Online research in philosophy

Comparisons between modally and amodally completed regions show that perceptual filling-in is not merely the ignoring of absences. Illusory filled-in colour arises for modal completion, but not for amodal completion in comparable displays. We find that attention spreads automatically to modally but not amodally completed regions from their inducers, revealing a functional effect of filled-in colour.

The target article discusses the classic blind spot, scotomas, subjective contours, and other so-called filling-in phenomena. Its purpose is to evaluate the idea that some theories of filling-in amount to tacit acceptance of Cartesian materialism and a form of psychophysical isomorphism. Pessoa et al. reject what is termed structural isomorphism as well as Cartesian materialism, but claim that neural processes adduced as underlying filling-in may be acceptable without implying isomorphism. The article supports the idea of perceiving as an active constructive process. However, the various subthemes are not clearly related to each other. Topological psychophysical isomorphism is indeed untenable, but the tacit assumption that filling-in enjoys any kind of unique status with respect to illuminating philosophical questions is doubtful.

This informative and conceptually stimulating target article is very useful. I merely query whether the term “illusory contours” is appropriate for gap filling; “illusory surfaces” seems better – and “fictional surfaces” better still. These seem to be rule based rather than knowledge based, suggesting indeed the importance of distinguishing rules (analogous to syntax in language) from knowledge (equivalent to semantics) for classifying perceptual phenomena.

(1) The purported evidence for neural filling-in is not evidence for filling-in, but just for long-range dynamic interactions. (2) Vision is perhaps not an “illusion,” but at any rate it is not “pictorial.” (3) The idea of the “world as an outside memory” as well as MacKay's “conditional readiness for action” may help approach an “enactive” theory of vision.

The question is broadened from isomorphism to invertible transformation and optimal representation. Motivations are drawn from image compression but with an emphasis on object segmentation. Filling-in is considered as the phenomenal side of the binding process with back-surface filling-in being important. Finally, re-normalization of local filtering by globally integrated context is emphasized.

Pessoa et al. (1998a) summarize a wide body of data suggesting that perceptual filling-in phenomena can be attributed to neural filling-in processes. However, they reject, on philosophical grounds, the hypothesis that filled-in representations in the brain are the immediate substrate of visual percepts. It is proposed in this commentary that resonant binding between distributed cortical areas may instead be the crucial ingredient for conscious visual percepts, and that filling-in processes may facilitate the interactions between behaving organisms and object surfaces. These suggestions circumvent some of the philosophical problems associated with the idea of localized visual representations.

Although Pessoa et al. imply that many aspects of the filling-in debate may be displaced by a regard for active vision, they remain loyal to naive neural reductionist explanations of certain pieces of psychophysical evidence. Alternative interpretations are provided for two specific examples and a new category of filling-in (of visual detail) is proposed.

A few findings from our laboratory are provided as evidence favoring “isomrphism” in filling-in. One is the responsivity of macaque-cortical area V1 cells to a stimulus designed for surface filling-in at the blind spot. Another is a phenomenological observation of motion aftereffect confined within a filled-in surface at the blind spot. Our recent study on the monkey's perception of surface filling-in at a scotoma is also mentioned.

“Finding out” about the visual world as approached from the organismic level may well include the “filling-in” type of perceptual completion if considered in terms of underlying neurophysiological mechanisms. But “filling in” can be interpreted not only as a result of within-level propagating of neural activity, but as a byproduct of the process that is necessary for modulating preconscious information about physically present objects or events so as to generate conscious quality in attending to them.

Implicit and explicit filling-in phenomena should be distinguished. Blind spot phenomena and mechanisms of boundary completion can be accounted for by implicit filling-in. Surface regions are “painted” with perceptual quantities, such as brightness, by explicit filling-in. “Filling-in” and “finding-out” relate to different computational tasks. Mechanisms of purposive computation (e.g., for navigation) evaluate local measurements, thus “finding out”; whereas mechanisms for grasping might require passive reconstruction, thus “filling in.”.