Vocal response times to real and imagined stimuli in spatial neglect: A group study and single-case report

Colors in the mind's eye

The famous "Piazza del Duomo" paper, published in Cortex in 1978, inspired a considerable amount of research on visual mental imagery in brain-damaged patients. As a consequence, single-case reports featuring dissociations between perceptual and imagery abilities challenged the prevailing model of visual mental imagery. Here we focus on mental imagery for colors. A case study published in Cortex showed perfectly preserved color imagery in a patient with acquired achromatopsia after bilateral lesions at the borders between the occipital and temporal cortex. Subsequent neuroimaging findings in healthy participants extended and specified this result; color imagery elicited activation in both a domain-general region located in the left fusiform gyrus and the anterior color-biased patch within the ventral temporal cortex, but not in more posterior color-biased patches. Detailed studies of individual neurological patients, as those often published in Cortex, are still critical to inspire and constrain neurocognitive research and its theoretical models.

Hemispheric asymmetries in visual mental imagery

Visual mental imagery is the faculty whereby we can "visualize" objects that are not in our line of sight. Longstanding evidence dating back over thirty years has shown that unilateral brain lesions, especially in the left temporal lobe, can impair aspects of this ability. Yet, there is currently no attempt to identify analogies between these neuropsychological findings of hemispheric asymmetry and those from other neuroscientific approaches. Here, we present a critical review of the available literature on the hemispheric laterality of visual mental imagery, by looking at cross-method patterns of evidence in the domains of lesion neuropsychology, neuroimaging, and direct cortical stimulation. Results can be summarized under three main axes. First, frontoparietal networks in both hemispheres appear to be associated with visual mental imagery. Second, lateralization patterns emerge in the temporal lobes, with the left inferior temporal lobe being the most common finding in the literature for endogenously generated images, especially, but not exclusively, when orthographic material is used to ignite imagery. Third, an opposite pattern of hemispheric laterality emerges when visual mental images are induced by exogenous stimulation; direct cortical electrical stimulation tends to produce visual imagery experiences predominantly when applied to the right temporal lobe. These patterns of hemispheric asymmetry are difficult to reconcile with the dominant model of visual mental imagery, which emphasizes the implication of early sensory cortices. They suggest instead that visual mental imagery relies on large-scale brain networks, with a crucial participation of high-level visual regions in the temporal lobes.

Evidence for a common mechanism of spatial attention and visual awareness: Towards construct validity of pseudoneglect

Present knowledge of attention and awareness centres on deficits in patients with right brain damage who show severe forms of inattention to the left, called spatial neglect. Yet the functions that are lost in neglect are poorly understood. In healthy people, they might produce “pseudoneglect”—subtle biases to the left found in various tests that could complement the leftward deficits in neglect. But pseudoneglect measures are poorly correlated. Thus, it is unclear whether they reflect anything but distinct surface features of the tests. To probe for a common mechanism, here we asked whether visual noise, known to increase leftward biases in the grating-scales task, has comparable effects on other measures of pseudoneglect. We measured biases using three perceptual tasks that require judgments about size (landmark task), luminance (greyscales task) and spatial frequency (grating-scales task), as well as two visual search tasks that permitted serial and parallel search or parallel search alone. In each task, we randomly selected pixels of the stimuli and set them to random luminance values, much like a poor TV signal. We found that participants biased their perceptual judgments more to the left with increasing levels of noise, regardless of task. Also, noise amplified the difference between long and short lines in the landmark task. In contrast, biases during visual searches were not influenced by noise. Our data provide crucial evidence that different measures of perceptual pseudoneglect, but not exploratory pseudoneglect, share a common mechanism. It can be speculated that this common mechanism feeds into specific, right-dominant processes of global awareness involved in the integration of visual information across the two hemispheres.

Botallo's error, or the quandaries of the universality assumption

One of the founding principles of human cognitive neuroscience is the so-called universality assumption, the postulate that neurocognitive mechanisms do not show major differences among individuals. Without negating the importance of the universality assumption for the development of cognitive neuroscience, or the importance of single-case studies, here we aim at stressing the potential dangers of interpreting the pattern of performance of single patients as conclusive evidence concerning the architecture of the intact neurocognitive system. We take example from the case of Leonardo Botallo, an Italian surgeon of the Renaissance period, who claimed to have discovered a new anatomical structure of the adult human heart. Unfortunately, Botallo's discovery was erroneous, because what he saw in the few samples he examined was in fact the anomalous persistence of a fetal structure. Botallo's error is a reminder of the necessity to always strive for replication, despite the major hindrance of a publication system heavily biased towards novelty. In the present paper, we briefly discuss variations and anomalies in human brain anatomy and introduce the issue of variability in cognitive neuroscience. We then review some examples of the impact on cognition of individual variations in (1) brain structure, (2) brain functional organization and (3) brain damage. We finally discuss the importance and limits of single case studies in the neuroimaging era, outline potential ways to deal with individual variability, and draw some general conclusions.

Representational pseudoneglect: a review

Pseudoneglect, the tendency to be biased towards the left-hand side of space, is a robust and consistent behavioural observation best demonstrated on the task of visuospatial line bisection, where participants are asked to centrally bisect visually presented horizontal lines at the perceived centre. A number of studies have revealed that a representational form of pseudoneglect exists, occurring when participants are asked to either mentally represent a stimulus or explore a stimulus using touch in the complete absence of direct visuospatial processing. Despite the growing number of studies that have demonstrated representational pseudoneglect there exists no current and comprehensive review of these findings and no discussion of a theoretical framework into which these findings may fall. An important gap in the current representational pseudoneglect literature is a discussion of the developmental trajectory of the bias. The focus of the current review is to outline studies that have observed representational pseudoneglect in healthy participants, consider a theoretical framework for these observations, and address the impact of lifespan factors such as cognitive ageing on the phenomenon.

Left-ear-driven representational pseudoneglect for mentally represented real-word scenes created from aural-verbal description

The current research explored pseudoneglect for the mental representation of real-world scenes generated from aural-verbal description in the complete absence of direct visual processing. Healthy participants listened binaurally or monaurally to aural-verbal descriptions of novel real-world scenes with familiar landmarks (e.g., 'shop', 'cafe', 'school') to be imagined on the left- or right-hand side. Participants were asked to mentally represent the street scene using a visuospatial template though it was up to participants how they mentally represented each individual landmark within the street (i.e., in terms of colour and size). There were two main tasks: a relative judgement task (which side of the street contains the most landmarks?) and a recall task (recall the landmarks on the left vs. right side of the street). When stimuli were presented monaurally to the left ear (favouring the activation of the right hemisphere) participants demonstrated representational pseudoneglect and showed a bias towards responding that there were more landmarks on the left compared to the right. However, this did not lead to enhanced recall for left side landmarks. When stimuli were presented binaurally or monaurally to the right ear, there was no evidence of representational pseudoneglect for the relative judgement or recall task. The current study discusses how the use of monaural presentation may boost right hemisphere activation in aural-verbal experimental paradigms designed to explore representational pseudoneglect.

Brain networks of visuospatial attention and their disruption in visual neglect

Visual neglect is a multi-component syndrome including prominent attentional disorders. Research on the functional mechanisms of neglect is now moving from the description of dissociations in patients' performance to the identification of the possible component deficits and of their interaction with compensatory strategies. In recent years, the dissection of attentional deficits in neglect has progressed in parallel with increasing comprehension of the anatomy and function of large-scale brain networks implicated in attentional processes. This review focuses on the anatomy and putative functions of attentional circuits in the brain, mainly subserved by fronto-parietal networks, with a peculiar although not yet completely elucidated role for the right hemisphere. Recent results are discussed concerning the influence of a non-spatial attentional function, phasic alertness, on conscious perception in normal participants and on conflict resolution in neglect patients. The rapid rate of expansion of our knowledge of these systems raises hopes for the development of effective strategies to improve the functioning of the attentional networks in brain-damaged patients.

Representational Pseudoneglect in an Auditory-Driven Spatial Working Memory Task

Two experiments explored lateralized biases in mental representations of matrix patterns formed from aural verbal descriptions. Healthy participants listened, either monaurally or binaurally, to verbal descriptions of 6 by 3 matrix patterns and were asked to form a mental representation of each pattern. In Experiment 1, participants were asked to judge which half of the matrix, left or right, contained more filled cells and to rate the certainty of their judgement. Participants tended to judge that the left side was fuller than the right and showed significantly greater certainty when judging patterns that were fuller on the left. This tendency was particularly strong for left-ear presentation. In Experiment 2, participants conducted the same task as that in Experiment 1 but were also asked to recall the pattern for the side judged as fuller. Participants were again more certain in judging patterns that were fuller on the left—particularly for left-ear presentation—but were no more accurate in remembering the details from the left. These results suggest that the left side of the mental representation was represented more saliently but it was not remembered more accurately. We refer to this lateralized bias as “representational pseudoneglect”. Results are discussed in terms of theories of visuospatial working memory.

Visual mental imagery: what the head's eye tells the mind's eye

The demonstration of an implication of attentional/eye gaze systems in visual mental imagery might help to understand why some patients with visual neglect, who suffer from severe attentional deficits, also show neglect for mental images. When normal participants generate mental images of previously explored visual scenes, their oculomotor behavior resembles that used during visual exploration. However, this could be a case of encoding specificity, whereby the probability of retrieving an event increases if some information encoded with the event (in this case its spatial location) is present at retrieval. In the present study, normal participants were invited to conjure up a mental image of the map of France and to say whether auditorily presented towns or regions were situated left or right of Paris. A perceptual version of the task was administered after the imaginal condition. Thus, in the imaginal condition participants had to retrieve information from long-term memory. Vocal response times and, unbeknownst to participants, also eye movements were recorded. Participants tended to produce similar eye movements on the imaginal and on the perceptual conditions of the task. We concluded that some mechanisms involved in spontaneous oculomotor behavior may be shared in exploration of visuospatial mental images. Deficits of these common processes participating in the oculomotor exploration might contribute to imaginal neglect.