Introduction

In the adult visual cortex, neurons are selectively responsive to particular stimulus features in the visual world. One of the most well-known functional properties of cortical neurons is orientation selectivity, in which cortical cells fire most intensely to lines and edges of specific orientations. Since the discovery of orientation-tuned cells in the primary visual cortex (Hubel and Wiesel, 1962), much attention has been devoted to elucidating the mechanisms underlying its development. Current evidence suggests that orientation selectivity develops in two stages: initial establishment followed by subsequent maturation and stabilization. The earliest emergence of orientation selectivity does not require visual experience, as indicated by the observation of well-tuned cells before or at the time of eye opening (Blakemore and Van Sluyters, 1975; Chapman and Stryker, 1993; Wiesel and Hubel, 1963). However, patterned vision after eye opening is essential to the maturation and maintenance of orientation specificity. Disrupting normal visual experience by binocular lid suture or dark rearing causes orientation tuning to deteriorate (Crair et al., 1998; White et al., 2001). Because visual deprivation significantly reduces the level of neural activity within the visual system, it has remained unclear whether natural physiological levels of activity simply permit the development of orientation selectivity to occur or whether patterns of activity have a deeper instructive role. If activity plays an instructive role in the development of orientation tuning, then spatiotemporally organized patterns of spontaneous or visually driven activity would be expected to drive this process. Correlations in firing between different neurons, when combined with Hebbian synaptic developmental processes, would lead to the strengthening or weakening of distinct synapses. Indeed, recent work reveals that manipulations in the precise correlational structure of neural activity block the maturation of orientation tuning (Chapman and Gödecke, 2000; Ramoa et al., 2001; Weliky and Katz, 1999). These results are consistent with correlation-based models of visual cortical development in which patterned spontaneous activity within the immature visual pathway serves to guide the development of orientation preference (Miller, 1992, 1994).

Historically, the cat has served as the model system for studying the role of visual experience on the development of orientation selectivity (Blakemore and Van Sluyters, 1975; Frégnac, 1979; Pettigrew, 1974; Wiesel and Hubel, 1963). However, research has been limited in the cat due to technical difficulties in performing experimental manipulations in young animals during the time when orientation tuning first develops. The recent shift toward the use of the ferret as an animal model in the study of the development of orientation specificity has enabled researchers to investigate activity-dependent processes at the earliest stages of development. This is because while the ferret visual system is quite similar to that of the cat (Law et al., 1988), the ferret is born approximately 3 weeks earlier in development (Linden et al., 1981). In this chapter, we review the literature on the development of orientation specificity in the cat and ferret in order to better understand the multifaceted role that neural activity plays in this process.