Specialized Representations in Visual Cortex A Role for Binding?

of neurons that represent elements of a given object, and that this correlated firing labels the neuronal activity associated with one object. Different frequencies or Seeing is deceptively simple. We perceive objects, sym-phases of modulation could be used to simultaneously bols, movements, and other aspects of the visual scene label different objects. With this approach, neurons dis-without effort or awareness of the mechanisms that pro-tributed across visual cortex, or across different sensory cess visual information. But our continuous and seam-and motor systems (Engel et al., 1997), could be dynami-less visual perceptions depend on the activity of billions cally and selectively bound whenever their activity was of individual neurons. The first step in seeing an object associated with a single object. is the generation of a pattern of activity that is distributed The temporal correlation hypothesis is based on many across hundreds, if not millions, of photoreceptors. neurophysiological studies that have shown that the These in turn activate many other neurons in more cen-spike rate of cortical neurons sometimes oscillates at tral structures. frequencies between 30 and 70 Hz, and that neurons How the activity of widely distributed neurons can driven by a single stimulus sometimes oscillate in syn-lead to unitary percepts has been a key question in chrony (reviewed by Engel et al., 1992b, 1997; Singer and neuroscience dating to the origins of the modern con-Gray, 1995). This synchronization has been observed cept of neurons. The apparent continuity of perception between neurons in different visual areas (Eckhorn et was one of the major philosophical arguments against al., 1988; Engel et al., 1991b; Roelfsema et al., 1997), Cajal's neuron doctrine. How could discrete anatomical between sites in different cerebral hemispheres (Engel units be responsible? It has been an enduring subject et al., 1991a; Nowak et al., 1995b), and between sensory of inquiry since then. For example, Kö hler and Held and motor regions (Bressler et al., 1993; Murthy and (1949) suggested that unitary visual perception depends Fetz, 1996a, 1996b). on currents that flow through the cerebral cortex as if The scope of the temporal correlation hypothesis is it were a volume conductor, a concept which led to limited. It does not address how a particular group of experiments that tested perceptual capability after em-neurons is segmented from other active cells, or how bedding wires or insulators into cortex to disrupt the synchrony is achieved. Nor does it attempt to explain hypothesized …