Why the "stimulus-error" did not go away.

Psychologists in the early years of the discipline were much concerned with the stimulus-error. Roughly, this is the problem encountered in introspective experiments when subjects are liable to frame their perceptual reports in terms of what they know of the stimulus, instead of just drawing on their perceptual experiences as they are supposedly felt. "Introspectionist" psychologist E. B. Titchener and his student E. G. Boring both argued in the early 20th century that the stimulus-error is a serious methodological pit-fall. While many of the theoretical suppositions motivating Titchener and Boring have been unfashionable since the rise of behaviourism, the stimulus-error brings our attention to one matter of perennial importance to psychophysics and the psychology of perception. This is the fact that subjects are liable to give different kinds of perceptual reports in response to the same stimulus. I discuss attempts to control for variable reports in recent experimental work on colour and lightness constancy, and the disputes that have arisen over which kinds of reports are legitimate. Some contemporary psychologists do warn us against a stimulus-error, even though they do not use this terminology. I argue that concern over the stimulus-error is diagnostic of psychologists' deep theoretical commitments, such as their conception of sensation, or their demarcation of perception from cognition. I conclude by discussing the relevance of this debate to current philosophy of perception.

Synthesis of contraries: Hughlings Jackson on sensory-motor representation in the brain.

This paper examines the concept of representation in the brain which occurs in the writings of the neurologist John Hughlings Jackson (1835-1911). Jackson was immersed in Victorian physiological psychology, a hybrid of British associationism and a reflex theory of the operation of the nervous system. Furthermore, Jackson was deeply influenced by Herbert Spencer, and I argue that Spencer's progressivist evolutionary ideas are in tension with the more mechanistic approach of the reflex theory. I also discuss Jackson's legacy in the 20th century and the longstanding debate about localisation of function in the brain.

The role of perceptual learning on modality-specific visual attentional effects.

Morrone et al. [Morrone, M. C., Denti, V., & Spinelli, D. (2002). Color and luminance contrasts attract independent attention. Current Biology, 12, 1134-1137] reported that the detrimental effect on contrast discrimination thresholds of performing a concomitant task is modality specific: performing a secondary luminance task has no effect on colour contrast thresholds, and vice versa. Here we confirm this result with a novel task involving learning of spatial position, and go on to show that it is not specific to the cardinal colour axes: secondary tasks with red-green stimuli impede performance on a blue-yellow task and vice versa. We further show that the attentional effect can be abolished with continued training over 2-4 training days (2-20 training sessions), and that the effect of learning is transferable to new target positions. Given the finding of transference, we discuss the possibility that V4 is a site of plasticity for both stimulus types, and that the separation is due to a luminance-colour separation within this cortical area.

Hughlings Jackson and the "doctrine of concomitance": mind-brain theorising between metaphysics and the clinic.

John Hughlings Jackson (1835-1911) is a major figure at the origins of neurology and neuroscience in Britain. Alongside his contributions to clinical medicine, he left a large corpus of writing on localisation of function in the nervous system and other theoretical topics. In this paper I focus on Jackson's "doctrine of concomitance"-his parallelist theory of the mind-brain relationship. I argue that the doctrine can be given both an ontological and a causal interpretation, and that the causal aspect of the doctrine is especially significant for Jackson and his contemporaries. I interpret Jackson's engagement with the metaphysics of mind as an instance of what I call meta-science-the deployment by scientists of metaphysical positions and arguments which help streamline empirical investigations by bracketing off unanswerable questions and focussing attention on matters amenable to the current tools of experimental research.

Accuracy of identification of grating contrast by human observers: Bayesian models of V1 contrast processing show correspondence between discrimination and identification performance.

This paper presents the results of a contrast identification study, where accuracy in identification is quantified as mutual information between stimulus contrast and observer's response. The stimulus was a set of 2-8 gratings, spanning the range of visible contrasts. Gratings from the set were presented individually for 500 ms, and the observer had to respond by giving the number label corresponding to the contrast of the grating presented. Mutual information increased with set size up to a maximum of around 2.35 bits, i.e., only 5 clearly identifiable contrasts. Set sizes greater than 5 showed a plateau or decline in performance. These data were well fit by Bayesian models of V1 contrast coding, with the parameters obtained by fitting the contrast discrimination results of Chirimuuta and Tolhurst [Chirimuuta, M., & Tolhurst, D. J. (2005). Does a Bayesian model of V1 contrast coding offer a neurophysiological account of human contrast discrimination? Vision Research].

Does a Bayesian model of V1 contrast coding offer a neurophysiological account of human contrast discrimination?

The dipper effect for contrast discrimination provides strong evidence that the underlying neural response is accelerating at low contrasts and saturating at high contrasts. The contrast-response functions of V1 neurons do have this sigmoidal shape, but individual neurons do not generally have a dynamic range wide enough to account for the dipper effect. This paper presents a Bayesian model of neurons in monkey V1, whose contrast-response function is described by a modified Naka-Rushton with multiplicative noise. It is shown that a model of groups of twelve or more neurons gives a reasonable explanation of the psychophysical data of two observers, but there is a large systematic error which is apparently due to the shape of the distribution of the monkey's sensitivity parameter, c50. A further model provides a better fit to the data by sacrificing strict adherence to V1 neuronal parameters and, instead using an arbitrary bimodal c50 distribution, perhaps reflecting differences between M- and P-cells.

Coding of the contrasts in natural images by populations of neurons in primary visual cortex (V1).

It is possible to discriminate between grating contrasts over a 300-fold contrast range, whereas V1 neurons have very limited dynamic ranges. Using populations of model neurons with contrast-response parameters taken from electrophysiological studies (cat and macaque), we investigated ways of combining responses to code contrast over the full range. One model implemented a pooling rule that retained information about individual response patterns. The second summed responses indiscriminately. We measured accuracy of contrast identification over a wide range of contrasts and found the first model to be more accurate; the mutual information between actual and estimated contrast was also greatest for this model. The accuracy peak for the population of cat neurons coincided with the peak of the distribution of contrasts in natural images, suggesting an ecological match. Macaque neurons seem better able to code contrasts that are slightly higher on average than those found in the natural environment.