomena but for paired pulses as well.Briefly, the model operates as follows: energy impinging on the retina traverses the optic nerve and tract and synapses on the lateral geniculate body. From the geniculates the impulses travel to the fourth layer of the occipital cortex where they synapse on the short axon Golgi cells. The Golgi cells in turn synapse on large pyramid cells of the same layer.From the pyramid cell the impulse follows two paths. One path is down the pyramidal axon to the thalamic cells. The second path involves recurrent branches of the pyramid cell. These branches arborize in the cortex and synapse on a second set of Goigi cells. The second path terminates by activating the dendrite of the pyramid cell and passing down the axon to the thalamus. The time of the first path to the thalamus is of the order of 20 msec. The second path is longer and it is thought that, from the first Goigi cell to the thalamus, the interval is equivalent to the alpha cycle of the specie involved.If an incoming impulse along the optic nerve arrives at the thalamus at the same instant that an impulse is returning to the thalamus along corticofugal loops, the result is a facilitation of the thalamic cells upon which these branches terminate. If the optic nerve activity reaches the thalamus at any other time there will be no facilitation and consequently lower cortical potential.Other investigators have offered data on the neurophysiological effects of paired photic pulses that are consistent with the Bishop and Clare model. Bartley (1941) has shown that the amplitude of the evoked potential of a second pulse waxes and wanes as a function of the separation between pulses. He found that, in order to obtain an amplitude at the cortex for the second pulse as great as that of the first pulse, the pulses had to be separated by an interval equivalent to the organism's alpha c...
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