Biomedical subjects

Parashkev Nachev

Publications and source records attributed to Parashkev Nachev.

9 recordsLinked to original sources

PubMed2006

Space and the parietal cortex.

Current views of the parietal cortex have difficulty accommodating the human inferior parietal lobe (IPL) within a simple dorsal versus ventral stream dichotomy. In humans, lesions of the right IPL often lead to syndromes such as hemispatial neglect that are seemingly in accord with the proposal that this region has a crucial role in spatial processing. However, recent imaging and lesion studies have revealed that inferior parietal regions have non-spatial functions, such as in sustaining attention, detecting salient events embedded in a sequence of events and controlling attention over time. Here, we review these findings and show that spatial processes and the visual guidance of action are only part of the repertoire of parietal functions. Although sub-regions in the human superior parietal lobe and intraparietal sulcus contribute to vision-for-action and spatial functions, more inferior parietal regions have distinctly non-spatial attributes that are neither conventionally 'dorsal' nor conventionally 'ventral' in nature.

Attention↗

PubMed2006

Role of the human supplementary eye field in the control of saccadic eye movements.

The precise function of the supplementary eye field (SEF) is poorly understood. Although electrophysiological and functional imaging studies are important for demonstrating when SEF neurones are active, lesion studies are critical to establish the functions for which the SEF is essential. Here we report a series of investigations performed on an extremely rare individual with a highly focal lesion of the medial frontal cortex. High-resolution structural imaging demonstrated that his lesion was confined to the region of the left paracentral sulcus, the anatomical locus of the SEF. Behavioural testing revealed that the patient was significantly impaired when required to switch between anti- and pro-saccades, when there were conflicting rules governing stimulus-response mappings for saccades. Similarly, the results of an arbitrary stimulus-response associative learning task demonstrated that he was impaired when required to select the appropriate saccade from conflicting eye movement responses, but not for limb movements on an analogous manual task. When making memory-guided saccadic sequences, the patient demonstrated hypometria, like patients with Parkinson's disease, but had no significant difficulties in reproducing the order of saccades correctly on a task that emphasized accuracy with a wide temporal segregation between responses. These findings are consistent with the hypothesis that the SEF plays a key role in implementing control when there is conflict between several, ongoing competing saccadic responses, but not when eye movements need to be made accurately in sequence.

Association Learning↗

PubMed2006

Attentional modulation of sensorimotor processes in the absence of perceptual awareness.

Attention modulates visual perception and is generally considered inextricably linked with conscious awareness: we become aware of stimuli as we attend to them, and we attend to stimuli as we become aware of them. Recent evidence suggests that attention can also modulate the effects of stimuli that remain invisible, and a natural explanation is that attention enhances weak perceptual representations, bringing them closer to conscious threshold even if they do not reach that threshold. However, there is also the possibility that attention may modulate neural processes that are entirely separate from those supporting conscious perception: sensorimotor mechanisms that do not create awareness however much they are enhanced. Here we provide evidence in support of this second hypothesis by showing that attentional cueing can modulate the behavioral response to invisible stimuli in a way that is distinct from enhancing their visibility. We used a masked-prime paradigm that produces a negative or positive compatibility effect depending on the perceptual strength (duration or brightness) of the prime. We found that attention enhanced the effect of both visible and invisible primes and also increased the likelihood of detecting the prime (i.e., boosted perceptual strength). Crucially, the pattern of attentional influence on priming could not be explained by attentional modulation of the prime's perceptual strength but was predicted by a direct attentional influence on the nonconscious priming process itself. Therefore, in addition to regulating what we perceive, attention seems to influence our behavior through sensorimotor processes that are not involved in conscious awareness.

Attention↗

PubMed2006

Space re-exploration in hemispatial neglect.

Exploration of the space around us is a fundamental part of human behaviour. When it breaks down there is an important opportunity to understand its underlying mechanisms. Here we show that many right-hemisphere patients with left neglect re-explore rightward locations, failing to keep track of them during search. Importantly, such re-exploration occurred despite leftward stimuli being indistinguishable in peripheral vision, so it is unlikely to result from implicit processing of neglected targets. Revisits generally occurred after visits to other targets and are therefore not immediate perseverations. Finally, manipulating the visual salience of found targets altered the degree of neglect, but not revisit rates. Space exploration appears to be modulated both by the ability to keep track of spatial locations and by stimulus salience.

Aged↗

PubMed2006

Disorders of visual attention and the posterior parietal cortex.

Traditionally, both the monkey and human posterior parietal cortex (PPC) have been considered to have a privileged role in spatial perception or action. Lesions to this region of the human brain, particularly of the right hemisphere, undoubtedly lead to spatially lateralised deficits such as visual extinction or neglect. However, although studies in monkeys have revealed much about the spatial functions of the parietal lobe, the monkey PPC may not be a good model system with which to understand fully the disorders of attention that follow damage to the human parietal cortex. Several lines of evidence, from functional imaging as well as investigations of patients with parietal damage, demonstrate that parts of the human inferior parietal lobe (IPL) have non-spatial functions. Here, we argue that it is important to distinguish spatially lateralised from spatial deficits. Both spatial and non-spatial impairments might, in principle, contribute to a spatially lateralised behavioural syndrome such as neglect. In this review, we discuss the evidence for such a proposal and suggest that a better understanding of human parietal syndromes may emerge from considering both the spatial and non-spatial functions of this region.

Animals↗

PubMed2006

Cognition and medial frontal cortex in health and disease.

PURPOSE OF REVIEW: Recent work on the role of the medial frontal cortex in cognition and its involvement in neurological disorders is critically reviewed. RECENT FINDINGS: The highly influential notion of conflict monitoring by the anterior cingulate has been called into question by monkey single-cell neurophysiology and lesion studies in monkeys and humans. An alternative role for this region in adapting behaviour in response to changing demands over time is gaining support. By contrast, the more dorsally placed presupplementary motor area and supplementary eye field have been implicated in direct executive control in situations of response conflict. Although more rostral medial areas have been linked to complex cognitive operations involving references to the self, conceptual obstacles make the evidence difficult to interpret. The role of the orbitofrontal cortex in guiding action based on value has been reinforced. SUMMARY: The role of the medial frontal cortex in cognition continues to generate both interest and controversy. A few striking discrepancies between data from functional imaging and interventional techniques illustrate the hazards of drawing strong conclusions from merely correlative evidence. More broadly, a case can be made for tempering the empirical enthusiasm here with a little more theoretical restraint.

Animals↗

PubMed2005

Which visual pathways cause fixation-related inhibition?

Visual stimuli can both inhibit and activate motor mechanisms. In one well-known example, the latency of saccadic eye movements is prolonged in the presence of a fixation stimulus, relative to the case in which the fixation stimulus disappears before the target appears. This automatic sensory-motor effect, known as the gap effect or fixation-offset effect, has been associated with inhibitory connections within the superior colliculus (SC). Visual information is provided to the SC and other oculomotor areas, such as the frontal eye fields (FEF), mainly by the magnocellular geniculostriate pathway, and also by the retinotectal pathway. We tested whether signals in these pathways are necessary to create fixation-related inhibition, by using stimuli invisible to them. We found that such stimuli, visible only to short-wave-sensitive cones (S cones), do produce fixation-related inhibition (including when warning effects were equated). We also demonstrate that this fixation-related inhibition cannot be explained by residual activation of luminance pathways and must be caused by a route separate from that of luminance fixation signals. Thus there are at least two routes that cause fixation-related inhibition, and direct sensory input to the SC or FEF by the magnocellular or retinotectal pathways is not required. We discuss the implications that there may be both cortical and collicular mechanisms.

Adolescent↗

PubMed2005

Volition and conflict in human medial frontal cortex.

Controversy surrounds the role of human medial frontal cortex in controlling actions. Although damage to this area leads to severe difficulties in spontaneously initiating actions, the precise mechanisms underlying such "volitional" deficits remain to be established. Previous studies have implicated the medial frontal cortex in conflict monitoring and the control of voluntary action, suggesting that these key processes are functionally related or share neural substrates. Here, we combine a novel behavioral paradigm with functional imaging of the oculomotor system to reveal, for the first time, a functional subdivision of the pre-supplementary motor area (pre-SMA) into anatomically distinct areas that respond exclusively to either volition or conflict. We also demonstrate that activity in the supplementary eye field (SEF) distinguishes between success and failure in changing voluntary action plans during conflict, suggesting a role for the SEF in implementing the resolution of conflicting actions. We propose a functional architecture of human medial frontal cortex that incorporates the generation of action plans and the resolution of conflict.

Adult↗

PubMed2004

Distinct cortical and collicular mechanisms of inhibition of return revealed with S cone stimuli.

Visual orienting of attention and gaze are widely considered to be mediated by shared neural pathways, with automatic phenomena such as inhibition of return (IOR)--the bias against returning to recently visited locations--being generated via the direct pathway from retina to superior colliculus (SC). Here, we show that IOR occurs without direct access to the SC, by using a technique that employs stimuli visible only to short-wave-sensitive (S) cones. We found that these stimuli, to which the SC is blind , were quite capable of eliciting IOR, measured by traditional manual responses. Critically, however, we found that S cone stimuli did not cause IOR when saccadic eye movement responses were required. This demonstrates that saccadic IOR is not the same as traditional IOR, providing support for two separate cortical and collicular mechanisms of IOR. These findings represent a clear dissociation between visual orienting of attention and gaze.

Adult↗