| The steady-state visual evoked potential(SSVEP)has been widely applied in brain-computer Interface technology(BCI)and clinical medical diagnosis and treatment,etc.The amplitude of SSVEP is an important factor that determines the accuracy of BCI control and medical diagnosis.Low-frequency SSVEP has a relatively high amplitude,but it is easy to cause eyestrain and even cause epilepsy.High frequency(greater than 25Hz)flash stimulation can relieve visual fatigue,but high frequency SSVEP amplitude is too small to be used effectively.How to increase the amplitude of high frequency SSVEP is a problem that people are working on.It has been agreed that the simultaneous action potential of many pyramidal neurons in the visual cortex stimulated by periodic flash is the reason for the formation of the equivalent dipole source in the brain of SSVEP.The number and location of the neurons in the visual cortex that produce the action potential determine the amplitude of the SSVEP.The change of neuron membrane potential under flash stimulation has the characteristic of time "superposition" and follows the law of "full" or "none" of action potential: when the membrane potential exceeds a certain threshold,the action potential is generated,otherwise,there is no action potential.Therefore,a question worth studying is:low-frequency(long duration of light action)and low-intensity light stimulation can make the neuron membrane potential exceed the threshold and induce the action potential,so can high-frequency(short duration of light action)but high-intensity light stimulation also make the neuron membrane potential exceed the threshold and induce the action potential? This question involves a key issue: whether the production of neuronal action potential is the result of the synergistic effect of the combination of parameters such as the frequency and intensity of light stimulation.In this paper,three methods including theoretical derivation,model simulation and experimental test were used to study the effects of different combinations of flash stimulation frequency and intensity on the emission of action potential and SSVEP amplitude of neurons in the visual cortex.The specific research work is as follows:1.From the microscopic Angle,according to the "all" or "none" theory of neuron action potential,the effects of different combinations of flash stimulation frequency and intensity on the emission of action potential of neurons in the visual cortex were studied.The results showed that: at low frequency,low-intensity flash stimulation can also make neurons generate action potential;At high frequencies,higher stimulation intensity is required to generate the action potential in the neuron.2.In addition to the number and location of neurons producing action potential are two important factors affecting SSVEP,the emission frequency of action potential of neurons in a cycle is also a key factor affecting the amplitude of SSVEP.In this paper,based on the classical neuron equivalent circuit model(Hodgkin-Huxley,H-H),the square wave current was taken as the stimulation current of the neuron,and the influence of the combination of different frequency and amplitude of the stimulation current on the number of neuron action potential emission was simulated.The simulation results show that under the condition of low frequency,the neurons can produce more action potentials under the condition of low intensity electric current stimulation.The number of action potentials induced by high frequency stimulation is less,but increasing the amplitude of square wave current can effectively increase the number of action potentials.Because the current in the neuron membrane corresponds to the frequency of the light pulse,and the stimulation intensity is proportional to the amplitude and light,the simulation results also indirectly reflect the influence of different combination of periodic light stimulation frequency and intensity influence the number of neurons action potential,and further shows that the low frequency,low intensity and high frequency,high intensity of flash stimulation can induce SSVEP to a certain amplitude.3.Based on the fact that every neuron producing action potential can be equivalent to a magnetic dipole,the SSVEP calculation model of brain scalp was established from a macroscopic perspective.According to the changing relation between the equivalent magnetic dipole current and the optical signal pulse,the effect of different combinations of the equivalent magnetic dipole current frequency and amplitude on the SSVEP amplitude was studied through the calculation model simulation,so as to realize the effect of different combinations of the light stimulation frequency and intensity on the SSVEP amplitude.Simulation results show that :(1)with the increase of the equivalent magnetic dipole current frequency,SSVEP amplitude first increases and then decreases;(2)with the increase of the amplitude of the equivalent magnetic dipole current,the stimulation frequency band of SSVEP can be effectively induced.Since the frequency of the equivalent magnetic dipole current of the neuron is consistent with the frequency of the flash stimulation pulse and the amplitude is proportional,the simulation results also indirectly indicate that the amplitude of high-frequency SSVEP is smaller than that of low-frequency SSVEP under the same intensity of flash stimulation,and increasing the amplitude of high-frequency SSVEP effectively can be achieved by increasing the intensity of flash stimulation.4.The effect of different combination of flash frequency and intensity on SSVEP amplitude was studied experimentally.The experimental results show that in the low frequency band,lower intensity flash stimulation can induce higher amplitude SSVEP.However,in high frequency band,higher intensity flash stimulation is needed to induce SSVEP of a certain amplitude.The conclusion is consistent with the simulation results of H-H neuron model.In addition,the experimental results also show that the amplitude of SSVEP varies with the frequency of stimulation at the same stimulus intensity,which is basically consistent with the simulation results of the SSVEP calculation model of the scalp of the brain,and verifies the rationality of the theory and the model.Finally,the influence mechanism of different combination of flash stimulation frequency and intensity on SSVEP amplitude was discussed.5.In this paper,the theoretical,simulation and experimental results all show that in the low frequency band,the light stimulation with lower intensity can induce the SSVEP with higher amplitude.In the high frequency band,the amplitude of SSVEP induced by low-intensity flash is very low and even difficult to induce SSVEP.However,if the stimulus intensity is increased,the amplitude of SSVEP induced by high frequency flash will increase to a certain extent.The results of this study change the traditional view that the amplitude of high frequency flash induced SSVEP is very small or even difficult to be induced,and provide a meaningful new idea for how to improve the amplitude of high frequency SSVEP and its effective application in BCI technology and clinical medicine. |
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