Flash evoked potentials in the ectostriatum of the zebra finch: a current source-density analysis

Recent research has demonstrated that ipsilaterally visually evoked potentials (VEPs) can be measured within the ectostriatum, the telencephalic target area of the tectofugal visual pathway in birds. In this paper we systematically measured contra- and ipsilateral VEPs within the ectostriatal complex to obtain more detailed information on the processing of contra- and ipsilateral stimuli. The similarity of neighbouring VEPs at equal depth and a comparison of a one dimensional and a three dimensional analysis of current source-densities (CSDs) for identical coordinates suggested that a one dimensional current source-density analysis might be applicable. The one dimensional current source-density analysis demonstrated largely corresponding patterns in the sink-source sequences of the current source-density depth profiles for the contra- and ipsilateral stimulus responses. The occurrence of a large sink in the centre of the ectostriatal core, together with the results of multiunit recordings, shows that the ectostriatal core is the location of the generators for both the contra- and the ipsilaterally evoked responses. The occurrence of macroscopic sinks and sources and the fact that VEPs can be recorded from the ectostriatum shows that there is a higher degree of order in the ectostriatum than has been previously demonstrated by anatomical methods. The time coincidence between the maximum spike rate of multiunit responses, the negative peak of the evoked potential, and the large central sink demonstrates that the influence of ipsi- as well as of contralateral stimuli is predominantly excitatory.