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Christopher D. Chambers
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Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2014) 26 (7): 1507–1518.
Published: 01 July 2014
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Following damage to the primary visual cortex, some patients exhibit “blindsight,” where they report a loss of awareness while retaining the ability to discriminate visual stimuli above chance. Transient disruption of occipital regions with TMS can produce a similar dissociation, known as TMS-induced blindsight. The neural basis of this residual vision is controversial, with some studies attributing it to the retinotectal pathway via the superior colliculus whereas others implicate spared projections that originate predominantly from the LGN. Here we contrasted these accounts by combining TMS with visual stimuli that either activate or bypass the retinotectal and magnocellular (R/M) pathways. We found that the residual capacity of TMS-induced blindsight occurs for stimuli that bypass the R/M pathways, indicating that such pathways, which include those to the superior colliculus, are not critical. We also found that the modulation of conscious vision was time and pathway dependent. TMS applied either early (0–40 msec) or late (280–320 msec) after stimulus onset modulated detection of stimuli that did not bypass R/M pathways, whereas during an intermediate period (90–130 msec) the effect was pathway independent. Our findings thus suggest a prominent role for the R/M pathways in supporting both the preparatory and later stages of conscious vision. This may help resolve apparent conflict in previous literature by demonstrating that the roles of the retinotectal and geniculate pathways are likely to be more nuanced than simply corresponding to the unconscious/conscious dichotomy.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2013) 25 (3): 465–483.
Published: 01 March 2013
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Neuropsychological, neurophysiological, and neuroimaging studies suggest that right frontoparietal circuits may be necessary for the processing of mental number space, also known as the mental number line (MNL). Here we sought to specify the critical time course of three nodes that have previously been related to MNL processing: right posterior parietal cortex (rPPC), right FEF (rFEF), and right inferior frontal gyrus (rIFG). The effects of single-pulse TMS delivered at 120% distance-adjusted individual motor threshold were investigated in 21 participants, within a window of 0–400 msec (sampling interval = 33 msec) from the onset of a central digit (1–9, 5 excluded). Pulses were delivered in a random order and with equal probability at each time point, intermixed with noTMS trials. To analyze whether and when TMS interfered with MNL processing, we fitted bimodal Gaussian functions to the observed data and measured effects on changes in the Spatial–Numerical Association of Response Codes (SNARC) effect (i.e., an advantage for left- over right-key responses to small numbers and right- over left-key responses to large numbers) and in overall performance efficiency. We found that, during magnitude judgment with unimanual key-press responses, TMS reduced the SNARC effect in the earlier period of the fitted functions (∼25–60 msec) when delivered over rFEF (small and large numbers) and rIFG (small numbers); TMS further reduced the SNARC effect for small numbers in a later period when delivered to rFEF (∼200 msec). In contrast, TMS of rPPC did not interfere with the SNARC effect but generally reduced performance for small numbers and enhanced it for large numbers, thus producing a pattern reminiscent of “neglect” in mental number space. Our results confirm the causal role of an intact right frontoparietal network in the processing of mental number space. They also indicate that rPPC is specifically tied to explicit number magnitude processing and that rFEF and rIFG contribute to interfacing mental visuospatial codes with lateralized response codes. Overall, our findings suggest that both ventral and dorsal frontoparietal circuits are causally involved and functionally connected in the mapping of numbers to space.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2006) 18 (3): 444–455.
Published: 01 March 2006
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In the course of daily living, humans frequently encounter situations in which a motor activity, once initiated, becomes unnecessary or inappropriate. Under such circumstances, the ability to inhibit motor responses can be of vital importance. Although the nature of response inhibition has been studied in psychology for several decades, its neural basis remains unclear. Using transcranial magnetic stimulation, we found that temporary deactivation of the pars opercularis in the right inferior frontal gyrus selectively impairs the ability to stop an initiated action. Critically, deactivation of the same region did not affect the ability to execute responses, nor did it influence physiological arousal. These findings confirm and extend recent reports that the inferior frontal gyrus is vital for mediating response inhibition.