September 2011
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Slowing perception down

According to one way of understanding perception, it would not be surprising if perception was completed before conscious awareness could contain the percept. Why is it important to examine this? So that experiment methods of assessing conscious awareness are valid. Gregori-Grgic, Balderi and de’Sperati look at this question (see citation below) by slowing the processes down.


Visual perception involves the establishment of edges, corners, textures, colours, location into objects in space. It also involves establishing the motion of these objects. A minimum of this work must be done before a ’scene’ can be experienced. By degrading motion with noise, limiting the duration of observation, and asking for the direction of motion (or a guess at it) at particular times, they separated the ability to discriminate the direction of motion from the ability to report seeing the direction of motion.


The flourishing of studies on the neural correlates of decision-making calls for an appraisal of the relation between perceptual decisions and conscious perception. By exploiting the long integration time of noisy motion stimuli, and by forcing human observers to make difficult speeded decisions – sometimes a blind guess – about stimulus direction, we traced the temporal buildup of motion discrimination capability and perceptual awareness, as assessed trial by trial through direct rating. We found that both increased gradually with motion coherence and viewing time, but discrimination was systematically leading awareness, reaching a plateau much earlier. Sensitivity and criterion changes contributed jointly to the slow buildup of perceptual awareness. It made no difference whether motion discrimination was accomplished by saccades or verbal responses. These findings suggest that perceptual awareness emerges on the top of a developing or even mature perceptual decision. We argue that the middle temporal (MT) cortical region does not confer us the full phenomenic depth of motion perception, although it may represent a precursor stage in building our subjective sense of visual motion.


The authors do not see conscious perception as a point in time.

…This does not imply strictly serial processes, as the processes underlying discrimination and awareness can coexist in time (race model). Note that we are not suggesting to take the saturation of perceptual awareness as the temporal marker of the perceptual delay; rather, its gradual buildup suggests that the notion of a precise point in time where conscious perception is realized may be too strict, at least with our degraded motion stimuli.


Other experimenters should take note of the result.

In disentangling decision from conscious perception, our study warns against an indiscriminate use of monkeys’ saccadic eye movement as a proxy for conscious visual perception (e.g., for what monkeys ‘‘see’’), even when accuracy is rewarded. More generally, our findings indicate that, especially when time is an issue, objective forced-choice responses may not provide a full account of visual perception, as the perceptual decision can be taken when the formation of perceptual awareness is still underway.


The decision of the direction of motion is not made on the basis of the conscious percept.

That perceptual awareness is more sluggish than motion discrimination may appear somewhat unsettling, as we tend often to assume that conscious perception precedes decision. However, phenomena such as blindsight and unconscious perception suggest that automatic decisions are indeed possible under certain conditions. In more ordinary contexts, many sensory- driven actions, as well as the stimuli that originated them, pass mostly unnoticed, as when driving or in sports. Similarly, we incessantly decide where to make the next gaze shift, despite poor or null awareness of peripheral - and sometimes also central - visual information. Awareness may just follow.


This ‘unsettling’ appears a bit naïve. There is no reason why an unconscious process creating part of a percept and some other unconscious process doing something else (moving the eye, navigating) should not share what information they are ‘wired’ to share. Sharing through working memory/consciousness is slow and limiting in the amount of information that be shared. And, indeed, the authors are not at all ‘unsettled’ but have a putative model of what is happening.

…the answer may lay in the particular mechanism that is thought to regulate the formation of the decision signal from the underlying visual signal. Several findings, both in humans and monkeys, indicate area MT (Middle Temporal Cortex) as a crucial node for motion processing. In recent years a growing body of data have disclosed also the role of area LIP (Lateral Intraparietal) in perceptual decisions involving motion stimuli. When a monkey is instructed to make an eye movement to report the direction of a random-dot kinematogram, neurons in LIP pick up sensory evidence, presumably from MT, and integrate it for some hundreds of ms until a decision bound is reached, and an oculomotor command issued. Importantly, the decision is reached even when the stimulus is still available or the response procrastinated, because LIP neurons exploit only the initial part of the discharge of MT neurons . Thus, monkey LIP seems to work as a device that implements a relatively quick rise-to- threshold mechanism for various types of visuo-motor responses when a perceptual decision is required. In this way, the decision is ready even though MT neurons are still processing the motion input. Note that structures other than LIP could be involved in motion discrimination when the response is verbal, perhaps as part of a circuit for more abstract decision-making, although the fact that we found a pattern of results virtually identical for saccades and verbal responses suggests that similar mechanisms may be at play.


They are careful to avoid leaving the impression that MT is the location of the motion conscious precept.

Clearly, a less simplistic view is that awareness is a large-scale, distributed property, in which case no single cortical structure may exhibit a macroscopic activation that co-varies on its own with the conscious percept.


One aspect that troubled me was the lack of discussion of the timing of consciousness ‘frames’. As awareness is not continuous but discretely updated, an awareness gradually increasing for times longer than the consciousness cycle should have been addressed. It would seem to affect the interpretation.

Gregori-Grgič, R., Balderi, M., & de’Sperati, C. (2011). Delayed Perceptual Awareness in Rapid Perceptual Decisions PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0017079

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