Binocular rivalry alternations and their relation to visual adaptation

When different stimuli are presented dichoptically, perception alternates between the two in a stochastic manner. After a long-lasting and rigorous debate, there is growing consensus that this phenomenon, known as binocular rivalry (BR), is the result of a dynamic competition occurring at multiple levels of the visual hierarchy. The role of low- and high-level adaptation mechanisms in controlling these perceptual alternations has been a key issue in the rivalry literature. Both types of adaptation are dispersed throughout the visual system and have an equally influential, or even causal, role in determining perception. Such an explanation of BR is also in accordance with the relationship between the latter and attention. However, an overall explanation of this intriguing perceptual phenomenon needs to also include noise as an equally fundamental process involved in the stochastic resonance of perceptual bistability

The effects of categorical and linguistic adaptation on binocular rivalry initial dominance

Binocular rivalry (BR) is a phenomenon in which visual perception alternates between two different monocular stimuli. There has been a long debate regarding its nature, with a special emphasis on whether low- or high-level mechanisms are involved. Prior adaptation to one of the two monocular stimuli is known to affect initial dominance in the subsequent dichoptic presentation. In the present work, we have used three different types of adaptation in order to investigate how each one affects initial dominance during BR. In the first adaptation type, adapting to a stimulus identical to the one used during rivalry has led to its consequent suppression, verifying previous findings. The binocular presentation which we have used excludes the possibility of eye-adaptation, suggesting that it is the specific stimulus that the brain adapts to. In the second adaptation type, we find suppression effects following adaptation to stimuli belonging to the same category (face or house) but are different from the specific ones used in the following BR presentation. In the final adaptation type, in which the words “face” or “house” are used as adaptors, no statistically significant effect was found. These results suggest that perceptual selection can be directly influenced by the prior presentation of visual stimuli different to the ones used during BR, and thus support a higher-level, cognitive influence on the latter

Brain activation and the locus of visual awareness

A major problem in visual neuroscience is to distinguish neuronal activity which is directly related to the conscious percept. The word “directly” is used here as opposed to an indirect relationship, as is for example the case with activity in the retina, which is produced by a stimulus in the outside world and will eventually lead to the perception of this stimulus. As for the word “related”, it is used to mean activity which creates the perceptual experience or, even more extremely, activity that is the perceptual experience. The distinction between the two (is vs. creates) is not straightforward, although there might be some differences between them. Philosophers would argue that they have a different phenomenology, the percept existing only for the perceiving person, whereas the underlying neuronal activation exists for all to observe. One could go on to argue that it is actually not the neuronal activation that “everybody” observes, but each one observes his own percept of it, which is also unique and subjective. Still, the content of this percept and the one of the original stimulus are quite different. The purpose of the present review is not to dig deep into such philosophical issues, but rather to give an overview of neuroscientific approaches trying to locate the neural correlate of conscious perception

The machine behind the stage: a neurobiological approach towards theoretical issues of sensory perception

The purpose of the present article is to try and give a brief, scientific perspective on several issues raised in the Philosophy of Perception literature. This perspective gives a central role to the brain mechanisms that underlie perception: a percept is something that emerges when the brain is activated in a certain way and thus all perceptual experiences (whether veridical, illusory, or hallucinatory) have a common cause behind them, namely a given brain-activation pattern. What distinguishes between different cases of perception is what has caused this activation pattern, i.e. something very separate and very different from the perceptual experience itself. It is argued that separating the perceptual event from its hypothetical content, a direct consequence of the way everyday language is structured, creates unnecessary ontological complications regarding the nature of the hypothetical ‘object’ of perception. A clear distinction between the physical properties of the real world on the one hand (e.g. wavelength reflectance), and the psychological properties of perceptual experiences on the other (e.g. colour) is clearly made. Finally, although perception is a way of acquiring knowledge/information about the world, this acquisition should be considered as a cognitive process which is separate to and follows perception. Therefore, the latter should remain neutral with respect to the ‘correctness’ or ‘truth’ of the knowledge acquired

Brain strategies of colour perception

This thesis deals with the problem of colour vision. Part I addresses the problem of specialisation for colour, with specific reference to area V2, interposed between V1, where cells selective for the wavelength of the stimulus are found, and V4, where cells selective for the colour of the stimulus are present. The responses of cells in V2 were studied for their selectivity to the wavelength, orientation, and direction of motion of the stimulus. Cells with particular selectivities were found in clusters which were directly related to the metabolic (cytochrome oxidase, CO) architecture of V2. Orientation selective cells were mostly found in the inter and thick stripes, direction selective cells (although generally rare) in the thick stripes, and wavelength selective cells in the thin stripes. Only very few cells were selective for more than one attribute. By studying the distribution of the receptive fields of the cells in the three different stripe compartments, it became clear that the visual field is independently mapped in each set of CO stripes. The visual field is thus separately mapped for each of the different attributes of vision in V2. Wavelength selective cells in V2 were tested for colour constancy. None was found to exhibit this property, but some were selective not only to the wavelength composition of the stimulus but also to the change in the relative amount of a particular wavelength. Part II addresses the general problem of how the brain binds the different visual attributes which are processed separately, and investigates the possibility that colour, motion, form, and stereoscopic depth are not perceived in precise temporal registration with one another. It describes the use of a psychophysical method to investigate the differences in time required to perceive colour and motion. By using a stimulus which rapidly and continuously changes in colour and direction of motion, it shows that subjects bind colour and motion incorrectly because colour is perceived before motion. The idea of functional segregation not only at the level of processing but also at the level of perception is introduced