Impaired implicit learning and reduced pre-supplementary motor cortex size in early-onset major depression with melancholic features

Abstract

Major depression is a heterogeneous disorder. Biological markers and cognitive tasks have been employed to distinguish clinical subtypes but results have been inconclusive. The current study assessed implicit learning with the Serial Reaction Time Task (SRTT) known to be sensitive to frontostriatal dysfunctions and regional brain volumes of the anterior supplementary motor area (pre-SMA) in participants with early-onset major depression (MD) of either melancholic (n=26) or non-melancholic (n=9) subtype, and 26 matched controls. Depressive subjects with melancholic features but not those with non-melancholic depression showed implicit learning deficits. This deficit could not be explained in terms of more severe depression or psychomotor retardation. Regional volumes of the right pre-SMA were reduced in depressive subjects with melancholic features. Medication effects in depressive subjects and the small size of the non-melancholic sample should be taken into consideration when reviewing the implications of these results. Deficits in implicit motor sequence learning seem to be an additional characteristic of the melancholic subtype of depression. It might be linked to dysfunction within structural or functionally altered frontostriatal circuits. Use of implicit sequence learning tasks could offer useful diagnostic and aetiological cues for future research.