Functional Organization and Plasticity of Visual Cortex over the Lifespan of the Ferret
An elaborate network of neurons along the visual pathway topographically maps fundamental stimulus features in visual space, such as stimulus location or orientation, into structured cortical representations within visual cortex. In carnivores and primates, these cortical representations are systematically distributed into functional maps of discrete columnar modules. We were interested in testing the hypothesis whether the functional responses of neural populations remain largely stable in cortical maps (Chapman et al., 1996; Godecke et al., 1997; Sengpiel et al., 1998) or undergo systematic functional changes, as is predicted by various computational studies modeling the self-organization of neuronal circuits (Wolf and Geisel, 1998; Shouval et al., 2000; Alexander et al., 2004; Wolf, 2005; Oster and Bressloff, 2006; Wright et al., 2006; Schnabel et al., 2008; Wright and Bourke, 2008).
First we investigated how the functional representation of visual cortex maps in ferrets matures during early postnatal development and into adulthood. We found that areal growth and shrinkage of the visual cortex corresponded with commensurate changes in cortical column spacing, which indicates a lack of functional reorganization in maps of the visual cortex; thus, the functional organization of cortical maps is normally stable throughout the lifespan of ferrets. Secondly, we sought to characterize the extent of neural plasticity in the ferret visual system using the classic monocular deprivation paradigm. We show a prolonged age-dependent decline in ocular dominance plasticity, lasting out to approximately a year, whereas previous reports indicated ocular dominance plasticity ended much earlier (Issa et al., 1999). We also show that chronic fluoxetine treatment could not reinstate ocular dominance plasticity in adult ferrets, unlike it does in adult rats (Bastos et al., 1999; Maya Vetencourt et al., 2008; Maya Vetencourt et al., 2011). And lastly, we studied whether the functional organization of neural circuits constrains plasticity in the visual system. We perturbed the orientation tuning of visual cortex neurons by pairing a visual stimulation with direct intracortical activation of neurons in the visual cortex, and show that plasticity near pinwheel centers is reduced compared to neurons located in linear zones of the orientation map.