Abstract
The properties of cortical cells depend to a certain extent on the type of their geniculate afferents. Since the first description of cortical RF organization (Hubel and Wiesel, 1962), attempts have been made to explain RF structure by the type of geniculate afferents (ON or OFF center). More recently, the discovery of different functional streams in the retino-geniculocortical pathway (X, Y, W) has promoted the view that many properties of cortical cells are the reflection of the X, Y or W type of geniculate afferents (see Stone et al., 1979 for review). In the extreme, labels of subcortical functional types have been used to categorize cortical cells (De Valois et al., 1982 b). More commonly, it has been believed that spatial frequency sensitivity or velocity sensitivity depend on the X, Y or W type of afferents. Recent evidence however suggests that velocity sensitivity depends mainly on intra-cortical mechanisms, leaving only spatial frequency sensitivity dependent on the type of LGN input. Only electrical stimulation of the visual pathways allows, within certain limits, direct identification of the type of geniculate afferents (X, Y, or W) of a cortical cell. This type of experiments relies on the different conduction velocities of the different types of afferents (see Chapter 3). All other attempts to identify the type of geniculate input to cortical cells provide only indirect evidience and should be interpreted with the greatest caution.
Published Version
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