Reply to Talis Bachmann on object substitution

In reply to the commentary by Talis Bachmann on a recent article by one of us entitled ‘‘Object substitution masking and its relation to other forms of masking’’ (Enns, 2004), we address the following issues. First, Bachmann s perceptual retouch theory concerns the role of nonspecific thalamic modulation in conscious visual experience. It is a reasonable theory, grounded in neurophysiological evidence, and it is likely relevant to many aspects of perception in which temporal factors play a critical role. But of greatest importance to the current discussion is that it makes no claims about the specific cortical representations that form the contents of consciousness. It simply takes cortical processes of object formation as a given. In contrast, object substitution theory is a framework for studying the emergence of specific cortical representations, but at the same time it stops short of claiming to be a theory of consciousness. As such, these theories should be seen as complementary to one another rather than as in competition. To illustrate the differences in the two approaches, consider the way in which perceptual-retouch theory accounts for common-onset masking. According to Bachmann, common-onset masking by four dots occurs because ‘‘. . . nonspecific modulation which is evoked by S1 and which is necessary for explicit perception is slow [and thus] reaches the cortex when mainly the specific signals from S2 drive cortical activity’’. What is overlooked in this account is that when the nonspecific modulation arrives, there are two relevant sources of cortical activity: one arising from the initial display containing the target and the distractors and persisting in the neural trace, the other from the four-dot mask that are still on view. Because the nonspecific modulation is, by definition, nonselective, it should enhance the decaying representation of the target as well as the representation of the mask. It must be emphasized that when the nonspecific modulation arrives, the target-related activity is clearly strong enough to mediate perception, as evidenced by