Are there unconscious perceptual processes?

Abstract

Blindsight and vision for action seem to be exemplars of unconscious visual processes. However, researchers have recently argued that blindsight is not really a kind of unconscious vision but is rather severely degraded conscious vision. Morten Overgaard and colleagues have recently developed new methods for measuring the visibility of visual stimuli. Studies using these methods show that reported clarity of visual stimuli correlates with accuracy in both normal individuals and blindsight patients. Vision for action has also come under scrutiny. Recent findings seem to show that information processed by the dorsal stream for online action contributes to visual awareness. Some interpret these results as showing that some dorsal stream processes are conscious visual processes (e.g., Gallese, 2007, Jacob and Jeannerod, 2003). The aim of this paper is to provide new support for the more traditional view that blindsight and vision for action are genuinely unconscious perceptual processes. I argue that individuals with blindsight do not have access to the kind of purely qualitative color and size information which normal individuals do. So, even though people with blindsight have a kind of cognitive consciousness, visual information processing in blindsight patients is not associated with a distinctly visual phenomenology. I argue further that while dorsal stream processing seems to contribute to visual awareness, only information processed by the early dorsal stream (V1, V2, and V3) is broadcast to working memory. Information processed by later parts of the dorsal stream (the parietal lobe) never reaches working memory and hence does not correlate with phenomenal awareness. I conclude that both blindsight and vision for action are genuinely unconscious visual processes.

Introduction

It is uncontroversial that there are phenomenally unconscious visual processes. For example, processes in the retina are visual processes that do not correlate with visual awareness. It is harder to say whether there are unconscious perceptual processes. An unconscious perceptual process, as I shall construe the term here, is a kind of representational mental process that occurs below the level of phenomenal awareness.

Before continuing it is important to point out that, strictly speaking, all processes of vision are unconscious. Only the representations computed can be conscious. So when we speak of a process correlating with visual awareness or a process being conscious, this should be understood as meaning that the product of the process is conscious or accessible to consciousness.

Blindsight has been thought to be an exemplar of an unconscious perceptual process. Blindsight occurs as the result of damage to the primary visual cortex which results in a scotoma, or region of blindness. Individuals with a scotoma typically report no visual awareness of visual stimuli represented to them in their blind field. But they nonetheless have a preserved ability to predict attributes of visual stimuli. They typically make above-chance predictions about the motion, location and colors of objects they report not seeing. The visual processes underlying these predictions thus seem to be good candidates to be unconscious perceptual processes. They certainly seem to be mental representational processes.

However, it has recently been called into question whether blindsight really is a form of unconscious vision. It has been argued that better methods of reporting are needed to determine whether blindsight patients are aware of stimuli in their blind field. Morten Overgaard and colleagues have recently developed such methods, and subsequent studies have shown that when these methods are used, reported visibility of stimulus correlates with accuracy in both normal individuals and blindsight patients. It has been concluded that blindsight is not a kind of unconscious vision, but rather highly degraded conscious vision.

Dorsal stream processes, too, seem to be exemplars of unconscious visual processes. Dorsal stream processes compute information about the absolute size of objects and the properties of objects in egocentric space. Goodale and Milner, 1992, Goodale and Milner, 2004, Milner and Goodale, 1996, Milner and Goodale, 2008 have argued that this information never reaches conscious awareness but is translated directly into online, or immediate, action. Milner and Goodale warn against confusing vision for action with perceptual processes. As they put it:

But what about visually elicited activity in the dorsal stream? This activity certainly does not give rise to visual awareness […], but that doesn’t mean that it has anything to do with unconscious perception. Use of that phrase carries the implication that such visual processing could, in principle, be conscious. The fact is that visual activity in the dorsal stream can never become conscious – so ‘perception’ is the wrong word to use. The dorsal stream is not in the business of providing any kind of a visual representation of the world: it just converts visual information directly into action (2004: 114).

Milner and Goodale’s claim that vision for action is not a kind of perceptual process is easily refuted. Elsewhere they argue that vision for action represents properties of objects in egocentric space. This suffices to make vision-for-action processes representational processes. Milner and Goodale are primarily concerned with arguing that these representations are not broadcast to working memory and hence do not reach visual awareness. However, independent evidence reviewed by, for example, Gallese, 2007, Jacob and Jeannerod, 2003 seems to show that part of the information processed by the dorsal stream does reach visual awareness. This indicates that dorsal stream processes are not genuinely unconscious processes.

The aim of this paper is to provide new support for the more traditional view that blindsight and vision for action are genuinely unconscious perceptual processes. I briefly review Block’s distinction between access consciousness and phenomenal consciousness. I then argue, against Block, that accessibility does correlate with phenomenal consciousness. So, to show that a process is genuinely unconscious it suffices to show that the information it computes is not cognitively accessible. In the final sections of the paper I revisit the empirical data aimed at demonstrating that blindsight and vision for action are not unconscious processes. I show that in the case of blindsight, researchers confuse global broadcasting of cognitive information and distinctly visual awareness. In the case of dorsal stream processing I argue that while information processed in the early dorsal stream contributes to visual awareness, representations computed in the later parts of the dorsal stream never reach global workspace. Hence, both blindsight and vision for action are genuinely unconscious perceptual processes.