‘An attempt to trace illusions to their physical causes’: atmospheric mirages and the performance of their demystification in the 1820s and 1830s

This article suggests that, during the 1820s and 1830s, Britain experienced a mirage moment. A greater volume of material was published on the mirage in scientific journals, treatises, travel literature and novels during these two decades than had occurred before in British history. The phenomenon was examined at the confluence of discussions about the cultural importance of illusions, the nature of the eye and the imperial project to investigate the extra-European natural world. Explanations of the mirage were put forward by such scientists and explorers as Sir David Brewster, William Wollaston and General Sir James Abbott. Their demystification paralleled the performance of unmasking scientific and magical secrets in the gallery shows of London during the period. The practice of seeing involved in viewing unfathomable phenomena whilst simultaneously considering their rational basis underwrote these different circumstances. I use this unusual mode of visuality to explore the ways the mirage and other illusions were viewed and understood in the 1820s and 1830s. Ultimately, this paper argues that the mirage exhibited the fallibility of the eyes as a tool for veridical perception in a marvellous and striking way, with consequences for the perceived trustworthiness of ocular knowledge in the period.

Neural dynamics of perceptual inference and its reversal during imagery

After the presentation of a visual stimulus, neural processing cascades from low-level sensory areas to increasingly abstract representations in higher-level areas. It is often hypothesised that a reversal in neural processing underlies the generation of mental images as abstract representations are used to construct sensory representations in the absence of sensory input. According to predictive processing theories, such reversed processing also plays a central role in later stages of perception. Direct experimental evidence of reversals in neural information flow has been missing. Here, we used a combination of machine learning and magnetoencephalography to characterise neural dynamics in humans. We provide direct evidence for a reversal of the perceptual feed-forward cascade during imagery and show that, during perception, such reversals alternate with feed-forward processing in an 11 Hz oscillatory pattern. Together, these results show how common feedback processes support both veridical perception and mental imagery.

Sensible Over-Determination

I develop a view of perception that does justice to Price's intuition that all sensory experience acquaints us with sensible qualities like colour and shape. Contrary to the received opinion, I argue that we can respect this intuition while insisting that ordinary perception puts us in direct contact with the mind-independent world. In other words, Price's intuition is compatible with naïve realism. Both hallucinations and ordinary perceptions acquaint us with instances of the same kinds of sensible qualities. While the instances in hallucination are mind-dependent, those in veridical perception are not. The latter are ontologically over-determined—they have their existence guaranteed both in virtue of having a material bearer and in virtue of being perceived by a mind. Such over-determined instances are mind-independent—they can continue to exist unperceived, because, in addition to the minds that perceive them, their existence is guaranteed by the material objects that are their bearers.

How non-veridical perception drives actions in healthy humans: evidence from synaesthesia

We continually perform actions that are driven by our perception and it is a commonly held view that only objectively perceived changes within the ‘real’ world affect behaviour. Exceptions are generally only made for mental health disorders associated with delusions and hallucinations where behaviour may be triggered by the experience of objectively non-existent percepts. Here, we demonstrate, using synaesthesia as a model condition (in N = 19 grapheme-colour synaesthetes), how objectively non-existent (i.e. non-veridical) but still non-pathological perceptions affect actions in healthy humans. Using electroencephalography, we determine whether early-stage perceptual processes (reflected by P1 and N1 event-related potential (ERP) components), or late-stage-integration processes (reflected by N2 component), underlie the effects of non-veridical perceptions on action control. ERP analysis suggests that even though the examined peculiarities and experimental variations are perceptual in nature, it is not early-stage perceptual processes, but rather higher-order executive control processes linking perceptions to the appropriate motor response underlying this effect. Source localization analysis implicates activation within medial frontal cortices in the effect of how irrelevant non-veridical perceptions modulate behaviour. Our results challenge common conceptions about the determinants of human behaviour but can be explained by well-established theoretical frameworks detailing the link between perception and action. This article is part of a discussion meeting issue ‘Bridging senses: novel insights from synaesthesia’.

Color in a Material World: Margaret Cavendish against the Early Modern Mechanists

Consider the distinctive qualitative property grass visually appears to have when it visually appears to be green. This property is an example of what I call sensuous color. Whereas early modern mechanists typically argue that bodies are not sensuously colored, Margaret Cavendish (1623–73) disagrees. In cases of veridical perception, she holds that grass is green in precisely the way it visually appears to be. In defense of her realist approach to sensuous colors, Cavendish argues that (i) it is impossible to conceive of colorless bodies, (ii) the very possibility of color experience requires that bodies are sensuously colored, and (iii) the attribution of sensuous colors to bodies provides the best explanation of color constancy. Although some passages might suggest that Cavendish endorses a reductive account of sensuous color, according to which sensuous color reduces to a body's microscopic surface texture (or some other mechanistically respectable property), I argue that she accepts a nonreductive account, on which sensuous color is not thus reducible.

Classification of Disjunctivism about the Phenomenology of Visual Experience

This paper proposes a classificatory framework for disjunctivism about the phenomenology of visual perceptual experience. Disjunctivism of this sort is typically divided into positive and negative disjunctivism. This distinction successfully reflects the disagreement amongst disjunctivists regarding the explanatory status of the introspective indiscriminability of veridical perception and hallucination. However, it is unsatisfactory in two respects. First, it cannot accommodate eliminativism about the phenomenology of hallucination. Second, the class of positive disjunctivism is too coarse-grained to provide an informative overview of the current dialectical landscape. Given this, I propose a classificatory framework which preserves the positive-negative distinction, but which also includes the distinction between eliminativism and non-eliminativism, as well as a distinction between two subclasses of positive disjunctivism. In describing each class in detail, I specify who takes up each position in the existing literature, and demonstrate that this classificatory framework can disambiguate some existing disjunctivist views.

How non-veridical perception drives actions in healthy humans - Evidence from Synaesthesia

We continually perform actions driven by our perception and it is commonly held that only objectively perceived changes within the ‘real’ world affect behaviour. Exceptions are usually only made for clinical conditions associated with hallucinations, where objectively non-existent percepts can influence behavior. Using synaesthesia as a model condition, we show that even in healthy populations irrelevant non-veridical precepts exert an effect on action. By non-veridical we refer to stimulus dimensions that are only subjectively perceived to be there. Applying electrophysiological (EEG) methods, we show that although these examined peculiarities are perceptual in nature, not primarily perceptual processes underlie the effects of irrelevant non-veridical perceptions on actions. Rather, high-order processes linking perceptions and motor control in medial frontal cortices reflect the underlying mechanism how irrelevant non-veridical perceptions modulate behaviour. Our results challenge assumptions about the determinants of healthy human behaviour but can be embedded within existing frameworks detailing perception action interactions.

Disjunctivism

The disjunctivist claims that the mental states involved in a case of successful – "veridical" – perception of an object differ from those involved in a hallucinatory experience of such an object, even in those cases in which the two experiences are indiscriminable for their subject. Among its supporters, there tend to be two main motivations for endorsing disjunctivism: because it is necessary if we are to hold that, in cases of successful perception, worldly objects and properties are literally constituents of our experiences, and because it offers us a way of responding to the challenge of skepticism. Among other things, its opponents argue that it is inconsistent with both empirical findings in, and the underlying commitments of, the psychology of vision, and challenge the disjunctivist to provide explanations of hallucination and illusion that explain how such states can be indiscriminable from veridical perceptions without positing some common factor – such as a common conscious / experiential core – that is shared by the different cases.