A conscious route to unconscious vision

Damage to the primary visual cortex can leave subjects with unconscious residual vision, or 'blindsight'. New research suggests that 'top-down' modulation by intact conscious visual processes can improve performance in the impaired visual domain, even though that domain still remains quite inaccessible to consciousness.     

The autonomy of the visual systems and the modularity of conscious vision.

Anatomical and physiological evidence shows that the primate visual brain consists of many distributed processing systems, acting in parallel. Psychophysical studies show that the activity in each of the parallel systems reaches its perceptual end-point at a different time, thus leading to a perceptual asynchrony in vision. This, together with clinical and human imaging evidence, suggests strongly that the processing systems are also perceptual systems and that the different processing-perceptual systems can act more or less autonomously. Moreover, activity in each can have a conscious correlate without necessarily involving activity in other visual systems. This leads us to conclude not only that visual consciousness is itself modular, reflecting the basic modular organization of the visual brain, but that the binding of cellular activity in the processing-perceptual systems is more properly thought of as a binding of the consciousnesses generated by each of them. It is this binding that gives us our integrated image of the visual world.     

Contributions of magno- and parvocellular channels to conscious and non-conscious vision

The dorsal and ventral cortical pathways, driven predominantly by magnocellular (M) and parvocellular (P) inputs, respectively, assume leading roles in models of visual information processing. Although in prior proposals, the dorsal and ventral pathways support non-conscious and conscious vision, respectively, recent modelling and empirical developments indicate that each pathway plays important roles in both non-conscious and conscious vision. In these models, the ventral P-pathway consists of one subpathway processing an object''s contour features, e.g. curvature, the other processing its surface attributes, e.g. colour. Masked priming studies have shown that feed-forward activity in the ventral P-pathway on its own supports non-conscious processing of contour and surface features. The dorsal M-pathway activity contributes directly to conscious vision of motion and indirectly to object vision by projecting to prefrontal cortex, which in turn injects top-down neural activity into the ventral P-pathway and there ‘ignites’ feed-forward–re-entrant loops deemed necessary for conscious vision. Moreover, an object''s shape or contour remains invisible without the prior conscious registration of its surface properties, which for that reason are taken to comprise fundamental visual qualia. Besides suggesting avenues for future research, these developments bear on several recent and past philosophical issues.     

Single unit approaches to human vision and memory

Research on the visual system focuses on using electrophysiology, pharmacology and other invasive tools in animal models. Non-invasive tools such as scalp electroencephalography and imaging allow examining humans but show a much lower spatial and/or temporal resolution. Under special clinical conditions, it is possible to monitor single-unit activity in humans when invasive procedures are required due to particular pathological conditions including epilepsy and Parkinson's disease. We review our knowledge about the visual system and visual memories in the human brain at the single neuron level. The properties of the human brain seem to be broadly compatible with the knowledge derived from animal models. The possibility of examining high-resolution brain activity in conscious human subjects allows investigators to ask novel questions that are challenging to address in animal models.     

Photic resetting of the human circadian pacemaker in the absence of conscious vision

Ocular light exposure patterns are the primary stimuli for entraining the human circadian system to the local 24-h day. Many totally blind persons cannot use these stimuli and, therefore, have circadian rhythms that are not entrained. However, a few otherwise totally blind persons retain the ability to suppress plasma melatonin concentrations after ocular light exposure, probably using a neural pathway that includes the site of the human circadian pacemaker, suggesting that light information is reaching this site. To test definitively whether ocular light exposure could affect the circadian pacemaker of some blind persons and whether melatonin suppression in response to bright light correlates with light-induced phase shifts of thecircadian system, the authorsperformed experiments with 5 totally blind volunteers using a protocol known to induce phase shifts of the circadian pacemaker in sighted individuals. In the 2 blind individuals who maintained light-induced melatonin suppression, the circadian system was shifted by appropriately timed bright-light stimuli. These data demonstrate that light can affect the circadian pacemaker of some totally blind individuals--either by altering the phase of the circadian pacemaker or by affecting its amplitude. They are consistent with data from animal studies demonstrating that there are different neural pathways and retinal cells that relay photic information to the brain: one for conscious light perception and the other for non-image-forming functions.     

Stereo vision and isoluminance.

Recent experiments have shown that stereo depth is given by fusion of illusory ('cognitive') contours. They occur across quite large gaps in figures, when these gaps are unlikely and form the shape of a probable (nearer) masking object or masking feature. Implications are that: (a) clearly defined contours and regions of brightness difference can be produced as postulates from sensory evidence, which may be surprising absence of stimulation; (b) each eye-system can derive its own postulates, or hypotheses, which (c) can be combined to give stereo vision. It has been shown that random-dot stereo depth does not occur when there is colour contrast but no brightness difference between the dots and their background. This we have confirmed by using a new technique for producing isoluminant pictures, of any complexity, with exact registration for any two colours. With this technique, we find large displacements of narrow borders bounding regions that are shifted across isoluminance. These displacements, which are clearly seen as movements, occur with or without colour differences. The direction of shift depends on whether the narrow border is light or dark. It is found that these dramatic shifts do not - when produced in opposite directions to the two eyes and fused - produce stereo depth. It is concluded, following Julesz's paradigm, that these contour displacements have their neural orgin not retinally, but after stereo fusion. Experiments combining the 'cognitive contours' stereo depth with isoluminance are described.     

Single units activities in ventral posterior and posterior group thalamic nuclei during nociceptive and non nociceptive stimulations in the cat.

The purpose of this study was to define, in hyperventilated and unanesthetized cats, the role of the posterior thalamic nuclei in pain mechanisms. Unit activities of these structures were compared to those of the ventro-posterior nucleus during non-noxious (touch, brushing) and noxious stimulations (pinches and intra-arterial injections of bradykinin into the limbs). 135 cells with somatic inputs and clear peripheral excitatory receptive field were studied. The cells driven by noxious stimulations were located in the posterior group nuclei as anatomically defined by Rinvik. These units, preferentially excited from contralateral receptive fields, were localized in POm, POl, suprageniculate nuclei, the magnocellular division of the medial geniculate body (Mgmc) and the ventral part of the lateral posterior nucleus. At this level two groups of units were found: those driven only by noxious stimulations and those driven by both noxious and non-noxious stimulations. On contrast, cells recorded at the levels of the VPm and VPl were not activated by noxious stimuli. These results emphasize the role of the posterior thalamic nuclei in pain processing.     

Single auditory evoked responses of the temporal and associative regions of the cerebral cortex in conscious cats

In awake cats single realizations of acoustic evoked responses (AER) from temporal, parietal and frontal cortex were registered and compared with averaged responses obtained by means of optic superposition of the same realizations. It is shown that the composition of these different realizations considerably varies due to inconstancy of manifestation of each component. The fact that the preceding component falls out does not exclude manifestation of the following one, which proves the functional independence of the mechanisms producing each component. The simultaneous registration of single realizations of AERs of different cortical areas shows that the reactions of frontal and parietal areas are independent of AERs of the temporal cortex.     

Coronary care units.

In the past 10 years with the development of improved methods, particularly radioisotope techniques, it has been demonstrated that a number of patients with gastrointestinal disease and depletion of plasma proteins become hypoproteinemic because of actual leakage of albumin and other plasma proteins into the lumen of the gastrointestinal tract. The site of protein leakage is variable depending on the underlying pathological state but the loss of protein-containing lymph through the gastrointestinal lymphatic channels seems to be the major mechanism for hypoproteinemia.It has become apparent that there exists a normal mechanism for secretion of plasma proteins into the gastrointestinal tract as part of the overall metabolism of the plasma proteins. When the process is exaggerated so that resynthesis of plasma protein cannot keep pace with its degradation, sometimes severe hypoproteinemia is the result. Such a pathological process has now been described in approximately 40 disease states. A review of all the techniques which can demonstrate gastroenteric protein loss reveals that there are no widely available quantitative tests but that accurate quantitation is not necessary for the diagnosis of protein losing gastroenteropathy.     

Knowledge-based vision and simple visual machines.

The vast majority of work in machine vision emphasizes the representation of perceived objects and events: it is these internal representations that incorporate the ''knowledge'' in knowledge-based vision or form the ''models'' in model-based vision. In this paper, we discuss simple machine vision systems developed by artificial evolution rather than traditional engineering design techniques, and note that the task of identifying internal representations within such systems is made difficult by the lack of an operational definition of representation at the causal mechanistic level. Consequently, we question the nature and indeed the existence of representations posited to be used within natural vision systems (i.e. animals). We conclude that representations argued for on a priori grounds by external observers of a particular vision system may well be illusory, and are at best place-holders for yet-to-be-identified causal mechanistic interactions. That is, applying the knowledge-based vision approach in the understanding of evolved systems (machines or animals) may well lead to theories and models that are internally consistent, computationally plausible, and entirely wrong.     

Binocular single vision and perceptual processing.

Stimuli with small binocular disparities are seen as single, despite their differing visual directions for the two eyes. Such stimuli also yield stereopsis, but stereopsis and single vision can be dissociated. The occurrence of binocular single vision depends not only on the disparities of individual stimulus elements, but also on the geometrical relation of different parts of the pattern presented to each eye. A pair of vertical bars with opposite binocular disparities is seen as single if the pair is moderately widely spaced but not if it is narrow. Vertical alignment and identity in length of such bars also increase the occurrence of double vision. It is argued that these effects reflect the extraction of features of the monocular patterns, with these detected monocular features determining the binocular percept. Single and double vision of bars differing in orientation can be similarly analysed. The occurrence of relatively elaborate processing of monocular signals does not exclude the possibility that binocular interaction can occur between signals that have not been so processed. Multiple sites or types of binocular interaction are likely.     

Evolution of color vision.

Color vision is achieved by comparing the inputs from retinal photoreceptor neurons that differ in their wavelength sensitivity. Recent studies have elucidated the distribution and phylogeny of opsins, the family of light-sensitive molecules involved in this process. Interesting new findings suggest that animals have evolved a strategy to achieve specific sensitivity through the mutually exclusive expression of different opsin genes in photoreceptors.