Thesis - Department of Electronic & Computer Engineering

inhibition. Due to the fact that each cell contributes to inhibit itself without any compensation,this circuit has a low resolution of 10% or so. The final outputs of the circuitare solely decided by the input conditions.2.4.2 Add self-excitationLazzaro’s circuit is enhanced through the addition of a current mirror to each cellto realize excitatory feedback (self-excitation). For example, J. A. Starzyk and X. Fangproposed such a circuit which can improve both the speed and resolution of the originalWTA circuit.To mediate the competition between potential winners, hysteresis can be added tothe WTA circuit by adding local feedback to the winning node in the array. The additionof hysteresis enhances the present winning stimulus so that it resists the selectionof a new winner unless the stimulus has a value much larger than the present winner[9]. This has been used for analog VLSI selective attention system.2.4.3 Add lateral excitationAfter lateral excitation is added to the WTA circuits through resistive networks, thewinner is no longer restricted to the one who has the largest input, but depends on thespreading inputs in the neighborhood around the neuron. This kind of competition israther close to the cortical competition. There are some implementations which canmake it difficult for the network to leave the WTA region [10][11].17