1. A method for determining stimulation signals in a cortical stimulation apparatus in which an electrode array, having at least one electrode adapted to be implanted in the cortex of a patient, is controlled by a control system, said method including at least the following steps: (a) a measuring step wherein a number n of electric signals is collected by said control system, each at a respective electrode of said electrode array, n being at least 1; (b) a processing step wherein said control system determines n stimulation signals, by a virtual neural field having a virtual array of n points corresponding to each electrode of the electrode array having collected an electric signal at the measuring step, said virtual array receiving the collected signal as an input on each of the n points and said virtual neural field being adapted to control the frequency spectrum of neural activity in the cortex, each stimulation signal being determined by a value of said virtual potential at each point of the virtual array using the following formula: LV a ⁡ ( x , t ) = I ⁡ ( x , t ) + β a ⁢ ∫ Ω ⁢ W a ⁡ ( ⅆ ( x , y ) ) · S ⁡ [ V a ⁡ ( y , t - ⅆ ( x , y ) v ) - θ ] · ⅆ y ( 1 ) where: L is an operator equal to λ · ∂ 2 ∂ t 2 + γ ⁢ ∂ ∂ t + 1 ; V a is a potential in the virtual neural field; x is a spatial position in the virtual neural field; t is time; Ω is a spatial domain of the neural field; d(x,y) is a distance between two spatial positions x, y in the spatial domain Ω; v is a propagating speed of a signal in the virtual neural field; β a is a synaptic strength in the virtual neural field; W a (d(x,y)) is a connectivity kernel of the virtual field; S(V) is a sigmoid function which provides a correspondence between a potential V and a corresponding firing rate of neurons; θ is a firing threshold; and I(x,t) is a function of the electric signals V r (x,t) collected through the electrodes ( 8 b ) and applied to the points of the virtual array in the virtual neural field, wherein: I ⁡ ( x , t ) = β ar · ∫ Ω ⁢ · W ar ⁡ ( ⅆ ( x , y ) ) · S ⁡ [ V r ⁡ ( y , t - τ ar ) - θ ] · ⁢ ⅆ y ( 1 ⁢ a ) W ( d ⁡ ( x , y ) = 1 π [ a e ⁢ exp ( - ⅆ ( x , y ) 2 - a i ⁢ r ⁢ exp ( - r 2 ⁢ ⅆ ( x , y ) 2 ) ] ( 1 ⁢ b ) S ⁡ ( V ) = f ma ⁢ ⁢ x 1 + exp ⁡ [ - λ ⁡ ( V - θ ) ] ( 1 ⁢ c ) where: β ar is a synaptic strength between the real and the virtual neural fields; W ar is a connectivity kernel between the real and the virtual neural fields; τ ar = d ⁡ ( x , y ) v is a delay; a e /a i are an excitatory/inhibitory synaptic weights respectively; r is a ratio of spatial ranges between excitatory and inhibitory fibers; f max is a maximum discharge rate of neurons in the neural field; and λ is a non-dimensional parameter; (c) a stimulation step wherein said stimulation signals are emitted by said control system, respectively at the electrodes of said electrode array corresponding respectively to the points of the virtual array.